the ecological planet - an introduction to earth’s major ecosystems (booklet)

Professor John KricherWHEATON COLLEGE

THE ECOLOGICAL

PLANET:AN INTRODUCTION TO

EARTH’S MAJOR ECOSYSTEMS

COURSE GUIDE

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The Ecological Planet:An Introduction to Earth’s Major Ecosystems

Professor John KricherWheaton College


Professor John Kricher

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COURSE GUIDE

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Lecture content ©2008 by John Kricher

Course guide ©2008 by Recorded Books, LLC

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All beliefs and opinions expressed in this audio/video program and accompanying course guideare those of the author and not of Recorded Books, LLC, or its employees.

Course Syllabus


About Your Professor/Introduction ...............................................................................4

Lecture 1 Ecology and the Big Picture ..................................................................6

Lecture 2 Earth and the Goldilocks Effect ...........................................................10

Lecture 3 Distribution of Global Ecosystems.......................................................15

Lecture 4 Climate and Ecology............................................................................19

Lecture 5 Biogeography and Evolution................................................................23

Lecture 6 Polar Ecosystems and Tundra ............................................................28

Lecture 7 Boreal Forest .......................................................................................33

Lecture 8 Temperate Deciduous Forest ..............................................................39

Lecture 9 Grassland and Savanna ......................................................................44

Lecture 10 Desert ..................................................................................................50

Lecture 11 Tropical Rain Forest ............................................................................55

Lecture 12 Marine Ecosystems .............................................................................61

Lecture 13 Unique Coastal Ecosystems................................................................67

Lecture 14 Current Issues in Global Ecology ........................................................75

Course Materials ........................................................................................................80

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John Kricher is a professor of biologyat Wheaton College, Norton,Massachusetts. His books includeGalapagos: A Natural History, ANeotropical Companion, three ecologyfield guides (Eastern Forests; RockyMountain and Southwestern Forests;and California and Pacific NorthwestForests), and First Guide to Dinosaurs.John is a fellow in the American

Ornithologists Union and past-president of both the Association of FieldOrnithologists and Wilson Ornithological Society. He resides with his wifeMartha Vaughan on Cape Cod.

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About Your Professor

John Kricher

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IntroductionEarth is the only known ecological planet, a place with life. But life is diverse:

plants, animals, microbes, millions of species, many interacting in complexways and all of them influenced in myriad ways by the environments in whichthey are found. Ecology is the study of organisms as they relate to their envi-ronments, the scientific study of natural history. But what does that reallymean? It means that because of the many physical, chemical, and atmos-pheric characteristics of Earth, diverse forms of life have evolved throughoutthe history of the planet, life that is shaped and reshaped by both physicaland biotic forces. Life is forced to evolve because of Earth’s diverse condi-tions and the fact that, over time, conditions change. Ecology is the searchfor broad general patterns evident in the distribution and abundance of life.Further, ecology attempts to explain such patterns with empirical reasoning.Ecology, a word taken from the Greek oikos, meaning home, is the same rootas the word economics. Like economics it studies complex systems, in thiscase ecosystems. Ecology is the broadest level of organization in biology, thediscipline that deals with interactions among populations, ecological commu-nities, and ecosystems.

Earth is a planet with a complex climate, one that varies dramatically fromequator to poles. As a consequence, life-forms adapted to polar regions arenot well suited to equatorial areas and thus climate alone forces a high diver-sity to evolve among Earth’s numerous and variable organisms. Polar bears,for all their magnificent adaptations, are not adapted to survive in tropicalconditions and the diverse array of trees comprising equatorial tropical rainforests would not long survive at higher latitudes.

This course explores exactly why Earth supports life. It examines the variousreasons why a small planet of about eight thousand miles diameter orbiting anaverage-sized star is uniquely suited to have a biosphere, a thin layer of living

matter surrounding itssurface. Life abounds onboth land and oceans buttakes many differentforms, mostly becauseclimate is so variable.

Beginning at the polarregions, the cold wind-swept Arctic andAntarctic, ProfessorKricher will devotelectures to each of theEarth’s major ecosys-tems, called biomes.These include the vari-ous major forest types,the northern coniferous(or boreal) forests thatsurround the higher lati-tudes like a vast belt of spruce and fir trees, the immense temperate decidu-ous forests of North America, Europe, and Asia, and the tropical rain forest,the ecosystem with more species than any other.

When moisture is insufficient to support forest, other kinds of biomes occur.These include grassland, called prairie in North America, as well as savanna,a combination of grassland and scattered trees that typifies much of EastAfrica. The driest of all biomes is desert, which is always water stressed butmay be hot or cold depending on where it is located.

The largest ecosystems on Earth are marine, the oceans and coastal areaswhere land meets sea. The ocean realm has ecosystems that differ by depth.The open ocean or pelagic zone is where the food webs of the sea begin,where billions of tiny plants, the phytoplankton, capture some of the Sun’senergy and thus support all of the other creatures of the seas. But most ofthe ocean below about six hundred feet is cold and dark, and the benthiczone, or sea bottom, is lit only by the bioluminescence of the life-forms thatinhabit it.

Coastal ecosystems such as salt marsh, mangrove swamp, and coral reefsare among the most ecologically valuable as they are highly productive. Atthe same time, they are among the most threatened.

The final lecture in the course will focus on how ecology has matured into apragmatic science that is as essential as economics for making sound judg-ments about how best to steward the ecological planet.

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cology is the scientific study of how life-forms interact and coexist.It is concerned not only with how living things adapt to each other,

but also how they interact with and adapt to the nonliving compo-nents of the environment. Ecology is often defined simply as thestudy of organisms as they relate to their environments. Another

definition is that ecology is the study of how various factors in theenvironment affect the distribution and abundance of organisms. Those “fac-tors” are divided into two broad categories, abiotic and biotic. Abiotic factorsare such things as temperature, precipitation, phosphorus, oxygen, salinity,and fire. Biotic factors are interactions such as predation, parasitism andpathogens, competition for resources, and mutually beneficial interactions.

Ecology is taken from the Greek word oikos, meaning “home,” the same rootfrom which the word “economics” is derived. Both disciplines, ecology andeconomics, study how things relate within complex systems. Economistsstudy the flow of money through economic systems as well as how materialsmove in the form of goods and services. Ecologists focus on energy flowthrough ecosystems and study how atoms combine and recycle as life usesenergy and material to maintain itself. The word “ecology” was coined in 1866by Ernst Haeckel. He was attempting to form a word describing the “economyof nature” that Charles Darwin described in his monumental book On theOrigin of Species (1859).

LECTUREONE

The Suggested Reading for this lecture is John Kricher’s A Field Guideto Eastern Forests (chapters 1 and 2).

Lecture 1:Ecology and the Big Picture

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Wildflowers at the edge of a wooded field.

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Nothing in ecology makes sense without environmental context. Forinstance, you can watch a kangaroo in a zoo cage but you will not be observ-ing ecology. Kangaroos live in Australia, in open grassland and savannahabitats. To really learn about the ecology of a kangaroo, it is necessary tostudy the creature in the wild, in the Australian outback where groups of kan-garoos eat, groom, defend their territories, mate, and reproduce. So it is withall other animals and plants as well. Ecology looks at living things in the con-text of the environment for which each is adapted.

Ecology is often described simply as the scientific study of natural history, itsroots reaching back to the early writings of the ancient Greek scholars whowere curious about such questions as why fish look as they do, have scales,and swim in water, while birds look quite different, have feathers, and fly inair. But ecology is far more than descriptive natural history. It encompassesbroader, more deeply penetrating questions. The living world does not yieldits secrets easily. Ecology is the science that attempts to uncover patterns innature and then to discover causal explanations for such patterns, explana-tions that account for the distribution and abundance of plants, animals, andmicrobes, and predict how various factors effect changes in such patterns.

Ecologists study the “big picture” within the biological sciences: how anopen field of grasses and wildflowers can, with time, develop into a closed,shadowy forest; why large predatory animals such as jaguars or great whitesharks are so much rarer than their prey species; why the loss or gain of buta single species in a habitat can radically affect and alter that habitat whileother species come or go with little, if any, discernible effects; why somespecies are abundant and broadly distributed and others are very local andrare; why some regions of the Earth are covered by rain forest while othersare deserts; how essential minerals such as phosphorus and nitrogen movefrom the nonliving to the living components of what we call “the environ-ment.” Ecologists today are concerned with such major global questionsas the potential for climate alteration, the complex effects caused by pollu-tants, the increasing prevalence of invasive species, and the decline ofglobal biodiversity.

At the base of ecological study is the organism. Ecologists study organismsin the context of both their present environment and their evolutionary histo-ries, meaning how they are adapted to survive within their environments.Organisms of the same species in nature are grouped into organizationalunits called populations; thus ecologists speak of the grizzly bear populationat Denali National Park in Alaska or the right whale population in the Gulf ofMaine in the Atlantic Ocean. When the focus is on questions pertaining towhole populations, the term “population biology” is often used. But no naturalenvironment consists of but a single kind, a single species of organism. Thusvarious populations, called an ecological community, coexist within the samelocality. Grizzly bears, along with caribou, Dall’s sheep, and arctic groundsquirrels, are each part of the arctic tundra animal community in Denali. Rightwhales, along with various oceanic birds such as gannets and greater shear-waters, plus other whale species such as humpback and minke whales, alongwith numerous fish species, oceanic invertebrates, and tiny plants called phy-toplankton, are all part of the Atlantic pelagic (open sea) community.

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Grizzly bear near a streambed in the Denali National Park, Alaska.

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Ecological communities, by necessity, interact with the nonliving, or abioticenvironment, the air, water, and substrate. The combination of the living, thebiotic, with the abiotic components of any habitat forms the most encompass-ing level of organization in the life sciences. The interactive associationbetween a community of organisms and their physical and chemical environ-ment is called an ecosystem.

When ecology was in its infancy it was described as physiology (alreadyan established laboratory science) applied to the environment, an attemptto explain how organisms function in an adaptive manner that permits someto survive in freezing cold and others to thrive in blazing heat. The ecosys-tem concept, first clearly articulated in 1935, was formulated to show thatthe interactions of multiple organisms with the physical environment inany given region forms a dynamic system worthy of its own study as alevel of organization.

Ecosystem boundaries are often fuzzy: a decaying log in a woodlot is anecosystem, but on a larger scale, so is the woodlot in which the log is found,and on a larger scale still, so too is the regional forest of which the woodlotforms but a part. The largest ecosystem known is called the biosphere orecosphere (the terms mean the same thing), the thin layer (perhaps twenty-five kilometers) of living matter that inhabits the crust and atmosphere of theEarth, thus far the only known area in the Universe to have an ecology, whichbrings us to the next lecture.

1. Is ecology the same thing as natural history study? How is ecology unique?

2. What are the differences among populations, communities, and ecosystems?

Kricher, John. A Field Guide to Eastern Forests. Boston: Houghton MifflinCompany, 1998.

Attenborough, David. The Living Planet. Boston: Little, Brown, 1984.

Bates, Marston. The Nature of Natural History. Princeton: PrincetonUniversity Press, 1990.

Wilson, Edward O. The Diversity of Life. New York: W.W. Norton, 1992.

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Suggested Reading

FOR GREATER UNDERSTANDING

Other Books of Interest

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The Suggested Reading for this lecture is Peter D. Ward and DonaldBrownlee’s Rare Earth.

Lecture 2:Earth and the Goldilocks Effect

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nce upon a time a little girl named Goldilocks was very selectiveabout eating porridge. It had to be “just right,” not too hot andnot too cold. Astrobiologists like to compare Earth with thefamous porridge of Goldilocks. Its characteristics, outlined in thislecture, make it “just right” to support life.

Earth is 4.5 billion years old, nearly as old as the Sun. Earthformed along with the rest of the Solar System, the other planets, comets,and asteroids. The universe itself began about 13.8 billion years ago with anevent physicists call a “singularity,” more popularly called the “Big Bang.”

Earth is the third planet from the Sun, about 93 million miles away. The Sun’scharacteristics, particularly its age and temperature, make Earth suitable forliquid water, the key to the origin of life. It is difficult to form scenarios for life’sevolution or for its persistence without liquid water because cells, which makeup organisms, are mostly composed of water and all biochemical reactionsoccur in aqueous solution. While water may have once flowed on the planetMars, it seems to be absent now and Mars appears to be a lifeless desert.

Evidence from a variety of sources suggests that Jupiter’s moon Europamay contain a liquid ocean, perhaps salty, beneath a dense overtopping layerof ice evident on the moon’s surface. What might be crawling or swimmingaround in such an ocean?

Earth’s only natural satellite, the Moon (the name is derived from the Greekfor menstrual cycle), is unusual as planetary satellites go and its size and

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presence may contribute to why life exists on Earth. It is proportionally largerelative to its planet, indeed, proportionally the second largest moon in theSolar System. Earth and the Moon are unusual in that astronomers character-ize them as virtually a biplanetary system, given the large proportional size ofthe Moon to the Earth. Thus, given Newton’s insights regarding gravity, theMoon has a strong effect on the Earth.

What if Earth had no moon? Yes, there would be no moonlit nights, manyromantic songs would not have been written, eclipses would not occur, andwolves would have nothing to howl at. But the consequences of moonless-ness could be more ecologically profound.

The Moon, which today is on average 238,860 miles (384,400 km) fromEarth, was considerably closer to Earth when it formed, though exactly howclose is a matter of conjecture. Today the Moon is becoming more distantfrom Earth, receding at about three centimeters annually. But the proximity ofthe Moon to the Earth, and the proportionally large size of the Moon to theEarth, means that throughout its existence, the Moon has exerted a stronggravitational effect on its planet. Most of us realize that Earth’s tidal cyclesare caused mostly by the influence of the Moon. Given that life may haveoriginated in conditions prevalent in tide pools and other coastal environ-ments, the Moon may have indirectly contributed to the first appearance oflife on the planet.

What is generally less well known, and what may be more important, is thatthe Moon likely stabilized the tilt of the Earth in space, what astronomers callEarth’s obliquity. If the Earth’s obliquity had undergone numerous substantialchanges, making the planet basically “wobble” unpredictably, Earth’s climatewould have undergone far more frequent, severe alterations, possibly toosevere to permit complex multicellular life to evolve. Our Moon’s gravitational“calming” effect on Earth may have been of utmost importance to its futureinhabitants. The Moon, a lifeless place, may have helped make life more pos-sible on its larger neighbor.

Earth has a strong magnetic field (the Van Allen belts) and that too is impor-tant for sustaining life. Earth is constantly being bombarded by potentiallyharmful radiation from space, most of it from the Sun, but also cosmic raysfrom space. The Sun emits what is called the “solar wind,” and radiation of thissort could certainly be harmful to life, particularly large multicellular life-forms.But we exist happily along with elephants and redwood trees, so how are weprotected from cosmic rays and the solar wind? The answer is that Earth gen-erates a strong magnetic field, called the magnetosphere. Like flak jacketsaround the planet, the belts intercept cosmic rays from space and solar windparticles from the Sun, affording a magnetic blanket of protection to Earth.

The huge planet Jupiter also helps protect Earth and thus helps Earth sup-port life. Jupiter is immense in comparison not only with Earth but with virtual-ly all other planets in the Solar System. Its huge mass means that Jupiterexerts a very strong gravitational attraction on things that pass reasonablyclose to it, things like asteroids, comets, and meteors. There are manyobjects in space that have trajectories that occasionally cross the orbital pathof Earth. The last of the dinosaurs, as well as many other life-forms, were vic-tims of one of these objects at the close of the Mesozoic Era. There have

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been other impacts on Earth, some major, both before and since the one thatended the Mesozoic Era. But it could have been worse and, to the degreethat it wasn’t, we probably have Jupiter to thank.

The presence of a large planet such as Jupiter likely reduces the number ofpossible collisions of Earth with potentially catastrophic space debris. Jupiteracts as a kind of cosmic “vacuum cleaner,” sweeping the inner Solar Systemof potentially dangerous objects. While it is true that previous devastatingevents on Earth have been followed by the evolution of new life-forms, ifmassive collisions were to be far more frequent, such calamities could pre-vent the formation of complex ecological communities.

Earth’s ecology is strongly seasonal. We take the seasons pretty much forgranted. Summer, fall, winter, spring: each is a part of our year. Even some-one with the most casual understanding of the natural world knows that therhythms of nature are closely attuned to seasonal cycles. Most ecological pat-terns on Earth are affected in some way by the existence of seasons. It isthus fair to conclude that seasonality has been an important determinant ofthe ecosystem and biodiversity patterns of the planet.

So why does Earth haveseasons? Earth is tilted23 degrees and 27 minutes,or 23.45º in its orbit. Planetsvary in their axial tilts. Axialtilt causes seasons. AsEarth progresses around itsorbit its tilt places theNorthern Hemispheretoward the Sun for part ofthe orbit while, at the sametime, the SouthernHemisphere points awayfrom the Sun. When this sit-uation occurs, more directsolar radiation falls onnorthern latitudes thansouthern latitudes. North ofthe equator, days are long,while south of the equator,days are short. It is thenorthern summer and the southern winter. During the northern winter the sit-uation is, of course, exactly reversed. More light falls on Buenos Aires thanon Minneapolis. Snow falls on Minneapolis.

Seasonal changes of Earth cause latitudinal differences in day length, affectpatterns of temperature and precipitation, and are, indeed, the major determi-nant of climate on the planet. These patterns are at the very essence of howecosystems function, a force profoundly influencing how species adapt. Thinkabout the annual migration of thousands of bird species around the globe aswell as overland migrations of caribou, wildebeest, and so many others. Thinkabout the amazing physiological changes that result in such events as fall

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The tilt of the Earth.

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color changes in leaves and hibernation in animals. Each of these elegantecological patterns, and many more, occurs because of a 23.5º axial tilt. It isindeed a significant tilt.

The position of Earth’s continents and the fact that their positions havechanged throughout history also has a strong effect on Earth’s ecology andthe evolution of organisms. Look at a map showing the positions of theoceans and the various landmasses. Look at South America and notice howthe East Coast of South America looks almost as though it could fit ratherwell against the West Coast of Africa. If the globe shows relief, notice howthe Himalayan Mountains form a rugged boundary at the border of India andAsia. Australia sits by itself, alone, an immense island continent in the south-ern Pacific Ocean.

The occurrence of cataclysmic events such as earthquakes and volcaniceruptions informs us that Earth’s geology is active. Unlike Earth’s Moon, orplanets such as Venus and Mars, Earth is dynamic, continuously rearrangingits surface because of processes occurring in its interior. One of the greatscientific discoveries of the twentieth century was that the crust of Earthitself is changeable, and that as it changes the continents that rest atop itactually move in relation to one another. The term for this process of changeis plate tectonics.

Continental movement caused by plate tectonics has mighty consequencesfor Earth’s ecology. Without such constant movement, Earth’s climatic historywould have been very different and less variable. As the continents moveabout the surface of the planet they effect changes in ocean currents, air cur-rents, and climate in general. As more coastline is exposed, coastal shallow-water species such as corals tend to proliferate. But when coastline is mini-mized, as when continents fuse, extinctions of such organisms seem common.

Separation of the continents acts to geographically separate organisms,stimulating evolutionary change, allowing evolution to proceed along variedand different pathways from continent to continent, island to island. The isola-tion of Australia, for example, led to the diversification of marsupial mammals,making Australia unique as the “land of marsupials.” Likewise, eucalyptustrees of over six hundred species occur in Australia and nowhere else(except when transplanted). Part of the great biodiversity of mammalsthroughout the Cenozoic may be attributable to continental separation stimu-lating high levels of speciation among groups isolated from one another.

The Earth has changed continuously and dramatically since its origin, anevolving planet in an evolving universe. The scale of change has varied bothwith time and in area, but any careful consideration of the history of life onEarth shows that change is the rule, not the exception. Life exists onlybecause it has the potential to change as circumstances around it change.

The tapestry through time that has been woven by life on Earth has generat-ed many millions of species, a temporal montage of fascinating ecosystems,most of which have forever been relegated to history.

1. Why might life be rare in the solar system, confined essentially to Earth?

2. What major characteristics of Earth are essential for supporting life?

Ward, Peter D., and Donald Brownlee. Rare Earth. New York: Copernicus, 2000.

Upgreen, Arthur. Many Skies: Alternative Histories of the Sun, Moon, Planets,and Stars. New Brunswick, NJ: Rutgers University Press, 2005.

Ward, Peter D., and Donald Brownlee. The Life and Death of Planet Earth:How the New Science of Astrobiology Charts the Ultimate Fate of OurWorld. New York: Owl Books, 2004.

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uppose you want to see all ten thousand or so of the world’sextant bird species. What would you have to do? In a word, trav-el. You would look for parrots in the tropics and penguins in theAntarctic oceans. And those are simply two extremes. Earth’sdistinct seasonality combined with ever-drifting continents over

millions of years has resulted in widely separat-ed landmasses where unique organisms have evolved tosurvive in varying climates.

The result of this reality is that ecosystems vary dramati-cally from one point in the world to another. The goal of thislecture will be to provide an overview of ecosystem distribu-tion and discuss how ecological processes vary from oneregion to another.

We begin with the most obvious characteristic of ecosys-tem distribution, which is that Earth has two major kinds ofecosystems, aquatic and terrestrial. The seas are vast butthey do not encompass the entire planet. This is no smallpoint. Because continents and islands exist, the evolution oflife on Earth evolved to be far more diverse than it wouldhave been had life been confined entirely to the seas.Consider insects, for example. Though just under a millioninsect species have been described, it is estimated thatbetween two and thirty million actually exist. Regardingbeetles alone (order Coleoptera) there is thought to be overa million species. Insects have a strong influence onnumerous terrestrial ecosystems, ranging from plague pro-portions in the case of outbreaks of locusts, to disease vec-tors, to essential plant pollinators. But there are no insectsin the oceans. Insects are terrestrial. They evolved fromearly terrestrial arthropods and today, as in the past, repre-sent the most abundant kind of organism on our planet.Without land, there would be no insects, and thus the mostdiverse kind of animal ever evolved would not exist.

The Suggested Reading for this lecture is Robert G. Bailey’s Ecoregions:The Ecosystem Geography of the Oceans and Continents.

Lecture 3:Distribution of Global Ecosystems

Meet the Beetles

Pictured at the right are five exotic members of the more than 30,000 speciesbelonging to the family scarabaeidae (scarab beetles). These specimens,found in Latin America, are from the subfamily rutelinae (shining leaf chafers). ©

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On the other hand, the most species-rich group of vertebrates is the groupknown as Osteichthyes, the bony fish. While some of these inhabit freshwater, and thus are in a way “terrestrial,” most are marine and there aresome twenty thousand known species. The vastness of the oceans hasserved to permit much speciation, and fishes are an example. The next mostdiverse group of vertebrates, as mentioned above, are the ten thousand birdspecies. There are about forty-five hundred mammal species. In marinewaters, the equivalent of insects is the Crustacea, the lobsters, crabs, andshrimp. But they are collectively far less diverse than insects, with about thirtythousand species, including some that inhabit fresh water.

The 23.5º tilt of Earth’s axis, responsible for its seasonality, combined withthe scattered positions of the continents, makes Earth’s climate quite variablefrom poles to equator. For example, Earth would be more temperate in gener-al was it not for the fact that the continent of Antarctica is directly over theSouth Pole. When Antarctica drifted to that position about 36 million yearsago, during the Tertiary period, it resulted in the world becoming more temper-ate, with areas of forest being gradually replaced by grassland and savanna.

We can generalize by identifying three broad climatic regions: polar (Arcticand Antarctic), temperate, and tropical. Of course these regions are notsharply separated but tend to meld together where they overlap. An exampleis southern Florida, where it is best to describe the climate as subtropical.

Physical conditions of climate, particularly temperature and patterns of pre-cipitation, largely determine characteristics of terrestrial ecosystems.Ecologists have long understood that if you know the average temperatureand average amount of annual precipitation, you get an accurate predictor ofthe kind of ecosystems that will dominate in that region. For example, aregion that experiences less than ten inches of rainfall annually will be desert.If that region is warm, it will be hot desert, usually dominated by various suc-culent shrubs and cactuses. But if the region experiences a cold winter,including regular snowfall, then it will be cold desert, dominated by cold-adapted shrubs such as sagebrush. The coldest of the desert-type ecosys-tems is so cold that we recognize it as unique, the tundra. True tundra isfound only in polar regions and atop high mountains.

Major terrestrial ecosystems are called biomes. Inside the Arctic Circle is theArctic Tundra biome, a vast ecosystem of sedges and lichens and numeroustiny flowering plants. This is the land of caribou and lemmings. Tundra is notcommon in Antarctica, a land more cold and desolate but with a thrivingmarine ecosystem offshore.

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Caribou graze on the Alaskan tundra.

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The temperate zone begins beyond the Arctic and Antarctic circles. In thenorth it begins with the Boreal Forest biome that extends like a ring ofspruce and fir trees around the globe, encompassing much of Canada,northern Europe, and Siberia. It is also found along mountain ranges, par-ticularly in the American West. This biome is dominated by cone-bearingconiferous trees, most of which are evergreen. It is often called the “spruce-moose” biome.

The next major biome is Deciduous Forest, comprising broad-leaved treesthat typically drop their leaves before the cold of winter sets in. The autumnalchanging colors of the leaves makes this biome one of the most aesthetic ofnature’s ecosystems. Much of eastern North America as well as Europe andparts of China are dominated by deciduous forests of oaks, maples,sycamores, and elms, as well as numerous other trees and shrubs.

The third major forest biome is tropical and it consists of a range of foresttypes from dry woodlands such as typify much of the Australian outback, torich Tropical Rain Forest such as is found throughout the Amazon Basin, aswell as parts of central Africa, Asia, Indonesia, and northeastern Australia.The Tropical Rain Forest is characterized by having more species of plantsand animals per square hectare than any other of the world’s terrestrial bio-mes. Tropical Rain Forest is equatorial, and occurs between the Tropic ofCancer and Tropic of Capricorn, encompassing about a 47º latitudinal band.

There are also biomes of grassland, savanna, and desert. These occur inareas with less precipitation or much more seasonal precipitation than is typi-cal of forest biomes. Grassland once typified the American prairie, but muchof the original prairie is now converted to agriculture. The steppes of Russiais another region dominated by mixed species of grasses. Savanna, which iswell developed in much of Africa and Australia, consists of grassland withvarious amounts of trees scattered within. Deserts are found where moistureis least available and are usually dominated by shrubs. Hot deserts have suc-culents, such as cactuses in the Western Hemisphere and euphorbias (whichclosely resemble cactus) in places such as Africa.

Fire-adapted shrubs and highly seasonal precipitation characterize a biomecalled Chaparral, which is found in much of central and southern California,Chile, and throughout the Mediterranean region.

The biome concept is normally not applied to the world’s oceans. Rather,their life zones differ by depth and proximity to land. For example, the littoralzone is found near coasts, where water is shallow. The pelagic zone is thearea of deep open ocean and the benthic zone is the life zone at the depthsof the seas, on the bottom of the ocean floor.

Why study biomes? Because they demonstrate how plants and animals areadapted for various kinds of climates. And because the amount of the Sun’senergy captured by vegetation differs remarkably from one biome to another.Since virtually all life on Earth depends on photosynthesis, such differences,and the reasons for them, are important. Climate and its effects on ecosys-tems will form the subject of the next lecture.

1. Why is the fact that Earth has continents important in the biodiversity of lifeon the planet?

2. What is a biome? Name the major terrestrial biomes.

Bailey, Robert G. Ecoregions: The Ecosystem Geography of the Oceans andContinents. New York: Springer, 1998.

Aber, John D., and Jerry M. Melillo. Terrestrial Ecosystems. San Diego:Harcourt Academic Press, 2001.

Barbour, Michael G., and William Dwight Billings. North American TerrestrialVegetation. Cambridge: Cambridge University Press, 1988.

Walter, Heinrich. Vegetation of the Earth: In Relation to Climate and theEco-physiological Conditions. New York: Springer-Verlag, 1973.

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hen I was a student in high school I was assigned TheGrapes of Wrath, the classic novel authored by JohnSteinbeck. It is a poignant tale of the Joad family and theirdaunting trek across 1930s America, following Route 66 from

dust-strickened Oklahoma to what they hoped would be thepromised land of southern California. The Dust Bowl, a term

describing the effects of a protracted and severe Midwestern drought, had aprofound effect on the ecology of the region. But why did it happen? In 2004,a team of researchers using a complex atmospheric-land general circulationmodel determined that before the onset of the drought, sea-surface tempera-tures both in the Pacific and Atlantic oceans had deviated significantly fromnormal: the Atlantic warmer than it should have been, and the Pacific cooler.In particular, the colder-than-normal Pacific sea-surface temperature alteredthe air circulation in the upper troposphere of the atmosphere such that rainfallwas suppressed throughout the Great Plains. The warmer-than-normal Atlanticsea-surface temperature created different conditions, but those resulted inblocking moisture from the Gulf of Mexico. The combination resulted in thesevere and protracted drought. Records from tree-ring data suggest that theGreat Plains has experienced similar droughts once or twice a century overthe past four centuries. Such a frequency is sufficient to impose significantecological stress on the ecosystems of the region. And that is why climate isthe number one factor in determining ecosystem characteristics.

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The Suggested Reading for this lecture is Heinrich Walter’s Vegetation ofthe Earth: In Relation to Climate and the Eco-physiological Conditions.

Lecture 4:Climate and Ecology

Dust storm approaching Stratford, Texas, April 18, 1935.

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The linkage between climate and ecology has long been understood. In Onthe Origin of Species Charles Darwin wrote, “Climate plays an important partin determining the average numbers of a species, and periodical seasons ofextreme cold or drought, I believe to be the most effective of all checks.” Thechecks to which Darwin referred were deaths of organisms that were notadapted to climatic shifts, the very essence of natural selection.

There are dust storms on Mars and immensely high winds on Jupiter. It canthus be said that both of those planets have a climate. Earth’s moon, in con-trast, has nothing happening above its rocky, dusty surface. It lacks a climatebecause it lacks an atmosphere. Climate results from the interaction of theatmospheric gases, whatever they might be, with solar radiation, landforms,and oceans. Because Earth has all of these things, climate is complex onEarth and strongly influences ecosystem composition.

Because Earth is a sphere, solar radiation strikes the planet most directlyat the equator and much more obliquely at the poles. Because of this fact,sunlight is more concentrated per unit area at the equator and thus thatregion of the world heats up more than regions to the north or south. Polarregions receive the least concentrated sunlight and thus are the coolestareas on the planet. The result of differential heating is the formation oflarge convection cells that characterize various latitudes. These cellsconverge in various places such as the Tropics of Cancer and Capricorn.Deserts are typical at latitudes 23.5º north and 23.5º south, becausewhere the convection cells converge, they are essentially devoid ofmoisture, having lost it before convergence.

The movement of convection cells as well as ocean currents is further com-plicated by the Earth’s rotation from west to east, a phenomenon known asCoriolis effect. A body on Earth moves most quickly at the equator, and as itmoves away from the equator it is thus deflected east or west as a result ofEarth’s rotation. If the object is moving north it will be deflected to the eastand, if it is moving south, to the west. This fact, for example, creates the gen-erally clockwise circulation of ocean currents in the Northern Hemisphere andthe counter-clockwise circulation in the Southern Hemisphere.

A look at the Earth from an ecological perspective will quickly reveal thatthere are several huge terrestrial areas on the planet that can be described inthe broadest sense: forests, deserts, grasslands, for example. A somewhatcloser look produces a finer resolution. Forests can be evergreen or decidu-ous, or a mixture of both kinds of trees, and trees may be predominantlybroad-leaved species or needle-leaved species. Grasslands can be inter-spersed with trees, in which case they are called savannas, or be composedentirely of herbaceous plants, grasses, and various wildflowers. In the UnitedStates, such grassland ecosystems are called prairies. Deserts are typicallyarid, often extremely so, with organisms such as succulent cacti and long-eared jackrabbits, whose very anatomies reflect obvious adaptations to envi-ronments where water is scarce and heat may be oppressive. Such large-scale ecosystems are called biomes and each is characterized by distinctivespecies of plants and animals. But what causes biomes?

The answer is climate. Biomes differ from one another primarily because ofclimatic variation from one place to another. Climate quite blindly selects for

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differing arrays of adaptations from one climatic region to another. Two vari-ables, temperature and precipitation, largely determine the type of biome.Amazingly, if one knows the average annual temperature and the averageannual amount of precipitation, one can predict with great accuracy just whattype of biome will be present.

For example, if the mean annual precipitation is somewhere between 300and 400 cm (118 to 157 inches) and the mean annual temperature is fromabout 20º to 30º C (68º to 86º F), the biome will be lush tropical rain forest.But if the mean annual precipitation is only, say, 100 cm, even if the meanannual temperature remains between 20º and 30º C, the biome will be savan-na. Deserts are uniformly arid, receiving less than 50 cm precipitation annual-ly, often much less. But some deserts are hot, as warm or warmer than rainforests while other deserts are cold, as cold as any place in the temperatezone. Arctic tundra, the realm of the caribou and snowy owl, is both dry andcold, so cold that permafrost (frozen soil) endures throughout the year.Tundra typically receives about 50 cm of precipitation per year and has amean annual temperature of only –10º C.

The importance of mean annual temperature and mean annual precipitationin determining which terrestrial biome will prevail in any one location was firstrevealed by Leslie R. Holdridge in 1947, who developed a system of ecologi-cal classification that he called “life zones.” The Holdridge system has passedthe test of time and is still used today to classify ecosystems.

Topological features such as mountains exert a strong influence on climate,creating what ecologists call rainshadows. Consider what happens in theCascade Mountains of Oregon, for example, if you were to travel from west toeast. You would begin in the cool, moist temperate rain forest of tall Douglas-firs and western redcedar trees, with a lush understory of bigleaf maple andvarious shrubs. In all likelihood, it would be raining. Ascending the mountainup its western slope, the trees would become shorter with increasing expo-sure to wind and cold. Spruces and firs would replace the temperate rain for-est, some stunted and twisted by exposure. Once on the eastern slope of themountain the predominant forest would be composed of ponderosa pines,and would be decidedly drier than the temperate rain forest. What is impor-tant is that the ponderosa pine forest occurs at exactly the elevation where,on the western slope of the mountain, there had been temperate rain forest.Finally, you descend into desert, the sagebrush desert of the Great Basin.

The marked difference between the west and east slopes of the mountain isdue to the fact that prevailing winds come from the Pacific Ocean, carryingevaporated moisture. As these winds encounter the tall Cascades, they areforced upward. As the air rises, it cools, condensing the moisture held withinand producing rainfall in generous amounts. This precipitation supports thelush temperate rain forest. By the time the wind has crossed over the peaksof the Cascades, it is largely depleted of its moisture. It is therefore not possi-ble to support temperate rain forest on the east slope, though there is suffi-cient moisture to support ponderosa pine forest. Finally, even this moisture isused and the air becomes so dry that only desert can occur.

In our next lecture we’ll see how separation of the continents, biogeography,also plays an essential role in global ecology.

LECTUREFOUR

1. What characteristics of Earth affect climate?

2. Why does climate vary with latitude?


Chapin, F. Stuart, Pamela A. Matson, and Harold A. Mooney. Principles ofTerrestrial Ecology. New York: Springer, 2002.

Odum, Eugene P., and Gary W. Barrett. Fundamentals of Ecology. 5th ed.Belmont, CA: Thomson, 2005.

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The Suggested Reading for this lecture is C. Barry Cox and Peter D.Moore’s Biogeography: An Ecological and Evolutionary Approach.

Lecture 5:Biogeography and Evolution

he famous ecologist G. Evelyn Hutchinson once authored abook with the intriguing title The Ecological Theatre and theEvolutionary Play. What he meant to convey was that an

understanding of the principles of organic evolution is prerequisiteto really comprehending the patterns evident in the world’s ecosys-tems. And it is essential to understanding that evolution results in

different organisms inhabiting different continents.

It is obvious that today’s world consists of several immense continents sepa-rated to various degrees by oceans. In addition to continents, there are numer-ous islands, ranging from large ones such as Madagascar and Borneo toextremely tiny islands whose names few would recognize. Some islands suchas South Georgia (near the Antarctic Circle) are rather isolated, while others,termed archipelagos, are in close proximity, such as the Hawaiian andGalapagos islands. Each continent and each island contains unique species,some of which migrated and colonized, and some of which evolved there. Thestudy of biogeography compares various regions of Earth that are geographi-cally distinct and examines the processes responsible for the differences. Insome cases, such as the famous Darwin’s finches of the Galapagos Islands,organisms evolve and are unique to a single region. These sorts of speciesare termed endemic. In other cases,such as the cattle egret, which colo-nized the Americas from Africa,species are recognized as invasive.In either case, they become part ofthe ecology of the region.

Consider a walk in a rain forestalong the Napo River in easternEcuador compared with a similarwalk in a rain forest in Queensland,Australia, half a world away. Initially,both rain forests would appear verysimilar. The climate would be hot and

Geospiza magnirostris(Large Ground Finch)

An illustration of two large ground finchesfrom the Charles and Chatham Islands,Galapagos Archipelago, as printed in JohnGould’s (1804–1881) The Zoology of theVoyage of H.M.S. Beagle, Part III: Birds(London: Smith, Elder & Co., 1841).

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muggy, possibly raining part or most of the day. The forest would be complexand dense, tall trees with wide buttresses at their bases. The crowns of thetrees would widen into a canopy and the tree branches would be covered bynumerous epiphytes, plants such as orchids that attach and grow on a treewithout parasitizing it. One’s impression of both of these forests would be thatthey are much alike. But upon closer inspection it would soon be learned thatthe trees, mammals, insects, birds, indeed even the fungi are quite distinct ineach region. The forests are both within the tropical rain forest biome, andtheir major structural characteristics evolved in response to the fact that bothare situated in a climate that is hot and rainy throughout the year.

The science of biogeography recognizes that the ecology and generalappearance of widely separated ecosystems with similar climates will be simi-lar even though the species have very different evolutionary histories.Tropical rain forest has been present since the time of the dinosaurs, theJurassic period of the Mesozoic Era, some 150 million years ago, but thoseforests would bear little in common with those of today other than the heat,humidity, and rainfall.

The geologic history of Earth reaches back some 4.5 billion years, but forour purposes, we may begin 248 million years ago at the beginning of theMesozoic Era. The Earth at that time consisted of one supercontinent,Pangaea. But soon after the Mesozoic began, Pangaea began to break apart,initially into two supercontinents, Laurasia to the north and Gondwana to thesouth. Evolutionary patterns began to take different trajectories betweenLaurasia and Gondwana and have been doing so ever since.

Australia has been long isolated from the rest of what was once part of thehuge southern continent of Gondwana. Near the end of the Mesozoic Era,some 66 million years ago, Australia split from Antarctica (Australia andAntarctica had split from the other southern continents earlier), and was aloneas a continent. Marsupial mammals such as kangaroos happened to be thriv-ing on Australia, but placental mammals had not colonized the now-isolatedcontinent. Thus marsupials were the sole mammalian occupants of the entirecontinent with millions of years to adapt to a wide range of habitats andlifestyles. Many species of marsupials proliferated on Australia. Placentalmammals (other than a few bat species that colonized by flying to Australia)remained largely absent until humans arrived with their dogs only about forty-five thousand years ago. Theabsence of placental mammalsresulted in the evolution of marsu-pials such as sugar gliders thatlook compellingly like placentalflying squirrels, a mole-like marsu-pial that looks and acts like a pla-cental mole, marsupial “mice,”and various marsupial bandicoots

A Sugar Glider (Petaurus breviceps) inflight at the Lone Pine Koala Sanctuary inBrisbane, Queensland, Australia.

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that resemble placental rabbits and hares. This evolutionary phenomenon oftwo distantly related kinds of organisms evolving similar phenotypes istermed convergence or convergent evolution.

Convergences are common throughout the world’s ecosystems. Indeed,convergence is so common that it often presents evolutionary biologists withdifficulty. Is apparent similarity among certain organisms a result of sharing arecent common ancestor, or is it a case of ecological convergence, a result ofnatural selection molding very different genotypes into similar phenotypes?Genetic analyses such as DNA sequencing help answer such questions.

Ratites are a group of large flightless birds: the ostrich, the rheas, the cas-sowaries, the emu, and the kiwis. The term “ratite” refers to the absence of abony carina or “keel” on the sternum (breastbone) for the attachment of flightmuscles. For many years it was debated as to whether or not the ratites areall genetically closely related or are an example of convergence. They do notfly, yet are widely separated, occurring on different southern continents. Theargument for convergence seemed strong. The argument weakens, however,when one realizes that cassowaries are forest dwellers, unlike ostriches,emus, and rheas that all live in open savannas, and kiwis are unique in beingnocturnal. In other words, the various ratites do not occupy such similar habi-tats that one would expect selection pressures to result in similar body forms.Recent work on DNA hybridization as well as DNA sequencing has shownconvincingly that the ratite birds are not convergent but all stem from a com-mon ancestor in ancient Gondwana. The ratite distributions offer convincingsupport for the reality of plate tectonics and resultant continental drift.

A key issue in ecology is the question of what, exactly, is a species. If youtravel throughout East and Central Africa you will surely see elephants. Aslarge and unmistakable as these ponderous beasts are, it may surprise you tolearn that, until quite recently, taxonomists were mistaken about exactly howmany species of elephants currently inhabit Africa. It was assumed that thereis but a single species, the African elephant, Loxodonta africana, when in factthere are two. Recent studies suggest strongly that there is a second speciesof African elephant, the African forest elephant, Loxodonta cyclotis. Comparedwith L. africana, now renamed the African savannah elephant, the forest ele-phant is smaller in body size, has rounded, not pointed ears, and straightertusks. The results of the molecular analysis suggest that forest and savannahelephants are each as distinct from one another as lions are from tigers.

Ecologists thus determine species by a combination of factors: anatomy,ecology, and genetics. The most widely used definition of a species is that itis a population that is reproductively isolated from other populations.

Cichlids are a group of colorful freshwater fish commonly kept by fish enthusi-asts in freshwater aquariums. These diverse bass-like fish are found in tropicalwaters around the world. There are about three hundred species in theAmericas, including one that occurs as far north as Texas. But, by far, mostspecies of cichlids are found in East Africa, clustered in three lakes thatformed in the Great Rift Valley: Lake Victoria (less than four hundred species),Lake Tanganyika (two hundred species), and Lake Malawi (three hundred tofive hundred species). Why are there so many species of African cichlids?

Two of the many varieties of cichlids found in Africa. At the left is a male Malawi cichlid found inLake Malawi. At the right is a male “Zaire Blue” found in the waters of Lake Tanganyika in theDemocratic Republic of the Congo (formerly Zaire), about 100 miles north.

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The oldest of the three African lakes is Lake Tanganyika, which formed 9 to12 million years ago, and the youngest is Lake Victoria, whose origin isbetween two hundred fifty thousand and seven hundred fifty thousand yearsago. Considering only Lake Tanganyika, if one assumes it to be 12 millionyears old, the highest estimate of its age, and it has two hundred cichlidspecies, that is a speciation rate of about one species every sixty thousandyears. However, researchers are in agreement that the amazing diversity ofcichlids in the African rift lakes arose recently, within the past few millionyears, demonstrating that speciation can occur with impressive rapidity.

Studies of DNA of the cichlids from Lake Victoria demonstrated that thiscluster of species is genetically very closely related, showing that they allrecently evolved from a common ancestor. What is stunning is that the totalgenetic variation among these four hundred species is less than that foundthroughout Homo sapiens, humans. In other words, the genetic distancebetween you and a stranger you might meet is greater than that between twoseparate species of cichlids from Lake Victoria. Given the estimated rates atwhich mutations occur, researchers believe the entire assemblage of cichlidspecies in Lake Victoria to have arisen within two hundred thousand years.That’s equivalent to about one species every five hundred years.

The evolutionary process whereby one kind of organism, in this case anancestral cichlid, evolves into numerous species, each adapted in such a wayas to be uniquely specialized, is called adaptive radiation. There are numer-ous examples of adaptive radiation both in the fossil record and amongextant animals and plants. Evolution is the process by which biodiversity isgenerated. Now it is time to see how it varies from one biome to another.

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1. What is convergent evolution, and how does the Earth’s current continentaldistribution affect convergent evolution?

2. What criteria are used to distinguish a species?

Cox, C. Barry, and Peter D. Moore. Biogeography: An Ecological andEvolutionary Approach. Oxford: Blackwell, 1993.

Adams, Douglas, and Mark Carwardine. Last Chance to See. New York:Harmony Books, 1990.

Few, Roger. The Atlas of Wild Places. Washington, DC: SmithsonianInstitution and Press, 1994.

Kingdon, Jonathan. Island Africa. Princeton: Princeton UniversityPress, 1989.

Weiner, Jonathan. The Beak of the Finch. New York: Alfred A. Knopf, 1994.

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LECTURESIX

uppose you are standing on the North Pole. And suppose atanother time you are standing on the South Pole? What, besidesthe fact that the first location is 90º north and the second 90ºsouth, is the difference between them? The answer is that at theNorth Pole you are standing on ice and at the South Pole you

are on a continent, Antarctica. The North Pole is located on theArctic Ocean, a sea of ice that because of global climate change is currentlymelting at its margins. Antarctica, once part of Gondwana, is the coldest con-tinent on Earth, located as it is at the southern pole. Ice that surrounds thispolar continent is also retreating.

Though bitterly cold and in the dark or twilight for much of the year, bothpolar regions have in common that they are ecologically productive. In otherwords, they each capture a respectable amount of the Sun’s energy by pho-tosynthesis. But neither does this terrestrially. The key to polar productivity isthe oceanic food web during the summer growing season.

Ocean currents bring up essential chemical nutrients to the surface whenthe sun shines for a few months on the surface waters. In both polar regionsgreat whales feast on the bounty of invertebrates and fish. Many seabirds,including various penguin species at southern latitudes and murres andpuffins at northern latitudes also partake of the seasonal bounty.

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The Suggested Reading for this lecture is E.C. Pielou’s A Naturalist’sGuide to the Arctic.

Lecture 6:Polar Ecosystems and Tundra

Earth’s Polar Regions

Two satellite composite images of the Earth’s polar regions are shown above. At left is theAntarctic continent; at right, a false-colored view of the Arctic Ocean. Both images were generatedin 2006.

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On land it is a different story. The growing season is short and in extremesituations, as are found in interior Antarctica, virtually no plants exist. Butthere is one kind of ecosystem found not only in the high Arctic but also athigh elevations on mountains that is permanent and diverse, adapted to theclimatic extremes of wind and cold. This ecosystem is called tundra.

Suppose you are standing in New Hampshire at an elevation of about fifty-three hundred feet on Mt. Washington. Or suppose you are visiting RockyMountain National Park in Colorado, on Trail Ridge Road at about the eleventhousand foot elevation. Or perhaps you are wandering along the gravel roadin Deadhorse, Alaska, well inside the Arctic Circle. Or maybe you are just lostin northern Siberia. Regardless, in each case you would be on tundra.Tundra is the biome that is found in the coldest, driest places on the planet,where conditions are sufficiently severe so as to prevent the existence oftrees, except in extreme dwarf form. Vegetation consists of lichens and moss-es, grasses and wildflowers, scattered shrubs, and little else. Tundra is openand wind swept, in a climate so dry that even the protective blanket of wintersnow, so common in the boreal forest, appears sparingly, if at all, on tundra.Organisms of this biome endure the worst weather conditions and enjoy thebriefest growing season of anywhere on Earth.

Latitudinally, the tundra biome, named “Arctic tundra,” occurs generallynorth of the Arctic Circle (at about 66º north latitude), though “low tundra,”where tundra mixes with boreal forest, begins at about 72 to 73º north lati-tude. Tundra exists as a band around the northernmost of the world’s conti-nents and is generally similar in North America and Eurasia. Tundra canalso be found above treeline at the summits of tall mountains. Montanetundra is usually called “Alpine tundra,” to distinguish it from latitudinallybased tundra, though the two are much alike in appearance. Tundra ishighly limited in Antarctica.

Obviously tundra regions are cold much of the year, with a markedly shortgrowing season. In the case of Arctic tundra, temperature is affected by thelow angle of solar radiation. Even during the warmest time of the year, mid-summer, temperatures reach only about 15º C. In the case of alpine tundra,exposure from elevation tends to keep the temperature low even though theangle of solar radiation may be much more direct than in the Arctic. Windsare a constant component of tundra climate. Wind speed is frequently high,especially in the Arctic, where it can routinely reach 65 km/hr. Snow is theusual form of precipitation for much of the year, but, perhaps surprisingly, inthe Arctic snowfall amounts tend to be small. Precipitation is sufficientlysparse as to be within the range of that of a desert. Alpine tundra areas expe-rience considerably more precipitation than Arctic areas.

The most notable characteristic of Arctic soils is permafrost. This means thatpart of the soil is permanently frozen, preventing its use by plants. Roots can-not penetrate into permafrost. In summer, the upper levels of the soil thaw,providing sufficient moisture for the rapidly growing plants. Because of thethawing and refreezing of the upper layers of soil, there is an intrinsic instabili-ty in Arctic soils that results in the formation of hummocks, which are elevatedareas, and polygons, geometric patterning that often spans wide areas.Polygons form from constant thawing and freezing of water in the upper layers

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of soil, a process that eventually sorts the larger fragments of rocks from thesmaller rubble, depositing the larger fragments on the borders of the polygon,which itself may be several meters in diameter.

Alpine tundra soils do not usually experience permafrost. They are general-ly young, having been created with the rising of the mountain range. Thesesoils typically are rocky with lots of gravel. Large rock fields, called fell-fields,are common, and form areas of habitation for such creatures as marmotsand pikas.

In general, the biodiversity of both alpine and Arctic tundra is markedly lessthan that found in other biomes, with the exception of certain hot deserts.Reduced biodiversity may result from the severity of physical conditions thatprevail in tundra regions. In the high Arctic, it is profoundly cold, windy, anddark for most of the winter, conditions that are generally unsuitable for plantsand animals. Nonetheless, the region is inhabited. Life-forms have evolvedthat succeed in the harsh climate.

In Arctic tundra, the presence of high winds and a contracted growing season(only sixty to one hundred days) selects for prostrate plants, literally lying lowto minimize evaporative water loss as well as to grow rapidly when conditionspermit. Many perennial wildflowers, some called “cushion plants” for theirshape, grow among the reindeer lichen (Cladonia spp.), dwarf willows, anddwarf birches. Sedges often dominate wet areas. These vegetation patternsare also generally true of alpine tundra. It is notable that numerous species ofperennial wildflowers found in the Arctic also occur in alpine tundra.

Animals abound during the summer growing season in the Arctic. Smallrodents called lemmings experience repeated cycles of abundance anddecline and, along with Arctic hares, serve as important food sources forwolves, arctic fox, grizzly bear, snowy owl, and rough-legged hawk. Manyspecies of shorebirds, sandpipers, and plovers nest during the short Arcticgrowing season, along with loons, grebes, and various species of ducks andgeese. Herds of caribou (called reindeer in Europe) graze on the tundra with

Geometrically shaped hummocks are clearly visible in this image of the arctic tundra area in theNorthwestern Territories, Canada.

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shaggy musk oxen. Many insects occur on tundra and any visitor to the Arctictundra in summer soon becomes familiar with just how abundant mosquitoescan become.

Tundra is fragile. This is because the growing season is so short that itrequires many years for vegetation to recolonize and grow in areas subjectedto disturbance. Unlike in the temperate zone or in the tropics, where vegeta-tion development following disturbance is generally quite rapid, the oppositeis true on tundra.

The fragility of the tundra as an ecosystem was cause for concern when apipeline was built to transport oil from the North Slope of Alaska, PrudhoeBay, several hundred miles south to the port city of Valdez. The pipeline wascarefully engineered to prevent damage to the landscape and, in variousareas, it was either raised or buried to permit caribou herds to pass unimped-ed during their annual migration. In general, the pipeline has been successfulin not damaging delicate tundra ecology. Proposals to drill for oil within theArctic Wildlife Refuge, an area entirely on tundra supported by permafrost,have met with skepticism because of the potential damage that could beinflicted onthe ecosystem.

In Russia andthe Ukraine,where environ-mental protec-tion laws arefar less thanthose of NorthAmerica, tun-dra areas havebeen subject-ed to damagefar in excessthan any expe-rienced inNorth America.Direct damagefrom humanuse has beenaugmented by air and water pollution from numerous factories that deal withmining and oil production.

Alpine tundra areas have been negatively affected by human use for activi-ties such as skiing and hiking. One of the worst threats to alpine tundra isthe increased use of off-road vehicles, which do significant damage to thefragile plants.

Polar ecosystems illustrate how organisms evolve diverse adaptations tocope with extreme climate. Deserts, as will be clear in another lecture, aresimilar in that regard.

The Alaska oil pipeline is shown snaking through Atigun Pass (elevation4,643 feet) in the Brooks mountain range. The Dawson Road is at the rightof the photo.

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LECTURESIX

1. Why does tundra exist primarily in the Arctic and not in Antarctica? Whatis tundra?

2. Why are polar areas ecologically productive even with the shortgrowing season?

Pielou, E.C. A Naturalist’s Guide to the Arctic. Chicago: University of ChicagoPress, 1994.

Miles, Hugh, and Mike Salisbury. Kingdom of the Ice Bear. Austin, TX:University of Texas Press, 1985.

Sage, Bryan. The Arctic and Its Wildlife. New York: Facts on File, 1986.

Shirihai, Hadoram. The Complete Guide to Antarctic Wildlife. Princeton:Princeton University Press, 2002.

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he Boreal, or Northern Coniferous Forest, is a vast biome thatencircles the far northern latitudes, covering just over 10 per-cent of Earth’s terrestrial area. It is strongly identified with the

wild, yodeling call of the common loon, which nests on the seeming-ly innumerable lakes that dot the boreal forest (thanks in large partto recent glaciation). It is a land of relatively low tree species rich-

ness, with millions upon millions of spruces, firs, and pines, intermingled withlarches (which are deciduous conifers) and a few broad-leaved tree speciessuch as aspens, birches, and willows. Often termed the “spruce-moose”biome, the largest of the world’s deer does, indeed, thrive throughout thebiome, feeding on aquatic vegetation within the numerous bogs that interruptthe unbroken vastness of the forest of “Christmas trees.” Species such asmoose, wolverine, lynx,goshawk, boreal owl, hawkowl, and red crossbill areamong those found not onlyin the boreal forests ofNorth America but also inOld World boreal forests aswell. Plant species are notthe same but are closelysimilar, with, for example,Norway spruce replacingwhite spruce in Europe.

The northern third of theboreal forest, where treesare stunted and shrub-likedue to the severity of cli-mate, is traditionally called“taiga” (from a Russianword), but the word “taiga” is now often used to name the entire biome, thus“boreal forest” and “taiga” are commonly considered interchangeable.

Much of the region of the northern coniferous forest has been subject torecent glaciation, resulting in poor soils and an abundance of lakes and bogs,created by the scraping action of the immense, heavy glaciers. A bog is dif-ferent from a lake in that it has no inflow and outflow of water but is, instead,an isolated basin that eventually fills with peat, becoming increasingly acidic.

In North America, the Boreal Forest begins roughly at the Canadian border,though it is also found in northern New England and northern Michigan,

The Suggested Reading for this lecture is E.C. Pielou’s The World ofNorthern Evergreens.

Lecture 7:Boreal Forest

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One of the many moose residents browsing in the woods atthe Algonquin Provincial Park in central Ontario, Canada.

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Minnesota, and Wisconsin. A unique extension of boreal forest, theAppalachian Extension occurs at high elevations southward from Maine tothe Great Smoky Mountains of North Carolina and south to Georgia. In theAmerican West, the boreal forest intermingles with the coniferous forestsfound within the Rockies, Cascades, and Sierra Nevada, as well as with thetemperate rain forest of the Pacific Northwest. For example, white spruce,abundant throughout the boreal forest, is replaced by closely relatedEngelmann spruce throughout the high elevations of the Rocky Mountains.These two species are so genetically similar that they occasionally hybridize.

The growing season becomes increasingly shortened as one moves northin latitude through the boreal forest, ranging from about 120 days in southernregions, to as short as 90 days or less at treeline, where the forest gives wayto arctic tundra. In general, mean annual temperature ranges from about 4º Cto –5º C. In a few places, temperature extremes from the heat of summer tothe cold of winter range as much as 100º C. In other words, summer couldbring a high temperature of, say, 30º C, and winter a low of, say, –70º C.Winters are almost always long and cold, the average daily low temperaturebeing about –15º C, with most precipitation falling as snow. In general, pre-cipitation averages between about 40 and 50 cm annually and is fairly con-stant from month to month, though falling as rain in summer and snow in win-ter. In winter, thefrozen ground pre-vents plants from tak-ing up moisture fromthe soil, thus thetrees must enter intowinter dormancy.Northern tree speciesundergo a physiologi-cal process (thatvaries somewhatamong species)called winter harden-ing, enabling them towithstand the subzerotemperatures withoutexperiencing damageto delicate tissues.The conical shape typical of spruces and firs acts to aid the trees in sheddinga burden of snow, which could potentially accumulate until branches breakfrom the weight of the snow.

Soils are typically acidic, thin, and nutrient poor. The former word used todescribe boreal soils was “podzol,” but that word has been replaced by spo-dosol in recent years. Spodosols are acidic due, in part, to the breakdown ofneedle leaves, which tends to add hydrogen ions to the upper soil layers.Boreal soils are well leached, the minerals deposited in a lower “B” horizonwell below the surface. The upper soil horizon is usually a grayish color.Because the growing season is short, there is a significant buildup of leaf lit-ter, making the ground soft underfoot. Bogs are particularly acidic due both to

Boreal landscape in northern Finland.

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the influx of decomposing litter as well as the prevalence of sphagnum moss,which enhances the acidic nature of bogs. Some boreal soils have per-mafrost, where the soil remains frozen throughout the year.

Throughout much of North America, two species of trees, white spruce andbalsam fir, are the dominant upland species of the boreal forest. In wet areas,black spruce abounds along with various willows and tamarack (Americanlarch). Areas subjected to recent fire support stands of aspens, birches,and/or jack pine. In the Eurasian boreal forest these species are replaced byothers, but the appearance of the forest is essentially the same.

Bogs, most created by glacial retreat, are generally abundant throughoutmuch of the boreal forest. Bogs provide habitat for species of insectivorousplants such as sundews and pitcher plants, as well as many other specializedplant species, including sphagnum moss, the dominant bog species through-out most of the biome.

In the most northern regions of boreal forest the trees are much reduced instature due to the severity of the winter weather. It is here that trees such asspruces and firs take on a shrub-like appearance called krummholz, a wordthat means “twisted wood.” The spreading, shrubby growth form is caused bythe fact that the tree branches beneath the snow survive, but those that pro-trude above it are killed by wind chill. Thus the tree grows mostly horizontally,producing a shrubby, often prostrate form. Beyond the zone of krummholz,trees cannot exist due to the severity of winter weather, and it is there thatthe boreal forest yields to arctic tundra.

Characteristic mammal species include red squirrel, ermine, varying hare,lynx, wolverine, grizzly and black bears, woodland caribou, and moose. Someof these, such as the ermine and the hare, change pellage seasonally, beingwhite in winter and mottled tan in summer. In winter, bears locate dens wherethey enter into a deep sleep, though not a true hibernation. Mammals such ashare and lynx remain active throughout winter months, the former trying toavoid predation by the latter. Beavers are particularly abundant in many bore-al forest regions. Porcupines are common rodents in boreal forests through-out North America, but are absent from Eurasia.

Many bird species, such as common loon, wood warblers, thrushes, and fly-catchers, are strongly migratory, but others, such as the pine grosbeak, bore-al chickadee, three-toed woodpecker, spruce grouse, and great gray owl areresident, even during the cold, snowy winter months.

Reptiles and amphibians experience a reduced biodiversity in the boreal for-est compared with more southern latitudes. This is most likely because theseanimals are ectothermic and cannot survive easily in an ecosystem wherecold temperatures predominate for most of the year.

There is a major insect flush in the spring, as many insect species emergefrom over-wintering eggs and pupae. Mosquitoes and black flies, a significantnuisance to humans as well as other animals, are often dramatically abundant.

Timber harvesting is widespread in boreal forests in North America andmuch of Eurasia. The wood is used for construction as well as for paper pulp.Clearcutting, a lumbering practice where a large section of forest is complete-ly cut, is a common practice in many regions. Clearcutting is controversial

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among conservation biologists because it destroys large tracts of forest,though in some regions it seems to succeed in allowing for sustainableharvest. Clearcut areas are typically reseeded to promote rapid regrowthof trees, which can then be harvested in as short a time as forty to sixtyyears. Clearcuts thus go through a process of ecological succession thatmakes them suitable habitats for species that require brushy areas. Lookingat the landscape on a large area scale, areas of clearcuts within otherwisemature boreal forest, each clearcut in a different state of regrowth, canactually enhance biodiversity by providing a “patchwork quilt” of differenthabitats. Because these habitats change with time, one area maturing, onenewly clearcut, ecologists call such a forest a “shifting mosaic” of habitats.The shifting mosaic concept is developing into a model for how timber har-vesting can be accomplished in such a way as to enhance preservation ofbiological diversity.

Tree harvesting, as noted above, should ideally ensure sustainable yield and,in North America, that is generally the case. The most serious loss of borealforest is currently in Russia, where far more forest is cut annually than is per-mitted to regrow, resulting in an overall loss of forest that is resulting in anexpansion of barren ground tundra into areas traditionally occupied by forest.

Boreal forests in North America are affected by increased human usage forrecreational purposes. Activities such as power boating may exert significantstresses on species such as common loon, which has experienced significantpopulation declines in some areas.

Boreal animals such as beaver and lynx have been historically exploited forthe fur trade. One boreal mammal, the caribou or “reindeer,” has beendomesticated in Europe.

Temperate Rain Forest: A Unique Coniferous Forest

The Pacific Northwest of North America, in the states of California, Oregon,and Washington, and the Canadian province of British Columbia, is the site ofa coastal forest of tall sitka spruce, western hemlock, western red cedar,grand fir, and common Douglas-fir, arguably the most majestic of the Earth’sforests, the temperate rain forest. In California, this forest also includes theimposing redwood, a tree that often exceeds 300 feet in height. This conifer-ous forest, composed of the most statuesque of all North American treespecies, is a land of tall trees and high rainfall, up to 350 cm (138 inches)annually. The combination of coastal fog along the California and Oregoncoasts as well as the high precipitation, particularly in Washington and BritishColumbia, provides the necessary moisture to support an unusually tallassemblage of tree species, collectively termed the temperate rain forest.

In many ways the temperate rain forest is ecologically similar to the borealforest, just larger and far more lush. Trees routinely exceed 200 feet in heightand may have trunks fully eight feet wide at their bases. It is normal for thetrees to live from between 400 to 700 years, though some may attain a lifespan of one thousand years. This forest is also unique in many areasbecause it is an old-growth forest, one in which the trees have not recently, ifever, been felled by ax or saw. The complex physical structure of an old-growth temperate rain forest makes it different from the small stature, more

rapidly growingspruce-fir forests thattypify most of theboreal region.

Because of the old-growth nature ofmuch of the temper-ate rain forest, therehas been significantcontroversy surround-ing logging practices.Tracts of forest havebeen clearcut inmany areas and theforest that is regrowneither from seed orby planting seedlingtrees will itself be har-vested in as short a time as sixty years, thus preventing the eventual reestab-lishment of old-growth forest. Clearcutting also has the potential to increaseerosion of soil, polluting streams that provide essential habitat for salmon andother species. The intrinsic beauty as well as ecological significance of old-growth forests have made many urge that they be conserved.

Because of the logging issue, old-growth temperate rain forests have beensubjects of much ecological study. The northern populations of spotted owlhave been at the center of intense debate (the owl made the cover of Timemagazine’s June 25, 1990, issue) because the species, which is consideredthreatened, apparently is restricted to old-growth forests for nesting sites. Inaddition, a small, robin-sized seabird called the marbled murrelet, commonon the offshore waters from British Columbia through northern California, fliesfrom the sea to nest atop the tall trees of the temperate rain forest. This bird’s

nesting site was utterlyunknown until a nest wasdiscovered atop aDouglas-fir in northernCalifornia in 1974. Thered tree vole, sometimescalled the redphenacomys, seems tolive its life in a singleDouglas-fir, feeding onthe needle leaves andnesting within the boughsof the huge tree. Wholepopulations and numer-ous generations of redphenacomys reside with-in a single tree.

The giant redwood tree named “General Sherman” dwarfs tourists atthe Sequoia National Forest near Visalia, California. The tree has amaximum diameter of 36.5 feet, is 274.9 feet tall, and is believed tobe about 2,200 years old.

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Moss-covered trees in the Olympic National Park near PortAngeles, Washington. The park boasts ocean shoreline and tallmountain peaks, including Mount Olympus at 7,965 feet. Theannual rainfall in the park is 56.5 inches.

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1. Why is much of the boreal forest dominated by needle-leavedevergreen trees?

2. What differences separate the boreal forest from the temperate rain forest,also a forest of needle-leaved trees?

Pielou, E.C. The World of Northern Evergreens. Ithaca, NY: Comstock, 1988.

Henry, J. David. Canada’s Boreal Forest. Washington, DC: SmithsonianInstitution and Press, 2002.

Kricher, John. A Field Guide to California and Pacific Coast NorthwestForests. Boston: Houghton Mifflin, 1998.

Mathews, Daniel. Cascade Olympic Natural History. Portland, OR: RavenEditions, 1988.

Storer, Tracy I., and Robert L. Usinger. Sierra Nevada Natural History.Berkeley: University of California Press, 1963.

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astern North America, as well as much of Europe and temperateAsia, has forest characterized by broad-leaved deciduous trees. Thesimilarity among these forests can be striking. Sycamores in China

are scarcely distinct from those in North America. This is not acoincidence but rather the historic result of continental drift, the

trees having evolved before the division of ancient Laurasia, thenorthern supercontinent.

Oaks, maples, sycamores, beeches, and hickories all typically lose theirleaves in synchrony as summer turns to autumn. Leaf drop is an adaptationto the impending winter, when the air temperature will become sufficientlycold to freeze the soil, making uptake of water impossible for the trees. Leafdrop is a precursor to physiological dormancy, permitting the trees to endurethe prolonged cold of winter without catastrophic water loss.

A temperate deciduous forest typically contains a mixed assemblage ofdeciduous trees along with some evergreens such as hemlocks and pines.Such a forest is annually cyclic, leaves gradually opening and unfolding inspring, with summer, the growing season, the time of maximum photosynthe-sis. As autumn arrives, chlorophyll, the pigment that reflects green (thus mak-ing the leaf look green to us) and is of crucial importance in photosynthesis,deteriorates, and other pigments, masked until then, make leaf colors turnvarious shades of red, yellow, and brown. The array of fall colors, from theblazing oranges of sugar maple to the subtle browns of oaks, makes this for-est one of the most splendid to behold. In winter, aside from the scatteredconifers that mostly retain their needle-like leaves, the forest has a stark look,the branches barren of their leaves.

Deciduous forests are typically stratified into fairly clear layers determined bythe heights ofthe residentplants. There isa canopy layerdefined by thecrowns of thetallest trees, typi-cally sixty toeighty feet above

The Suggested Reading for this lecture is John Kricher’s A Field Guideto Eastern Forests.

Lecture 8:Temperate Deciduous Forest

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Colorful leavesenhance a beautifulautumn day inNew England.

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the ground. Usually there will be a subcanopy or understory of trees such asdogwoods or sassafras. Beneath them will be a shrub layer of various vibur-nums, blueberries, huckleberries, rhododendrons, or other shrubs. There isalso a fern/herb layer where various wildflowers grown among the fern speciesand other plants collectively called ground-cover.

This biome is represented in the northern hemisphere throughout most ofeastern North America, Europe, much of China, and Japan. A similar forest isalso found, with very different species, in southern Argentina, southernAustralia, Tasmania, and New Zealand.

The key to understanding the Temperate Deciduous Forest is growing sea-son. This is the number of days when conditions of temperature and moistureare favorable for sustained growth. As you might expect, the growing seasonbecomes shorter in the northern hemisphere as you move in latitude fromsouth to north. For example, the growing season is about 250 days in muchof the Deep South (Mississippi, Georgia, Louisiana), about 200 days in NorthCarolina, 175 days in New York, and only 140 days in Ontario, Canada. Themost important variable of climate is temperature, which varies from high inthe summer months to often below freezing for much of the winter months.Precipitation is less variable, but, because of the cold air temperatures of win-ter, it often falls as snow, which is unavailable to plant roots until it melts andthe ground thaws. Temperate forest is found through a fairly wide range ofmean annual temperatures, from about 3º C to 18º C. Mean annual precipita-tion, depending upon locality, ranges from about 150 cm to 250 cm.

Deciduous forests drop leaves in fall and thus there is a substantial buildup ofground litter that is then worked on by decomposer organisms. This “litter layercommunity” is active in mulching theleaves, releasing minerals that will enterthe soil and subsequently be takenback into the vegetation, the essenceof recycling. Soils are usually fairlywell drained, typically mixtures ofsand, silt, and clay. As water movesthrough the soils it leaches mineralsfrom the top layers of soil anddeposits them in deeper layers. Thusthe soil has its own pattern of stratifi-cation, called horizons, due to theinteraction of climate,precipitation, and veg-etation. Tree rootseasily penetrate intothe deeper horizonswhere minerals aremost concentrated.Because of leaching,as well as the chem-istry of the vegetationitself, the soils aretypically acidic.

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The biodiversity of temperate deciduous forests is impressive, though itnever rivals that of equatorial rain forests. Permanent residents include manynut consumers, such as squirrels, chipmunks, raccoons, wild turkeys, andblue jays. Deer, skunks, and foxes are abundant and wide ranging, and bearsare common in certain areas. In most regions, however, large predators suchas bears, bobcats, and wolves have been eliminated.

Vegetation diversity is complex in deciduous forests. In Eastern NorthAmerica, for example, different regions are dominated by different treespecies. In the Southeast, for example, is the Southern Hardwood Forest, arich forest of species such as various magnolias, Virginia live oak, commonpersimmon, pecan, redbay, and pawpaw. This species assemblage is instrong contrast to that of the Northern Hardwood Forest comprising mostlyyellow birch, sugar maple, American beech, eastern hemlock, and easternwhite pine. The most species-rich area of deciduous forest is in theAppalachians, the Cove Forests of the Great Smoky Mountains of easternTennessee. Over thirty tree species may occur in close proximity, includingsuch species as white basswood, Carolina silverbell, tulip tree, yellow buck-eye, and sugar maple.

Ecologists have struggled to understand what factors determine speciescomposition in various regions within the overall deciduous forest biome. Oneprevalent view was that such forest associations were interdependent andthus strong interactions among, say, American beech and sugar maple forcedboth species to associate. A school of study called “phytosociology,” largelystarted by European ecologists, struggled to find order among plant associa-tions and reveal predictive “rules of assembly.” But other ecologists objected,suggesting that the co-occurrence of such species was largely coincidental, aresult of each species having similar physiological requirements and similardistributions. If anything, many species likely competed among themselvesand with other species to reach adult size and successfully reproduce. Thisview of the plant community was termed “individualistic,” meaning that no twoplant communities are more than superficially similar.

Studies of pollen profiles in bogs and lakes, as well as carefully done statisti-cal studies on plant distributions, have confirmed the individualistic model.Plant pollen accumulates (without decomposing) through thousands of yearsin the layers of mud found in bogs and lakes. Such accumulations allow alook back through time, reaching back to the time of glaciation, some 20,000years ago. What is seen is that today’s plant associations did not move as aunit when glaciation forced northern species to the south and, when glacierseventually melted, plants did not migrate north in the same associations asnow exist.

Forests throughout much of eastern North America were extensively cutduring the years of European colonization, but these areas, used for agricul-ture and pasture, were largely abandoned when the West opened to colo-nization in the mid-1800s. After abandonment, vegetation strongly tended tofollow a generally predictable series of changes as weedy invader specieswere progressively replaced by various grasses and shrubs, then colonizingtrees, and then so-called climax tree species typical of the mature forest.This process is called ecological succession and has been the subject of

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much study. Ecologists have learned that succession results from numerousspecies having evolved life cycles that are optimal at various levels of lightintensity and disturbance. This view has resulted in viewing deciduousforests as patchworks of varying disturbance histories rather than asmassive, homogeneous tracts.

Biodiversity is strongly seasonal in deciduous forests, with wildflowers andferns evident only in summer months, though they survive as root systemsbelow ground during the winter. The most pronounced seasonality is found inthe animal community. Some species, such as many insects and mammals,overwinter in a dormant or semi-dormant stage. Insects survive in egg casesand as dormant pupae and some mammals may hibernate or enter deep tor-por. Pond animals such as frogs and toads lie dormant in the mud throughoutthe winter chill. Bird species offer the most dramatic example of seasonalchanges in biodiversity. With the opening of leaves in spring comes a flush ofinsects, especially caterpillars, flies, and beetles. It is then that many speciesof migrant birds, the thrushes, warblers, orioles, and tanagers, return fromtheir tropical wintering grounds to nest in the broad-leaved forests.

Many of the most industrialized areas of the planet are in the latitudinalrange of temperate deciduous forest. As such, the forest is subject to cuttingto make room for commercial ventures or housing as well as subjected to airand water pollution. Acid rain, for example, is a byproduct of industrialization,coming from emissions of industrial plants as well as automobiles and othervehicles with internal combustion engines.

As mentioned earlier, the Eastern Deciduous Forest of North America wasdramatically cut and cleared during the European settlement of the continent,as agriculture pushed westward. Much of what was pasture and farmland acentury ago has reverted back into forest cover today. However, much of thatarea is being subdivided for shopping malls, industrial parks, golf courses,and housing developments, all of which represent relatively permanent alter-ations to the landscape. Ecologists use the term “fragmentation” to describethe process of chopping a sizeable tract of forest into ever-smaller parcels,thus isolating populations on small forest “islands.” This realization has led tothe emergence of a branch of ecology called landscape ecology, and hascontributed to yet another growing field called restoration ecology.

Landscape ecology focuses on how landscape patches interact to affectsuch things as biodiversity. For example, if two woodlots are isolated by frag-mentation, they become “forest islands,” and each may be too small to sup-port animal species that typically require large areas. But if a woodland “corri-dor” is left intact connecting the two fragmented forest lots, in effect theyfunction as though the actual area is much larger. As another example, ecol-ogists have observed the nest failure of birds such as wood thrush fromsmall, fragmented woodlots. But in spite of repeated nest failure, thrushesremain in such woodlots. This is because they move as surplus individualsfrom “source” populations in large forests and attempt to nest in “sink” habi-tats such as fragmented woodlots.

Restoration ecology attempts to mimic nature’s processes and thus restoredamaged or fragmented habitats.

1. In what ways does length of growing season influence the adaptations oftemperate forest trees?

2. What was phytosociology and what was it attempting to do? Why is it nolonger used in ecology?


Braun, E. Lucy. Deciduous Forests of Eastern North America. New York:Hafner, 1972.

Brooks, Maurice. The Appalachians. Boston: Houghton Mifflin, 1965.

Robichaud, Beryl, and Murray F. Buell. Vegetation of New Jersey. NewBrunswick, NJ: Rutgers University Press, 1973.

Sutton, Ann, and Myron Sutton. Eastern Forests. New York: Alfred A.Knopf, 1985.

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LECTURENINE

he sight of millions of bison grazing on endless prairie, thegrasses waving in the gentle breeze, portrays, for the mind’seye, a vision of the grassland biome in North America, a “sea of

grass.” Grassland appears monotonous, populated with variousgrasses and wildflowers, with little in the way of trees and shrubs.

Grassland

Grasslands do not all look alike. Because of differences in annual moisture,some grasslands contain predominantly tall grasses, others mixed, and oth-ers short. This pattern is evident in North America, where the tallest grassestypify the most easternregions. Approaching theRocky Mountains, grasslandbecomes predominantlyshort, and eventually mixeswith desert.

Grassland, wherever itoccurs, gives the impressionof vast openness. There is agood reason why Montana,which is mostly grassland, isoften called “Big SkyCountry.” In grassland, thehorizon is more sweepingthan any other place exceptthe open ocean.

In North America, grasslanddominates the central part ofthe continent, from south-central Canada into easternMexico. States such asMontana, the Dakotas,

The Suggested Reading for this lecture is Stephen R. Jones and RuthCarol Cushman’s A Field Guide to the North American Prairie.

Lecture 9:Grassland and Savanna

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Top: The Flint Hills of Kansas containthe largest tract of remaining tallgrassprairie in the United States.

Bottom: Tallgrass prairie is dominated by three main grasses: Big Bluestem, Little Bluestem, andIndiangrass. Subdominant grasses and forbs (non-woody, non-grass, flowering plants), though lessabundant, contribute to the diversity of the prairie.

Inset: A bee and a spider take advantage of the flowering grasses during the pollination cycle.

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Kansas, Nebraska, and Oklahoma are historically grassland dominated, andnatural prairie occurs as far east as Ohio. Grassland is also the natural vege-tation of the Central Valley of California. The biome occurs in many otherplaces in the world: in South America, where, in Chile and Argentina, thegrassland is called pampas. Throughout central Europe grassland is calledsteppe. In central and southern regions in Africa it is veld. In North America, itis called prairie.

In general, mean annual precipitation is less in grassland than in forest, butthere is a wide range. In the short grass prairie of Colorado, for example, themean annual precipitation is a meager 10 inches (25 cm), but it ranges from25 to 40 inches (65 to 100 cm) in the most eastern regions. In some prairieregions, mean annual precipitation is as low as 6 inches (15 cm). There is agradient in precipitation from west to east, with increasing precipitation mov-ing eastward. Thus tallgrass prairie is found in eastern regions, and much ofthe central prairie is mixed grassland.

Because North American grasslands are confined to the interior of the conti-nent, there are no mitigating climatic effects by the oceans, such as occuralong the eastern and western seaboards. Extremes of temperature, indeed,extremes of weather, are common. Prairie experiences severe winter bliz-zards with drifting snow blown by incessant high winds. One of the character-istics of prairie most recorded in the journals of settlers and pioneer travelersduring the nineteenth century was the constancy of wind. Tornadoes arecommonplace throughout the summer months. In spring, one day may bemild while the next features a major snowfall.

Natural fire, set by lightning (many thunderstorms occur throughout the sum-mer months), is a major factor influencing prairie ecology. Without regularfires, in many areas prairie would eventually be replaced by forest. This pat-tern is evident in places such as Wind Cave National Park in South Dakota.The area where the park is located is called “the Black Hills,” named for thedark foliage of ponderosa pine forests that occur there. Without prescribedburning by the National Park Service, Wind Cave National Park, one of thelast vestiges of natural grassland in the region, would be invaded by pon-derosa pine and associated species. There is anthropological evidence thatsuggests that Native Americans regularly set fire to the grasslands, apparent-ly understanding that doing so would maintain the area as grassland, whichwas desirable for hunting game and for moving from one place to another.

Soils are typically rich in nutrients, with abundance of such elements as cal-cium, potassium, and phosphorus. The dense roots of the grasses effective-ly tap into this abundance of nutrients, allowing continual growth through the120- to 200-day growing season. Because of the natural richness of thegrassland soils, it is hardly surprising that agricultural activities dominateregions of grassland.

Many species of grasses occur throughout the biome, but they differ from oneregion to another depending on how much precipitation the region receives.Tallgrass prairie in North America is dominated by species such as bigbluestem (Andropogon gerardi ), which is a bunch grass. Bunch grasses, asthe name implies, have stems that radiate from a central cluster. Big bluestem,under ideal growing conditions, can reach a height of just over three meters,

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about ten feet. When General Custer’s troops were easily defeated at theBattle of the Little Bighorn in Montana, Sitting Bull’s braves were totally hiddenin the tall grass that Custer’s cavalry rode through, making it easy for thebraves to attack the soldiers.

Many grasses are “sod forming,” rather than bunch grasses. Species suchas blue grama (Bouteloua gracilis) and buffalo grass (Buchloe dactyloides),which occur abundantly in shortgrass prairie, are similar to grasses that makeup lawns, growing as a dense mat covering the soil.

Grasses are adapted for rapid growth after fire or grazing. They often spreadby underground stems called rhizomes, soon carpeting an area. The root sys-tem contains as much or more tissue as the above-ground shoots, making iteasy for grasses to resprout after fire or grazing (consider how often one hasto cut the lawn in summer—a lawnmower is essentially a form of “grazing”).Grasses are wind-pollinated, which is an effective means of pollen dispersalin an open land where wind normally prevails.

Many wildflower species are mixed among grasses, including legumes suchas various clovers, which take nitrogen from the atmosphere and incorporateit into usable form. The abundance of legumes further enhances the high fer-tility of the soil. Numerous species of composites, daisy-like plants, many ofthem with wonderfully colorful flowers, grow among the prairie grasses. Oneof the great pleasures of exploring real prairie is to enjoy the myriad of differ-ent kinds of wildflowers encountered there.

In North America, several animals are closely associated with prairie, but thetwo that are most obvious are the American bison and prairie dogs. Bison,once among the most abundant of North American mammals, were huntednearly to extinction in the nineteenth century. Bison were important grazers inthe central regions of the prairie and many ecologists believe that regulargrazing by bison helped to conserve the grassland, preventing invasion bynon-grass species. Thus it is the combination of low precipitation, periodicfire, and grazing that combine in determining if grassland will persevere.

There are two prairie dog species, the black-tailed and the white-tailed, andthey are both keystone species in maintaining prairie ecology. Prairie dogs arereally a kind of ground squirrel, a member of the rodent family, and otherground squirrels also are common throughout grassland, as are many speciesof mice. The black-footed ferret, a member of the weasel family and the rarestNorth American mammal, closely associates with prairie dog colonies. Othergrassland mammals include coyotes, badgers, and pronghorns.

Scattered throughout much of the central regions of North American prairieare lakes and marshes called prairie potholes. These are essential nestinggrounds for many duck species such as canvasback and redhead, as well asnumerous herons and shorebirds.

Most natural prairie in North America has been replaced by agriculture. Whatwas once natural grassland are now fields of wheat, rye, corn, sorghum, andother essential crops. Thus it is difficult to find natural prairie where once itmay have seemed endless. There are scattered preserves that sustain naturalprairie, but the total areas of these places are generally small. In addition,many alien species of grasses and wildflowers (“weeds”) have invaded naturalprairie, diminishing the biodiversity that would otherwise prevail in these areas.

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The spread of cattle and sheep ranching in North America has made amajor impact on grasslands. In areas that are particularly sensitive, such asthe short grass regions found in the southwest, natural grassland hasbecome desert as a result of selective grazing by cattle. Cattle eat the grass-es but ignore such noxious species as cacti and creosote bush, allowingthese species to eventually replace the grasses.

SavannaSavanna is an open habitat with scattered trees among various grasses.

Trees, particularly acacias, may be scattered rather evenly among the grass-es or be more restricted to certain areas within the expanse of grasses. Windis often a major factor and periodic fire, set by lightning, is common through-out savannas. In Africa, large animal herds are an obvious feature of savan-na, but an abundance of large animals is not a universal characteristic ofsavannas elsewhere. The vast herds of antelope, gazelles, wildebeest, zebra,and elephants that serve as food for lions, cheetahs, leopards, and hyenasare unique to Africa.

Most of us associate savanna with east and southern Africa. But savannaoccurs elsewhere. In North America, the Everglades of Florida is a type ofwet savanna. There is much savanna throughout parts of South America,particularly the llanos of Venezuela, the cerrados of northern Brazil, and thepantanal of southern Brazil. Savanna can also be found in parts of CentralAmerica such as Nicaragua and Belize. Savannas are also found in parts ofIndia and southeast Asia, and Australia.

Savannas occur in regions with a strongly developed dry season. Theseverity of the dry season varies among savannas, but in East Africa, thedry season is sufficiently severe to require annual migrations of the largeanimals, as they move in search of water and fodder. Savannas generallyreceive between 50 and 150 cm of precipitation annually. During wetmonths, the grasses and trees are green and lush, but during dry season,the landscape turns brown.

Tropical savannas experience a mean annual temperature from about 18º to30º C, the same temperature range as is found with rain forests. The lack ofrainfall during dry season and overall high temperature result in significantphysiological stress on both plants and animals.

It is unclear to ecologists exactly what factors determine the existence ofsavannas. Not all savannas occur in areas with a protracted dry season. As aresult, some ecologists believe that savannas are also caused by nutrient-poor soils that, for various reasons, are insufficient to support lush forest.Prolonged human agricultural activities have been blamed for causing savan-na formation. The argument is that human use has depleted nutrients in thesoil, thus preventing regrowth of rain or seasonal forest. Savanna vegetationclaims such depleted soils by “default.”

Fire is another strong factor in savanna ecology. Periodic natural fires, justas they do with grasslands, aid in promoting regrowth of savanna grassesand sedges, at the expense of woody species that, in the absence of fire,might eventually come to dominate the region. Because most savannas arealso areas of strong human influence, fire frequency may have increased dueto human activity.

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Historically, savannas have been increasing in area since the middle part ofthe Cenozoic Era. Global climate has become drier and more temperate,causing the spread of savanna where once there was unbroken rainforest. It isthis climatic shift that was likely the major selection pressure in the emergenceof hominids, ancestors of human beings, from East and Southern Africa.

Without question the large herds of hoofed (ungulate) mammals that inhabitthe African savanna are the animals most associated with savanna ecology.Savannas elsewhere in the world lack this amazing mammalian diversity. It isinteresting to note that African rainforests do not support nearly the samemammal biomass as savannas.

The large herbivorous mammals of African savannas feed either by brows-ing or grazing. Browsers, like giraffes and Thompson’s gazelle, for example,feed mostly on leaves from shrubs and trees, while grazers, like zebra andwildebeest, rely mostly on grasses. There are numerous species of antelopesand gazelles throughout African savanna as well as zebra, wildebeest,giraffe, elephant, and wart hog.

Elephants exert strong effects on savanna ecology. They damage trees suchas baobobs as well as prevent their regeneration, thus eventually convertinga savanna rich in trees to one of mostly grassland.

Though vertebrates command most attention on African savanna, numerousinsect species are ecologically important, ranging from locusts, which periodi-cally experience population eruptions, to termites, whose tall mounds charac-terize savannas worldwide.

Human beings, as mentioned above, likely owe their very existence tosavannas. If one habitat deserves to be called “natural” as far as our species’evolution is concerned, it is savanna. It has even been suggested that ourfondness for lawns, for grassy areas with scattered trees, stems from adeeply ingrained familiarity with a savanna-like habitat.

Savannas support a high biodiversity of unique species. In addition, theseanimals, as is the case in Africa as well as other areas, are dependent onbeing able to move around, to migrate, with the seasons. It is thus impera-tive that large areas of savanna be maintained as sanctuaries for the ungu-lates and associated species. These must be separated from areas wherecattle and other non-native animals are grazed as the native animals can actas vectors for diseases such as sleeping sickness (vectored by the tsetsefly) and malaria (vectoredby mosquitoes). Such dis-eases can devastate cattle,as they are not nearly asresistant to the pathogensas the native species thathave shared a long evolu-tion with the pathogenand thus developednatural immunity.

Elephants on the move along thesavanna during the African spring. ©

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1. How do variables such as precipitation, fire, and grazing combine toaffect grassland?

2. What is the essential difference between grassland and savanna?

Jones, Stephen R., and Ruth Carol Cushman. A Field Guide to the NorthAmerican Prairie. Boston: Houghton Mifflin, 2004.

Brown, Lauren. Grasslands. New York: Alfred A. Knopf, 1985.

Gleason, Henry A., and Authur Cronquist. The Natural Geography of Plants.New York: Columbia University Press, 1964.

Reichman, O.J. Konza Prairie. Lawrence: University of Kansas Press, 1987.

1. The TIEE (Teaching Issues and Experiments in Ecology) journalwebsite provides an excellent abstract of an article by professorsHarmony J. Dalgleish (Kansas State University) and Teresa M. Woods(Utah State University) entitled “The Effects of Bison Grazing on PlantDiversity in a Tallgrass Prairie” —http://tiee.ecoed.net/vol/v5/practice/dalgleish/abstract.html

2. Konza Prairie Biological Station (LETR) Long-Term Ecological Researchat Kansas State University, Manhattan, KS, provides a comprehensivewebsite about prairie ecology — http://climate.konza.ksu.edu/konza

Websites to Visit

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eserts look dry. The very word “desert” seems to convey a senseof aridity, of a land where water is at a premium. Indeed that istrue. But there is much variety evident in the world’s deserts,which can range in appearance from undulating sand dunes with

little apparent life, to colorful landscapes awash with the color ofmyriad blooming flowers. Some deserts, like the Atacama along the

coast of Peru and Chile, resemble what we imagine the surface of a planetsuch as Mars to be, seemingly devoid of any life. Others, like the Great BasinDesert of western North America, are populated by a monotonous-lookingassemblage of shrubs, such as Big Sagebrush. Still others, like the SonoranDesert of Mexico, Arizona, and southern California abound with succulents,various cacti, and other species, some the size of trees.

Deserts contain many unique species of plants and animals because inhabi-tants of deserts are subjected to strong selection pressures resulting inremarkable adaptations to the constant aridity. Some adaptations of desertorganisms, like the water-holding barrel-shape of spiny cactuses, are readilyevident while others, like the subtle behaviors of lizards to control their bodytemperatures or the ability of kangaroo rats to subsist without drinking liquidwater, are much more subtle.

Desert biomes occur in many places on Earth. Deserts found in the temper-ate zone are called cold deserts because they experience snow and coldtemperatures in the winter months. Deserts closer to the equator are calledhot deserts because they rarely experience snow. Shrubs tend to be the

The Suggested Reading for this lecture is James A. MacMahon’s Deserts.

Lecture 10:Desert

The vast Atacama Desert, near San Pedro de Atacama, Chile.

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dominant form of plants in cold deserts, but succulents join shrubs in hotdeserts. In general, biodiversity is highest in hot deserts.

Globally, many deserts occur around 30º north or 30º south latitude. This isbecause of major convection cells in the atmosphere that converged at thoselatitudes and that are previously depleted of moisture. Thus dry air is forceddown, but precipitation does not occur, so deserts result.

There are four deserts in North America: the Great Basin, the Chihuahuan,the Sonoran, and the Mojave. Of these, only the Great Basin is cold desert. Itranges from the Canadian border and Washington south to Nevada and partsof Arizona and eastern California. It is called the Great Basin as it is borderedto the east by the Rocky Mountains and to the west by the Cascade andSierra Nevada mountain ranges. The Chihuahuan Desert is found throughoutmuch of northern and central Mexico, crossing the U.S. border in Texas andeastern New Mexico. The Sonoran Desert, perhaps the most picturesque ofany of the four North American deserts, contains many cactus species includ-ing the giant saguaro cactus and organpipe cactus. It is found from north-western Mexico and the Baja Peninsula through southern Arizona and partsof southern California. The Mojave Desert is the smallest of the NorthAmerican deserts, confined to southeastern California, including Death Valleyand the Salton Sea. It is famous for its joshua trees, which belong to a groupof desert-dwelling plants called yuccas.

In South America, one finds the Atacama Desert along the coast of Peruand Chile, blocked from obtaining moisture from the huge Amazon Basin bythe high Andes Mountains. There is desert in Africa both in the northern partof the continent, where the great Sahara Desert extends from Moroccothrough Egypt, as well as in the south, where the Karroo Desert is found.Much of interior Australia is desert, a huge area. The largest desert on Earthis the vast Gobi desert found throughout most of central Asia, throughoutmuch of China and Mongolia.

Deserts typically experience below 10 inches (25 cm) of annual precipitation.What moisture they do receive is seasonal, falling during a brief wet season.In the Sonoran Desert in southern Arizona, for example, rain falls briefly inthe spring, often resulting in a flush of annuals, many species of colorful flow-ers that bloom in synchrony. Rain also occurs again sparingly in mid to latesummer, when late afternoon thunderstorms are typical. The temperaturerange of deserts is wide, with annual mean temperatures from 23º F to 86º F(–5º C to 30º C). The warmest temperatures of any place in the United Statesare typically recorded for Death Valley, California, in the Mojave Desert,where a temperature of 134.5º F (57º C) was once recorded, ranking as thesecond highest temperature recorded on Earth.

Desert soils tend to be fertile, with an abundance of essential minerals. Thesparse plant growth that typifies deserts is thus the result of a continual short-age of water. Some desert plants, such as honey mesquite, have long tap-roots that grow sufficiently deep to reach ground water. Others, like the manyspecies of succulents, absorb large quantities of water during the brief timeswhen it is available and then utilize it as needed during the long dry spells. Inareas of high temperature, evaporative water loss from desert soils can resultin the deposition of minerals on the soil surface. Minerals, dissolved in the

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water, remain behind as the water evaporates. Salt lakes, where minerals areconcentrated in bodies of standing water, are common in some desert regions.

Because there is relatively little plant cover, erosion by wind and floodwa-ters is common in deserts. Such erosion sometimes exposes ancient sedi-mentary rocks that contain an abundance of fossils. In North America, such isthe case in many locations, including Dinosaur National Monument in Utahand the Petrified Forest in Arizona. One of the richest fossil hunting groundsin the world is the Gobi Desert in China and Mongolia, where the remains ofmany dinosaurs and early mammals have been excavated.

Given the natural fertility of many desert soils, it is little surprise that irrigationcan convert a desert into productive agricultural land. Doing so, however,involves importing a great deal of water and thus tends to be costly. The highcost is increased even more in hot desert irrigation because of the high evapo-rative water loss caused by high temperature. Thus crops such as corn raisedin such areas require even more water than they would in a cooler location.

Deserts have a surprisingly high biodiversity. Many species of plants haveadapted to thrive in deserts. Shrubs as well as large cactus species tend tobe widely spaced, with shallow, extensive root systems that absorb moisturefrom a large area of soil. Leaves are typically small and waxy, traits that mini-mize water loss from heat and wind. Some plants have an abundance ofthorns as well as toxic chemicals in their leaves to discourage herbivores.The pungent odors of shrubs such as creosote bush and big sagebrush areproduced by such chemicals. Species such as cholla cactus actually use theirdense thorns as a means of dispersal. These plants have long, chain-likebranches divided in to sections called joints. A joint can easily becomeattached to a passing animal such as a deer or peccary. The joint, transport-ed by the animal, eventually drops off or is picked off by the creature. Onceon the ground, the joint can root and grow. Many desert plants, most particu-larly cactus and Old World euphobias, photosynthesize mostly through theirstems. The palo verde tree of the Sonoran Desert is so named for its greenbark. Though it is leafless for much of the year, it still can photosynthesizewithout experiencing the evaporative water loss that would come with leaves.

Many desert plants, including creosote bush, mesquite, palo verde, andcacti, are pollinated by animals. Some, like creosote bush, produce small flow-ers, but others, like honey mesquite, become amazingly colorful when flower-ing, a real beacon for pollinators. In the case of the giant saguaro cactus, batsare the usual pollinators, visiting the large white flowers at night. Many insectsare important desert pollinators, as are some hummingbird species.

Though harsh physical conditions typify deserts, many intriguing biotic inter-actions are essential to ecosystem structure. For example, a seed of asaguaro cactus will not germinate unless it is on a slope and has some shadefrom a nearby rock or plant. The best place for a saguaro seed to germinateand grow is in the shade of a shrub or tree such as a palo verde. It is com-mon to see giant cactuses emerging from the low crown of a shrub or paloverde, which has served as a “nurse tree,” furnishing essential shade duringthe early development of the cactus.

Deserts harbor a high biodiversity of animals. Up to thirty bird species nestin cavities in saguaro cactuses, for example. Many desert animals are active

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only at night when the air temperature is much cooler, minimizing evaporativewater loss. Numerous rodent species inhabit deserts, some elegantly adaptedanatomically and physiologically to the harsh desert conditions. The kangaroorat subsists on water obtained from the seeds it digests. Jackrabbits havelarge ears that obviously serve to increase evaporation and cool the animalduring the heat of the day. Wood rats, sometimes called pack rats, assemblelarge middens containing mostly seeds as well as other objects unrelated tonutrition. Reptiles adapt well to deserts, and numerous species of lizards andsnakes can be found throughout the world’s deserts. One of the more spe-cialized species is the sidewinder, a rattlesnake that moves in a sidewaysmanner rather than forward, an adaptation to unstable, shifting desert sands.

People have adapted throughout history to living in deserts and continue todo so. Many of the major oil-supplying nations of the world are located in theMiddle East, now a region that is primarily desert. Thanks to sophisticatedthough costly irrigation techniques, countries such as Israel are able to growmany crops in some of the world’s most arid lands.

Human use of deserts in North America is potentially stressful to regionalwater supplies. The only way to bring water to deserts is to tap what is deeplybelow ground or transport the water, via pipelines, from other areas. Thus ashuman populations grow in desert regions, as agricultural activities in suchareas increase through irrigation, water is depleted from some other location.

Because of increasing human populations in desert regions, encroachmentby housing developments, malls, and industrial parks is threatening manydeserts with fragmentation and species loss. Immigrants to desert regions fre-quently attempt to replace natural vegetation with lawns and other water-costlylandscaping. Such luxuries as swimming pools, which tend to be common-place among residents of desert regions, require significant amounts of water.

Another threat to deserts is the increasing use of off-road vehicles. Like theArctic tundra, desert plants are not able to recover easily from disturbance. Itrequires about eighty years for a saguaro cactus to attain a height of 6 to 8feet, at which point it will produce its first blossoms. Full maturity of the plantis not reached until it is well over a centuryin age, and it can survive for up to 250years. Nonetheless, the root system is veryshallow, and a minor impact from an SUV orpickup truck can topple the tree in seconds.

Unfortunately, many desert plant speciesare in demand as ornamentals. The illegalcollection of various cactuses has becomean increasing problem, requiring legal pro-tection for species such as organpipe cac-tus and giant saguaros. Deserts, oncethought of as “wastelands,” must be recog-nized for their unique ecological character-istics and afforded reasonable conserva-tion protection.

A saguaro cactus emerged from the scant shadeprovided by a palo verde plant. ©

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1. What are the primary adaptations of desert plants?

2. Why are deserts fairly different in different places?

MacMahon, James A. Deserts. New York: Alfred A. Knopf, 1985.

Jaeger, Edmund C. Desert Wildlife. Stanford, CA: Stanford UniversityPress, 1961.

Kirk, Ruth. Desert: The American Southwest. Boston: Houghton Mifflin, 1973.

Phillips, Steven J., and Patricia Wentworth Comus, eds. A Natural Historyof the Sonoran Desert. Tucson: Arizona Sonoran Desert MuseumPress, 2000.

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ropical rain forest isthe most lush anddiverse of the

world’s terrestrial bio-mes. The word “jungle”is closely associated

with tropical rain forest, but rainforest and jungle, though related,are really different habitats withinthe same biome. Jungles are dis-turbed areas where rain foresthas been temporarily destroyed,either by cutting, windthrow, orsome other factor, to be replacedby a thick tangle of vegetationthat often grows so densely thata machete is necessary to cut apath through it. Jungles, if leftalone, will eventually transformback into rain forest, a processcalled ecological succession.

Tropical rain forests are muchmore open inside and deeply shaded than jungles, composed of manyspecies of trees, some of which grow to great stature, often in excess of 100feet. Trees, which are broad-leaved and usually evergreen, are often ratherslender, accentuating their apparent height. Branches radiate out high aboveground level, like spokes on an umbrella, and roots emerge in large buttress-es at the bases of the trees. At ground level rain forests are well shaded, asup to 99 percent of the light striking the canopy fails to reach the forest floor.

Rain forest trees are often abundantly laden with an impressive assemblageof epiphytes, plants that live on other plants. These include orchids, variouscacti, and bromeliads (with a pitcher-like array of spiky leaves resemblingthose of a pineapple). Vines also typify rain forests, including various figs(such as strangler fig), philodendrons, and looping, twisting lianas. The com-bination of numerous tree species plus diverse epiphytes and vines makesthe physiognomy, or physical structure, of the rain forest perhaps the moststructurally complex of any terrestrial ecosystem.

Animals are generally hard to see well within rain forest, as the complexphysical structure of the forest shelters its animal inhabitants very well.Mammals, ranging from canopy-dwelling monkeys to tapirs and tigers, can be

The Suggested Reading for this lecture is John Kricher’s A NeotropicalCompanion: An Introduction to the Animals, Plants, and Ecosystems ofthe New World Tropics.

Lecture 11:Tropical Rain Forest

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Tropical rain forest in southern Venezuela.

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very difficult to detect. Butterflies are conspicuous in places where sunlightstrikes blooming flowers, attracting the colorful insects. Birdsong reveals thepresence of many bird species and insect sounds include the almost constantwhine of cicadas and other stridulating insects.

Rain forest is found in equatorial regions throughout the world. Rain forestsare best developed in proximity to the equator, where true rain forest prevails.Moving north or south from the equator, forests become more subject tostrong seasonality and are called tropical moist forests. Rain forests andmoist forests, taken together, can be found within about a 47º latitudinal belt,bounded by the Tropic of Cancer to the north and the Tropic of Capricorn tothe south.

The only actual rain forest in the United States is on the Hawaiian Islands,though the Everglades in south Florida rank as subtropical. The largestexpanse of rain forest in the world is found in South America, in the AmazonBasin. Rain forest also occurs in Central Africa, Southern India, andSoutheast Asia, Thailand, Indonesia, Malaysia, New Guinea, as well asNortheastern Australia.

Constant heat and relatively constant high humidity characterize all rainforests. The growing season, the time in which plants can actively add tissue,is usually year-round. Typical daily high temperatures are around 88º F (31ºC), sometimes higher, and nightly low temperature is usually around 72º F(22º C). This temperature range prevails during all months of the year. Inother words, temperature is basically constant in rain forests, not seasonal,with cold winters and hot summers, as it is in the temperate zone. At groundlevel, humidity can reach as high as 95 percent, making the air feel veryoppressive. All precipitation, and there is lots of it, falls as rain. In any area ofrain forest or moist forest, it is usual for rain to fall anywhere from 130 to 250days per year. In the Amazon Basin, precipitation ranges between 150 cm(nearly 60 inches) to 300 cm (118 inches) annually. Some areas experienceup to 460 cm (180 inches) or more precipitation annually. Rain is typicallyseasonal, most of it occurring during the “rainy season,” though, in manyplaces, it rains substantially during the “dry season” as well. North of theEquator, rainy season is usually from late summer through early January.The opposite prevails south of the Equator.

Soils are often heavily eroded of their minerals in rain forest areas, due tothe constant precipitation washing the minerals from the soil, a process calledleaching. As water passes through soils it adds hydrogen atoms, making thesoil increasingly acidic. Adding to the low soil fertility is the fact that manytropical soils are geologically ancient, and their great age has also resulted inaccumulated leaching. It may seem ironic that lush areas of rain forest sitatop some of the Earth’s most infertile soils. However, recycling of minerals israpid and efficient in rain forests and most of the minerals are rapidly pulledfrom the litter (leaves, twigs, roots, dead animal bodies) by the vegetationeven before the chemicals have a chance to enter the soil and be washedaway in erosion.

Infertile tropical soils are often deep reddish in color due to the accumula-tion of iron and aluminum oxides, chemicals for which plants have little use.Most tropical soils also have a high content of clay, a microscopic kind ofsoil particle that makes the soil feel compacted, slippery, and gummy when

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wet. Not all tropical soils are infertile. In areas where there is recent geologi-cal activity, such as mountain raising or volcanism, tropical soils are youngand mineral rich.

More species of organisms comprise tropical rain forests than do any otherecosystem on Earth. One of the unique characteristics of rain forest is thehigh species richness that typifies it. Indeed, more than 50 percent of theworld’s total species are thought to inhabit rain forests. Nearly 300 differentspecies of trees have been identified within a single hectare (approximately2.5 acres, not a large area) of some Amazonian rain forests. In contrast, inthe most species-rich forests in the temperate zone, such as one finds in theGreat Smoky Mountains in Tennessee, at most thirty species of trees will beencountered within a hectare. Typically, the number is lower than that. Addedto the diversity of trees is the diversity of such groups as epiphytes and vari-ous vines, all adding to the profligate diversity of plant life in the forest.

Animal life is also stunningly diverse. There are more bird species, moreinsect species, more frog species, more snake species in rain forest than inother kinds of ecosystems. In one area in southwestern Amazonia, 1,234species of butterflies have been identified from within a two kilometer area.The total species richness of insects and other arthropods is unknown, butone estimate, based on carefully collected samples from single trees, sug-gests that as many as 20 to 30 million arthropod species may reside withinthe world’s rain forests.

Vertebrate diversity is high as well. Within a single reserve in AmazonianEcuador, eighty-one species of frogs have been documented, the samenumber of frog species found in the entire United States. Bats are highlydiverse in rain forests around the world. In African, Asian, and Australianrain forests there are large bats often called “flying foxes,” for their dog-likefaces. These impressive creatures dine mostly on fruit and are essential indispersing the seeds of many fruit trees. In South and Central American rainforests, bats are found that consume insects, fruit, nectar, fish, frogs, birds,other bats, and even blood. The infamousvampire bat, which makes small, painlessincisions in sleeping mammals and thenlaps the flowing blood, is found throughoutthe American tropics. In the tiny country ofBelize, in Central America, a land withapproximately the area of the state ofMassachusetts, there are eighty-four batspecies, compared with a total of fortyfound within the entire United States.

Birds are among the most species rich ofvertebrate groups in rain forests. Roughlytwice as many species of birds, 1,695, arefound in Colombia, in northern SouthAmerica, than in all of North America. Thereare far too many species of rain forest birdsto even attempt a summary but, like bats,they demonstrate many modes of feeding.

One of many species of Poison-ArrowFrog (Dendrobates ventrimaculatus)found in the Amazonian rain forests ofBrazil, Colombia, Ecuador, FrenchGuiana, and Peru. Adults reach a lengthof a little more than a half inch. It feedsmainly on ants. This photo was taken inthe Parque Nacional Yasuní in Ecuador.

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The harpy eagle of South America, the largest of the world’s eagles, feedson tree sloths and monkeys, which it plucks from the forest canopy during itsswift flight. These are similar eagles to the harpy in Africa, tropical Asia, andNew Guinea. The dinosaur-like flightless cassowarys of Australia and NewGuinea feed mostly on large fruits taken from the rain forest floor, thoughthey mix their vegetarian diet with occasional small animals. Colorful birds-of-paradise inhabit New Guinea and Australia, while equally brilliant manakinsand cotingas, such as the Guianan cock-of-the-rock, are widely spreadthroughout the American tropics. Both feed largely on fruit and both groups ofbirds include species where males concentrate in arenas of their own con-struction and perform elaborate courtship “dances” to entice females. Manykinds of parrots live within rain forests, most highly colorful though often hardto see in the dense green foliage. Not all rain forest birds are gaudy: manyspecies of antbirds are found in South America, most of them subtle shadesof gray and brown. Some antbirds actually accompany army ant swarms andfeed on the many arthropods attempting to flee the marauding ants.

Many kinds of primates, virtually all of them arboreal, dwell within the world’srain forests, ranging from lowland gorillas in Africa, orangutans in Sumatraand Borneo, to howler monkeys in the American tropics. Various hoofedmammals such as deer, pigs (or peccaries in the American tropics), andtapirs roam the forest interior, preyed on by jaguars (American tropics) andtigers (India and Asia).

Snakes, including constrictors such as pythons and boas, are common with-in rain forest and surrounding ecosystems, and many species of poisonoussnakes, pit vipers, true vipers, and cobras are found within rain forests. Incentral Africa, the lush rain forest is the haunt of the Gaboon viper, whichreaches five feet in length but is very thick, sometimes over twelve inches incircumference, weighing as much as eighteen pounds. The snake has fangstwo inches in length and very toxic venom. It is magnificently camouflagedamong the leaf litter of the rain forest floor.

A chestnut-mandibled toucan (Ramphastos swainsonii) photographed in the Honduran rain forest.

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Most people are aware that rain forests are being cut down around theworld, often for wood to be used as fuel or for building, often for such activi-ties as agriculture or cattle ranching. Loss of rain forest is of major concern tothose who value biodiversity and believe it to be essential for ecosystemfunctioning. As rain forest is lost, so are species. Many rain forest species arerare, their distributions limited, and thus they are quite vulnerable to loss ofhabitat. Most remaining rain forest is in the vast Amazon Basin, an areawhere increasing human encroachment is ongoing, but one that is soimmense that the majority of the overall forest remains, at least for now,intact. In contrast, rain forests of Central America, central Africa, and Asia arebeing lost very rapidly. Less than 5 percent of remaining rain forests on Earthare protected as national parks or reserves, so rain forests remain vulnerableto the chainsaw and bulldozer.

When rain forest is cut, it is typically burned, releasing carbon dioxide intothe atmosphere and contributing to global warming through GreenhouseEffect. Beyond that, rain forest is often replaced with ecosystems that take infar less carbon dioxide. Indeed, rain forest is often called a “sink” for carbondioxide, as it takes in so much of it in the process of photosynthesis. Thusrain forest removal is a “double-edged sword” in the steady accumulation ofcarbon dioxide in the atmosphere.

Rain forest loss is unlikely to be lessened, as the most rapidly growinghuman populations on Earth are in tropical regions. The conflict betweenhumanity’s perceived needs and the utility of preserving natural ecosystemsis nowhere more challenging than in tropical regions.

Rainfall is distributed quite unevenly over the course of a year in some tropi-cal areas. In such places, the dry season is so severe that tropical rain forestcannot exist on the site. In its place is a type of ecosystem called eitherTropical Dry Forest, Thorn Scrub Woodland, or Tropical Seasonal Forest. Inmarked contrast to lush rain forest, Tropical Seasonal Forest typically has adecidedly arid look to it. Trees are far smaller in stature than those that typifyrain forest and the species richness, not only of trees, but of other life-formsas well, is less, often far less, than in rain forest. Woodlands consist of rela-tively few tree species, typically those with thorns, such as acacias, for exam-ple. Mean annual temperature ranges from about 20º to 30º C, the normalrange for tropical ecosystems, but mean annual precipitation can range fromas low as 50 to about 250 cm. What this means is that there is a moisturegradient from moderate (250 cm) to low (50 cm) and thus there is a range ofecosystem types within the overall biome itself. At the wettest end of the gra-dient, forests exist, ranging from broad-leaved evergreen to seasonally decid-uous. The canopy is always low, typically no higher than 10 to 12 m (roughly30 or 40 feet) and epiphytes and vines, so common within rain forests, are farless abundant, if present at all. The ground is often covered with grasses andsmall shrubs, many of which have leathery leaves. On the driest end of themoisture gradient, bordering that of a typical desert, the ecosystem consistsof low stature trees often referred to as thorn scrub. Many animal inhabitantsof Tropical Seasonal Forest, from insects to large mammals, are migratory,their perambulations determined by their need to find water.

LECTUREELEVEN

1. What factors combine to make tropical rain forests the most species rich ofall the world’s terrestrial ecosystems?

2. What are the major threats to rain forest biodiversity?

Kricher, John. A Neotropical Companion: An Introduction to the Animals,Plants, and Ecosystems of the New World Tropics. 2nd ed. Princeton:Princeton University Press, 1999.

Forsyth, Adrian, and Ken Miyata. Tropical Nature: Life and Death in the RainForest of Central and South America. NY: Touchstone, 1987.

Janzen, Daniel H. Costa Rican Natural History. Chicago: University ofChicago Press, 1983.

Primack, Richard, and Richard Corlett. Tropical Rain Forests: An Ecologicaland Biogeographical Comparison. Oxford: Blackwell, 2005.

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he world’s oceans are the largest and most voluminous ofEarth’s ecosystems. The major variables of importance in deter-mining ocean ecology are the amount of solar radiation received

and the abundance of available elemental nutrients such as phos-phorus and nitrogen. The topic of oceanic productivity will be dis-cussed in the next chapter. What is important here is to understand

the basic organization of oceanic ecosystems. Oceans do not exhibit lifezones that are comparable to those found terrestrially. It is helpful in under-standing the great diversity of oceanic life to recognize patterns in zonationas determined by depth.

Consider that the oceans’ average depth is about 2.33 miles (3.7 km), andthat the deepest trenches in the oceans exceed 7 miles (11.2 km), substan-tially deeper than Mt. Everest is tall. Physical conditions change radically fromsurface waters to deep waters. In addition, conditions near shore and overthe continental shelf are different from those far from shore, in open ocean.

Pelagic Zone

When you are on a ship on the open ocean, far from shore, and you look atthe rolling waves of the sea, you are looking at the Pelagic Zone. Supposeyou had access to a submersible, such as a diving bell, a device that could

The Suggested Reading for this lecture is Sean Connell’s Marine Ecology.

Lecture 12:Marine Ecosystems

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The Major Zones in the Marine Ecosystem

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take you many fathoms (a fathom is a nautical term that means six feet)below the surface. What would you find?

You begin your journey at the ocean surface, where sunlight strikes the sea.This is where photosynthesis is occurring, so it is called the euphotic zone,the zone of light. If you were to drag a plankton net you would collect manykinds of phytoplankton, much of it single-celled diatoms, dinoflagellates, andother minute plants. You would need a microscope to see these tiny plantswell, though they are ecologically equivalent in what they do to the trees offorests and the grasses of prairies. They are the primary producers of theoceanic realm. Some phytoplankton, collectively called nannoplankton, are sosmall that they elude capture in plankton nets and must be centrifuged fromwater to concentrate them for microscopic observation.

Feeding on the phytoplankton hordes are various kinds of zooplankton, tinyanimals, some single-celled, but most multicellular. Some, like copepods,which are crustaceans related to lobsters and crabs, are permanent membersof the zooplankton community, but others are larval stages of animals such asbarnacles, crabs, and jellyfish. Zooplankton form the food base for numerouskinds of small fish, which in turn support larger fish and other organismsincluding tuna, mackerel, sharks, sea turtles, whales, and porpoises. The term“plankton” is taken from the same Greek root as the word “planet,” meaning“wanderer.” Plankton organisms are so small that they have limited mobility, ifany at all, against the currents, placing them, for all intents and purposes, atthe mercy of the currents. All of the fish, cetaceans, turtles, and larger inverte-brates such as squid and shrimp are collectively referred to as “nekton,” aterm that refers to their ability to move effectively against the currents.

Much of the life in the ocean is confined to about the first 600 feet or so(about 200 meters), where light penetrates. If phytoplankton is abundant, lightwill attenuate at a more shallow depth and, if phytoplankton is in low density,such as in some tropical seas, light will penetrate more deeply. Animals,some of which migrate to the surface from deeper waters on a nightly basis,concentrate in the surface waters because that’s where most available ener-gy is found. Phytoplankton (or any other plants) cannot photosynthesizebeneath the euphotic zone.

Once your diving bell passes through the last twilight of the euphotic zoneyou have entered the permanent darkness of the mesopelagic zone. Youwould notice far fewer kinds of fish, but what you would see would astoundyou. Even in the darkness you would see some of the fish, because they lightup. Numerous species are bioluminescent, illuminated by tiny lights similar tothose found on fireflies (which are not really flies at all, but beetles).Bioluminescence is a form of “cold light” created by the oxidation of certainprotein molecules. Most of the bioluminescent fish are tiny, no more than thesize of a quarter. These include schools of lantern fish and hatchetfish.Joining them in the water column would be various squid and shrimp. Withluck, a huge sperm whale might dive past you as it plumbs the depths insearch of its principal prey, the giant squid.

Deeper still, your diving bell is withstanding many hundreds of pounds ofpressure per square inch as you enter the bathypelagic zone, the deepestpart of the ocean, profoundly dark and very cold. As is the case with the

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mesopelagic zone, the permanent inhabitants of this zone must rely onorganic matter from the euphotic zone to eventually drift downward. A fishthat dies in the surface waters is consumed well before it strikes the bottomseveral miles below. You might see the skeleton of a tuna or swordfish drift-ing past your diving bell but little more of the deceased creature. Energy is ata premium at such a great depth.

Some amazing fish inhabit the deepest depths of the oceans. Many aresmall, their color predominantly black, though with bioluminescent pattern-ing, particularly in the head region. Some, popularly called swallowers andgulpers and viperfish, have immense jaws with needle-like teeth. A swallow-er has an abdomen that can expand to hold a fish longer than it is. Thereare also species of deepwater sharks and rays as well as deepwater squidand octopus.

Deep-sea anglerfish are chunky, with huge mouths that snap up prey attract-ed to the bioluminescent lure that they wiggle to entice preyto within capture distance. Somespecies of deep-sea anglerfish have acurious life cycle such that males trans-form into parasitic worm-like animals thatattach to the body of a female anglerfish.This odd characteristic is adaptive andpoints to an interesting ecological char-acteristic of the bathypelagic zone. It isadaptive because these animals, likemost of the bathypelagic creatures, arerelatively rare, existing in small popula-tions within an immense volume ofhabitat. In an environment with so littleorganic matter, most populations will belimited. When a male anglerattaches to a female andbecomes an ectoparasite, at leastthe female has a source of spermaccompanying her. When shesheds eggs, she need not search the vastness of the ocean depths for amate. He’s already attached to her.

Benthic Zone

The R.M.S. Titanic, subject of several popular motion pictures, many books,and an ambitious and successful underwater search, rests on the benthiczone of the North Atlantic Ocean. The term “benthic” refers to bottom. It canbe applied widely, to lakes, shallow bays, or the ocean depths.

Once it was believed that no life could exist on the ocean bottom. It seemedincredulous that an environment so deep, so permanently cold, with no light,and under thousands of pounds per square inch of water pressure, couldsupport living things. Such a view was, to say the least, naïve. Another fanci-ful idea was that the ocean bottom was uniformly covered by a curious organ-ic “jelly” called “Bathybius,” a living organic slime of sorts. Though such asuggestion may sound like the plot of an old episode of Star Trek, it required

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Illustration of a humpback anglerfish(Melanocetus johnsonii)

Source: Brauer, A. Die Tiefsee-Fische: Tiefsee-ExpeditionValdivia, 1898–99. Jena: Berlin, 1906.

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a serious oceanic expedition, the “Challenger Expedition,” from 1872 to 1876,to disprove it.

When your diving bell has reached bottom, you observe the soft sedimentsthat cover the ocean floor, sediments that are far younger than the oceanitself, many of them the countless tiny shells of pelagic forms of amoebascomprising part of the permanent zooplankton. There are organisms movingabout or protruding from the sediments. You might see such things as bra-chiopods, clam-like creatures that were far more diverse and abundant in thePaleozoic Era, before the dinosaurs evolved. Or waving serenely in the slowbottom currents, you might see crinoids, plant-like, stalked echinoderms relat-ed to sea stars and sand dollars, whose ancestry also reaches back to thevery appearance of multicellular animals in the fossil record. The ocean floormay be virtually covered with active brittle stars or bizarre-looking sea cucum-bers, all roaming the ocean floor. Perhaps you might see the odd-looking tri-pod fish, which supports itself on two spiny extensions of its pectoral fins andone from its caudal (tail) fin, making it look like a tripod. Should you find adead whale or other carcass on the sea bottom, it would be swarming withopportunistic scavengers. Among the oddest of the deep sea fish, hagfish arejawless fishes that may be among the only remaining lineages of some of thefirst of the vertebrates. Their closest relatives are lampreys, also jawless,some of which inhabit fresh water. Hagfish are also called slime eels becausethey exude a dense mucous through skin pores. They are extraordinarily flex-ible, capable of tying themselves in knots (and untying themselves). Hagfishmass at carcasses of dead organisms and the thick mucous that sometimesclouds the water probably is adaptive in discouraging other competitors suchas sharks from consuming the carcass.

In addition, perhaps you would see elongate fish called grenadiers or ratfishthat compete with hagfish for access to whatever organic matter they canfind. Even at the greatest depths of the oceans, you can study ecology.

Littoral Zone

Coastal areas, where the presence of continents makes the sea shallow,support complex ecosystems. The littoral zone, a term that can apply to lakesand estuaries as well as to oceans, is the shallow area at the edge of a bodyof water, where light penetrates all the way to the bottom, making photosyn-thesis possible throughout the water column. For this reason, macroscopicplants such as kelp can grow in abundance, joining phytoplankton in captur-ing solar radiation.

The littoral zones of the world’s oceans support many kinds of ecosystems.These include kelp “forests,” prime fishing grounds such as the Grand Banksof the North Atlantic, and even tropical coral reefs.

Littoral zones are ecologically diverse and productive because physical con-ditions are conducive to biodiversity. Not only is there light throughout most, ifnot all, of the zone, but proximity to shore means that nutrients are regularlyintroduced, transported from rivers to the sea, fertilizing the shallow waters,making them more productive. Currents and tides mix the water, ensuringavailability of nutrients and an abundance of dissolved oxygen.

Many kinds of fish and invertebrates inhabit littoral waters, as well ascetaceans and seabirds. The littoral zone is a prime area of reproduction for

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many forms of life in the sea, and shallow waters can serve as ideal “nurs-eries” for juvenile fish. Offshore currents help distribute larval forms of fishand invertebrates, including many that inhabit the intertidal zone.

Intertidal Zone

The place where sea meets land is the intertidal zone. The definition refersto the area exposed between low and high tide. Intertidal zones are com-posed primarily of marine organisms,including some of the hardiest exam-ples. Like the littoral zone, it is an areaof diverse habitats. It may be rocky,sandy, or composed mostly of mud,exposed at high tides. Intertidal zonesmay be associated with such coastalecosystems as salt marshes and tropi-cal mangrove swamps.

Rocky intertidal zones have manykinds of algae, often called seaweeds,among which are various barnacles,mussels, crabs, snails, and other life-forms. There is usually a fairly clearzonation from low to high water, withonly the hardiest creatures able to tol-erate prolonged exposure to air. Justlook at a dock piling exposed at lowtide, for example, and you will likelysee brown and red algae still partlysubmerged and partly mixed with mus-sels, above which, totally exposed, willbe a zone primarily of barnacles.

Sandy and muddy intertidal zones arefrequently too unstable to supportmany organisms on their surfaces, butthey both have a diverse infauna ofburrowing worms and mollusks as wellas others. In some areas, however, there are impressive forests of kelpsamong which there are many kinds of fish and invertebrates. Shallowerwaters support plants such as eelgrass, one of the only vascular plants (thusnot an alga) that thrives in salt water. Eelgrass flats are often associated withsalt marshes and support many kinds of marine organisms, including suchodd fish as seahorses.

Intertidal zones have many of the same ecological advantages as littoralzone habitats: an abundance of oxygen, light, and elemental nutrients wellmixed by the tides. But intertidal zones are also subject to intense natural dis-turbances, as tides fluctuate and as storms batter coastal areas. Thus thereare always areas of disturbance and recolonization within intertidal zones.The life cycles of organisms have evolved to adapt to such realities.

A rock on a beach near Kalaloch, Washington.The rock, seen at low tide, exhibits typical inter-tidal zonation.

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1. What are the major zones of the sea and how does zonation by depthcompare with factors that determine terrestrial biomes?

2. What areas of the oceans are most productive and support the highest bio-mass, and why?

Connell, Sean. Marine Ecology. Oxford: University of Oxford Press, 2007.

Bertness, Mark D., Steven D. Gaines, and Mark E. Hay, eds. MarineCommunity Ecology. Sunderland, MA: Sinaur, 2000.

Hardy, Sir Alister. The Open Sea: Its Natural History. Boston: HoughtonMifflin, 1970.

Russell, F.S., and C.M. Yonge. The Seas. London: Frederick Warne, 1975.

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here are a number of marine ecological systems that appearonly in specialized places on the Earth. Their uniqueness is dueto such forces as the latitude, landform, wind, and ocean cur-

rents. Other factors may be the nearness of freshwater estuaries,extreme tidal variations, and human intervention.

Salt Marsh

A marsh is defined as a wetland where grasses and sedges predominate.Freshwater marshes are often characterized by such species as cattails(Typha spp.). A salt marsh, as the name implies, is a coastal marsh wheretidal influence is strong. Consequently the marsh is regularly inundated bysalt water. However, a salt marsh is also affected by fresh water carried byrivers as they flow to the sea. Thus a salt marsh has brackish water, itswaters carrying variable concentrations of salts, depending upon the degreeto which fresh and salt water have mixed in the marsh and depending uponthe tidal cycle. Salt marshes are usually in close proximity to estuaries. Thecombination of marsh and estuary is the ideal habitat for many kinds ofmarine organisms, particularly juvenile life-cycle stages of fish and inverte-brates that will later join the fauna of the open sea. Thus salt marshes have avaluable role in the conservation of marine fisheries.

The ecological worth of salt marshes has not been appreciated until relative-ly recently. Both on the East and West coasts, salt marshes were historicallyregarded merely as havens for mosquitoes. Consequently, they were exten-sively drained and converted to various human uses. The so-called Back Bayregion of Boston, Massachusetts, now home to many thousands of people, iswell named. It was once a bay, extensively bordered by salt marsh. Those

The Suggested Reading for this lecture is Mark D. Bertness’s AtlanticShorelines: Natural History and Ecology.

Lecture 13:Unique Coastal Ecosystems

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Bride Brook and Coastal Salt Marsh, near East Lyme, Connecticut

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ecosystems were long ago either filled or drained or some combination ofboth. A visitor to Boston today would never suspect that much of “the city”was once bay and salt marsh. The same story is true of much of SanFrancisco, California.

Salt marshes are characterized by grasses and sedges. These species arecollectively called halophytes, as they are tolerant of high concentrations ofsalt. Salt marshes in eastern North America consist mostly of Spartina grass-es. One species, Spartina alterniflora, called salt marsh cordgrass, is themost physiologically hardy of any of the marsh grasses and thus tends todominate areas of highly salty water, where tidal influence is greatest. Saltmarsh cordgrass grows well under virtually anoxic conditions and with highsalt exposure. No other salt marsh plant can survive under such oxygen limi-tation, thus salt marsh cordgrass is able to persist by virtue of its physiologi-cal ability to adapt to extremely low oxygen levels. When in competition withother salt marsh grass species under controlled conditions, where the sub-strate is not anoxic, cordgrass is easily outcompeted.

A closely related species, S. patens, called salt meadow hay, is usuallyfound growing at higher elevation, beyond alterniflora. Mixed among the S.patens is spikegrass (Distichlis spicata) and various glasswort species(Salicornia spp.). Other species grow in zones higher in elevation, beyondpatens, thus, like is the case with the intertidal zone, salt marshes show apattern of zonation as tolerance levels to saline water varies among the resi-dent species. Beyond the salt hay zone there is typically a zone of black rush(Juncus gerardi) followed by a zone of marsh elder (Iva frutescens). Saltmarshes of the Pacific coast also are characterized by Spartina at the water’sedge, but the species is S. foliosa. It is followed by a similar zonation patternto that seen in the East, though the species are somewhat different.

Salt marshes, though low in plant biodiversity, are highly productive ecosys-tems, accomplishing much photosynthesis throughout the growing season. Inaddition to the salt marsh grasses and related plants, many minute diatomscoat the mud with a golden sheen. Their combined photosynthesis furtherenhances the overall marsh productivity.

The high productivity of the salt marsh supports many kinds of invertebrates(crabs, shrimp, snails, mussels, worms) as well as animals such as minks,raccoons, and numerous bird species such as herons, egrets, rails, andshorebirds. Many insect species, particularly such nuisances as mosquitoesand biting flies, also abound in salt marshes, where they are fed upon by var-ious dragonflies and birds such as tree swallows (Tachycineta bicolor) andpurple martins (Progne subis).

Fiddler crabs are common in salt marshes. Males have one claw that is muchlarger than the other, giving the group its general name “fiddler crab.” Studieshave demonstrated that fiddler crabs have a mutualistic relationship with saltmarsh cordgrass. The extensive burrow network of the crab colony aids inbringing aeration to the roots as well as slowing down the accumulation ofdense sediment (called peat), which limits the growth of the cordgrass. Thepresence of the stand of cordgrass acts to stabilize the mud, a huge advan-tage for the crabs. Without the cordgrass, the tidal currents would continuallyagitate the mud, making it difficult for the crabs to have a stable colony.

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Salt marshes export much of their productivity to neighboring estuaries.Grasses that are products of the growing season die back in winter, survivingthe cold and ice of winter as roots in the mud. The above-ground shoots,often sheared off by ice or tidal forces, are moved by tidal action, eventuallyreaching estuaries where they are colonized by a host of microbial organismsthat form a food resource for larval invertebrates and fish. Thus the marshgrasses are also an important energy resource for the estuary.

Salt marshes act as natural sponges, absorbing nutrients, which are recy-cled to the growing marsh plants. Some organisms such as the ribbed mus-sel (Geukensia demissa) are keystone species in concentrating nutrients.These filter-feeding mollusks, which grow abundantly among the salt marshcord grasses, feed by filtering suspended particles such as plankton fromwater during periods of immersion. In doing so, they remove and concentratenutrients such as nitrates and phosphates. These are deposited on the marshmud in the form of compact “pseudofeces” that serve as small pellets of fertil-izer. Many marsh species, including various clams and oysters, also filterwater to remove suspended food particles. Pollutants, ranging from pesticidessuch as DDT to heavy metals such as mercury, are also removed by organ-isms and concentrated in the marsh. This is the mode by which these eso-teric and often highly dangerous compounds begin to bioconcentrate as theyenter natural food webs. Natural “pollution products” such as the toxins pro-duced by minute algae (dinoflagellates) responsible for red tide also can bio-concentrate in marshes due to activities of filter feeders.

Mangrove Forest

Mangroves are a diverse assemblage of coastal tropical tree species, alltolerant of high salt concentration. There are thirty-four species worldwide,mostly distributed throughout the vast tropical Pacific. A few species occurabundantly in tropical America. Mangroves are not a taxonomic group anymore than all salt marsh plants are, instead representing numerous plantfamilies. Mangroves are all defined by their physiological tolerance to immer-sion in salt water and thus are the tropical equivalent of salt marsh grasses,sedges, and associated plant species. Like salt marsh plant species, man-groves are halophytes.

Mangrove forests typify tropical coastlines and may line rivers where thereis tidal influence. Like salt marsh grasses, they thrive in high saline environ-ments, but are outcompeted by plants in low salinity areas, where otherwisemangroves could survive. In general, mangrove species are excellent colo-nizers and quickly invade and grow in areas subjected to the effects of tropi-cal storms such as hurricanes. Most mangroves are small trees, rarelyexceeding 10 to 20 meters (33 to 66 feet) in height. Some grow as denseshrubs. Mangrove root systems grow well in soft coral sands as well asthick, anoxic mud. The forest itself provides a complex coastal habitat formany species of animals.

Red mangrove (Rhizophora mangle) is an abundant species in tropicalregions throughout the world. It typically has extensive prop roots anchoring itfirmly in the shifting sands. The roots are abundantly equipped with lenticels,openings that admit air needed for the roots to survive when immersed. Asuperb colonizing species, red mangrove seeds actually germinate while still

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attached on the parent plant. The seedlings look like long, pencil-like podshanging vertically from the tree. Once dropped in water, they can survive fora long period, transported by the currents. As a seedling absorbs seawater itsoon floats vertically rather than horizontally, bobbing with the current until itstrikes land, where it immediately grows a root to anchor it. Red mangrovesform dense stands on exposed sand and mudflats, taking advantage of sedi-ment deposition by coral reefs. As other mangroves colonize, a new man-grove island, or caye, is formed.

Black mangrove (Avicennia germinans) often grows densely in thick, anoxicmud. It manages to secure sufficient oxygen because its shallow, horizontalroots send up vertical shoots, called pneumatophores, which allow adequateamounts of air to enter the root system.

The prop roots of mangroves provide excellent habitat for many marine ani-mals. Various sponges, anemones, tunicates, mollusks, and worms find pur-chase among the roots. Numerous fish and crabs find shelter and food aswell. Many species of coral reef fish spend part of their juvenile life cycleamong the mangroves. In this manner, mangrove swamps function like saltmarshes and estuaries, providing nurseries for juvenile marine animals.Similarly, mangrove leaves, thick and waxy, eventually drop on to the sedi-ment or into the sea, where they provide an essential energy source. Redmangrove is highly productive, its rate of photosynthesis so high that it canadd up to eight grams dry organic matter per square meter per day. How isthis energy utilized? Once leaves drop, they act as sites for microbial coloniz-ers (bacteria, fungi, various protozoans) that, in turn, serve as a food base forlarval sea animals. A fish can ingest a particle of mangrove leaf, digest thevarious organisms on it, and spit out the leaf particle or allow it to passthrough its alimentary canal. In either case, the leaf fragment now back in thewater is soon recolonized and serves yet again as a food base for microbes.Thus the energy of the mangroves is moved, in small and continuousamounts, into sustaining the marine food web of the coral reef and surround-ing seagrass. It is essential to recognize how the food webs of what appearto be various different kinds of ecosystems are, in reality, closely linked.

Above- and below-water view of mangrovesin Bangladesh.

Mangrove “knees” (pneumatophores) at low tideon the Yap Islands of Micronesia in the Pacific.

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Mangroves, like salt marshes, have high ecological value. It is thus regret-table that throughout the world mangrove forests are being cleared by suchactivities as dredging, channeling, or cutting as a source of wood. Huge tractsof Southeast Asian mangrove forest were destroyed by herbicide sprayingduring the Vietnam War, over three decades ago. In addition to serving asessential nurseries for many marine species, and in buffering the land againstthe forces of tropical storms, mangrove forests form important nesting areasfor tropical waterbirds such as pelicans, boobies, herons and egrets, spoon-bills, and frigatebirds. In addition, long distance migrant landbirds often over-winter in the food-rich mangrove forests.

Mangrove forests persist in North America in southern Florida and along theFlorida Keys. The endangered American crocodile (Crocodylus acutus) is oneof many species dependent on the mangrove ecosystem.

Coral Reef

Coral reefs are found throughout the clear marine waters of the world’s trop-ics. Among the oldest ecosystems on Earth, coral reefs, though not with thesame assemblages of species found today, have existed virtually since theCambrian Period, over 500 million years ago. Reef organisms are among themost abundant in the marine fossil record. Today, coral reefs support the high-est species richness of any marine ecosystem. Like their terrestrial counter-parts, tropical rainforests, corals reefs support myriad species and accomplishhigh rates of photosynthesis.Reefs are both diverse andproductive. All coral reefs areconfined to clear, warm tropi-cal seas. Cool water or sedi-ment deposition will kill coralreefs. Coral reefs require atemperature of at least 18º C(64.4º F).

Most coral species are inIndo-Pacific waters, whereover 700 species can befound. In comparison, Atlanticreefs are much less speciesrich, with only about sixtyspecies. This difference inspecies richness between theolder and larger Indo-Pacifictropical oceans and theAtlantic/Caribbean is true offish as well. There are about500 fish species in theBahamas, compared with2,000 in the Philippines and1,500 in Australia’s GreatBarrier Reef. The differencein richness between the two A pillar coral at the Florida Keys National Marine Sanctuary.

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oceans likely is scale related with regard to both time and area. The Indo-Pacific is a much older ocean, with considerably greater area than the Atlantic.Both of these factors would suggest more speciation in the Indo-Pacific.However, while the total species richness of fish is lower in the Caribbean thanin the Pacific, the number of fish species that can be found, per unit area,within a given reef in either region is about the same. In other words, youwould find approximately equal numbers of fish species per 100 meters ofcoral reef in the Caribbean and Pacific, but in the Pacific, as you moved fromone reef to another, you would find new species. In the Caribbean, you wouldfind many of the same species youhad already encountered.

Coral is a kind of animal related tosea anemones and, somewhat moredistantly, to creatures such as jellyfishand hydras. They are part of the phy-lum Cnidaria (sometimes calledCoelenterata), known for their tenta-cles that possess cnidoblasts or sting-ing cells. All reef corals are colonial,growing together in matrixes that arecomposed mostly of calcium carbon-ate secreted by the animals them-selves. The reef structure is also aug-mented by algae that secrete calciumcarbonate and, in the Indo-Pacific, bygiant clams. Corals are cup-like, tinyanimals called polyps that captureminute organisms suspended in thewater column, using their tentacleswith stinging cells.

Reef corals have an extraordinarilyhigh primary productivity, fixing about as much carbon as rain forests thathave huge amounts of plant biomass. But where are the plants on the reef?Corals are, after all, animals. The answer to why it is that coral reefs are soproductive can be sensed by looking at the shapes of the corals themselves.In shallow, well-lit waters, they resemble the branching patterns so typical ofplants. A close look within the coral matrix reveals the presence of numer-ous tiny, one-celled plants called zooxanthellae. These plants and their coralhosts are mutualistic in their interaction toward one another. Zooxanthellaeaugment the growth of the coral colony and supply much carbohydratebecause they are active photosynthesizers. Measurements of the intake andoutput of carbon dioxide on a reef indicate that corals with zooxanthellae,which are known as hermatypic corals, are, for all intents and purposes,functioning physiologically as plants. They take in more carbon dioxide thanthey respire, all due to the presence of the zooxanthellae.

Coral reefs are structurally complex ecosystems, characterized by both hori-zontal and vertical zonation patterns. Horizontal zonation is determined bythe intensity of wave action from the windward to the leeward side of the reef.Vertical zonation is a result of depth and light penetration. Reefs may form as

Anatomy of a coral polyp.©

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fringing reefs, barrier reefs, or atolls. Fringing reefs typically surround islands,built by the corals in the shallow sediments where the island rises. Barrierreefs run parallel to coastlines, and atolls are circular reefs that remain wherean island, now subsided, once existed. In one of his first major scientificworks, Charles Darwin correctly surmised that the various kinds of coral reefsall form from the same basic process of coral growth. In other words, an atollwas once a fringing reef but, as the island subsided, the activity of the mil-lions of coral animals maintained the reef at the water’s surface, even as theisland disappeared beneath the sea.

Ecologists have learned that corals are highly competitive with one another.Like plants, some corals overtop and shade out other species. Still otherspoison their competitors when in direct contact with them. Given the clearcompetition among coral species, it may seem surprising that reefs can main-tain a high species richness of corals, but they do. The reason is that distur-bance is a relatively constant feature of coral reefs.

Disturbance by wave action or storms (such as hurricanes) can quicklychange the dynamics of competitive interactions on reefs. In this way, coralreefs are similar to ecosystems such as rainforests, where periodic distur-bances of varying magnitudes are a key component to the persistence ofhigh species richness. Some disturbance agents can reek havoc on coralreefs. One in particular is the sea star (starfish) Acanthaster planci, the so-called crown-of-thorns. Population outbreaks of this species can result in thedestruction of whole reefs. It is unclear what causes such outbreaks.

Like rainforests, coral reefs are being lost due to various human activities. Inmany areas, over-fishing is a major problem, but other activities such asdredging and mining can cloud the water to the degree that corals can nolonger endure. Chemical pollution is also of major concern. In recent years ithas been learned that meteorological events such as El Niño negativelyaffect coral reefs. El Niño is a periodic and generally unpredictable change inglobal climate caused by the migration of a high-pressure system in the cen-tral Pacific. Patterns of current flow change, precipitation patterns change,and the result is that some ecosystems suffer serious negative impact.Unfortunately,coral reefs dobadly in El Niñoyears. Globally,many scientistsbelieve coralreefs are gener-ally in decline.

Colorful reef fish—Pennantfish,Pyramid butterflyfish,and Milletseed but-terflyfish—school ingreat numbers atRapture Reef, offthe northwesternHawaiian islands.

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1. What are the most essential ecological functions of salt marsh and how arethese functions similar to mangrove forest processes?

2. Why is the coral reef so species rich and so highly productive? What arethe current threats to coral reef ecosystems?

Bertness, Mark D. Atlantic Shorelines: Natural History and Ecology.Princeton, NJ: Princeton University Press, 2006.

Kaplan, Eugene H., and Susan L. Kaplan. A Field Guide to Coral Reefs:Caribbean and Florida. Boston: Houghton Mifflin, 1999.

———. A Field Guide to Southeastern & Caribbean Seashores. Boston:Houghton Mifflin, 1999.

Shumway, Scott. The Naturalist’s Guide to the Atlantic Seashore: BeachEcology from the Gulf of Maine to Cape Hatteras. Helena, MT:Falcon, 2008.

Teal, John, and Mildred Teal. Life and Death of the Salt Marsh. New York:Ballantine, 1991.

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cology is sometimes called a“subversive science” because itseems to commonly espouse

views antithetical to well-established disciplines such

as economics. Conservationcan historically be viewed as a sociopoliti-cal movement that places intrinsic value onnature. Ecological science was not found-ed to promote conservation, though ecolo-gists typically have sympathies toward thenatural environment. As ecology has matured as a predictive discipline,approaches to conservation have become increasingly based on science andthus ecology has been at the forefront of such approaches. The field of con-servation biology is now recognized as one of the sub-disciplines of ecology.Ecologists have also developed applied and restoration ecology as sub-disci-plines of ecology, so ecological principles are being widely applied in waysthat were not imagined a few decades ago.

Conflicts arise about how best to utilize, value, and preserve what are per-ceived as natural environments. Humans, at least in Western culture, oftenconsider themselves as somehow apart from nature, rather than part of the

The Suggested Reading for this lecture is Edward O. Wilson’s TheFuture of Life.

Lecture 14:Current Issues in Global Ecology

“We must find new waysto provide for a humansociety that presently hasoutstripped the limits ofglobal sustainability.”

~Peter Raven, PresidentAmerican Association for the

Advancement of Science, 2002

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evolutionary process from which nature results. The philosophical rootsof the Western view of nature are historically deep and may be at leastpartly explained by the traditional Judeo-Christian theological view of manand nature.

Technology driven by science and economics coupled with human popula-tion growth have exerted strong anthropogenic effects on global ecosystems.This has resulted in a global loss of topsoil and agricultural land, depletion ofthe ozone layer, addition to the atmosphere of greenhouse gases, and ongo-ing major losses of biodiversity. Humans demand a uniquely large percent-age of Earth’s net primary productivity and renewable fresh water.

Earth can be viewed as a global ecological commons, where resources suchas soil, air, water, and wildlife are the common resources of humanity.Concerns are focused on how best to manage global resources for sustain-ability. The world’s fisheries are in dramatic decline because, as fundamental-ly a commons-type resource, there has been inadequate study, regulation,and agreement about how best to manage fisheries. Different nations takedifferent approaches, but stocks continue to decline. It is necessary to aban-don the concept of a free commons and replace it with one in which all inter-ested parties are subject to strong regulation regarding degree of exploitation.

The major environmental problem facing citizens in this century is how bestto ensure that Earth’s ecosystems remain sustainable such that we continueto enjoy the diverse ecosystem services essential to the functioning of thebiosphere. Economic analysis of natural ecosystem function compared withgains derived from human development demonstrates that it is often in thelong-term economic best interest of society to preserve natural ecosystems.

Environmental ethics has emerged from the pragmatic need to recognizeand value the natural environment and its component species. Still in itsinfancy as a discipline, environmental ethics attempts to rigorously provide arationale for how applied conservation biology ought to be practiced to thecollective good of humanity.

Biodiversity loss is arguably the single greatest problem in conservation biol-ogy. Biodiversity can be expressed as numbers of species, genetic diversityamong populations, or ecosystem diversity. It is not known how many speciesactually inhabit Earth today, but it is clear that biodiversity is in decline. Thecurrent rate of extinction far exceeds the current rate of speciation, so Earthis losing species.

There are many causes for biodiversity loss, including habitat loss, pollutionand contamination, and over-exploitation, such as is seen in modern industri-alized fisheries operations or the decline of great ape populations in Africa.Another major cause for biodiversity loss is the increasing number of invasivespecies that, by various means, are entering ecosystems in which they didnot evolve. These exotic species often out-compete or predate native speciesto the point of driving them to extinction. Global climate change is also affect-ing diversity patterns.

Nature’s services, the natural ecosystem functions upon which all life,including humanity, depends, are diverse and numerous, but their importancewas not really recognized until recently. Ranging from purification of air and

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water, cycling and movement of nutrients, generation and preservation ofsoils, and renewal of soil fertility, to seed dispersal, pollination of crops andother vegetation, to maintenance of biodiversity, it is clear that the functioningof natural ecosystems is essential to human welfare. But how essential? Ifput in the light of economic analysis, just how much, in dollar value, arenature’s services worth?

A team of researchers led by Robert Costanza (Costanza, et al., 1997) usednumerous databases to estimate that nature’s services, presuming thathumans had to somehow “pay” for them, would be worth about 33 trillion dol-lars annually. It is noteworthy that this estimate is about double the grossworld product, the total of all gross national products, which, at that time, wasabout $18 trillion. In 2002, updated estimates suggested a rough average of$38 trillion, with a range of between $18 and $61 trillion. The wide rangereflects the extraordinary difficulty of attempting to measure the macroeco-nomic worth of all ecosystem goods and services.

Some economists have criticized the effort by Costanza et al. for a numberof reasons, including the assertion that the extrapolations made were incon-sistent with principles of microeconomic theory (Balmford, et al., 2002). Inresponse, a team of nineteen researchers, including economists and ecolo-gists, examined over three hundred specific case studies in an attempt tocompare what they termed “marginal values of goods and services deliveredby a biome when relatively intact, and when converted to typical forms ofhuman use.” Robert Costanza, who authored the previous study, was part ofthe research team. Their results were consistent in showing that naturalecosystems have the potential for greater societal economic gain than doecosystems converted for narrow economic objectives.

In one example it was clear that reduced impact logging in Malaysian tropi-cal forests did not provide the immediate economic benefits to individuals thatwould be obtained by high-intensity unsustainable logging, which is what isnormally done there. However, unsustainable logging reduced social andglobal benefits through loss of forest products (other than timber), flood pro-tection, carbon stocks, and endangered species. The total economic value ofthe forest was about 14 percent greater when managed to be sustainable,using reduced impact logging techniques.

In a second example, a mangrove ecosystem in Thailand was converted toaquaculture (shrimp farming), something that is occurring in many places inthe world’s tropics. There was no question that short-term economic interestswere well served by such a conversion. However, when social benefits of leav-ing the mangrove ecosystem intact were included in the economic analysis,the result changed. Benefits such as the sequestration of carbon, storm pro-tection, and protection for fish added much more value to the intact mangroveforest than to the aquaculture ponds. The estimate showed that the intactmangrove forest was worth about 70 percent more than the aquaculture.

Most ecologists agree that climate change is the most significant factor nowinfluencing Earth’s ecology. Climate change has always occurred throughoutEarth’s history for many reasons that have nothing whatsoever to do withhumans. But today’s climate change is unique in that human activities do, in

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fact, seem to be forcing much of the climate change, much of which is due togreenhouse effect.

It is well known that greenhouses admit light and trap heat. Some atmos-pheric gases have an analogous effect to that of the glass in a greenhouse.Gases such as water vapor, methane, nitrous oxide, and carbon dioxide actto block heat energy from passing easily through the atmosphere. Ratherthan a quick, easy transit from Earth to space, the heat is “trapped,” retainedwithin the atmosphere for a relatively long period. The more greenhousegases there are, the more this effect of trapping heat is manifest. This phe-nomenon, the retention of heat energy by certain atmospheric gases, hascome to be termed “the greenhouse effect.” It is tremendously important inmitigating rapid temperature fluctuations on Earth and it has contributed in anessential way to making the Earth a habitable planet.

Recall that Earth is an example of the “Goldilocks effect,” situated at precise-ly the right distance from the Sun for water to exist in liquid form. One pro-found benefit of the presence of oceans of liquid water is that greenhousegases, and in particular, carbon dioxide, can be absorbed into the oceans,and, in the case of carbon dioxide, by a series of purely physical reactionsconverted to insoluble carbonate, taken out of circulation. The importance ofthis reality cannot be overemphasized. Without the oceans, any buildup incarbon dioxide, such as from volcanic emissions, for example, would not becorrectable and the atmosphere would continually increase in carbon dioxideconcentration that, in turn, would trap more and more heat. Eventually, thisprocess would “run away” and the amount of heat trapped would be sufficientto raise the temperature of the planet beyond that which life could endure.Such is apparently the case with the planet Venus.

Although the oceans can and do absorb carbon dioxide, it is clear that sincethe onset of the Industrial Revolution, atmospheric carbon dioxide concentra-tion has increased steadily and today it is about 385 ppm (parts per million)by volume (compared with 315 ppm in 1960). This increase has correlatedwith the growing use of fossil fuel and, particularly in the latter part of thetwentieth century, with increased global deforestation. These two factors havecombined to release a significant amount of carbon dioxide, a process that isongoing and which is altering the atmosphere to the degree that Earth iswarming and climate changing.

The result of climate change will be to alter ecosystem properties andspecies interactions globally. Some species, such as the polar bear of theArctic, may be seriously endangered because of its dependency of shelf iceon which to hunt seals and give birth. Penguins in the Antarctic are becomingisolated between inland breeding colonies and coastal feeding areas, reduc-ing their reproductive potential. Everywhere on the planet where ecologistsmeasure populations or ecological processes, changes seem to be occurring,many of them negative, though not all. In a sense, this is an exciting time forecology as it is challenged to respond to the realities of human culture and itscollective and powerful influence on this, the ecological planet.

1. Why was ecology once described as the subversive science? What shouldbe the relationship between ecology and economics?

2. What are the major factors that are today altering ecosystems on Earth?Which of these is likely to have the greatest influence?

Wilson, Edward O. The Future of Life. New York: Alfred A. Knopf, 2002.

Botkin, Daniel B. Discordant Harmonies: A New Ecology for the Twenty-FirstCentury. Oxford: Oxford University Press, 1992.

Levin, Simon A. Fragile Dominion: Complexity and the Commons. NY: BasicBooks, 2000.

Balmford, A., et al. “Economic Reasons for Conserving Wild Nature.” Science.Vol. 297, pp. 950–953, 2002.

Costanza, Robert, et al. “The Economic Value of the World’s Ecosystems.”Nature. Vol. 387, pp. 253–260, 1997.

Articles of Interest

�Questions

Suggested Reading



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COURSEMATERIALS

COURSE MATERIALS

Suggested Readings:

Bailey, Robert G. Ecoregions: The Ecosystem Geography of the Oceans andContinents. New York: Springer, 1998.

Bertness, Mark D. Atlantic Shorelines: Natural History and Ecology.Princeton, NJ: Princeton University Press, 2006.

Connell, Sean. Marine Ecology. Oxford: University of Oxford Press, 2007.

Cox, C. Barry, and Peter D. Moore. Biogeography: An Ecological andEvolutionary Approach. Oxford: Blackwell, 1993.

Jones, Stephen R., and Ruth Carol Cushman. A Field Guide to the NorthAmerican Prairie. Boston: Houghton Mifflin, 2004.


———. A Neotropical Companion: An Introduction to the Animals, Plants,and Ecosystems of the New World Tropics. 2nd ed. Princeton: PrincetonUniversity Press, 1999.

MacMahon, James A. Deserts. New York: Alfred A. Knopf, 1985.

Pielou, E.C. A Naturalist’s Guide to the Arctic. Chicago: University of ChicagoPress, 1994.

———. The World of Northern Evergreens. Ithaca, NY: Comstock, 1988.


Ward, Peter D., and Donald Brownlee. Rare Earth. New York: Copernicus, 2000.

Wilson, Edward O. The Future of Life. New York: Alfred A. Knopf, 2002.

These books are available online through www.modernscholar.comor by calling Recorded Books at 1-800-636-3399.

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the ecological planet - an introduction to earth’s major ecosystems (booklet)

Documents

books includegalapagos

john krichercourse guide

trademark ofrecorded

major ecosystemsabout

global ecology

polar ecosystems

marine ecosystems

course materials