Chapter 1: What Is Ecology?

There are many different definitions of what ecology is. They all share the notion that ecology is the study of the factors that explain distribution and abundance of organisms.

Distribution and abundance are straightforward ideas. How many of a species are there, and where are they? The causes for those distributions and abundances are more complex. They involve the physical and biological environment including other species present and their abundance, history of both the physical and biological conditions over space and time, and relationships among these factors.

The basic ideas in this definition are not new. The word, based upon the Greek word oikos, meaning home, was first applied to the scientific study of organisms and environment by Ernst Haeckel in 1870.

"By ecology, we mean the body of knowledge concerning the economy of nature - the investigation of the total relations of the animal to both its organic and to its inorganic environment. … In a word, ecology is the study of all the complex interrelationships referred to by Darwin as the conditions for the struggle for existence."

As these factors are complex individually, and even more complex collectively, ecology is a rapidly growing and evolving discipline. To achieve its goals of understanding relationships and causes, it draws on many other disciplines. Geology is useful because, for example, soil chemistry is important in determining plant distributions, and therefore species dependent on the plants for food and/or homes. Geology also teaches us about continental drift, which is important in understanding the distributions of many species. Physiology is important to understand the mechanisms involved in energy assimilation and use. Behavior is important in understanding population structure and mating strategies. Genetics, because genes determine what the organisms are like, how they function physiologically, thus where they can potentially live, and, at least in part, how they will behave is critical to modern ecology. An argument could easily be made that ecology draws on every major area of biology to achieve its goals.

Short Biography of Ernst Haeckel:Ernst Haeckel was born in 1834 and died in 1919. Haeckel wasan early and ardent supporter of Darwin's theory of evolution. However, some of his ideas might today be regarded as racist.He thought that humans comprised 12 different species. He expected the "superior" Germanic group would eventually driveother "inferior" groups to extinction. In 1866, Haeckel publishedGeneral Morphology, a genealogical tree of vertebrates, thatrepresented the first ordering of life according to the principles of Darwinism. The definition of ecology we use is drawn from that book. In 1868, in History of Creation, he argued for an evolutionary scheme in 22 steps, of which the 21st step was the'missing link' between apes and humans. In 1899 he published The Riddle of the Universe, which attacked the religious view of life, and attempted to place mankind in a dynamic universe. Haeckel had chosen a career in science, rather than art. Late in life he combined his two passions, using his own illustrations in Art Forms in Nature.

The History of Ecology

The history of ecology can, in one sense, be suggested to begin with observations of the ancient philosophers of Egypt and Greece. However, their observations fall into what we would now class as "natural history". The data gathered was about what species were found and where they were found. Associations with physical conditions of the environments of the species were noted, but quantitative information was not a part of these data. Evolution generally was not a part of the story. Plato's writings were metaphysical: there was a clear distinction between the "real" world of ideal species and the imperfect observations we make with our senses. Aristotle believed in a "scala naturae", which was a scale of increasing complexity along which species could be ordered. Species remain unchanging on their rungs of this ladder, and evolution does not occur. The philosophical notions were abstract, and clearly did not consider the abundance of species and the reasons underlying either distribution or abundance.

Quantification in "ecology" can probably be traced to observations collected during the middle ages, at the time of the Black Plague. The disease aroused such fear that it became important to document its occurrence in neighborhoods. In England, matrons in each parish acted as amateur coroners, trying to determine the causes of death. The parish records, many still preserved in the Guildhall Museum Library in London, record the numbers of births and christenings and the probable causes of death for each corpse on a regular and an annual basis. They, of course, indicated any sign of deaths due to the plague so that whatever protective action was possible could be taken. The records are still a sort of quantitative natural history.

The Origins of Population Biology

The Bills of Mortality and Christening provided the data for the first calculations of population growth rate. In 1662, John Graunt published "Natural and Political Observations", in which he estimated the doubling time for the population of London from rates of birth and death in the bills. His estimate was that the population of London should have been doubling every 56 years. As a religious man, this produced a conundrum. Working through the generations reported in the bible, chroniclers figured that creation occurred in 3948 BC.

Biography of John Graunt:John Graunt was born in 1620 and died in 1674. Graunt, like many of the figures important in the early history of Ecology and Population Biology, was not primarily a scientist. He had no university education. He had been apprenticed to a haberdasher, and he earned his living in that trade. He was, however, influential in London, holding various political offices, and was made a member of the Royal Society. It is clear from this that his scientific endeavors were respected by his peers. He wrote only one major work, Observations on the Bills of Mortality (1662). In it he established that more female babies were born than males, as well as, on average, longer lifespans for females. (There are no known portraits of John Graunt. The closest we can come is this photograph of an actor portraying him.)

Assuming that biological rates had not changed, that meant there had been 87 doublings from the time of Adam and Eve. If that were true, the population would have reached 1026 individuals, or about 100 million per square centimeter of habitable ground. Even Graunt knew this could not be; he recognized that a pattern of regular doubling could not continue indefinitely. This was the first formal recognition of limits to growth.

Regular doubling has a name as a pattern of growth. It is called exponential or geometric increase. This term is not one that originated in 20th century quantitative ecology. Instead, the term was coined by Sir Matthew Hale in 1677.

His career is indicative of the breadth of interests and education at the time. Population biology was at most a hobby for him. His 'real' job was as Chief Justice of the King's Bench, the equivalent in North American courts of being chief justice of the Supreme Court.

Another of Graunt's friends recognized the implications of Graunt's calculations. William Petty, in "Another Essay in Political Arithmetic" (1683) established the notion of a maximum sustainable population size. We call that population size the carrying capacity, K.His estimate of sustainable population density was supported three centuries later by the best and brightest thinkers assembled by the Rockefeller Foundation, whose work was published as "The Limits to Growth" in 1960. He estimated the K as a density of 2 people for every 5 acres of habitable land. However, he did not know the total amount of habitable land on earth, and mistakenly estimated that area as 50 billion acres. He over-estimated by approximately a factor of two, and, in parallel, his estimate of a K of 20 billion was similarly too large by the same factor. The Rockefeller Foundation estimate of K is 10 billion. It is interesting to think about what the similarity of modern and historical estimates of K for the human population means. Is it that our resource needs and our resource base per person and per unit area are still the same? Is the reason our much improved standard of living (and associated energy costs) have had no impact is because we are living on a historical energy surplus (fossil fuels) that cannot be indefinitely maintained?

Biography of Sir Matthew Hale:Sir Matthew Hale lived from 1609 to 1676. After studying lawat Oxford, he was admitted to Lincoln's Inn, where he studiedmathematics, physics, anatomy and architecture. His law careeradvanced rapidly, from being a judge in the court of common pleas beginning in 1653 to Lord Chief Justice in 1664. He heldthat position until retiring due to ill health in 1676. Hale presidedover a famous and controversial trial of witches in Suffolk, and wrote what remains a basic treatise on the common law associated with criminal offences, History of the Pleas of the Crown. He apparently was able to serve free of political, religious, or social biases, and thus remained an important jurist through Oliver Cromwell's parliament and the rule of Charles II. His mathematical training led to his contribution of the terminology for geometric increase in population size.

Narration continues:

Petty also did some corrective surgery on Graunt's doubling time. He recognized that a portion of the increase in London's population was not due to the balance between birth and death. Instead, an important part was due to the immigration into London associated with the onset of the industrial revolution. His revised estimate of the inherent doubling time was 360 years.

That is far slower than Graunt had figured. Petty went further, and suggested that the rate of population growth had slowed since biblical times. That slowing is a preliminary hint on the path to describing logistic population growth, which has a maximum population size, the carrying capacity K.

The Origins of the Theory of Evolution

Historically, the next important development was Thomas Malthus' recognition that population growth may potentially be exponential, but the growth of needed resources is generally linear.

He suggested, in "An Essay on the Principle of Population" that population growth would eventually overwhelm the amount of resource available. When that happened, there would be "misery and vice". The misery part seems apparent: starvation and disease in a population weakened by lack of food. By vice he meant artificial means of reducing the number of children born, as parents could not adequately feed them. In later editions, under pressure from his peers, he added a third means of restricting population growth, moral restraint. The importance of Malthus's ideas was not recognized widely until they underpinned a major part of Darwin's theory of evolution.

Biography of William PettyWilliam Petty was born in 1623 and died in London in 1687.His family apparently was poor, and he was sent to sea as a cabin boy at age 15. After he returned, he studied at a number of universities, eventually earning an M.D. at Oxford in 1650, though he apparently never earned a Bachelor's degree. While he is considered the father of quantitative demography, based on Natural and Political Observations (1662), he may not have been the author. Most of his works were in the field of economics, in which he was apparently the first to use statistics to arrive at his conclusions. Among his works in economics are Treatise of Taxes and Contributions (1662), Political Anatomy of Ireland (c.1672), and Political Arithmetick (1690). Some believe he was the author of the book we have listed as John Graunt's, but that is not widely accepted. He, too, was a member of the Royal Society. While a student, he earned his living as a jeweller. Later he prepared anatomical specimens and was a professor of Anatomy at Oxford. He left Oxford and became a professor of Music at Gresham College. He surveyed and drew maps of Ireland that are considered the foundation of Irish geography. Late in life he became wealthy, but, possibly because of his heritage, never became powerful, even though he contributed so much to economics, demography and cartography.

Short biography of Thomas Malthus:Thomas Malthus was born in 1766 just south of London. Like many people known today for their contributions to science, he was educated to be a cleric. In 1805 he took a position as professor of Political Economy at the college of Haileybury. The college educated those who worked in the East India Company. He completed his working life at the college. His key work was An Essay on the Principle of Population…, initially published in 1798, but revised and expanded a number of times during the first years of the 19th century. Two key principles were presented: 1) Populations in crease geometrically, while the resources needed to support them increase arithmetically (by which he meant linearly) and 2) mankind has two basic drives,for food and for sex. A key quotation from the book indicates where Darwin drew one of his major ideas: "Through the animal and vegetable kingdoms, nature has scattered the seeds of life abroad with the most profuse and liberal hand. She has been comparatively sparing in the room, and the nourishment necessary to rear them… The race of plants and the race of animals shrink under this great restrictive law. And the race of man cannot, by any efforts of reason, escape from it."

Short biography of Georges Cuvier:Georges Cuvier lived from 1769 to 1832. He contributed to geology, but also added to paleontology and comparative anatomy. He could well be regarded as the father of this last subject. He was one of the first to recognize that species had gone extinct during the history of the earth, and also to suggest the great age of the planet. In findingfossils in sediments, he indicated that those sediments were "thousands of centuries old". He believed (correctly) that life in the sea had preceded life on land, and (correctly) that reptiles had preceded mammals. However, he also believed that species do not change during their spans of existence, that the earth had undergone a series of geological catastrophes.Short biography of Charles Lyell:

Lyell lived from 1797-1875. His most important contribution to science was a painstakingly established principle of uniformitarianism. The principle, published in The Principles of Geology: Being an Attempt to Explain the Former Changes of the Earth's Surface, by Reference to Causes now in Operation (1830), was that what is happening geologically in the present can explain the history of the earth. Geological processes are ongoing. Darwin took a copy of this book along on the expedition of the Beagle, and it helped him understand how he could find marine fossils on the slopes of the Andes. Lyell was influential in persuading Darwin to publish his theory of evolution in the same issue that also contained Alfred Russell Wallace's essentially identical theory.

Narration continues: Soon after this there was a revolution in geology, from a notion of catastrophic change in the biological realm that was evident in changes in the fossil record to a concept of gradual change. The former view was one developed by Georges Cuvier, who basically founded paleontology. Scottish geologists developed the latter view. First, James Hutton believed that what had occurred in the past, leading to current geological and fossil forms, was still operating as he wrote. He introduced the notion of gradualism, i.e. that major changes are the cumulative result of many slow and small changes. Then came Charles. It was his books that strongly influenced Darwin. He established the idea called uniformatarianism. This is the name given the idea that geological processes have not changed in any basic way over the whole history of the earth.

Short biography of James Hutton:James Hutton (1726-1797) was the first to recognize the dynamic geological history of the earth. He found layers of sedimentary rock set one on top of another, and saw this as evidence of sequential deposition and a vast age for the earth. In Scotland, he found intrusions of granite into sedimentary layering, and took this as evidence of enormous heat in the core of the earth. In both these observations and interpretations he was correct, but, without modern knowledge, took the age of the earth to be limitless. His ideas of geological dynamism were a critical influence on Lyell, and thus on Darwin.

Biography (and partial bibliography) of Darwin:Charles Robert Darwin was the first evolutionary biologist. That comes not only from the publication of The Origin of Species by Means of Natural Selection (1859) and The Descent of Man (1871), but also from his many other books that establish the effects of natural and artificial selection on the traits of species. A list of some of those other books ends this short essay. He found the information that would lead to his theory of natural selection during the five-year voyage of the Beagle. He was selected to go on that voyage not for his abilities as a naturalist, but because he could be an appropriate companion for the young captain, Robert Fitzroy. The admirals who chose him nevertheless, recognized the opportunities for natural observations. He served as geologist, botanist, and zoologist, as well as well-bred companion to the captain. Julian Huxley stated Darwin's impact succinctly: "Darwin's work…put the world of life into the domain of natural law."

The diversity in his writings is amazing. Here is a short list of some of his other books:On the Various Contrivances by Which Orchids Are Fertilized by Insects (1862)The Movements and Habits of Climbing Plants (1865)The Variation of Animals and Plants Under Domestication (1868)The Expression of the Emotions in Man and Animals (1872)Insectivorous Plants (1875)The Effects of Cross and Self-Fertilization in the Vegetable Kingdom (1876)Volcanic Islands (1876)Different Forms of Flowers (1877)The Power of Movement in Plants (1880)

Charles Darwin did not put these ideas together with his own insights until years after his return to England following the voyage of the Beagle. Even as a young man, he had been intensely interested in nature, collecting insects and observing nature. His father, a physician, saw no future in becoming a naturalist, and sent Charles to the University of Edinburgh to study medicine. He dropped out, but returned to university at Oxford to study theology. It is not likely that Charles wanted to be a clergyman. Rather, the most common university background for naturalists was theology. Darwin became a protégé of Reverend John Henslow, a botany professor at Oxford. Soon after graduation he was recommended to the commander of the Beagle to serve as a naturalist for its five year research voyage.

The voyage carried Darwin along the Atlantic and Pacific coasts of South America, to tropical forest on the coast of Brazil, the pampas of Argentina, the barren land of Tierra del Fuego, the slopes of the Andes in Chile and Peru, and to the Galapagos.

In all these places Darwin collected samples of plant and animal life. He noted, even as he collected, that many specimens bore a basic similarity to species from Europe, but there were clear and evident differences. Frequently there were unique characteristics to samples from particular places. He could tell from characteristics of tortoise shells from the Galapagos Islands from which island a given tortoise had come.

He noted the enormous diversity of finches from the Galapagos, all similar to but differing from those of South American mainland.

In addition to the evidence of differences in closely related species Darwin observed during the Beagle's voyage, there was important evidence that came from some of his interests in England. He was an avid breeder of pigeons, and was well aware of the potential to produce exotic looking patterns of feathers, their colors, and similar character selection in other species. The evidence that careful selection of characteristics and matings could produce cows that gave more milk, different breeds of dogs, or flowers with showier petals and colors, which we call artificial selection, indicated the potential for inheritance of parental characteristics, which is called descent with modification. Combined with observations of differences in natural settings from the voyage, Darwin argued that natural selection could produce the varieties we see in the world.

Darwin was masterful in assembling evidence for his ideas, but he was very slow in putting those ideas into print. He had constructed a manuscript expressing them by 1844, but had never submitted it for publication. The voyage of the Beagle was completed in 1836. Publication of the theory of evolution was delayed, possibly by the controversy that might have been expected from such a paradigm shift, until Alfred Russell Wallace had independently achieved the same synthesis.

In 1858 Wallace sent Darwin a letter and a manuscript. His research leading to the hypothesis was mostly about insects in the East Indies. He had sent an earlier manuscript to Darwin not because he knew of Darwin's ideas, but because Darwin was widely respected as a naturalist. Darwin sent this new paper on to Lyell to be published, feeling that the ideas were no longer his alone. It was Lyell, his friend, who arranged publication of both portions of Darwin's old manuscript from 14 years earlier and Wallace's article together in Nature.

There was an ongoing controversy about the occurrence of evolution, particularly as it relates to the origin of humankind, for decades after. Aspects of the controversy continue today. Some states have passed legislation requiring the teaching of creation alongside evolution in high school. Other jurisdictions permit the teaching of evolution without mention of the creationist view, but severely restrict how much time can be devoted to the study of evolution.

An Input from Genetics

Part of Darwin's problem in explaining the mechanism of evolution was the lack of knowledge and understanding of genetics as he constructed the theory. It was less than a decade later that Mendel developed the laws of inheritance, even though he, too, lacked knowledge of the sub-cellular mechanisms involved.

Short biography of Alfred Russell Wallace:Alfred Russell Wallace was born in 1823 and died in 1913. His father had been a man of leisure until ill-advised investments left the family quite poor, and Alfred had to leave school at 14. While in London living with his brother he learned map making and drafting. After a time in London and exposure to bothnatural history and the utopian socialism of Robert Owen, he taught at post-secondary institutions for a time using both his knowledge of natural history andtrades. After forming a sort of partnership with an experienced entomologist, Henry Bates, in 1848 he took a position as a commercial collector of insects inthe Amazon basin. Bates' observations of mimicry among insects in the Amazon gave his name to one form of the evolved similarity. During this period Wallace began to observe patterns among the insect species he collected that were the initial basis of his ideas about evolution. However, on the return voyage a fire on the ship destroyed everything he had collected. After a time in England, he went back to collecting, this time in the Indonesian archipelago. He traveled and collected for eight years, adding more than 1,000 new species to science. He also developed a theory for the evolution of new species, and published it. The more fully developed version that forced Darwin to finally publish his work followed further studies and an attack of malaria. Wallace's paper was titled On the Tendency of Varieties to Depart Indefinitely from the Original Type." With the publication of Darwin's and Wallace's papers, the concept of natural selection was made public.Wallace did more than just collect insects. After four additional years in the Indonesian archipelago, he also published in zoogeography, and Wallace's line established the boundaries separating faunas identified with Australia and associated islands from those of islands associated with Asia.

The Law of Segregation and the Law of Independent Assortment predict the inheritance of many traits, but not all. T.H. Morgan found white-eyed male Drosophila, and was able to discern that some traits are sex-linked, inherited from the X chromosome, which is not present in diploid condition.

Short biography of Gregor Mendel:Gregor Mendel (1822-1884) was an Austrian monk who taught and studied genetics. His most famous and most successful work was on the common garden pea plant. Until Mendel's work, there was the common sense realization that offspring resembled parents. Mendel quantified and predicted traits that would be observed in offspring peas and the plants grown from them. Mendel carefully chose traits that that are passed, whole and intact, to offspring. He performed meticulous experiments that permitted him to determine both male and female parents of peas, and counted the frequencies with which different kinds of offspring were produced. The work took him from 1857 to 1865. From it he recognized dominant and recessive traits, but because of the traits he chose, he did not encounter measurable recombination. In 1866 he published his results in a regional naturalists' journal, but they were effectively lost until re-discovered in 1900 by geneticists, including de Vries, who found the same sort of results in studies of Drosophila.

Short biography of Thomas Hunt Morgan:Thomas Hunt Morgan (1866-1945) studied the genetics of the fruitfly, Drosophila melanogaster. He is one of the key figures in the history of genetics, wrote, with other geneticists, the first landmark textbook of genetics, The Mechanism of Mendelian Heredity, and won a Nobel Prize for his work that established the chromosomal mechanism of inheritance. Early in this work he discovered sex-linked inheritance. The initial phase of this discovery was an observation that only males inherited a white-eyed condition, then exploring the inheritance of the trait.

The chromosome that 'pairs' with the X chromosome is the Y that, in part, determines maleness. Still the molecular mechanisms weren't known. A trait carried on the X chromosome of a male has no paired allele on the Y. That knowledge did not develop until the 1940's and 50's. Now ecological genetics and concepts like expression of genes dependent on environmental conditions complicate the subject, but add to our ability to understand the importance of genetics to ecology.

Mathematical Models of Population Growth

In quantitative ecology, the basic equation for population growth and models of species interaction is the logistic.

Logistic equation:

Pierre Francois Verhulst first published papers expressing the logistic equation as a measure of growth between 1838 and 1847. At the time, scientists apparently were unsatisfied because the mathematical model of growth lacked a physical analogy. As a result, the logistic was rediscovered and finally accepted as a result of its use in Pearl and Reed's studies of population growth in the United States around 1920. They also found one of the key difficulties in applying the logistic model to real data. It doesn't work well when the carrying capacity, K, of a population in some environment changes over time.

Short biography of Pierre Verhulst:Pierre Francois Verhulst was born in 1804 in Brussels, Belgium and died there in 1849. He was educated at the University of Ghent, and was, both there and afterward, influenced by Quetelet, a foremost French mathematician of the time. A part of that influence was to become interested in what was termed social statistics. However, the older and younger mathematicians differed in their views about population growth. Quetelet believed populations grew geometrically, but that a force proportional to the square of the rate of growth prevents indefinite geometric increase. Verhulst believed that the opposing force was proportional to a ratio of the difference between a maximum supportable population size and the current size to that maximum size. Expressed in mathematical terms, the differential equation that Verhulst developed is the logistic equation. The logistic equation, like the genetics of Mendel, disappeared from sight for an extended period. This basic equation was re-discovered by Pearl and Reed in studying American population growth early in the 20th century.

Community Ecology

In addition to developments in population biology, early in the 20th century there were also important developments in what we now term community ecology. One of the key questions was how communities were assembled. Are communities much like organisms, with specific species having critical functions? Or are communities much more random groups of species, where specific functions must be achieved, but there are many alternative species that can do the various jobs (or fill the niches), and it doesn't matter much which species end up filling the niches? What matters in which species we find is that they all can tolerate the physical conditions in the place under study. These are relatively extreme statements of positions taken by ecologists F.E. Clements and H.A. Gleason.

Frederic E. Clements (1874-1945) did decades offield research under the auspices of the CarnagieInstitute of Washington. He believed that his "study of vegetation reveals that it is an organicentity and that, like an organism, each part isinterdependent upon every other part…" This hasbeen phrased as the 'superorganism' view ofplant communities.

Henry Allen Gleason (1882-1975) had a very different view of plant community structure, termed the 'individualistic' or 'continuum' concept of associations. "Vegetation, in its broader aspects, is composed of a number of plant individuals. The development and maintenance of vegetation is therefore merely the resultant development and maintenance of the component individuals…"

Biography of Raymond Pearl:Raymond Pearl (1879-1940), with L.J. Reed, a biostatistician also at Johns Hopkins University, re-discovered the mathematical model of logistic growth, and published the model in a paper titled "On the rate of growth of the population of the United States since 1790 and its mathematical representation." (PNAS, USA [1920] 6:275-288). He founded one of the premier journals in biological research, Quarterly Review of Biology, but, during the 1920s, he also supported the eugenics movement. Apparently he rescinded that support when eugenics was scientifically discredited.

Clements believed that communities bore important similarities to organisms. His view is called the interactive or, more usually, the organismal hypothesis for community structure. Gleason's view is termed the individualistic hypothesis. As is usual in such conflicts, the truth lies somewhere between the extreme views.

Neither Clements nor Gleason phrased their hypotheses for community structure in terms of the niches occupied by species. Yet during the middle of the 20th century ecology was vitally concerned with understanding how species pack themselves into an environment, and, thus, better ways of thinking about their niches. G. Evelyn Hutchinson made a critical contribution to this debate by introducing the notion of the niche as a multidimensional hypervolume.G. Evelyn Hutchinson (1903-1991) grew up with the sub-

discipline called limnology, but his contributions extended farbeyond that. Among the minor ones, he persuaded the City ofNew York to cancel a construction project that would havedestroyed Belvedere Lake in Central Park. In it lived 19different species of dragonflies. The lake is now a dragonflyreserve. He accomplished the same end in preventing theconstruction of a military base on Aldabra Island in the Indian ocean. It is now a reserve for sea turtles. His academic careerbegan in South Africa at the University of Witwatersrand, but moved to Yale University for a post-doctoral position after a dismissal. He stayed at Yale for a long academic career, duringwhich he Published extensively in limnology, including a 3-volume text, A Treatise on Limnology. His most lasting contributions may well lie outside limnology, in papers such as “Homage to St. Rosalia, or Why are there so Many Kinds of Animals” and “Concluding Remarks” for the Cold Spring Harbor symposium of 1957, where he introduced the concept of the multidimensional niche, as well as in a classic population biology text, An Introduction to Population Ecology.

That complicated terminology simply means that each species occupies a range of conditions along each niche axis. The many axes that may be important include physical variables like temperature, humidity, and soil composition, as well as biological axes like food, frequently in the form of the size of food items, and the abundances of the other species with which it lives. Since we usually think of a volume as involving only three axes, height, width, and depth, Hutchinson termed the volume of this multidimensional niche a hypervolume. With it, many different ecologists contributed ideas about how species packed into the environment.

Species Interactions

Initially, competition among species was thought to be the principle organizing force for communities. Probably the most important contribution to understanding competition as a structuring force was the research, both in the field and in the development of theory, of Robert MacArthur.

He studied the use of spruce trees by a number of warbler species living together in New Jersey. All the species fed on insects, but they lived and caught insects in different parts of the tree.

One species lived in the topmost branches of trees, another about midway up, and a third in the lower branches; some species live in the interior, and others live on the outer branches. The species divide space and limit the intensity of competition among them through this partitioning.

The belief that communities were structured by competition among species held sway in ecology for many years. Eventually, ecologists recognized that in many studied examples predation was important in determining which species survived. One of the most important of these examples is Robert Paine's studies of the effect of the starfish Pisaster on the community of invertebrates living on the rocky shore along the coast of Washington State.

Make Paine's name come on as a clickable object. If clicked, fade out MacArthur. Bring up a portrait of Paine on the left. With or without the click, bring up a picture of the intertidal community on the right.

Robert MacArthur (1930-1972) was one of the key fathers of modern ecology. G. Evelyn Hutchinson supervised his graduate studies. It was MacArthur’s work, both in field observations and in the development of mathematical theory, that gives us much of our modern view of species interactions and community ecology. He came by his interest in nature from observations on birds, mammals and trees in Ontario and rural Vermont. He switched from mathematics to biology in graduate school, and his Ph.D. thesis, supervised by G. Evelyn Hutchinson, resolved the mechanism of coexistence of a number of warbler species living in the same trees in New Jersey pine forests: niche partitioning. Later he developed core theoretical ideas in species diversity, the relative abundance of species in communities, species (niche) packing, competition, and, finally, island biogeography. In all these sub-disciplines his work is seminal to our modern view of how communities, and nature in general, are structured.

In the absence of the predatory starfish, a mussel, Mytilus, dominated the community through its success in competing for space. Being a 'smart' predator, the starfish fed most intensively on this mussel, opening space for other species to colonize the shoreline. The difference in the number of species living on the shoreline with and without the starfish was dramatic. After a decade in a test area where Paine had removed all starfish there were only 2-3 species present, while in areas where starfish had been allowed to feed naturally approximately 20 species coexisted. Where the effect of a single species on the structure of a community is so dramatic, that species is called a keystone.

Island Biogeography and Non-Equilibrium Ideas

In addition to achieving a balance in considering interactions among species as critical in understanding how communities are assembled, over the last two decades ecology has come to recognize the importance of disturbance and colonization of sites in determining what we observe. When a habitat is bare, it is those species that successfully colonize the site that we will observe there. It may be a newly formed site from volcanic eruption or it may have been cleared as a result of some catastrophe like a hurricane, tornado, flood, or tidal wave. The colonization process and the kinds of species we will observe on the site are similar whatever has caused the habitat to be empty. Robert MacArthur and E.O. Wilson developed a theory of island biogeography to quantify the number of species that should be found on these islands of habitat during the colonization process.

Robert Paine retired in 1999 from a long career at theUniversity of Washington. For most of his career he studiedthe ecology of the rocky intertidal along the coast ofWashington. He studied the impact of wave disturbance and the impact of biological interactions on intertidal community structure. One of his most lasting contributions was recognizingthat a single species could have a major impact on a wholetrophic level with which it interacted. The concept of a keystonespecies came from Paine's studies of the starfish, Pisaster, andits effects on the community of invertebrates on which it fed. WithPisaster present, that community was diverse, since the dominantspecies, a mussel, Mytilus, was preferentially removed by feeding.When the starfish was absent, the community was severely reduced in diversity, and Mytilus occupied most of the shorelinespace in the intertidal zone.

How isolated an island is and the dispersal capabilities of potential colonists determine the rate of immigration, and the number of species already present determines the rate at which resident species should go extinct.

Rather than limiting the model to oceanic islands, ecologists quickly recognized that the same ideas should apply to isolated areas of habitats on mainland. These isolates might be lakes, forests isolated by farmed areas on all sides, or, of greater impact, parks. Though the basic model is so simplified that application to real world situations was difficult and dangerous, it formed a key basis for the development of the field known as conservation biology. It was also used to construct an early set of guidelines for designing parks and preserves to conserve the largest number of species possible. Research into 'island' biology led to the recognition that their edges are altered by incursion of species from surrounding areas and changes in the climate (temperature, humidity, etc.) in edge areas. Since the island biogeography models had also pointed to the importance of island isolation, the idea of connecting 'islands' by corridors was introduced. Developing corridors has become a controversial subject, and the balance between benefit and cost is still under intense study.

More modern evolution of these ideas has led to an approach called metapopulation biology, in which a continuous flow individuals among islands of habitat leads to the preservation of a species, even if individual sub-populations on particular islands may not long persist. The metapopulation view seems closer to real world conditions, where individual parks are rarely large enough to ensure long term persistence of a population, but a number of smaller parks close enough together for dispersal among them to be a reasonable expectation may together conserve the species.

Models of island biogeography and the ideas that have developed from them are among the first ecological models to recognize that the real world does not persist in equilibrium. The very events that lead to loss of

Short biography of E.O.WilsonE.O.Wilson (1930 -) has worked from a laboratory atthe Museum of Comparative Zoology at Harvard for many years. Much of his research has been concerned with the sociobiology of ants. However, from that research arose a seminal and controversial book, Sociobiology: A New Synthesis. Some took that book to be racist, and read the text to suggest that genetic variation among humans left some superior to others. In what is now the middle term, the concepts presented in the book are recognized to represent a 20th century Darwinian approach, and not an advocacy of eugenics or other ill-conceived notions. With Robert MacArthur, he 'invented' island biogeography, which was also seminal to the development of a sub-discipline we now call conservation biology. The two books about biodiversity and its conservation that he in part wrote and also edited have been extremely important in making the call for biodiversity conservation the subject of major international conventions. He has never avoided controversy, and sees a need for genetically modified organisms (GMOs) to protect humankind from starvation. One writer, Ed Douglas in the Guardian, has placed Wilson intellectually at Darwin's right hand, and that is not a bad place, given the many major contributions Wilson has made to modern views of evolution, genetics, and conservation.

species from communities are themselves now recognized as important. Disturbances take many forms. They include those that damage communities, like floods, overgrazing, and other human activities, as well as some events that may be important in the survival of species and communities. Fire, for example, is necessary for the success of seeds of some pine species, and is critical to the persistence of a number of prairie plants. Disturbances provide openings for species to become established. However, disturbances can occur at different frequencies. If they occur too frequently, only 'weedy' species can complete their life cycles and successfully reproduce. If they occur too infrequently, there is little open space, and competitively dominant species will come to occupy most space, preventing the existence there of other species. This led, in the 1970s, to the proposal of an intermediate disturbance hypothesis. Joseph Connell proposed that the maximum diversity or richness of species would persist at an intermediate frequency and intensity of natural disturbance.

Other ecologists have had a lasting impact on the direction and approach research in ecology has taken. Among them, six who contributed in very different ways to the current views of the subject are Ronald Lindeman, whose paper on “The trophic-dynamic aspect of ecology” underlies our view of the way energy moves among organisms in communities, Robert H. Whittaker, who showed the way out of the conflict between Clements and Gleason, as well as applying statistical methods to the analysis of community structure, C.S. Holling, who neatly categorized the different ways predators may relate to the densities of their prey, E.P. Odum, who instituted a larger view of ecological processes that drew in some of the methods of analysis more traditionally seen in engineering, an approach called systems ecology, Charles Elton, who developed the concept of trophic structure that we still call an "Eltonian pyramid", as well as anticipating the importance of invading species, and John L Harper, who is the progenitor of applying demographic tools to studies of plant populations. Short biographies of each of these eminent ecologists are available by clicking on their names. The ecologists whose biographies are presented represent an idiosyncratic list from among many others that different authors might have felt more important.

Joseph Connell has made two vitally important contributions to ecology. In the 1960s he published a series of studies of the rocky intertidal zone on the coast of Scotland. His study of interactions between the barnacles Balanus balanoides and Chthalmalus stellatus are the classic example in every text. Later he studied the impact of disturbance on community diversity. A figure reflecting the pattern he found,that led to the intermediate disturbance hypothesis, is shown on the right.

Joseph Connell has made two vitally importantcontributions to ecology. In the 1960s he pub-lished a series of studies of the rocky intertidalzone on the coast of Scotland. His study ofinteractions between the barnacles Balanusbalanoides and Chthalmalus stellatus are the classic example in every text. Later he studied the impact of disturbance on community diversity. A figure reflecting the pattern he found,that led to the intermediate disturbance hypothesis, is shown on the right.

Crawford S. (Buzz) Holling is still activeas an emeritus scholarat the University of Florida. Canadian by birth and training, he took his degrees at the University of Toronto and the University of British Columbia. For many years he worked in a government of Canada lab. During that time he published papers recognizing different kinds of predator-prey interactions. Type I is a filter-feeding animal. Type II hunts and feeds on specific prey species. Type III is an adaptive predator that shifts the prey taken based on the relative abundances of its prey species. Holling's work is also important in the development of adaptive management strategies for conservation, and in integrating economic, social and cultural factors into management within a framework he terms "resilience".

R.H. Whittaker (1920-1980) was a distinguished professor at Cornell University in Ithaca, New York for most of his professional career. During that career he advocated a 5-kingdom classification of life to replace the earlier 3 kingdom view. More recently, a six-kingdom system has replaced his advance. He was a major force in the acceptance of Gleason’s view of an individualistic species continuum through studies on the spatial distribution of species in the Siskyou Mountains of northern California, the Great Smoky Mountains of Tennessee and the Santa Catalina Mountains of Arizona in a series of classic papers. He developed the tools of ordination to assess plant community spatial structure that made it possible to quantify community structure. He was the author of a classic ecology text titled Communities and Ecosystems. In addition to these contributions to ecology, he was active in Mycology and other areas of Microbiology and plant biology.

Eugene P. Odum is also still active as an emeritus scientist at the University of Georgia, where he spent the largest part of his career. He is theauthor of one of thefirst modern texts inecology, Fundamentalsof Ecology, and theintellectual father of large scale ecology, some-times called systems ecology. He studied wetland ecology in Georgia and the energy flows through food chains in these systems. More recently he has been involved in assessment of the relations between society and environmental and environmental conditions.

Raymond Lindeman (1915-1942) died before his critical paper, initially rejected as being “too theoretical”, was published in Ecology. That paper was titled “The Trophic-Dynamic Concept in Ecology”, and forms the basis for ecological limnology and the modern methods for studying aquatic food chains. In it he studied the relationships among nutrient cycling, productivity, energy flow, and nutritional efficiency. He could well be considered the father of ecosystem ecology, in part because he coined the term ecosystem to describe the system of species he studied in Cedar Bog Lake. He, too, was a student supervised by G. Evelyn Hutchinson, and it was Hutchinson’s belief in Lindeman’s work that led him to persuade the editors of Ecology to publish what became a seminal work in fields as widely divergent as aquatic ecology, ecosystem ecology, and ecological energetics.

Broadening the Base: Landscape Ecology

Landscape ecology is a recent offshoot of ecologists' attempts to integrate the real world into answers for important questions in conserving species and habitats. Landscape ecology recognizes that the artificial division of large areas into separate ecosystems is ineffective in designing strategies for conservation. Borders between habitats and ecosystems are inherently fuzzy. Human land use patterns are critically important in both political and ecological decision making. Landscapes cross the ecological boundaries, integrating what has happened, what is happening, and what we predict will happen over regions, rather than ecosystems. It is, therefore, an important element in the goal of preserving species, biodiversity, and communities in all parts of the earth.

Charles Elton taught at Oxford, and established its Bureau of Animal Populations. At many points in this text we will refer to research done by scientists who became associated with that institution. His recognition of the numerical and biomass relationships among the trophic levels comprising and ecosystem led to that structure being called an Eltonian pyramid. He also anticipated the importance and impact of invasions by non-native species, and wrote the seminal book on that subject, The Ecology of Invasions by Animals and Plants.

John L. Harper can reasonably be given credit for beginning the study of plant demography. He taught a large number of the first generation of plant demographers at the University of North Wales at Bangor. For plant population biologists, his 1977 book, The Population Biology of Plants, is the bible of all that was known at the time it was written. Later in his career he was a co-author of texts in ecology and introductory biology that were widely used and appreciated.