how ecosystems 5 work

LAKE VICTORIA’S ECOLOGICAL IMBALANCE

Until relatively recently, the world’s secondlargest fresh water lake, Africa’s Lake Vic-

toria, was home to about 400 species of small, col-orful fishes known as cichlids (pronounced sik´lids).The cichlids in Lake Victoria had remarkably differ-ent eating habits. Some grazed on algae; othersconsumed dead organic material at the lake bot-tom; and still others ate insects, shrimp, or othercichlids. These fishes thrived throughout the lakeecosystem and provided much needed protein tothe diets of 30 million humans living near the lake.

Today, the aquatic community is different inLake Victoria. More than half of the cichlids andother native fish species are now extinct. As a result of the disappearance of most of the algae-eating cichlids, the algal population has increasedexplosively. When these algae die, their decomp-osition uses up the dissolved oxygen in the water.The bottom zone of the lake, once filled withcichlids, is empty because it contains too littledissolved oxygen. Any fishes venturing into theoxygen-free zone suffocate. Local fishermen,who once caught and ate hundreds of differenttypes of fishes, now catch only a few types.

When, in the early 1960s, the Nile perch(inset) was introduced into the lake, proponentsthought its successful establishment would stimu-late the local economy and help the fishermen.For about 20 years, as its population slowly in-creased, the Nile perch didn’t have an appreciableeffect on the lake. But in 1980, fishermen noticedthey were harvesting increasing quantities of Nileperch and decreasing amounts of native fishes. By1985, most of the annual catch was Nile perch,which was increasing in number because it hadan abundant food supply—the cichlids.

In 1990 the water hyacinth, a South Americanplant, invaded Lake Victoria, adding to the eco-logical havoc (see large photo). The World Bankhas established the Lake Victoria EnvironmentalManagement Program to control the spread ofthe water hyacinth, but so far little progress hasoccurred.

How EcosystemsWork 5

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CHAPTER OUTLINE

■ What Is Ecology? p. 96

■ The Flow of Energy Through

Ecosystems p. 99

■ The Cycling of Matter in

Ecosystems p. 104

■ Ecological Niches p. 110

■ Interactions Among Organisms p. 113

■ CASE STUDY: Global Warming p. 120

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n the nineteenth century the German biologist Ernst Haeckel first developed theconcept of ecology. He devised its name—eco from the Greek word for “house” and

logy from the Greek word for “study.” Thus, ecology

literally means “the study of one’s house.” The envi-ronment—one’s house—consists of two parts, the biotic (living) environment, which includes all organisms, andthe abiotic (nonliving, or physical) environment, whichincludes such physical factors as living space, tempera-ture, sunlight, soil, wind, and precipitation.

Ecology is the broadest field within the biologi-cal sciences, and it is linked to every other biological dis-

cipline. The universality ofecology also links it to otherfields. Geology and earth sci-ence are extremely importantto ecology, especially whenecologists examine the phys-ical environment of planetEarth. Chemistry and physicsare also important. Humansare biological organisms, andall of our activities have a bear-ing on ecology. Even econom-ics and politics have profoundecological implications, as waspresented in Chapter 3.

Ecologists are most in-terested in the levels of biolog-ical organization including orabove the level of the individ-ual organism. Individuals ofthe same species occur in pop-

ulations. (A species is a groupof similar organisms whose

members freely interbreedwith one another in the wild to produce fertile offspring;members of one species don’tinterbreed with other speciesof organisms.) A populationecologist might study a popu-lation of walruses or a popula-tion of marsh grass.

Populations are orga-nized into communities. Thenumber and kinds of speciesthat live within a community,along with their relationshipswith one another, characterizecommunities. A communityecologist might study how or-ganisms interact with one an-other—including feeding rela-tionships (who eats whom)—in a coral reef community or inan alpine meadow community(Figure 5.1).

Ecosystem is a moreinclusive term than commu-nity. An ecosystem includes allthe biotic interactions of acommunity as well as the in-teractions between organisms and their abiotic envi-ronment. In an ecosystem, all of the biological, physi-cal, and chemical components of an area form anextremely complicated interacting network of energyflow and materials cycling. An ecosystem ecologistmight examine how energy, nutrient composition, orwater affects the organisms living in a desert commu-nity or a coastal bay ecosystem.

The ultimate goal of ecosystem ecologists is to understand how ecosystems function. This isn’t asimple task, but it is important because ecosystemprocesses collectively regulate global cycles of water,carbon, nitrogen, and oxygen essential to the survivalof humans and all other organisms. As humans increas-ingly alter ecosystems for their own uses, the natural

96 CHAPTER 5 How Ecosystems Work

LEARNING OBJECTIVES

Define ecology.

Distinguish among the following ecological levels: population,

community, ecosystem, landscape, and biosphere.

What Is Ecology?

I

population A

group of organ-

isms of the same

species that live

together in the

same area at the

same time.

ecology The

study of the inter-

actions among

organisms and be-

tween organisms

and their abiotic

environment.

community A

natural association

that consists of all

the populations of

different species

that live and inter-

act together within

an area at the

same time.

landscape Aregion that includes

several interacting

ecosystems.

ecosystemA community

and its physical

environment.

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functioning of ecosystems is changed, and we mustlearn if these changes will affect the sustainability of ourlife-support system.

Landscape ecology is a sub-discipline in ecologythat studies the connections among ecosystems. Con-sider a landscape consisting of a forest ecosystem lo-cated adjacent to a pond ecosystem. One possible connection between these two ecosystems is the greatblue heron, which eats fish, frogs, insects, crustaceans,

and snakes along the shallow water of the pond but of-ten builds nests and raises its young in the secluded tree-tops of the nearby forest (Figure 5.2 on the follow-ing page). Landscapes, then, are based on larger landareas that include several ecosystems.

The organisms of the biosphere—Earth’scommunities, ecosystems, and landscapes—dependon one another and on the other realms of Earth’sphysical environment: the atmosphere, hydrosphere,

What Is Ecology? 97

STOPSTOP

Alpine flowers put on a colorful show

during their brief blooming period.

Photographed in National Park Bercht-

esgaden, Germany. (Inset) The golden-

mantled ground squirrel, larger than a

chipmunk, is a burrowing mammal

common in alpine meadows in west-

ern North America.

An alpine meadow community Figure 5.1

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tinuously supplies them with energy. You will revisit en-ergy as it relates to human endeavors in many chaptersthroughout this text.


A A great blue heron has caught a frog in a pond. B Herons usually nest in trees adjacent to the pond ecosystem.

CONCEPT CHECK

What is the difference

between a community

and an ecosystem?


between an ecosystem

and a landscape?

STOPSTOP

A connection between two ecosystems within a landscape Figure 5.2

A B

and lithosphere. The atmosphere is the gaseous enve-lope surrounding Earth; the hydrosphere is Earth’s sup-ply of water—liquid and frozen, fresh and salty; andthe lithosphere is the soil and rock of Earth’s crust. Ecol-ogists who study the biosphere examine the globalinterrelationships among Earth’s atmosphere, land,water, and organisms.

The biosphere teemswith life. Where do these or-ganisms get the energy to live?And how do they harness thisenergy? Let’s now examine theimportance of energy to or-ganisms, which survive only aslong as the environment con-

biosphere The

layer of Earth

containing all

living organisms.

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nergy is the capacity or ability to do work.Organisms require energy to grow, move,reproduce, and maintain and repairdamaged tissues. Energy exists as stored

energy—called potential energy—or as kinetic energy,the energy of motion. Think of potential energy as an ar-row on a drawn bow (Figure 5.3). When the string isreleased, this potential energy is converted to kinetic en-ergy as the motion of the bow propels the arrow. Simi-larly, the grass a bison eats has chemical potential energy,some of which is converted to kinetic energy and heat asthe bison runs across the prairie. Thus, energy changesfrom one form to another.

THE FIRST AND SECOND LAWS OF THERMODYNAMICS

Thermodynamics is the studyof energy and its transforma-tions. Two laws about energyapply to all things in the uni-verse: the first and secondlaws of thermodynamics. Ac-cording to the first law of

thermodynamics, an organ-ism may absorb energy fromits surroundings, or it may

The Flow of Energy Through Ecosystems 99

LEARNING OBJECTIVESF

Define energy, and state the first and second

laws of thermodynamics.

Distinguish among producers, consumers,

and decomposers.

Summarize how energy flows through a

food web.

The Flow of Energy Through Ecosystems

Efirst law of ther-modynamicsEnergy cannot

be created or de-

stroyed, although

it can change from

one form to

another.

Potential energy is stored in the drawn bow (A) and is converted to kinetic energy (B) as the arrow speeds toward its target.

Photographed in Athens, Greece, during the 2004 Summer Olympics.

BA

Potential and kinetic energy Figure 5.3

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from the biological point of view. However, it isn’t gonefrom a thermodynamic point of view because it still ex-ists in the surrounding physical environment. The useof food to enable us to walk or run doesn’t destroy thechemical energy once present in the food molecules.After you have performed the task of walking or run-ning, the energy still exists in the surroundings as heatenergy.

According to the second law of thermody-

namics, the amount of usable energy available to dowork in the universe decreases over time. The secondlaw of thermodynamics is consistent with the first law—that is, the total amount of energy in the universe isn’tdecreasing with time. However, the total amount of en-ergy in the universe available to do biological work isdecreasing over time.

Less usable energy is more diffuse, or disorga-nized. Entropy is a measure of this disorder or random-ness. Organized, usable energy has low entropy,whereas disorganized energy such as heat has high en-tropy. Another way to explain the second law of ther-modynamics is that entropy, or disorder, in a systemtends to increase over time. As a result of the secondlaw of thermodynamics, no process requiring an en-ergy conversion is ever 100 percent efficient becausemuch of the energy is dispersed as heat, resulting inan increase in entropy. For example, an automobileengine, which converts the chemical energy of gaso-line to mechanical energy, is between 20 and 30 per-cent efficient: Only 20 to 30 percent of the originalenergy stored in the chemical bonds of the gasolinemolecules is actually transformed into mechanical en-ergy, or work.

PRODUCERS, CONSUMERS, AND DECOMPOSERS

The organisms of an ecosystem are dividedinto three categories based on how theyobtain nourishment: producers, con-sumers, and decomposers. Virtually allecosystems contain representatives of allthree groups, which interact extensivelywith one another, both directly and indirectly.


The sun powers photosynthesis, producing chemical energy

stored in the leaves and seeds of this umbrella tree. Photo-

graphed in Hanging Rock State Park, North Carolina.

Energy flow in the environment Figure 5.4

give up some energy into its surroundings, but the totalenergy content of the organism and its surroundings isalways the same. An organism can’t create the energy itrequires to live. Instead, it must capture energy fromthe environment to use for biological work, a processinvolving the transformation of energy from one formto another. In photosynthesis, plants absorb the radiantenergy of the sun and convert it into thechemical energy contained in the bonds ofsugar molecules (Figure 5.4). Later, ananimal that eats the plant may transformsome of the chemical energy into the me-chanical energy of muscle contraction, en-abling the animal to walk, run, slither, fly,or swim.

As each energy transformation oc-curs, some of the energy is changed to heatenergy that is released into the cooler sur-roundings. No organism can ever use thisenergy again for biological work; it is “lost”

second law ofthermodynam-ics When energy is

converted from one

form to another,

some of it is de-

graded into heat, a

less usable form

that disperses into

the environment.

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Plants and other photosynthetic organisms areproducers and manufacture large organic moleculesfrom simple inorganic substances, generally carbon diox-ide and water, usually using the energy of sunlight. Pro-ducers are potential food resources for other organismsbecause they incorporate the chemicals they manufac-ture into their own bodies. Plants are the most significantproducers on land, and algae and certain types of bacte-ria are important producers in aquatic environments.

Animals are consumers—they consume otherorganisms as a source of food energy and bodybuild-

ing materials. Consumers that eat producers are pri-mary consumers, or, herbivores. Grasshoppers, deer, andrabbits are examples of primary consumers (Figure

5.5A). Secondary consumers eat primary consumers,whereas tertiar y consumers eat secondary consumers.Both secondary and tertiar y consumers are carni-vores that eat other animals. Lions, spiders, andlizards are examples of carnivores (Figure 5.5B).Other consumers, called omnivores, eat a variety oforganisms. Bears, pigs, and humans are examples ofomnivores.


A The arctic hare is a herbivore, or pri-

mary consumer. The chemical energy

stored in flowers and leaves transfers to

the juvenile hare as it eats. B A Madagas-

car day gecko (a tertiary consumer) feeds

on a spider (a secondary consumer). Both

the gecko and the spider are carnivores.

C Ghost crabs forage in the sand for detri-

tus. Photographed along the West African

coast. D These mushrooms are growing

on a dead beech tree in Ostmuritz/Serrahn

National Park, Germany. The mushrooms

you see are reproductive structures; the in-

visible branching, threadlike body of the

mushroom grows un-

derground, decom-

posing dead organic

material.

A

C D

B

Consumers and decomposers Figure 5.5

www.wiley.com/college/berg

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Some consumers, called detritus feeders ordetritivores, consume detritus, organic matter that in-cludes animal carcasses, leaf litter, and feces (Figure

5.5C). Detritus feeders, such as snails, crabs, clams,and worms, are especially abundant in aquatic envi-ronments, where they consume the organic matter inthe bottom muck. Earthworms are terrestrial (land-dwelling) detritus feeders, as are termites, beetles,snails, and millipedes. Detritus feeders work togetherwith microbial decomposers to destroy dead organismsand waste products.

Bacteria and fungi are importantexamples of decomposers, organisms thatbreak down dead organisms and wasteproducts (Figure 5.5D). Decomposersrelease simple inorganic molecules, suchas carbon dioxide and mineral salts,which producers can then reuse.

THE PATH OF ENERGY FLOW IN ECOSYSTEMS

In an ecosystem, energy flow occurs in food chains,in which energy from food passes from one organismto the next in a sequence (Figure 5.6). Each level,or “link,” in a food chain is a trophic level (the Greektropho means nourishment). Producers form the firsttrophic level, primary consumers form the secondtrophic level, secondary consumers the third trophic

level, and so on. At every step in a foodchain are decomposers, which respire or-ganic molecules in the carcasses and bodywastes of all members of the food chain.

Simple food chains rarely occur innature because few organisms eat just onekind of organism. The flow of energythrough an ecosystem typically takes place


Energy enters ecosystems from the sun,

flows linearly—in a one-way direction—

through ecosystems, and exits as heat

loss. Much of the energy acquired by a

given level of the food chain is used and

escapes into the surrounding environment

as heat. This energy, as the second law of

thermodynamics stipulates, is unavailable

to the next level of the food chain.

Energy

from sun

Heat

First trophic level:

Producers

Second trophic level:

Primary consumers

Third trophic level:

Secondary consumers

Fourth trophic level:

Tertiary consumersDecomposers

Heat Heat Heat Heat

energy flow The

passage of energy

in a one-way direc-

tion through an

ecosystem.

Energy flow through a food chain Figure 5.6

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Decomposers

Blackberry

Grasshopper

Spider

Shepherd's-purseCrabgrass

American

robin

Eastern

cottontail

Deer

mouse

Red

fox

Barred owl Black

rat snake

Meadow

mushroom

Bacteria

This food web is greatly simplified

compared to what actually happens

in nature. Many species aren’t in-

cluded, and numerous links in the

web aren’t shown. (The energy-flow

diagram along the left side of the fig-

ure does not correspond exactly to

the food web diagram. For example,

deer mice and cottontails are primary

consumers, and the spider is a

secondary consumer.)

A meadow food web Figure 5.7



697028_ch05_3rd.qxd 9/20/06 3:24 PM 3

have carbon available to them. Carbon makes up ap-proximately 0.04 percent of the atmosphere as a gas,CO2. It is present in the ocean in several chemicalforms, such as carbonate (CO2

3–) and bicarbonate

(HCO–3) and in rocks such as limestone. The global

movement of carbon between the abiotic environment,including the atmosphere and ocean, and organisms isthe carbon cycle (Figure 5.8).

Photosynthesis removes carbon (as CO2) from theabiotic environment and fixes it into the biological com-pounds of producers. Carbon dioxide is returned to theatmosphere when living organisms—producers, con-sumers, and decomposers—respire. During respiration,sugar is broken down to carbon dioxide, and energy is re-leased. A similar carbon cycle occurs in aquatic ecosystemsinvolving carbon dioxide dissolved in the water.

Sometimes the carbon in biological molecules isn’t recycled back to the abiotic environment for sometime. A large amount of carbon is stored in the wood of

in accordance with a range of food choices for each or-ganism involved. In an ecosystem of average complexity,numerous alternative pathways are possible. An owl eat-ing a rabbit is a different energy pathway than an owl eat-ing a snake. A food web, a complex of interconnectedfood chains in an ecosystem, is a more realistic model ofthe flow of energy and materials through ecosystems(Figure 5.7 on previous page).

The most important thing to remember aboutenergy flow in ecosystems is that it is linear, or one-way.Energy moves along a food chain or food web from oneorganism to the next as long as it isn’t used for biologicalwork. Once an organism uses energy, it is lost as heat andis unavailable for any other organism in the ecosystem.

n contrast to energy flow, matter, the ma-terial of which organisms are composed,moves in numerous cycles from one partof an ecosystem to another—from one

organism to another and from living organisms to theabiotic environment and back again. We call these cyclesof matter biogeochemical cycles because they involve biological, geological, and chemical interactions. Fourdifferent biogeochemical cycles of matter—carbon, hydrologic, nitrogen, and phosphorus—are representa-tive of all biogeochemical cycles. These four cycles areparticularly important to organisms, for these materialsmake up the chemical compounds of cells.

THE CARBON CYCLE

Proteins, carbohydrates, and other molecules essentialto living organisms contain carbon, so organisms must


LEARNING OBJECTIVEF

Diagram and explain the carbon, hydrologic, nitrogen, and phosphorus cycles.

The Cycling of Matter in Ecosystems

I

CONCEPT CHECK

What is the first law of

thermodynamics? the

second?

Could you construct a

balanced ecosystem that

contains only producers

and consumers? only

consumers and

decomposers? Explain

your answers.

How does energy move

through a food web?

STOPSTOP

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The Cycling of Matter in Ecosystems 105

Photosynthetic organisms

remove carbon dioxide

from air and incorporate it

into chemical compounds

such as sugar.

Sugar and similar compounds

are used as fuel by

producer that made them,

by consumer that eats producer,

or by decomposer that

breaks down remains of

producer or consumer.

Carbon in coal, oil,

natural gas, and wood

is returned to atmosphere

by burning, or combustion.

Millions of years ago,

vast coal beds formed

from bodies of ancient trees

that did not decay fully

before they were buried.

Unicellular marine organisms

probably gave rise to underground

deposits of oil and natural gas

that accumulated in geological past.

A lot of carbon is incorporated

into shells of marine organisms.

When they die, their shells

sink to ocean floor and

form thick seabed deposits.

Chemical and physical weathering

processes slowly erode limestone,

returning carbon to

water and atmosphere.

Air (CO2)

Coal

Natural gasOil

Burial and compaction

form rock

(limestone).

The carbon cycle Figure 5.8

trees, where it may stay for several hundred years or evenlonger. Coal, oil, and natural gas, called fossil fuels be-cause they formed from the remains of ancient organ-isms, are vast deposits of carbon compounds—the endproducts of photosynthesis that occurred millions ofyears ago. In combustion, organic molecules in wood, coal,oil, and natural gas are burned, with an accompanyingrelease of heat, light, and carbon dioxide.

The thick deposits of shells of marine organ-isms contain carbon. These shells settle to the oceanfloor and are eventually cemented together to form thesedimentary rock limestone. The crust is dynamicallyactive, and over millions of years, sedimentary rock onthe bottom of the seafloor may lift to form land sur-faces. The summit of Mt. Everest, for example, is com-posed of sedimentary rock.

The movement of carbon between the

abiotic environment (the atmosphere

and oceans) and living organisms is a

process known as the carbon cycle.

Because proteins, carbohydrates, and

other molecules contain carbon, the

process is essential to life.


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Pro

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m

Water moves from

atmosphere to ocean

as precipitation –

rain, snow, sleet,

or hail.

Water moves from

atmosphere to land

as precipitation –

rain, snow, sleet,

or hail.

Atmosphere

Runoff to

ocean

Movement of

moist air

Transpiration, or loss

of water vapor from

land plants, adds

water to atmosphere.

Condensation

(cloud formation)

When water evaporates

from ocean surface and

soil, streams, rivers, and

lakes on land, it forms

clouds in atmosphere.

Water seeps down through

soil and rock to become

groundwater. Groundwater

eventually supplies water to

soil, streams, rivers,

and ocean.

The hydrologic cycle Figure 5.9

Water may evaporate from land and reenter theatmosphere directly. Alternatively, it may flow in riversand streams to coastal estuaries. The movement of waterfrom land to rivers, lakes, wetlands, and, ultimately, theocean is runoff, and the area of land that runoff drainsis a watershed.

Regardless of its physical form—solid, liquid, orvapor—or location, every molecule of water eventually

THE HYDROLOGIC CYCLE

You are probably most familiar with the hydrologic cycle. Wa-ter continuously circulates from the ocean to the atmo-sphere to the land and back to the ocean. It provides a re-newable supply of purified water for terrestrial organisms.This cycle results in a balance among water in the ocean,on the land, and in the atmosphere (Figure 5.9).

In the hydrologic cycle, water moves among the ocean, the atmosphere, the land, and back to the ocean in a continuous

process that supports life.

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moves through the hydrologic cycle. Tremendousquantities of water are cycled annually between Earthand its atmosphere. The volume of water entering theatmosphere each year, estimated at about 389,500 cubickm (95,000 cubic mi), is incomprehensibly large. Ap-proximately three-fourths of this water reenters theocean directly as precipitation over water; the remain-der falls on land.


Nitrogen fixation is conversion of gaseous nitrogen (N2)

to ammonia (NH3). Nitrogen fixation gets its name

because nitrogen is fixed so organisms can use it.

Combustion, volcanic action, lightning discharges,

and industrial processes also fix

considerable nitrogen.

Ammonification is conversion of

biological nitrogen compounds into

ammonia (NH3). Decomposers

perform ammonification.

Atmospheric

nitrogen (N2)

Denitrification is reduction of nitrate (NO3)

to gaseous nitrogen (N2). Denitrifying bacteria

reverse the action of nitrogen-fixing and

nitrifying bacteria by returning nitrogen

to the atmosphere as nitrogen gas.

Nitrate (NO3)Ammonia (NH3)

Plant roots absorb nitrate (NO3) or ammonia (NH3)

and assimilate nitrogen into plant proteins

and nucleic acids. When animals consume

plant tissues, they assimilate nitrogen

by converting plant proteins to animal proteins.

Nitrification is conversion of

ammonia (NH3) to nitrate (NO3).

Soil bacteria perform nitrification.

The movement of nitrogen between the

abiotic environment (primarily the at-

mosphere) and living organisms is

known as the nitrogen cycle. The five

steps of the nitrogen cycle are nitrogen

fixation, nitrification, assimilation, am-

monification, and denitrification.

The nitrogen cycle Figure 5.10

THE NITROGEN CYCLE

Nitrogen is an essential part of biological moleculessuch as proteins and nucleic acids (for example, DNA).At first glance a shortage of nitrogen for organisms appears impossible. The atmosphere is 78 percent nitrogen gas (N2). But atmospheric nitrogen is so stablethat it does not readily combine with other elements.

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Atmospheric nitrogen must first break apart before thenitrogen atoms combine with other elements to formproteins and nucleic acids. There are five steps in thenitrogen cycle, in which nitrogen cycles between the abi-otic environment and organisms: nitrogen fixation, nitrification, assimilation, ammonification, and denitri-fication (Figure 5.10 on previous page).

Nitrogen-fixing bacteria carry out nitrogen fixa-tion in soil and aquatic environments. Nitrogen-fixingbacteria split atmospheric nitrogen and combine the re-sulting nitrogen atoms with hydrogen. Some nitrogen-fixing bacteria, Rhizobium, live inside swellings, or nod-ules, on the roots of legumes such as beans or peas and some woody plants (Figure 5.11A). In aqua-tic environments, photosynthetic bacteria called cyano-bacteria perform most of the nitrogen fixation (Figure 5.11B).

During nitrification, soil bacteria convert ammo-nia to nitrate. The process of nitrification furnishesthese bacteria, called nitrifying bacteria, with energy. Inassimilation, plants absorb ammonia or nitrate throughtheir roots and convert the nitrogen into plant com-pounds such as proteins. Animals assimilate nitrogen


A Bacteria carry out nitrogen fixation in the nodules of a pea plant’s roots. B Nostoc is a cyanobacterium that fixes nitrogen.

BA

Nitrogen fixation Figure 5.11

when they consume plants or other animals and con-vert the proteins into animal proteins.

Ammonification occurs when organisms producenitrogen-containing waste products such as urine.These substances, plus the nitrogen compounds thatoccur in dead organisms, are decomposed, releasingthe nitrogen into the abiotic environment as ammonia.The bacteria that perform this process are called am-monifying bacteria. Other bacteria perform denitrifica-tion, in which nitrate is converted back to nitrogen gas.Denitrifying bacteria prefer to live and grow wherethere is little or no free oxygen. For example, they arefound deep in the soil near the water table, an environ-ment that is nearly oxygen-free.

THE PHOSPHORUS CYCLE

Unlike the biogeochemical cycles just discussed, thephosphorus cycle doesn’t have an atmospheric compo-nent. Phosphorus cycles from the land into living organ-isms, then from one organism to another, and finallyback to the land (Figure 5.12). Phosphates are

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Plant roots take up soil

phosphorus as inorganic phosphates.

Animals obtain most of their phosphate

from the food they eat, although

drinking water may supply phosphate

in some localities.

Streams and rivers carry

some phosphate to ocean,

where it is deposited on seafloor

and remains for millions of years.

Geological process of

uplift may someday expose

seafloor sediments as new land,

from which phosphate

will again be eroded.

Phosphorus released by decomposers

becomes part of the soil's pool of

inorganic phosphate that plants reuse.

Some rock is weathered,

becoming soil

Phosphate

mining

Fertilizer

containing

phosphates

As water runs over

phosphorus-containing rocks,

it erodes and carries off

inorganic phosphate (PO4 ) molecules.3

Rock containing

phosphorus

Phosphorus moves from the land

through aquatic and terrestrial com-

munities, between organisms in these

communities, and back to the land in

a process known as the phosphorus

cycle. Unlike other biogeochemical

cyles, the phosphorus cycle does

not involve the atmosphere.

The phosphorus cycle Figure 5.12



697028_ch05_3rd.qxd 9/20/06 3:24 PM Page 109


ou have seen that a diverse assortmentof organisms inhabits each communityand that these organisms obtain nour-ishment in a variety of ways. You have

also considered energy flow and biogeochemical cycles.Now let’s examine the way of life of a given species inits ecosystem. An ecological description of a species typ-ically includes whether it is a producer, consumer, ordecomposer. However, we need other details to providea complete picture.

Every organism is thought to have its own role,or ecological niche, within the structure and functionof an ecosystem. An ecological niche is difficult to de-fine precisely because it takesinto account all aspects of the organism’s existence—all physical, chemical, and biological factors the organ-ism needs to survive, re-main healthy, and reproduce.Among other things, theniche includes the local envi-ronment in which an organ-ism lives—its habitat. An organism’s niche also encom-passes what it eats, what eats it, what organisms itcompetes with, and how the abiotic components of itsenvironment, such as light, temperature, and moisture,interact with and influence it. A complete descriptionof an organism’s ecological niche involves numerousdimensions (Figure 5.13).

When two species are similar, their ecologicalniches may appear to overlap. However, many ecolo-gists think no two species indefinitely occupy the sameniche in the same community. Resource partitioning isone way some species avoid or at least reduce nicheoverlap. Resource partitioning is the reduction in com-petition for environmental resources such as food


Describe the factors that contribute to an organism’s ecological niche.

Explain the concept of resource partitioning.

Ecological Niches

ecologicalniche The totality

of an organism’s

adaptations, its

use of resources,

and the lifestyle

to which it is fitted.

How is photosynthesis

involved in the carbon

cycle?

How are living

organisms involved in

the hydrologic cycle?

What are the five steps

of the nitrogen cycle?

What four steps of

the nitrogen cycle

exclusively involve

bacteria?

used in biological molecules such as nucleic acids andATP, a compound important in energy transfer reac-tions in cells. Like carbon and nitrogen, phosphorusmoves through the food web as one organism con-sumes another.

Phosphorus cycles through aquatic communi-ties in much the same way it does through terrestrialcommunities. Dissolved phosphorus enters aquaticcommunities as algae and plants absorb and assimilateit; plankton and larger organisms obtain phosphoruswhen they consume the algae and plants. A variety offishes and mollusks eat plankton in turn. Ultimately,decomposers release inorganic phosphorus into thewater, where it is available for aquatic producers to useagain. Phosphate can be lost from biological cycles. Somephosphate is carried from the land by streams and riversto the ocean, where it can be deposited on the seafloorand remain for millions of years.

A small portion of the phosphate in the aquaticfood web finds its way back to the land. A few fishes andaquatic invertebrates are eaten by seabirds, which maydefecate on land where they roost. Guano, the manureof sea birds, contains large amounts of phosphate andnitrate. Once on land, these minerals may be absorbedby the roots of plants. The phosphate contained in guanomay enter terrestrial food webs in this way, although theamounts involved are small.

Y

CONCEPT CHECK STOPSTOP

How does the

phosphorus cycle differ

from the other three

cycles presented in

this chapter?

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feeding, location of feeding, nest sites, and other aspectsof an organism’s ecological niche (see the featureWhat a Scientist Sees on the following page).

Ecological Niches 111

CONCEPT CHECK

What are three aspects

of the wildebeest’s

ecological niche?

What is resource

partitioning?

STOPSTOP

among coexisting species as a result of each species’niche differing from the others in one or more ways.Evidence of resource partitioning in animals is well doc-umented and includes studies in tropical forests of Cen-tral and South America that demonstrate little overlapin the diets of fruit-eating birds, primates, and bats thatcoexist in the same habitat. Although fruits are the pri-mary food for several hundred bird, primate, and batspecies, the wide variety of fruits available has allowedfruit eaters to specialize, thereby reducing competi-tion. Resource partitioning also includes timing of

Wildebeests live in herds on the open

plains of East Africa and graze on the

grasses. When the dry season begins,

wildebeests living in East Africa’s

Serengeti Plain migrate more than 1,200

km in search of water and grass. Females

usually bear one calf at a time. Adult

wildebeests are swift runners. They are

preyed on by lions and, when swimming

across rivers, by crocodiles. Hyenas, lions,

cheetahs, leopards, and wild dogs prey

on the calves.

The wildebeest’s ecological niche Figure 5.13

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Yellow-rumped Warbler Bay-breasted Warbler Cape May Warbler Black-throated Green Warbler Blackburnian Warbler

Wh

at a

Scie

ntist S

ee

sR e s o u rc e Pa r t i t i o n i n g

Robert MacArthur’s study of five American warbler species is

a classic example of resource partitioning. Although it initially

appeared that their niches were nearly identical, MacArthur

determined that individuals of each species spend most of

their feeding time in different portions of spruces and other

conifer trees. They also move in different directions through

the canopy, consume different combinations of insects, and

nest at slightly different times. The photo shows a male

black-throated green warbler in a spruce tree.


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Interactions Among Organisms 113

o organism exists independently ofother organisms. The producers, con-sumers, and decomposers of an ecosys-tem interact with one another in a vari-

ety of ways, and each forms associations with otherorganisms. Three main types of interactions occuramong species in an ecosystem: symbiosis, predation,and competition.

SYMBIOSIS

In symbiosis one species usually lives in or on anotherspecies. The partners of a symbiotic relationship maybenefit, be unaffected, or be harmed by the relationship.

Symbiosis is the result of coevolution, the inter-dependent evolution of two interacting species. Flower-ing plants and their animal pollinators are an excellentexample of coevolution. Bees, beetles, hummingbirds,bats, and other animals transport pollen from one plantto another. During the millions of years over which theseassociations developed, flowering plants evolved severalways to attract animal pollinators. One of the rewards forthe pollinator is food—nectar (a sugary solution) and

pollen. Plants possess a varietyof ways to get the pollinator’sattention, most involving showypetals and scents.

While plants were ac-quiring specialized features toattract pollinators, animals co-evolved specialized body partsand behaviors to aid pollina-tion and obtain nectar andpollen as a reward (Figure

5.14). Coevolution is respon-

Interactions Among Organisms


Distinguish among mutualism,

commensalism, and parasitism.

Define predation and describe predator–

prey relationships.

Define competition and distinguish between

intraspecific and interspecific competition.

Discuss an example of a keystone species.

N

symbiosis Any

intimate relation-

ship or association

between members

of two or more

species; includes

mutualism, com-

mensalism, and

parasitism. This Hawaiian honeycreeper uses its gracefully curved bill to

sip nectar from long, tubular flowers of the lobelia.

Coevolution Figure 5.14

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sible for the hairy bodies of bumblebees, which catchand hold the sticky pollen for transport from one flowerto another. Coevolution is also responsible for the long,curved beaks of certain Hawaiian birds that insert theirbeaks into tubular flowers to obtain nectar.

The thousands, or even millions, of symbiotic associations that result from coevolution fall into threecategories: mutualism, commensalism, and parasitism.The association between pollinators such as the hon-eybee (Figure 5.15) and plants such as the flowerit is obtaining nectar and pollen from is an example ofmutualism, in which both organisms benefit.


Commensalism is a symbiotic relationship inwhich one species benefits and the other is neitherharmed nor helped. One example of commensalism isthe relationship between a tropical tree and its epi-phytes, smaller plants, such as mosses, orchids, andferns that live attached to the bark of the tree’sbranches (Figure 5.16). The epiphyte anchors it-self to the tree but doesn’t obtain nutrients or waterdirectly from the tree. Its location on the tree enablesit to obtain adequate light, water (as rainfall drippingdown the branches), and required nutrient minerals(which rainfall washes out of the tree’s leaves). The

The honeybee is obtaining nectar and pollen from the flower and pollinating it at the same time. This is an example of pollination

mutualism, which is a major category of mutualism.

Mutualism Figure 5.15

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epiphyte benefits from the association, whereas thetree is apparently unaffected.

Parasitism is a symbiotic relationship in whichone species (the parasite) benefits at the expense of theother (the host). Parasitism is a successful lifestyle; morethan 100 parasites live in or on the human species (Fig-

ure 5.17). The parasite, usually much smaller than itshost, obtains nourishment from its host, but although aparasite may weaken its host, it rarely kills it quickly. (Aparasite would have a difficult life if it kept killing off itshosts!) Some parasites, such as ticks, live outside the

Epiphytes are small plants that attach to the branches and

trunks of larger trees. Photographed in Costa Rica.

Close-up of body lice feeding on a human arm. Each louse is

about 3 mm (0.12 in.) long.


Commensalism Figure 5.16 Parasitism Figure 5.17

host’s body; other parasites, such as tapeworms, livewithin the host.

PREDATION

Predation includes both animals eating other animals(for example, herbivore–carnivore interactions) andanimals eating plants (producer–herbivore interac-tions). Predation has resulted in an “arms race,” with thecoevolution of predator strategies—more efficient ways

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to catch prey—and preystrategies—better ways to es-cape the predator. An effi-cient predator exerts a strongselective force on its prey, andover time the prey speciesmay evolve some sort of coun-termeasure that reduces theprobability of being cap-tured. The countermeasurethat the prey acquires in turnmay act as a strong selectiveforce on the predator.


The cheetah is the world’s fastest animal andcan sprint at 110 kilometers per hour for short distances (Figure 5.18). Killer whales, which huntin packs, often herd salmon or tuna into a cove so that they are easier to catch. Any trait that increaseshunting efficiency, such as the speed of a cheetah or the intelligence of killer whales, favors predatorsthat pursue their prey. Ambush is another effectiveway to catch prey. The goldenrod spider is the samecolor as the white or yellow flowers in which it hides.This camouflage prevents unwary insects that visit theflower for nectar from noticing the spider until it istoo late.

predation The

consumption of one

species (the prey)

by another (the

predator).

Global Locator

A cheetah is in a full sprint as it pursues possible prey. Photographed in Masai Mara National Park, Kenya.

Predation Figure 5.18

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Many potential animal prey, such as wood-chucks, run to their underground burrows to escapepredators. Others have mechanical defenses, such as the barbed quills (in the porcupine) and a shell(in the pond turtle). Some animals live in groups—a herd of antelope, colony of honeybees, school ofanchovies, or flock of pigeons. This social behaviordecreases the likelihood of a predator catching oneof them unaware because the group has so many eyes,ears, and noses watching, listening, and smelling for predators (Figure 5.19A).

Chemical defenses are common among ani-mal prey. The South American poison arrow frog haspoison glands in its skin and bright warning colors

that experienced predators avoid. Some animalsblend into their surroundings and so hide frompredators. Certain caterpillars resemble twigs soclosely you would never guess they are animals untilthey move (Figure 5.19B).

Plants possess adaptations that protect themfrom being eaten. The presence of spines, thorns,tough leathery leaves, or even thick wax on leaves discourages foraging herbivores from grazing. Otherplants produce an array of protective chemicals that are unpalatable or even toxic to herbivores. Thenicotine found in tobacco is so effective at killing in-sects that it is an ingredient in many commercial insecticides.


BA

A Two adult meerkats stand watch

over a young meerkat pup. If one of

the sentries spies a predator such as

an eagle or a hawk, they will alert the

other meerkats, and all will scramble

into their burrows. Photographed

in the Kalahari Desert, South Africa.

B Predators often overlook caterpillars

that closely resemble small branches. Can

you find the caterpillar? (Hint: The cater-

pillar is mimicking an adjacent twig.)

Avoiding predators Figure 5.19

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COMPETITION

Competition occurs when two or more individuals at-tempt to use an essential common resource such asfood, water, shelter, living space, or sunlight.

Resources are often in limited supply in the envi-ronment, and their use by one individual decreases theamount available to others. If a tree in a dense forest growstaller than surrounding trees, it absorbs moreof the incoming sunlight (Figure 5.20).Less sunlight is available for nearby treesthat the taller tree shades. Competition oc-curs among individuals within a population(intraspecific competition) and between differ-ent species (interspecific competition).

Competition isn’t always a straight-forward, direct interaction. Consider a vari-ety of flowering plants that live in a young

pine forest and compete with conifers for such resourcesas soil moisture and soil nutrient minerals. Their rela-tionship is more involved than simple competition. Theflowers produce nectar that some insect species con-sume; these insects also prey on needle-eating insects, re-ducing the number of insects feeding on pines. It istherefore difficult to assess the overall effect of floweringplants on pines. If the flowering plants were removed

from the community, would the pines growfaster because they were no longer compet-ing for necessary resources? Or would theincreased presence of needle-eating insects(caused by fewer omnivorous insects) in-hibit pine growth?

Short-term experiments in whichone competing plant species is removedfrom a forest community often have demon-strated improved growth for the remaining

competition The

interaction among

organisms that vie

for the same re-

sources in an

ecosystem (such

as food or living

space).

Competition Figure 5.20

Plants compete for light and growing space in a forest. Taller trees reduce the amount

of sunlight available for shorter ones. Photographed in Kerala State, India.

697028_ch05_3rd.qxd 9/20/06 3:24 PM Page 118

species. However, few studies have tested the long-termeffects on forest species of removing one competingspecies. These long-term effects may be subtle, indirect,and difficult to assess. They may lower or negate the neg-ative effects of competition for resources.

KEYSTONE SPECIES

Certain species are more crucial to the maintenance oftheir ecosystem than others. Such keystone species arevital in determining an ecosystem’s species compositionand how the ecosystem functions. Keystone species areusually not the most abundant species in the ecosystem.Although present in relatively small numbers, keystone

species exert a profound influence on the entire ecosys-tem because they often affect the available amount offood, water, or some other resource.

Identifying and protecting keystone species arecrucial goals of conservation biologists because if a key-stone species disappears from an ecosystem, other or-ganisms may become more common or more rare, oreven disappear. One example of a keystone species is atop predator such as the gray wolf (Figure 5.21).Where wolves were hunted to extinction, the popula-tions of deer and other herbivores increased explo-sively. As these herbivores overgrazed the vegetation,plant species that couldn’t tolerate such grazing pres-sure disappeared. Smaller animals such as insects werelost from the ecosystem because the plants they de-pended on for food were now less abundant. Thus, thedisappearance of the wolf resulted in an ecosystem withconsiderably less biological diversity.

Some scientists think we should abandon theconcept of keystone species because it is problematic.For one thing, most of the information about keystonespecies is anecdotal. Scientists have performed fewlong-term studies to identify keystone species and to de-termine the nature and magnitude of their effects onthe ecosystems they inhabit.


The wolf is considered a keystone species in its ecosystem.

CONCEPT CHECK

What is an example of

mutualism? of

parasitism?

What is an example of a

predator–prey

interaction? of

competition?


between interspecific

and intraspecific

competition?

STOPSTOP

Gray wolf Figure 5.21

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During the past two centuries,the level of carbon dioxide(CO2) in the atmosphere

has increased dramatically, causedby the burning of fossil fuels such as coal, oil, and natural gas, and theclearing and burning of forests. En-vironmental scientists are increas-ingly concerned that the rising lev-els of CO2 may change Earth’sclimate. CO2 levels rose from 315parts per million (ppm) in 1958 to377 ppm in 2004—a 20 percent increase. CO2 in the atmosphere al-lows solar radiation to pass throughbut does not allow heat to radiateinto space. Instead, the heat is radi-ated back to Earth’s surface. As theCO2 accumulates, it may trapenough heat to warm the planet.

Observations confirm that ourclimate is changing rapidly, particu-larly in the past few decades. The1980s, 1990s, and early 2000s sawsome of the warmest years since scientists started keeping records

in 1820. Nine of the 10 warmestyears have occurred since 1990 (theother occurred in 1988). Earth’stemperature in 2005 was the warm-est in the weather records; the sec-ond warmest temperature on record occurred in 1998, and the thirdwarmest in 2002.

Environmental scientists esti-mate that if trends don’t change,Earth’s mean temperature could rise1.4° to 5.8°C (2.5° to 10.4°F) by theend of the 21st century. This temper-ature increase would make the atmo-sphere warmer than tree ring andglacial ice analyses estimate for thepast 1,000 years. This warming couldproduce major shifts in patterns ofrainfall and initiate melting of theWest Antarctic and Southern Green-land ice sheets, as did the last warmperiod 120,000 years ago. Such melt-ing would cause the ocean level torise, an alarming scenario, as it mightput many major cities at least partlyunder water.

An increasing number of envi-ronmental scientists are concernedthat human-induced global climatewarming may have started. The U.S.National Academy of Science hasstated that global climate warmingmay be the most pressing interna-tional issue of the 21st century. In2001 the U.N. IntergovernmentalPanel on Climate Change (IPCC)released a report based on the con-sensus of hundreds of climate scien-tists worldwide. The IPCC declaredthat the human-produced increase in

greenhouse gas concentrations arelikely responsible for most of the ob-served warming in the last 50 years.

An international climate warm-ing conference was held in Kyoto,Japan, in December 1997. Althoughmany of the participating countrieshad conflicting positions, an initialtreaty known as the Kyoto Protocolwas formulated. This stipulated thathighly developed countries must cuttheir emissions of CO2 and othergases that cause warming by an aver-age of 5.2 percent by 2012. Unlessmany countries ratify and observethe treaty, CO2 emissions are ex-pected to continue to increase. (The United States signed the pro-tocol in 1998 at a follow-up meetingin Buenos Aires, but the Bush ad-ministration withdrew the UnitedStates from that commitment.)

Energy experts at the U.S. De-partment of Energy say that energyconservation and efficiency mea-sures could accomplish much of the U.S. emissions reductions re-quired if we were to observe the Kyoto Protocol. We could save theequivalent of 60 million metric tonsof CO2 emissions each year if auto-motive vehicles were designed to get better gas mileage (see photo). Installing high-efficiency wind tur-bines to generate electricity in statessuch as Texas and California couldsave as much as 20 million metrictons of CO2 emissions per year. Wesay more about global warming inChapter 9.

This vehicle averages perhaps 10 miles

per gallon, which translates into a great

deal of CO2

emissions.

CASE STUDYGlobal Warming: Is There an Imbalance in the Carbon Cycle?


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community is a natural association

that consists of all the populations of

different species that live and interact

together within an area at the same

time. An ecosystem is a community

and its physical environment. A land-

scape is a region that includes several

interacting ecosystems. The biosphere

is the layer of Earth containing all living

organisms.

CHAPTER SUMMARY

Chapter Summary 121

1What is

Ecology?

1. Ecology is the study of the interac-

tion among organisms and between

organisms and their abiotic

environment.

2. A population is a group of organisms

of the same species that live together

in the same area at the same time. A

3The Cycling of Matter

in Ecosystems

1. Biogeochemical cycles are the

processes by which matter cycles from

the living world to the nonliving, physi-

cal environment and back again. Carbon

dioxide is the important gas of the car-

bon cycle; carbon enters the living world

by photosynthesis and returns to the

abiotic environment when organisms

respire. The hydrologic cycle continu-

ously renews the supply of water and

involves an exchange of water among

the land, the atmosphere, and organ-

isms. There are five steps in the nitro-

gen cycle: nitrogen fixation, nitrifica-

tion, ammonification, assimilation, and

dentrification. The phosphorus cycle

has no biologically important gaseous

compounds; phosphorus erodes from

rock and is absorbed by plant roots.

2The Flow of Energy

Through Ecosystems

1. Energy is the capacity or ability to do

work. According to the first law of ther-

modynamics, energy can’t be created or

destroyed, although it can change from

one form to another. As a result of the

second law of thermodynamics, when

energy is converted from one form to an-

other, some of it is degraded into heat, a

less usable form that disperses into the

environment.

2. A producer manufactures large organic

molecules from simple inorganic sub-

stances. A consumer can’t make its own

food and uses the bodies of other organ-

isms as a source of energy and body-

building materials. Decomposers are

microorganisms that break down dead

organic material and use the decomposi-

tion products to supply themselves with

energy.

3. Energy flow is the passage of energy

in a one-way direction through an

ecosystem.

See Figures 5.8, 5.9, 5.10, and 5.12 for

additional details.

4Ecological

Niches

1. An ecological niche is the totality of

an organism’s adaptations, its use of

resources, and the lifestyle to which it

is fitted. An organism’s ecological niche

includes its habitat, its distinctive life-

style, and its role in the community.

2. Resource partitioning is the reduction

in competition for environmental re-

sources, such as food, that occurs

among coexisting species as a result

of each species’ niche differing from

the others in one or more ways.

697028_ch05_3rd.qxd 9/20/06 3:24 PM Page 121


CHAPTER SUMMARY

1. To function, ecosystems require an input of energy. Where does

this energy come from?

2. After an organism uses energy, what happens to it?

3. In ecosystems, matter cycles, whereas energy flows in a linear

fashion. Explain the distinction.

4. What is a biogeochemical cycle? Why is the cycling of matter

essential to the continuance of life?

5. Describe how organisms participate in each of these bio-

geochemical cycles: carbon, nitrogen, and phosphorus.

6. How are food chains important in biogeochemical cycles?

7. How are the many cichlid species in Lake Victoria an example of

resource partitioning?

8. In both parasitism and predation, one organism benefits at the

expense of another. What is the difference between the two

processes?

9. Some biologists think protecting keystone species would help

preserve biological diversity in an ecosystem. Do you agree?

Explain your answer.

10. How do human effects on climate warming make it more

difficult to reach the goal of environmental sustainability?

CRITICAL AND CREATIVE THINKING QUESTIONS

■ ecology p. 96

■ population p. 96

■ community p. 96

■ ecosystem p. 96

■ landscape p. 96

■ biosphere p. 98

■ first law of thermodynamics p. 99

■ second law of thermodynamics p. 100

■ energy flow p. 102

■ ecological niche p. 110

■ symbiosis p. 113

■ predation p. 116

■ competition p. 118

5Interactions Among

Organisms

1. Symbiosis is any intimate relationship or

association between members of two or

more species. Mutualism is a symbiotic

relationship in which both species bene-

fit. Commensalism is a symbiotic rela-

tionship in which one species benefits

and the other species is neither harmed

nor helped. Parasitism is a symbiotic

relationship in which one species (the

parasite) benefits at the expense of

the other (the host).

2. Predation is the consumption of one

species (the prey) by another (the preda-

tor). During coevolution between preda-

tor and prey, the predator evolves more

efficient ways to catch prey (such as pur-

suit and ambush), and the prey evolves

better ways to escape the predator

(such as flight, association in groups,

and camouflage).

3. Competition is the interaction among

organisms that vie for the same re-

sources in an ecosystem (such as food

or living space). Competition occurs

among individuals within a population

(intraspecific competition) or between

species (interspecific competition).

4. A keystone species is crucial in deter-

mining the nature and structure of the

entire ecosystem in which it lives.

Though present in relatively small num-

bers, keystone species have a dispro-

portionate effect on the ecosystem.

KEY TERMS

697028_ch05_3rd.qxd 9/20/06 3:25 PM Page 122

This figure shows the components of a simple food chain.

Use it to answer Questions 11–13.

11. Identify the producers, consumers, and decomposers in the

food chain. How many trophic levels are represented?

12. Describe or indicate the flow of food and energy within this

system.

13. Which forms of energy are present within this chain?

■ This caterpillar has

inflated its thorax to

make its body look

like the head of a

snake. Suggest a

possible reason the

caterpillar does this.

■ Note the two large

spots. What do they

resemble? Why would

this animal have such

conspicuous spots?

■ If a hungry bird saw

this caterpillar, do you

think it would have

second thoughts

before eating it?

Why or why not?

What is happening in this picture ?

Critical and Creative Thinking Questions 123

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how ecosystems 5 work

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