90 Chapter 4

1 FOCUSObjectives4.2.1 Explain how biotic and abiotic

factors influence an ecosystem.4.2.2 Identify the interactions that

occur within communities.4.2.3 Describe how ecosystems

recover from a disturbance.

Vocabulary PreviewThe text provides pronunciations for most of the new Vocabulary termsin this section. Encourage students to use a dictionary to look up thephonetic spellings of the words forwhich pronunciations are not given,convert those spellings to the systemused in this text, and include thephonetic spellings when they list thehighlighted, boldface terms.

Reading StrategyStudents often confuse the terms sym-biosis, mutualism, and commensalism.To help them understand and remem-ber the distinctions, have them lookup the derivations of the words in adictionary and note the derivationswhen they list the highlighted, bold-face terms.

2 INSTRUCT

Biotic and Abiotic FactorsBuild Science SkillsApplying Concepts Point out tostudents that in any ecosystem,removing biotic elements can dra-matically affect the ecosystem’sabiotic conditions. For example, thetrees in a forest hold topsoil withtheir roots, shade the soil, contributeorganic matter to the soil in the formof dead leaves, and return water tothe atmosphere through evaporationand transpiration. Removing treesfrom the forest ecosystem reducesthese benefits. Ask students to sug-gest other examples of removingbiotic elements from an ecosystem.

4–2 What Shapes an Ecosystem?

Key Concepts• How do biotic and abiotic

factors influence an ecosystem?

• What interactions occur withincommunities?

• What is ecological succession?

Vocabularybiotic factorabiotic factorhabitatnicheresourcecompetitive exclusion principlepredationsymbiosismutualismcommensalismparasitismecological successionprimary successionpioneer speciessecondary succession

Reading Strategy: Building Vocabulary Beforeyou read, preview new vocabu-lary terms by skimming thesection and making a list of thehighlighted, boldface terms.Leave space to make notes asyou read.

� Figure 4–4 Like allecosystems, this pond is shapedby a combination of biotic andabiotic factors. The bullfrog,plants, and other organisms in thepond are biotic factors. The water,the air, and the rock on which thebullfrog sits are abiotic factors.

If you ask an ecologist where a particular organism lives, that

person might say the organism lives on a Caribbean coral

reef, or in an Amazon rain forest, or in a desert in the American

Southwest. Those answers provide a kind of ecological address

not unlike a street address in a city or town. An ecological

address, however, tells you more than where an organism lives.

It tells you about the climate the organism experiences and

what neighbors it is likely to have. But what shapes the ecosys-

tem in which an organism lives?

Biotic and Abiotic FactorsEcosystems are influenced by a combination of biological and

physical factors. The biological influences on organisms within

an ecosystem are called These include the

entire living cast of characters with which an organism might

interact, including birds, trees, mushrooms, and bacteria—in

other words, the ecological community. Biotic factors that influ-

ence a bullfrog, for example, might include the tiny plants and

algae it eats as a tadpole, the herons that eat the adult frog, and

other species that compete with the bullfrog for food or space.

Physical, or nonliving, factors that shape ecosystems are

called (ay-by-AHT-ik) For example, the climate

of an area includes abiotic factors such as temperature, precipi-

tation, and humidity. Other abiotic factors are wind, nutrient

availability, soil type, and sunlight. For example, the bullfrog in

Figure 4–4 is affected by abiotic factors such as the availability of

water and the temperature of the air. Together, biotic and

abiotic factors determine the survival and growth of an

organism and the productivity of the ecosystem in which

the organism lives. The area where an organism lives is called

its A habitat includes both biotic and abiotic factors.

Give an example of an abiotic factor.

habitat.

factors.abiotic

biotic factors.

Section 4–2

SECTION RESOURCES

Print:

• Laboratory Manual A, Chapter 4 Lab• Laboratory Manual B, Chapter 4 Lab• Teaching Resources, Lesson Plan 4–2,

Adapted Section Summary 4–2, AdaptedWorksheets 4–2, Section Summary 4–2,Worksheets 4–2, Section Review 4–2,Enrichment

• Reading and Study Workbook A, Section 4–2

• Adapted Reading and Study Workbook B,Section 4–2

• Lab Worksheets, Chapter 4 Exploration

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eSaver

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The NicheAddress MisconceptionsStudents sometimes misunderstandwhat a niche is, believing it to be apart of an ecosystem. Use simpleanalogies to clarify the meaning ofthe term. For example, each playeron a baseball team has a specificniche—a different role to play.

Ecosystems and Communities 91

How do abiotic factors affectdifferent plant species?

Materials presoaked rye and rice seeds, sand,potting soil, 4 paper cups

Procedure

1. Use a pencil to punch three holes in the bottom ofeach cup. Fill 2 cups with equal amounts of sandand 2 cups with the same amount of potting soil.

2. Plant 5 rice seeds in one sand-filled cup and 5 riceseeds in one soil-filled cup. Plant 5 rye seeds ineach of the other 2 cups. Label each cup with thetype of seeds and soil it contains.

3. Place all the cups in a warm, sunny location. Eachday for 2 weeks, water the cups equally and recordyour observations of any plant growth. CAUTION:Wash your hands well with soap and warm waterafter handling plants or soil.

Analyze and Conclude1. Analyzing Data In which medium did the rice

grow best—sand or soil? Which was the bettermedium for the growth of rye?

2. Inferring Soil retains more water than sand,providing a moister environment. What can youinfer from your observations about the kind ofenvironment that favors the growth of rice? Thegrowth of rye?

3. Drawing Conclusions Which would competemore successfully in a dry environment—rye orrice? In a moist environment?

The NicheIf an organism’s habitat is its address, its niche is its occupa-

tion. A (NITCH) is the full range of physical and biologi-

cal conditions in which an organism lives and the way in which

the organism uses those conditions. For instance, part of the

description of an organism’s niche includes its place in the food

web. Another part of the description might include the range of

temperatures that the organism needs to survive. The combina-

tion of biotic and abiotic factors in an ecosystem often deter-

mines the number of different niches in that ecosystem.

A niche includes the type of food the organism eats, how it

obtains this food, and which other species use the organism as

food. For example, a mature bullfrog catches insects, worms,

spiders, small fish, or even mice. Predators such as herons,

raccoons, and snakes prey on bullfrogs.

The physical conditions that the bullfrog requires to survive

are part of its niche. Bullfrogs spend their lives in or near the

water of ponds, lakes, and slow-moving streams. A bullfrog’s

body temperature varies with that of the surrounding water and

air. As winter approaches, bullfrogs burrow into the mud of pond

or stream bottoms to hibernate.

The bullfrog’s niche also includes when and how it repro-

duces. Female bullfrogs lay their eggs in water during the

warmer months of the year. The young frogs, called tadpoles,

live in the water until their legs and lungs develop.

niche

Answer to . . . The example should be

any nonliving factor, such as air, water,soil, or rocks.

Less Proficient ReadersSome students learn best when material isorganized. Help these students better under-stand succession in a marine ecosystem byhaving them create a flowchart of the processdiscussed on pages 96 and 97. You could havestudents work in small groups to make thesegraphic organizers. When groups finish, displaythe flowcharts so that groups can compare theirapproaches.

Advanced LearnersEncourage students who need an extra chal-lenge to research the various types of defensesthat have evolved in prey species as protectionagainst predators. Such defenses include cam-ouflage, mechanical defenses such as quills andthorns, chemical defenses such as toxins, warn-ing coloration, and mimicry. Have studentsshare their findings with the class in oral reports,posters, or displays.

Objective Students will be able todescribe how abiotic factors affectgrowth in different species ofplants.Skill Focus Drawing ConclusionsTime 15 minutes for setup; briefobservation and recording each dayfor 2 weeksAdvance Prep Soak the rice andrye seeds in water overnight.Strategies• Have students place the cups in

trays or other shallow containersto hold any water, sand, and soilthat may leak from the holes.

• Emphasize that except for the typeof soil and type of seeds in eachcup, all variables must be kept thesame for all four cups.

Alternative Materials If rye isnot available, you may use wheatseeds instead.Expected Outcome Rice seeds willnot grow well in the sand-filled cupbecause the water drains out andleaves the sand too dry. Rye seedsshould grow well in soil or sand.Analyze and Conclude1. Both types of seeds will be moresuccessful in soil. However, the effecton rice will be more pronounced.2. Rice requires a moister environ-ment than rye does. Rye cantolerate dry conditions better thanrice can but also benefits from anample supply of water.3. Dry environment: rye; moist environment: rice

UNIVERSAL ACCESS

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92 Chapter 4

CommunityInteractionsDemonstrationMark off a 3-meter-square area withmasking tape or string. Scatter 25uncolored toothpicks (or other smallobjects) and 25 colored toothpicksover the area. Explain that the tooth-picks represent two different speciesof insects. Choose two students torepresent different species of lizardsthat eat both types of insects. At asignal from you, the two lizards startcatching insects of both types. Signalthe lizards to stop after 5–10 sec-onds, and ask them to count theirinsects. Rescatter the toothpicks andrepeat the activity, but this time haveone lizard eat only uncolored insectsand the other lizard eat only coloredinsects. Compare the insect countsfrom the two different methods. Askthe “lizards”: Was it easier to catchinsects when you were competingwith each other or when you eachhad a different food? (When eachhad a different food) Also ask theother students to describe competi-tive behaviors they observed.

Build Science SkillsProblem Solving Emphasize thatany given ecosystem has only a cer-tain amount of space, food, water,and other life essentials. Ask: If oneorganism is involved in direct com-petition for life essentials, what arethe possible outcomes for thatorganism? (The organism may winthe struggle and survive, or it may losethe struggle and die.) Is there anyother alternative for organismsthat are in competition with otherorganisms? (If the competition isbetween different yet similar species,the organisms may change in waysthat will decrease competition. In thisway, both species may survive.)

As you will see, no two species can share the same niche in

the same habitat. However, different species can occupy niches

that are very similar. For instance, the three species of North

American warblers shown in Figure 4–5 live in the same spruce

trees but feed at different elevations and in different parts of

those trees. The species are similar, yet each warbler has a

different niche within the forest.

What is a niche?

Community Interactions When organisms live together in ecological communities, they

interact constantly. These interactions help shape the ecosystem

in which they live. Community interactions, such ascompetition, predation, and various forms of symbiosis,can powerfully affect an ecosystem.

Competition Competition occurs when organisms of the

same or different species attempt to use an ecological resource

in the same place at the same time. The term refers

to any necessity of life, such as water, nutrients, light, food, or

space. In a forest, for example, broad-leaved trees such as oak or

hickory may compete for sunlight by growing tall, spreading out

their leaves, and blocking the sunlight from shorter trees.

Similarly, two species of lizards in a desert might compete by

attempting to eat the same type of insect.

Direct competition in nature often results in a winner and a

loser—with the losing organism failing to survive. A fundamen-

tal rule in ecology, the

states that no two species can occupy the same niche in the

same habitat at the same time. Look again at the distribution of

the warblers in Figure 4–5. Can you see how this distribution

avoids direct competition among the different warbler species?

competitive exclusion principle,

resource

Fee

ding

hei

ght (

met

ers)

18

12

6

0Spruce tree

Bay-Breasted WarblerFeeds in the middle

part of the tree

Yellow-Rumped WarblerFeeds in the lower part of the tree andat the bases of the middle branches

Cape May WarblerFeeds at the tips of branchesnear the top of the tree

� Figure 4–5 Each of thesewarbler species has a different nichein its spruce tree habitat. By feedingin different areas of the tree, thebirds avoid competing with oneanother for food. Inferring Whatwould happen if two of the warblerspecies attempted to occupy thesame niche?

The competitive exclusion principleThe competitive exclusion principle was first pos-tulated by Russian ecologist G. F. Gause in 1934.In laboratory experiments, Gause studied theeffect of interspecific competition on two closelyrelated species of protists. When he cultured thetwo species separately, both populations grewrapidly and then leveled off at the culture’s

carrying capacity. When he cultured the twospecies together, however, one species apparentlyhad a competitive edge in obtaining food, andthe other species was driven to extinction in theculture. Gause concluded that two species so similar that they compete for the same limitedresources cannot coexist in the same place. Hisconclusion was later confirmed by further studies.

HISTORY OF SCIENCE

4–2 (continued)

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Build Science SkillsProblem Solving Explain thatunder normal conditions, prey popu-lations seldom become extinct as aresult of predation. Tell students toimagine an ecosystem in which apredator species has killed off anentire prey species. Ask: What wouldthe possible consequences be forthe predators? (They would run outof food and die, they would have tochange their eating habits and findother prey, or they would have to moveto another area where that preyspecies still survives.)

Build Science SkillsApplying Concepts Have each stu-dent list examples of symbioticrelationships that he or she knowsfrom previous learning or hasresearched. In a class discussion, askvolunteers to describe exampleswithout identifying the type of sym-biosis each example represents. Aftereach description, challenge other stu-dents to identify the relationship asmutualism, commensalism, or para-sitism.

Ecosystems and Communities 93

Predation An interaction in which one organism

captures and feeds on another organism is called

(pree-DAY-shun). The organism that does

the killing and eating is called the predator (PRED-

uh-tur), and the food organism is the prey. Cheetahs

are active predators with claws and sharp teeth. Their

powerful legs enable them to run after prey. Other

predators, such as anglerfishes, are more passive. An

anglerfish has a fleshy appendage that resembles a

fishing lure, which it uses to draw unsuspecting prey

close to its mouth.

Symbiosis Any relationship in which two species

live closely together is called (sim-by-OH-

sis), which means “living together.” Biologists recog-

nize three main classes of symbiotic relationships in

nature: mutualism, commensalism, and parasitism.

Examples of these three symbiotic relationships are

shown in Figure 4–6.

Mutualism In (MYOO-choo-ul-iz-um),

both species benefit from the relationship. Many flow-

ers, for example, depend on certain species of insects

to pollinate them. The flowers provide the insects with

food in the form of nectar, pollen, or other substances,

and the insects help the flowers reproduce.

Commensalism In (kuh-MEN-

sul-iz-um), one member of the association benefits

and the other is neither helped nor harmed. Small

marine animals called barnacles, for example, often

attach themselves to a whale’s skin. The barnacles

perform no known service to the whale, nor do they

harm it. Yet, the barnacles benefit from the constant

movement of water past the swimming whale,

because the water carries food particles to them.

Parasitism In (PAR-uh-sit-iz-um),

one organism lives on or inside another organism and

harms it. The parasite obtains all or part of its nutri-

tional needs from the other organism, called the host.

Generally, parasites weaken but do not kill their host,

which is usually larger than the parasite. Tapeworms,

for example, are parasites that live in the intestines

of mammals. Fleas, ticks, and lice live on the bodies of

mammals, feeding on the blood and skin of the host.

parasitism

commensalism

mutualism

symbiosis

predation

Figure 4–6 Three examples of symbiosis are shown: mutualism,commensalism, and parasitism. Predicting What would happen to the aphids if the ant died?

Mutualism The ant cares for the aphids andprotects them from predators. The aphidsproduce a sweet liquid that the ant drinks.

Commensalism The orchid benefits from itsperch in the tree as it absorbs water and miner-als from rainwater and runoff, but the tree is notaffected.

Parasitism A tick feeds on the blood of its hostand may also carry disease-causing microorganisms.

Answers to . . . The full range of physical

and biological conditions in which anorganism lives and the way in whichthe organism uses those conditions

Figure 4–5 Most likely, one warblerspecies would be more successful inthat niche, and the other species wouldnot survive.

Figure 4–6 Without the ants, theaphids could be eaten by predators.

Predation and diversityIn nature, predator species rarely kill and eat alltheir prey species, which would reduce commu-nity diversity. In fact, studies have shown thatpredation can actually help maintain diversity.One example of this process involves the graywolf, a top predator in its ecosystem. Wherewolves were hunted to extinction, such as inmany parts of North America, populations of

deer and other herbivores increased dramatically.As these populations overgrazed the vegetation,many plant species that could not tolerate suchgrazing pressure disappeared from the ecosys-tem. In turn, many insects and small animals thatdepended on the plants for food also disap-peared. The elimination of wolves thus producedan ecosystem with considerably less speciesdiversity.

BACKGROUND

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94 Chapter 4

Ecological SuccessionDemonstrationThe following activity models stagesof succession described on this page.Elicit student volunteers to help youcreate the model. Put a 2.5-cm layerof gravel in the bottom of a dishpan,and cover with a 10-cm layer of soil.Make a pond by sinking a shallowdish into the soil so its top is evenwith the soil surface. Put a 1-cm layerof soil in the bottom of the pond.Slowly pour water into the dishpanuntil the pond is completely full andthe soil around it is wet. Sprinkle ahandful of grass seeds over the entiredishpan. Leave the dishpan near asunny window. Every 3 to 4 days,sprinkle grass seeds over the dishpanagain. Lightly water the soil to keep itdamp, but do not refill the pond orclean it out. Over time, the pond willbecome shallower and will eventuallyfill in with growing grass. When thishas occurred, sprinkle a handful ofmixed birdseed over the dishpanonce a week for two weeks. The bird-seed plants, which will be larger thanthe grass plants, represent the grad-ual invasion of shrubs and trees andthe succession from a meadow to aforest.

Build Science SkillsProblem Solving Ask: What typesof human activities can disturb anecosystem and cause succession?(Examples include logging, strip mining,draining a marsh, clearing woodland togrow crops or graze livestock, removinga beaver dam, and the like.)

� �� �

Ecological SuccessionOn the time scale of a human life, some ecosystems may seem

stable. The appearance of stability is often misleading, because

ecosystems and communities are always changing. Sometimes,

an ecosystem changes in response to an abrupt disturbance,

such as a severe storm. At other times, change occurs as a more

gradual response to natural fluctuations in the environment.

Ecosystems are constantly changing in response tonatural and human disturbances. As an ecosystemchanges, older inhabitants gradually die out and neworganisms move in, causing further changes in the com-munity. This series of predictable changes that occurs in a

community over time is called Some-

times succession results from slow changes in the physical

environment. A sudden natural disturbance from human activi-

ties, such as clearing a forest, may also be a cause of succession.

Primary Succession On land, succession that occurs on

surfaces where no soil exists is called

For example, primary succession occurs on the surfaces formed

as volcanic eruptions build new islands or cover the land with

lava rock or volcanic ash. Primary succession also occurs on

bare rock exposed when glaciers melt.

In Figure 4–7, you can follow the stages of primary succes-

sion after a volcanic eruption. When primary succession begins,

there is no soil, just ash and rock. The first species to populate

the area are called The pioneer species on

volcanic rocks are often lichens (LY-kunz). A lichen is made up of

a fungus and an alga and can grow on bare rock. As lichens

grow, they help break up the rocks. When they die, the lichens

add organic material to help form soil in which plants can grow.

What are pioneer species?

pioneer species.

primary succession.

ecological succession.

� Figure 4–7 Primary successionoccurs on newly exposed surfaces,such as this newly deposited volcanicrock and ash. (1) A volcanic eruptiondestroys the previous ecosystem. (2) The first organisms to appear arelichens. (3) Mosses soon appear, andgrasses take root in the thin layer ofsoil. (4) Eventually, tree seedlings andshrubs sprout among the plantcommunity. Predicting What typesof animals would you expect to appearat each stage, and why?

Succession and chanceMany ecologists once believed that successionwas an orderly and predictable process. Today,they realize that random, unpredictable eventsmay influence which species succeed and whichdie off after a disturbance. For example, randomvariables such as the season of year the distur-bance occurs, the wind direction and rainfallimmediately after the disturbance, and which

organisms are in an active stage of their breedingcycle can change the succession of a communityafter a fire, volcanic eruption, or other major dis-turbance. Ecologists have also found that climaxcommunities are often not as stable as they wereonce thought to be. In studying coral reefs andtropical rain forests, for example, researchers findshifting patchworks of early, middle, and late suc-cessionary communities.

BIOLOGY UPDATE

4–2 (continued)

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Use Community ResourcesInvite students to cite examples ofnatural or human disturbances theyhave seen in their area. Emphasizethat the disturbance need not be alarge-scale event, such as cuttingdown all the trees in an area ofwoodland, but could be on a smallscale—for example, homeownersremoving thorny bushes from a nar-row strip of land between theirhouses. Ask students to describe anychanges they observed after the dis-turbance or, if the disturbance is veryrecent, to predict changes they thinkwill occur over time. Take the class toa disturbed site, if feasible, or ask stu-dents to visit a site on their ownperiodically to observe changes thatoccur during the school year.

Ecosystems and Communities 95

Secondary Succession Components of an ecosys-

tem can be changed by natural events, such as fires,

or by human activities, such as farming. These

changes may affect the ecosystem in predictable or

unpredictable ways. When the disturbance is over,

community interactions tend to restore the ecosys-

tem to its original condition through

For example, secondary succession

occurs after wildfires burn woodlands and when land

cleared for farming is abandoned. Figure 4–8 shows

trees regrowing after a wildfire. In fact, fires set by

lightning occur in many ecosystems, and some plants

are so adapted to periodic fires that their seeds won’t

sprout unless exposed to fire!

Ecologists used to think that succession in a

given area always proceeded through predictable

stages to produce the same stable “climax commu-

nity.” Old-growth forests in the Pacific Northwest, for

example, were considered climax communities. But

natural disasters, climate change, and human activ-

ity such as introduction of nonnative species pro-

foundly affect these communities today. Healthy

ecosystems usually recover from natural distur-

bances because of the way components of the system

interact. Ecosystems may or may not recover from

long-term, human-caused disturbances.

succession.secondary

� Figure 4–8 Ten years after wildfires burnedregions of Yellowstone National Park, small evergreentrees have begun to regenerate the forest. PredictingHow do you think this region will look 20 years after thefires?

Forestry TechnicianJob Description: work outdoors to helpmaintain, protect, and develop forests (byplanting trees, fighting insects and diseasesthat attack trees, and controlling soil erosion)

Education: two- or four-year college degree inforestry, wildlife, or conservation; summerwork in parks, state and national forests; andprivate industry provides on-the-job training

Skills: knowledge of the outdoors and basicsafety precautions; communication skills forworking with the public; keen observationalskills; physical fitness for jobs that requirewalking long distances through forests

Highlights: help to manage and conserveforest biomes by analyzing data, planting trees,and managing fires when necessary; contributeto people’s enjoyment of outdoor recreation

Careers in Biology

For: Career linksVisit: PHSchool.comWeb Code: cbb-2042

To help students understand ecological succes-sion, have them interview an older familymember or neighbor who has lived in theirneighborhood for a long time. Ask the person todescribe how the neighborhood has changedover time. Make sure that students ask the fol-lowing questions. (1) Have areas that wereformerly grassy been paved or developed?

(2) Have any farms, parks, or lots returned totheir wild state? Ask students to write a summaryof their interview and then share it with the class.

—Deidre GalvinBiology TeacherRidgewood High SchoolRidgewood, NJ

TEACHER TO TEACHER

Encourage students to use libraryresources to find out how to getstarted in a forestry career. Suggestthat they present their findings inposters that include information suchas the geographic locations whereforestry professionals work, specificcourses that are necessary, areas ofresearch, and issues in the industry.

Resources Society of AmericanForesters; International Union ofForestry Research Organizations; U.S.Department of Agriculture ForestService

Careers in Biology

You can have students write amore extensive job description aswell as list the educational require-ments for a career in this field.

Answers to . . . The first species to popu-

late an area at the beginning of theprocess of primary succession

Figure 4–7 Stage 2: insects and spi-ders are carried in by the wind; stage3: rodents that feed on these arthro-pods and new grasses; stage 4: largermammals and birds that feed onrodents

Figure 4–8 Answers may vary. Moststudents will suggest that succession willhave proceeded in specific and pre-dictable stages and the community in20 years might be mature and stable.

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96 Chapter 4

Use VisualsFigure 4–9 After students have stud-ied the figure and read the caption,ask: How is the situation shown inthis illustration an example of eco-logical succession? (Ecologicalsuccession is the series of predictablechanges that occurs in a communityover time. In this situation, the pre-dictable changes that occur are thedifferent stages in the consumption ofthe dead whale.) What is the causeof succession in this situation? (Thedeath and sinking of the whale causesthe succession.) Is the succession in amarine ecosystem more like pri-mary or secondary succession onland? (Students may compare it favor-ably to either kind of succession onland. Like primary succession, succes-sion in a marine ecosystem begins withan event, a cause. It’s also like second-ary succession in that certain oceanorganisms are adapted to the “distur-bance” of a sinking dead whale, just asland organisms are adapted to the dis-turbance of fire.)

Build Science SkillsInterpreting Graphics Collect, orask students to find, several sets ofphotographs that show areas soonafter a disturbance and at intervals assuccession occurs. Good subjects forsuch photos include areas affected bya volcanic eruption or forest fire, suchas the areas mentioned in the stu-dent text. Photos of succession afterthe eruption of Mount St. Helens arealso widely available. Display each setof photos in random order, and havestudents identify the correct order.For each set, also ask: How were youable to determine the correctorder? (Sample answer: by the typesand sizes of the plants growing in thearea)

Succession in a Marine Ecosystem Succession can

occur in any ecosystem—even in the permanently dark, deep

ocean. In 1987, scientists found an unusual community of

organisms living on the remains of a dead whale in the deep

waters off the coast of southern California. At first, ecologists

did not know what to make of this extraordinary community.

After several experiments and hours of observation, the

ecologists found that the community represented a stage in

succession amid an otherwise stable and well-documented

deep-sea ecosystem. Since that discovery, several more whale

carcasses have been found in other ocean basins with similar

organisms surrounding them. Figure 4–9 illustrates three

stages in the succession of a whale-fall community.

The disturbance that causes this kind of succession begins

when a large whale, such as a blue or fin whale, dies and sinks

to the normally barren ocean floor. The whale carcass attracts a

host of scavengers and decomposers, including amphipods

(inset), hagfishes, and sharks, that feast on the decaying meat.

Within a year, most of the whale’s tissues have been eaten.

The carcass then supports only a much smaller number of fishes,

crabs, marine snails (inset), and other marine animals. The

decomposition of the whale’s body, however, enriches the sur-

rounding sediments with nutrients, forming an oasis of sediment

dwellers, including many different species of marine worms.

2

1

Figure 4–9 Ecosystems areconstantly changing in responseto disturbances. In natural environments, succession occurs in stages. A dead whale that falls to the ocean floor is soon coveredwith scavengers. After a time, onlybare bones are left. The bonescontain oil that supports severaltypes of deep-sea bacteria. In thenext stage of succession, thebacteria provide energy andnutrients for a different communityof organisms that live on the bonesand in the surrounding sediments.

1

4–2 (continued)

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3 ASSESSEvaluate UnderstandingHave each student select an exampleof succession—either one describedin the student text or one of his orher own choice—and write a briefdescription of the sequence ofchanges that the ecosystem couldundergo. As an alternative, studentscould draw sketches to show thechanges.

ReteachUsing photographs of ecosystemsthat you have selected from otherchapters in this textbook or fromother sources, have students list thebiotic and abiotic factors in eachecosystem.

Ecosystems and Communities 97

When only the whale’s skeleton remains, a third community

moves in. Heterotrophic bacteria begin to decompose oils inside

the whale bones. In doing so, they release chemical compounds

that serve as energy sources for other bacteria that are

chemosynthetic autotrophs. The chemosynthetic bacteria, in

turn, support a diverse community of mussels, limpets, snails,

worms, crabs, clams, and other organisms that live on the bones

and within the nearby sediments.

3

2

3

1. Key Concept What is thedifference between a biotic factorand an abiotic factor?

2. Key Concept Name threetypes of community interactionsthat can affect an ecosystem.

3. Key Concept What is thedifference between primary suc-cession and secondary succession?

4. How is an organism’s nichedetermined?

5. Critical Thinking Comparingand Contrasting How are thethree types of symbiotic relation-ships different? Similar?

6. Critical Thinking ApplyingConcepts Summarize the roleof organisms, including micro-organisms, in maintaining theequilibrium of a marine ecosys-tem while a dead whale decayson the ocean floor.

Creative WritingUse the information from thissection to write a short storyabout an ecosystem that isdisturbed and undergoessuccession. Hint: Include aflowchart with your story toshow the main stages ofchange.

4–2 Section Assessment

If your class subscribes to theiText, use it to review the KeyConcepts in Section 4–2.

4–2 Section Assessment1. A biotic factor is a living organism. An abiotic

factor is nonliving.2. Competition, predation, and symbiosis3. Primary succession occurs on surfaces where

no soil exists. Secondary succession occurswhen a disturbance of some kind changes anexisting community without removing the soil.

4. An organism’s niche is determined by thephysical and biological conditions in its envi-ronment and how it uses those conditions.

5. In mutualism, both species benefit. In com-mensalism, only one species benefits; the otheris neither helped nor harmed. In parasitism,one species benefits; the other is harmed. Inall three, two species live closely together.

6. Students should describe how scavengers anddecomposers eat the decaying meat, how thedecomposition of the whale’s body forms anoasis for sediment dwellers, and how bacteriadecompose the oils inside the whale’s bones.

Let students use any ecosystemand any kind of disturbance oftheir choice. In general, students’stories should include informationabout a change in the biotic orabiotic factors in a stable ecosys-tem, resulting in succession.Students should chronicle thegradual changes that return theecosystem to a climax community.If students have a hard time get-ting started, suggest that theybegin by making a flowchart ofsteps in succession for a particularecosystem and then use the flow-chart as an outline for the story.

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