20CHAPTER COMMUNITY ECOLOGY - WordPress.com · 2019. 2. 25. · COMMUNITY ECOLOGY 399...

A large variety of organisms interactwithin this coral reef community.

SECTION 1 Species Interactions

SECTION 2 Patterns in Communities

Unit 7—Ecosystem DynamicsTopic 3

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20CHAPTER COMMUNITY ECOLOGYCOMMUNITY ECOLOGY

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399C O M M U N I T Y E C O L O G Y

S P E C I E S I N T E R A C T I O N SJust as populations contain interacting members of a single

species, communities contain interacting populations of many

species. Many species have specific types of interactions with

other species. This chapter introduces the five major types of

interactions among species: predation, competition, parasitism,

mutualism, and commensalism. These categories are based on

whether each species causes any benefit or harm to the other

species in a given relationship.

PREDATIONIn predation (pree-DAY-shuhn), an individual of one species, calledthe predator, eats all or part of an individual of another species,called the prey. Predation is a powerful force in a community. Therelationship between predator and prey influences the size of eachpopulation and affects where and how each species lives.Examples of predators include carnivores—predators that eat ani-mals—and herbivores—predators that eat plants. Many types oforganisms can act as predators or prey. All heterotrophs are eitherpredators or parasites or both.

Predator AdaptationsNatural selection favors the evolution of predator adaptations forfinding, capturing, and consuming prey. For example, rattlesnakeshave an acute sense of smell and have heat-sensitive pits locatedbelow each nostril. These pits enable a rattlesnake to detect warm-bodied prey, even in the dark. Many snakes usevenom to disable or kill their prey. A venomous rat-tlesnake is shown in Figure 20-1.

Other predator adaptations include the stickywebs of spiders, the flesh-cutting teeth of wolves andcoyotes, the speed of cheetahs, and the striped pat-tern of a tiger’s coat, which provides camouflage in agrassland habitat. Many herbivores have mouthpartssuited to cutting and chewing tough vegetation.

A predator’s survival depends on its ability tocapture food, but a prey’s survival depends on itsability to avoid being captured. Therefore, naturalselection also favors adaptations in prey that allowthe prey to escape, avoid, or otherwise ward offpredators.

SECTION 1

O B J E C T I V E S● Identify two types of predator

adaptations and two types of preyadaptations.

● Identify possible causes and resultsof interspecific competition.

● Compare parasitism, mutualism,and commensalism, and give oneexample of each.

V O C A B U L A R Ypredationinterspecific competitionsymbiosisparasitism mutualismcommensalism


www.scilinks.orgTopic: Predator/PreyKeyword: HM61205

A rattlesnake has several adaptationsthat make it an effective predator.

FIGURE 20-1

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Adaptations in Animal Prey Animals may avoid being eaten by carnivores in different ways.Some organisms flee when a predator approaches. Others escapedetection by hiding or by resembling an inedible object, as shownin Figure 20-2a. Some animals use deceptive markings, such as fakeeyes or false heads, to startle a predator. Some animals have chem-ical defenses. Animals such as the frog shown in Figure 20-2b pro-duce toxins and use bright colors to warn would-be predators oftheir toxicity.

In mimicry (MIM-ik-ree), one species closely resembles anotherspecies. For example, the harmless king snake is a mimic of thevenomous coral snake, as shown in Figure 20-3. This form of mim-icry is called Batesian mimicry. Another form of mimicry calledMüllerian mimicry, exists when two or more dangerous or distaste-ful species look similar. For example, many kinds of bees andwasps have similar patterns of alternating yellow and black stripes.This kind of mimicry benefits each species involved becausepredators learn to avoid similar-looking individuals.

Adaptations in Plant Prey Plants cannot run away from a predator, but many plants haveevolved adaptations that protect them from being eaten. Physicaldefenses, such as sharp thorns, spines, sticky hairs, and toughleaves, can make plants more difficult to eat. Plants have alsoevolved a range of chemical defenses that are poisonous, irritating,or bad-tasting. These chemicals are often byproducts of the plants’metabolism and are called secondary compounds. Some examplesof secondary compounds that provide a defensive function arestrychnine (STRIK-nin), which is produced in plants of the genusStrychnos, and nicotine, which is produced by the tobacco plant.Poison ivy and poison oak produce an irritating chemical thatcauses an allergic reaction on some animals’ skin.


Modeling PredationMaterials 4.1 m white string,4 stakes, 40 colored toothpicks,stopwatch or timer, meterstick

Procedure1. Use the stakes and string to

mark off a 1 m square in agrassy area.

2. One partner should scatter thetoothpicks randomly throughoutthe square. The other partner willhave 1 minute to pick up asmany toothpicks as possible, oneat a time. This procedure shouldbe repeated until each team hasperformed five trials.

3. Record your team’s results in adata table.

Analysis Toothpicks of which col-ors were picked up most often?Which toothpicks were picked upleast often? How do you accountfor this difference?

Quick Lab

(a) Green leaf mantid, Choeradodus rhombicollis (b) Amazonian poison frog, Dendrobates ventrimaculatus

Coloration is an adaptation in prey aswell as predators. In (a), the mantidcannot readily be detected among theleaves. In (b), the frog’s bright colorswarn other organisms that the frog istoxic if eaten.

FIGURE 20-2


COMPETITIONInterspecific competition is a type of interaction in which two ormore species use the same limited resource. For example, bothlions and hyenas compete for prey such as zebras. Likewise,many plant species compete for soil or sunlight. Some species ofplants prevent other species from growing nearby by releasingtoxins into the soil. If two populations compete for a resource,the result may be a reduction in the number of either species orthe elimination of one of the two competitors. More often, onespecies will be able to use a resource more efficiently than theother. As a result, less of the resource will be available to theother species.

Competitive ExclusionEcologist George F. Gause was one of the first scientists to studycompetition in the laboratory. In test tubes stocked with a foodsupply of bacteria, Gause raised several species of paramecia invarious combinations. When grown in separate test tubes,Paramecium caudatum and Paramecium aurelia each thrived. Butwhen the two species were combined, P. caudatum always died outbecause P. aurelia was a more efficient predator of bacteria.Ecologists use the principle of competitive exclusion to describe sit-uations in which one species is eliminated from a communitybecause of competition for the same limited resource. Competitiveexclusion may result when one species uses the limited resourcemore efficiently than the other species does.

Joseph Connell’s study of barnacles along the Scottish coast inthe 1960s documented competition in the wild. Connell studiedtwo species, Semibalanus balanoides and Chthamalus stellatus.These barnacles live in the intertidal zone, the portion of theseashore that is exposed during low tide.


(a) Scarlet king snake, Lampropeltis triangulum (b) Eastern coral snake, Micrurus fulvius

The king snake in (a) may avoidpredators because of its mimicry of thecolor patterns of the coral snake in (b).A closer look reveals the differences: theking snake has a red snout and a blackring separating its red and yellow rings,and the coral snake has a black snoutand adjacent red and yellow rings. Thering patterns of other species of coralsnakes may differ from the patternsshown in the photo above.

FIGURE 20-3

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As Figure 20-4 shows, each species of bar-nacle occupied a distinct band within theintertidal zone. Chthamalus lived higher onthe rocks than Semibalanus did. Connelldemonstrated that this difference was partlydue to competition. When a rock coveredwith Chthamalus was transplanted to thelower zone, Chthamalus was clearly able totolerate the conditions in the lower zone.However, Semibalanus would soon settle onthe rock and eventually crowd outChthamalus. Connell concluded that compe-tition restricted the range of Chthamalus.Although Chthamalus could survive lower onthe rocks, competition from Semibalanusprevented it from doing so. Higher on therocks, Chthamalus is free from competitionbecause it can tolerate drying out more thanSemibalanus can.

Reduced Niche SizeRecall that a species’ niche (NICH) is the role that the species playsin its environment. But this role may vary due to interactions withother species. So, ecologists differentiate between the fundamentalniche and the realized niche of a species. The fundamental niche ofa species is the range of conditions that it can potentially tolerateand the range of resources that it can potentially use. Often, pre-dation and competition limit the species’ use of these ranges.Thus, the realized niche is the part of the niche that the speciesactually uses, as shown in Figure 20-4.

Character DisplacementCompetition has the potential to be an important influence on thenature of a community. The composition of a community maychange as competitors win, lose, or evolve differences that lessenthe intensity of competition. Natural selection favors differencesbetween competitors, especially where the niches of the competi-tors overlap. This evolution of differences in a characteristic dueto competition is called character displacement. Character dis-placement is a way of reducing niche overlap.

The beaks of Galápagos Island finches provide an example ofcharacter displacement. Many of these closely related finchspecies eat seeds that come from the small number of plantspecies on the islands. Furthermore, the size of each bird’s beakdetermines the size of the seeds that the bird can eat. When theaverage beak size of each species is compared with the averagebeak size of each of the other species, the greatest difference inbeak size is observed between species that share an island.Differences in beak size reduce competition by enabling speciesthat share an island to favor different food sources.

www.scilinks.orgTopic: CompetitionKeyword: HM60326

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Semibalanusbalanoides

The realized niche of the Chthamalusbarnacle species is smaller than itsfundamental niche because ofcompetition from the Semibalanusspecies. Although Chthamalus cansurvive at all levels of the intertidalzone, it is usually crowded out at thelower level by the faster-growingSemibalanus. But Semibalanus cannotsurvive in the upper level, which is leftdry for longer periods.

FIGURE 20-4



Resource PartitioningAs Charles Darwin noted, competition is likely to be most intensebetween similar species that require the same resources. Whensimilar species coexist, each species may avoid competition withothers by using a specific part of an available resource. This pat-tern of resource use is called resource partitioning. For example, thepioneering ecologist Robert MacArthur (1930–1972) studied sev-eral species of warblers that feed on insects in spruce and fir trees.MacArthur discovered that when more than one species of warbleris foraging within the same tree, each species hunts for insectsonly in a particular section of the tree. As a result, competitionamong the species is reduced.

SYMBIOSISA symbiosis (SIM-bie-OH-sis) is a close, long-term relationshipbetween two organisms. Three examples of symbiotic relation-ships include: parasitism, mutualism, and commensalism.Parasitism (PAR-uh-SIET-IZ-UHM) is a relationship in which one indi-vidual is harmed while the other individual benefits. Mutualism(MYOO-choo-uhl-IZ-uhm) is a relationship in which both organismsderive some benefit. In commensalism (kuh-MEN-suhl-IZ-uhm), oneorganism benefits, but the other organism is neither helpednor harmed.

ParasitismParasitism is similar to predation in that one organism, calledthe host, is harmed and the other organism, called the parasite,benefits. However, unlike many forms of predation, parasitismusually does not result in the immediate death of the host.Generally, the parasite feeds on the host for a long time ratherthan kills it. Parasites such as aphids, lice, leeches, fleas, ticks,and mosquitoes that remain on the outside of their host arecalled ectoparasites. Parasites that live inside the host’s bodyare called endoparasites. Familiar endoparasites are heart-worms, disease-causing protists, and tapeworms, such as theone shown in Figure 20-5. Natural selection favors adaptationsthat allow a parasite to exploit its host efficiently. Parasites areusually specialized anatomically and physiologically for a par-asitic lifestyle.

Parasites can have a strong negative impact on the healthand reproduction of the host. Consequently, hosts haveevolved a variety of defenses against parasites. Skin is animportant defense that prevents most parasites from enteringthe body. Tears, saliva, and mucus defend openings throughwhich parasites could pass, such as the eyes, mouth, andnose. Finally, the cells of the immune system may attack para-sites that get past these defenses.

parasite

from the Latin word parasitus,meaning “one who eatsat the table of another”

Word Roots and Origins

Tapeworms are endoparasites that cangrow to 20 m or greater in length.Tapeworms are so specialized for aparasitic lifestyle that they do not havea digestive system. They live in thehost’s small intestine and absorbnutrients directly through their skin.Tapeworms reproduce by producingegg-filled chambers, which are releasedin their host’s feces to be unknowinglypicked up by a future host.

FIGURE 20-5


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MutualismMutualism is a relationship in which two species derivesome benefit from each other. Some mutualistic relation-ships are so close that neither species can survive withoutthe other. An example of mutualism, shown in Figure 20-6,involves ants and some species of Acacia plants. The antsnest inside the acacia’s large thorns and receive food fromthe acacia. In turn, the ants protect the acacia from herbi-vores and cut back competing vegetation.

Pollination is one of the most important mutualistic rela-tionships on Earth. Animals such as bees, butterflies, flies,beetles, bats, and birds that carry pollen between floweringplants are called pollinators. A flower is a lure for pollina-tors, which are attracted by the flower’s color, pattern,shape, or scent. The plant usually provides food—in theform of nectar or pollen—for its pollinators. As a pollinator

feeds in a flower, it picks up a load of pollen, which it may thencarry to other flowers of the same species.

CommensalismCommensalism is an interaction in which one species benefits andthe other species is not affected. Species that scavenge for leftoverfood items are often considered commensal species. However, arelationship that appears to be commensalism may simply be mutu-alism in which the mutual benefits are not apparent.

An example of a commensal relationship is the relationshipbetween cattle egrets and Cape buffaloes in Tanzania. The birdsfeed on small animals such as insects and lizards that are forcedout of their hiding places by the movement of the buffaloesthrough the grass. Occasionally, the cattle egrets also feed onectoparasites from the hide of the buffaloes, but the buffaloes gen-erally do not benefit from the presence of the egrets.

1. List the five major kinds of species interactions.

2. Describe one adaptation of a herbivore and oneadaptation of a carnivore for obtaining food.

3. Identify two possible results of interspecificcompetition.

4. What is the competitive exclusion principle?

5. Identify the ways that parasites are similar to predators.

6. In the relationship between cattle egrets andCape buffaloes, if the egrets regularly removedticks from the buffaloes, would this relationshipstill be considered commensal? Explain youranswer.

CRITICAL THINKING7. Analyzing Concepts Explain how two similar

species of birds could nest in the same tree andyet occupy different niches.

8. Making Comparisons How does predation onplants differ from predation on animals, in termsof the usual effect on the prey?

9. Analyzing Concepts Species A and B have avery similar niche. Species A recently arrived in a location where species B previously lived andcarried a disease that killed all members ofspecies B. Is this situation an example of com-petitive exclusion? Explain your answer.

SECTION 1 REVIEW

Acacia trees in Central America have amutualistic relationship with certaintypes of ants. The trees provide food andshelter to the ants, and the ants defendthe tree from insect herbivores.

FIGURE 20-6



P A T T E R N S I NC O M M U N I T I E SThe investigation of community properties and interactions is an

active area of ecology. Which properties are most significant in

structuring a community? What determines the number of species

in a community? How do communities recover from disturbance?

These questions are central to a study of communities.

SPECIES RICHNESSOne characteristic of a community is species richness, the numberof species in the community. A related measure is speciesevenness, which is the relative abundance of each species. Thesetwo measures provide slightly different information. Species rich-ness is a simple count of the species in the community. Eachspecies contributes one count to the total regardless of whetherthe species’ population is one or 1 million.

In contrast, species evenness takes into account how commoneach species is in the community. To calculate the species even-ness of a community, ecologists must measure or estimate the pop-ulation size of all species in the community. In general, ecologistsstudy both species richness and species evenness when theyinvestigate communities.

Latitude and Species RichnessSpecies richness varies with latitude (distance from the equator).As a general rule, the closer a community is to the equator, themore species it will contain. Species richness is greatest in thetropical rain forests. For example, entomologists Edward O. Wilsonand Terry Erwin identified nearly as many species of ants in a sin-gle tree in Peru as can be found in the entire British Isles.

Why do the Tropics contain more species than the temperatezones do? One hypothesis is that temperate habitats, havingformed since the last Ice Age, are younger. Therefore, tropicalhabitats were not disturbed by the ice ages, but habitats closer tothe poles were disturbed. Also, the climate is more stable in theTropics. This stability allows species to specialize to a greaterdegree than they could in temperate regions, where the climate ismore variable.

SECTION 2

O B J E C T I V E S● Describe the factors that affect

species richness in a community.● Explain how disturbances affect

community stability.● Distinguish between types of

succession, and explain whysuccession may not be predictable.

V O C A B U L A R Yspecies richnessspecies evennessspecies-area effectdisturbancestabilityecological successionprimary successionsecondary successionpioneer speciesclimax community

www.scilinks.orgTopic: Species

Interactions andRichness

Keyword: HM61435


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Another hypothesis suggests that because plants can photo-synthesize year-round in the Tropics, more energy is available tosupport more organisms. The high richness of species in theTropics, as shown in Figure 20-7, is likely the result of several factors.

Habitat Size and Species RichnessAnother pattern of species richness is that larger areas usuallycontain more species than smaller areas do. This relationship iscalled the species-area effect. The species-area effect is most oftenapplied to islands, where area is clearly limited by geography. Inthe Caribbean, for example, more species of reptiles and amphib-ians live on large islands, such as Cuba, than on small islands, suchas Redonda, as shown in Figure 20-8. Because all of these islandsare close together, differences in species richness cannot be due to

differences in latitude. Why does species richnessincrease as area increases? Larger areas usuallycontain a greater diversity of habitats and thus cansupport more species.

The species-area effect has one very importantpractical consequence: reducing the size of a habitat reduces the number of species that thehabitat can support. Today, natural habitats areshrinking rapidly under pressure from the ever-growing human population. About 2 percent of theworld’s tropical rain forests are destroyed eachyear, for example. The inevitable result of thedestruction of habitats is the extinction of species.


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CubaJamaica

Puerto Rico

MontserratSaba

Redonda

This species-richness map of NorthAmerican and Central American birdsshows that fewer than 100 species ofbirds inhabit arctic regions, whereasmore than 600 species occupy sometropical regions. This evidence suggeststhat species richness increases closer to the equator. Equatorial rain forestsare biologically the richest habitats on Earth.

FIGURE 20-7

In a 1971 survey, the large islands ofCuba and Hispaniola each had about100 species of reptiles and amphibians,whereas the small island of Redondaonly had about 5 species. In general,species richness increases as availablehabitat increases. This principle is truefor inland forests as well as islands.

FIGURE 20-8


Species Interactions and Species RichnessInteractions among species sometimes affect species richness.Several studies have demonstrated that predators can preventcompetitive exclusion among their prey. In the 1960s, zoologistRobert Paine showed the importance of the sea star Pisasterochraceus, shown in Figure 20-9, in maintaining the species rich-ness of communities on the Washington coast. Paine removedall Pisaster individuals from one site and for several years pre-vented any new Pisaster individuals from settling there. Thischange caused a dramatic shift in the community. The musselMytilus californianus, which had previously coexisted with sev-eral other species, became much more abundant, spread overthe habitat, and crowded out other species. The number ofother species fell from almost 20 to fewer than 5 within adecade. Evidently, Mytilus was the superior competitor forspace on the rocks, but its population was normally held incheck by predation from Pisaster.

Community Stability and Species RichnessOne of the most important characteristics of a community is howit responds to disturbance. Disturbances are events that changecommunities, remove or destroy organisms from communities, oralter resource availability. Examples of abiotic disturbances aredroughts, fires, floods, volcanic eruptions, earthquakes, andstorms. Examples of animal disturbances include elephants tearingup trees while feeding and prairie dogs moving soil around whiledigging their burrows. Human disturbances include bulldozing,clear-cutting, paving, plowing, and mowing land.

Disturbances affect practically all communities at some point. Anumber of organisms may even depend on a certain type of distur-bance in order to survive. For example, the lodgepole pine, shownin Figure 20-10, depends on periodic fires to disperse its seeds. Thecones of this tree contain a tough resin seal that is cracked openby intense heat. Disturbances may also create opportunities forspecies that have not previously occupied a habitat to becomeestablished.

Stability is the tendency of a community to maintain relativelyconstant conditions. Stability therefore relates to the community’sresistance to disturbances. Ecologists suspect that a community’sstability is also related to its species evenness and species rich-ness, because communities that have more species contain morelinks between species. These links may spread out the effects ofthe disturbance and lessen disruption of the community. One lineof evidence cited in support of this view is the vulnerability of agri-cultural fields to outbreaks of insect pests, given that these fieldsusually consist of one species of plants. Further evidence camefrom a study of grassland plots in the 1980s. Ecologists observedthat during a drought, species-rich plots lost a smaller percentageof plant mass than did species-poor plots. The species-rich plotsalso took less time to recover from the drought.

Predation by the sea star Pisasterochraceus on the mussel Mytiluscalifornianus promoted diversity ofmussel species by controlling theMytilus population.

FIGURE 20-9

These young lodgepole pines havestarted growing after a devastating forest fire. The heat of the fire helpedrelease the pine seeds from their cones,which allowed the seeds to germinate.

FIGURE 20-10


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SUCCESSIONAL CHANGES INCOMMUNITIES

During the summer and early fall of 1988, fires burned large areasof Yellowstone National Park. Approximately 320,900 hectares(793,000 acres) were affected. If you visit Yellowstone today, you will find that regrowth is well underway in the burned areas. In time, if no further major disturbances take place, the burned areas of Yellowstone National Park will undergo aseries of regrowth stages. The gradual, sequential regrowth of acommunity of species in an area is called ecological succession(EK-uh-LAHJ-i-kuhl suhk-SESH-uhn). You can see early stages of succes-sion in vacant lots, along roads, and even in sidewalks or parkinglots where weeds are pushing up through cracks in the concrete.Figure 20-11 shows the successional changes that took place afterthe eruption of Mount St. Helens in 1980.

Ecologists recognize two types of succession. Primary successionis the development of a community in an area that has not sup-ported life previously, such as bare rock, a sand dune, or an islandformed by a volcanic eruption. In primary succession, soil is notinitially present. Secondary succession is the sequential replace-ment of species that follows disruption of an existing community.The disruption may stem from a natural disturbance, such as aforest fire or a strong storm, or from human activity, such as farm-ing, logging, or mining. Secondary succession occurs where soil isalready present.

Any new habitat, whether it is a pond left by heavy rain, a freshly plowed field, or newly exposed bedrock, is an invitation tomany species that are adapted to the new conditions. The species oforganisms that predominate early in succession are called pioneerspecies. Pioneer species tend to be small, to grow quickly, and toreproduce quickly, so they are well suited for invading and occupy-ing a disturbed habitat. They also may be very good at dispersingtheir seeds, which enables them to quickly reach disrupted areas.


Successional changes in communitiesare most apparent after a majordisturbance, such as a volcanic eruption.The aftermath of the 1980 MountSt. Helens eruption is shown in (a). Thephoto in (b) was taken 12 years afterthe eruption. Many herbaceous plantsand young trees had grown up.

FIGURE 20-11

(a) (b)

succession

from Latin succedere,meaning “to go beneath” or

“to follow after”

Word Roots and Origins


Primary SuccessionPrimary succession often proceeds very slowly because the soil istoo thin or lacks enough minerals to support plant growth. Forexample, when glaciers last retreated from eastern Canada, about12,000 years ago, they left a huge stretch of barren bedrock fromwhich all the soil had been scraped. This geologic formation, calledthe Canadian Shield, was a place where plants and most animalscould not live. Repeated freezing and thawing broke this rock intosmaller pieces. In time, lichens—mutualistic associations betweenfungi and either algae or cyanobacteria—colonized the barren rock.Acids in the lichens and mildly acidic rain washed nutrient mineralsfrom the rock. Eventually, the dead organic matter from decayedlichens, along with minerals from the rock, began to form a thinlayer of soil in which a few grasslike plants could grow. These plantsthen died, and their decomposition added more organic material tothe soil. Soon, mosses and then larger plants began to grow. Today,much of the Canadian Shield is densely populated with pine, bal-sam, and spruce trees, whose roots cling to soil that in some areasis still only a few centimeters deep. A similar series of changes hasbeen documented at Glacier Bay, Alaska, shown in Figure 20-12.

Secondary SuccessionSecondary succession occurs where an existing community hasbeen cleared by a disturbance, such as agriculture, but the soil hasremained intact. In secondary succession, the original ecosystemreturns through a series of well-defined stages. In eastern temper-ate regions, secondary succession typically begins with weeds,such as annual grasses, mustards, and dandelions, whose seedsmay be carried to the site by wind or by animals. If no major dis-turbance occurs, succession in these regions proceeds with peren-nial grasses and shrubs, continues with trees such as dogwoods,and eventually results in a deciduous forest community. The com-plete process takes about 100 years.


Ecologists study the process of primarysuccession by examining a variety ofareas at different successional stages.These photos were taken at differentlocations at Glacier Bay, Alaska; thechanges that occur between the stageshown in (a) and the stage shown in (c)take about 200 years. Shown in (a) islifeless glacial “till” (pulverized barerocks) left in the wake of the retreatingglacier. Shown in (b) is an early stage ofsuccession in which small plants andshrubs are growing on the site. Amature forest is shown in (c), an endstage of succession.

FIGURE 20-12

www.scilinks.orgTopic: SuccessionKeyword: HM61475

(a) (b) (c)

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Human disturbances such as mining, logging,farming, and urban development often start theprocess of succession. A recently abandoned farmfield is shown in Figure 20-13. After areas such aswoodlands and prairies are cleared by humans,grasses and weeds often begin to grow, thus begin-ning the process of secondary succession.

THE COMPLEXITY OFSUCCESSION

The traditional description of succession is that the community pro-ceeds through a predictable series of stages until it reaches a stableend point, called the climax community. The organisms in each stagealter the physical environment in ways that make it less favorable fortheir own survival but more favorable for the organisms that eventu-ally succeed them. In a sense, each stage paves the way for the next,leading ultimately to the climax community, which remains constantfor a long period of time.

When ecologists began to study and document many instancesof succession, they found a more complex picture. Some so-calledclimax communities, for example, are not stable and continue tochange. Instead of proceeding inevitably toward the climax com-munity, succession may be regularly “reset” by disturbances. Forexample, many grasslands give way to forests, but periodic firesprevent the forests from developing. There may be many possiblesuccessional pathways in a particular area. The actual path fol-lowed may depend on the identities of the species present, theorder in which the species arrive, the climate, and many otherfactors. Ecologists agree that the idea of a single successionalpathway ending in a stable climax community is too simple todescribe what actually occurs in nature.

1. What is the difference between species richnessand species evenness?

2. Explain the relationship between species richness and latitude.

3. Identify examples of how animal and humandisturbances can affect a community.

4. Explain how species richness can affect community stability.

5. Identify differences between primary and secondary succession.

CRITICAL THINKING6. Applying Information Why is the species-area

effect important in efforts to conserve species?

7. Distinguishing Relevant Information Explainhow the example of agricultural fields relatesspecies richness to stability.

8. Evaluating Viewpoints Do you agree with thefollowing statement: “Communities are usuallyin a state of recovery from disturbance”?Consider both animals and plants in youranswer.

SECTION 2 REVIEW


A recently abandoned agricultural fieldis being pioneered by weeds. Eventually,taller plants and shrubs will competewith the pioneers. If no furtherdisturbances occur, a forest of pine orcottonwood may follow, succeeded by ahardwood forest. The whole process willtake about 100 years.

FIGURE 20-13

CHAPTER HIGHLIGHTS


Species InteractionsSECTION 1

predation (p. 399) interspecific competition (p. 401)

symbiosis (p. 403)parasitism (p. 403)

mutualism (p.403)commensalism (p. 403)

Vocabulary

Patterns in CommunitiesSECTION 2

species richness (p. 405) species evenness (p. 405)species-area effect (p. 406)

disturbance (p. 407)stability (p. 407) ecological succession (p. 408)

primary succession (p. 408)secondary succession (p. 408)pioneer species (p. 408)

climax community (p. 410)Vocabulary

● Species richness is the number of species in a community.Species evenness is the relative abundance of eachspecies.

● In general, species richness is greatest near the equator,and larger areas support more species. Speciesinteractions such as predation can promote speciesrichness.

● Disturbances can alter a community by eliminating orremoving organisms or altering resource availability.

● Species richness may improve a community’s stability.Areas of low species richness may be less stable in theevent of an ecological disturbance.

● Ecological succession is a change in the speciescomposition of a community over time. Primarysuccession is the assembly of a community on newlycreated habitat. Secondary succession is the change in an existing community following a disturbance.

● Primary succession occurs in areas that have beenrecently exposed to the elements and lack soil. Primarysuccession typically proceeds from lichens and mosses toa climax community.

● Secondary succession occurs in areas where the originalecosystem has been cleared by a disturbance. Secondarysuccession typically proceeds from weeds to a climaxcommunity.

● Ecologists recognize five major kinds of speciesinteractions in communities: predation, parasitism,competition, mutualism, and commensalism.

● Predation is an interaction in which one organism (thepredator) captures and eats another organism (the prey).

● Predators have adaptations to efficiently capture prey,whereas prey species have adaptations to avoid capture.Mimicry is an adaptation in which a species gains anadvantage by resembling another species or object.

● Competition may cause competitive exclusion, theelimination of one species in a community. Competitionmay also drive the evolution of niche differences amongcompetitors.

● In parasitism, one species (the parasite) feeds on, butdoes not always kill, another species (the host).

● In mutualism, both interacting species benefit.● In commensalism, one species benefits, and the other is

not affected.


C H A P T E R 2 0412

CHAPTER REVIEW

USING VOCABULARY1. For each pair of terms, explain how the meanings

of the terms differ. a. mutualism and commensalismb. parasitism and predationc. species richness and species evennessd. primary succession and secondary successione. pioneer species and climax community

2. Word Roots and Origins The word niche isderived from the Old French word nichier, whichmeans “to nest.” Using this information, explainwhy niche is a good term for the role of an organism in its environment.

UNDERSTANDING KEY CONCEPTS3. Compare the five major types of relationships

between species by creating a table. 4. Describe two evolutionary adaptations that

enable organisms to be efficient predators.5. Identify two evolutionary adaptations that enable

prey species to avoid being eaten. 6. Explain some of the adaptations that may enable

a host species to defend itself against parasites. 7. Describe the experiments that Gause conducted

on competition in paramecia, and explain whatthe results demonstrated.

8. State why, in the study of competition betweentwo species of barnacles, Semibalanus balanoideswas the superior competitor, yet Chthamalusstellatus was not excluded from the community.

9. Identify the benefits that certain ants derive fromtheir relationship with Acacia plants and the benefits that the plants receive from the ants.

10. Explain the possible consequences of habitat lossin terms of the species-area effect.

11. Describe how a disturbance could benefit somespecies in a community.

12. Summarize one view on how species richnessaffects community stability.

13. State some reasons why the process of succes-sion may be more complex than once thought.

14. Write a short report summarizinghow artificial ecosystems used inthe management and treatment

of wastewater and pollutants can demonstratesuccession.

15. CONCEPT MAPPING Use the following terms to create a concept map that com-

pares the two types of succession: primarysuccession, secondary succession, pioneerspecies, climax community, bare rock, lichens,organic matter, soil, weeds, shrubs, and trees.

CRITICAL THINKING

16. Interpreting Graphics Examine the figure above.These three warbler species each feed oninsects in spruce trees at the same time.However, each species tends to forage in adifferent area of the tree.a. What ecological process is demonstrated by

the feeding patterns of these species?b. In some areas, there are five species of war-

blers that feed in spruce trees. Form ahypothesis about the feeding patterns of theother two species.

17. Analyzing Concepts Can two species that nevercome in contact with each other compete for thesame resource? Explain your answer.

18. Analyzing Relationships Some plants are polli-nated by only one pollinator. Explain why this situation might benefit the plant. How could thisrelationship be a danger to the plant species?

19. Making Inferences Explain why measuring thespecies evenness of a community is usuallyharder than measuring species richness.

Blackburnian warbler

Bay-breasted warbler

Myrtle warbler



Standardized Test PreparationDIRECTIONS: Choose the letter of the answer choicethat best answers the question or completes thesentence.

1. A certain tropical tree has a fruit that is eaten byonly one species of bats. As the bat digests thefruit, the seeds are made ready to sprout. Whenthe bat excretes the wastes of the fruit, it dropsseeds in new locations. Which of the following isthe correct term for the relationship between thebat and the tree?A. predationB. mutualismC. competitionD. commensalism

2. Which of the following is a parasite?F. a lion hunting a zebraG. a deer grazing on grassH. a tick sucking blood from a dogJ. a snake swallowing a bird’s egg

3. Three species of birds forage for insects in thesame tree. However, each species tends to foragein different parts of the tree. This pattern of forag-ing is best explained as an adaptation to which ofthe following relationships?A. predationB. mutualismC. competitionD. commensalism

INTERPRETING GRAPHICS: The map below showstwo islands. Use the map to answer the question that follows.

4. What can you infer about the number of specieson each of these islands?F. Island A has more species.G. Island B has more species.H. Island A and Island B will have the same

number of species.J. Both islands will have fewer species than

islands that are located farther north.

DIRECTIONS: Complete the following analogy.5. predator : prey :: herbivore :

A. carnivoreB. plantC. parasiteD. predation

INTERPRETING GRAPHICS: The shading in the graphbelow indicates the frequency with which a certainbird species obtains prey, by prey size and location.Use the graph to answer the question that follows.

6. Which of the following statements is best sup-ported by this graph?F. Most often, the bird eats insects.G. Most often, the bird nests above ground.H. Most often, the bird finds prey at ground level.J. Most often, the bird eats prey that is

between 3 and 5 mm long.

SHORT RESPONSESome plants produce chemicals that are irritating orpoisonous to some animals.

Explain the role of these adaptations in an ecologicalcommunity.

EXTENDED RESPONSEThe gradual, sequential change in species in an areais called ecological succession.

Part A Describe the stages of primary succession.

Part B Compare primary succession and secondarysuccession.

Carefully read all instructions,questions, and answer options completely beforeattempting to choose an answer.

Hei

ght

abov

egr

ound

(m)

Prey length (mm)

Prey Size and Location

2 4 6 9 11 13 151 3 5 8 10 12 147

2

4

6

9

11

1

3

5

8

10

7

Low High

Frequency

Island AArea = 150 km2

Island BArea = 1,000 km2

Equator


C H A P T E R 2 0414

Observing Symbiosis: Root Nodules

■ Examine root nodules in legumes.■ Investigate the differences between a legume (bean)

and a nonlegume (radish).■ View active cultures of symbiotic Rhizobium.

■ recognizing relationships■ hypothesizing■ comparing and contrasting

■ protective gloves■ 2 three-inch flowerpots■ 2 cups of soil■ 2 mixing sticks■ 1.2 mL (1/4 tsp) of Rhizobium bacteria per pot■ 2 bean seeds■ 2 radish seeds■ 2 microscope slides■ 2 coverslips■ 1 prepared slide of a soybean root nodule with

symbiotic Rhizobium bacteria■ compound light microscope■ stereoscope or magnifying glass■ scalpel

Background1. Define symbiosis, and give examples.2. Nitrogen-fixing bacteria and leguminous plants have

a symbiotic relationship.3. Rhizobium is a genus of nitrogen-fixing bacteria.

Rhizobium species exist in soil and in the root nodules of leguminous plants such as soybeans.

4. Green root nodules indicate actively reproducingbacteria that are not fixing nitrogen. Pink nodulesindicate bacteria that are actively fixing nitrogenbut are not reproducing.

5. The process of nitrogen fixation is the basis of thenitrogen cycle. Only a few kinds of organisms arecapable of converting nitrogen gas into a form thatis usable by other organisms.

Growing the Test Plants1. Fill two flowerpots with soil. Using a mixing stick,

stir approximately 1/4 tsp of the Rhizobium mixtureinto each pot.

2. Plant two bean seeds in one pot, and label the pot“Bean.” Plant two radish seeds in the other pot,and label this pot “Radish.” Water each pot so thatthe soil is moist but not saturated.

3. Place the plants where they will receive direct sun-light. Water the soil when necessary to keep the soilmoist but not saturated. Do not fertilize the plants.

4. After approximately one week, check to see if bothseeds germinated in each pot. If they did, remove thesmaller seedling. The plants will be ready to be exam-ined after six to eight weeks. Monitor the plants eachday. (Note: The radish seeds will germinate faster thanthe bean seeds.)

5. Clean up your materials, and wash yourhands before leaving the lab.

Observing the Roots of Beans6. Prepare a data table similar to the one below. As you

work, record your observations in your data table.7. CAUTION Wear disposable gloves while

handling plants. Do not rub any plant partor plant juice on your eyes or skin. Remove thebean plant from the pot by grasping the bottom ofthe stem and gently pulling the plant out. Be carefulnot to injure the plant. Carefully remove all dirtfrom the roots.

8. View the roots of the bean plant under a stereoscope.Compare the appearance of the bean root systemwith the photograph above. Note the formation of any

PART B

PART A

MATERIALS

PROCESS SKILLS

OBJECTIVES

EXPLORATION LAB

OBSERVATIONS OF ROOT NODULES

Color of nodule

Shape of nodule

Number of nodules

Number of pink nodules

Number of green nodules



nodules on your bean plant’s roots. Draw a root witha nodule in your lab report, and label each structure.

9. CAUTION Use the scalpel with care. Ascalpel is a very sharp instrument. When

cutting, make sure that the blade faces awayfrom your body. If you cut yourself, quickly applydirect pressure to the wound, and call for yourteacher. Remove a large nodule from the bean root,and carefully cut the nodule in half with a scalpel.The pink nodules contain active nitrogen-fixing bac-teria. The green nodules contain bacteria but cannotfix nitrogen because they are actively reproducing.Rhizobium bacteria will begin fixing nitrogen onlyafter they stop reproducing. View the cross sectionunder a stereoscope. Note the arrangement of sym-biotic cells within the nodule. Draw and label thisarrangement.

Observing the Roots of Radishes

10. CAUTION Wear disposable gloves whilehandling plants. Do not rub any plant part

or plant juice on your eyes or skin. Remove theradish plant from the pot by grasping the bottom ofthe stem and gently pulling the plant out. Be carefulnot to injure the plant. Carefully remove all dirtfrom the roots.

11. Examine the roots of the radish plant under a stere-oscope. Compare the radish roots with the roots ofthe bean plant that you have already examined. Dothe roots of the radish plant contain any nodules?Draw the root of the radish plant in your lab report.Label the drawing “Radish.”

Preparing a Wet Mount of Rhizobium

12. CAUTION Handle the slide and coverslipcarefully. Glass slides break easily, and the

sharp edges can cut you. Prepare a wet mount byplacing part of a pink nodule on a microscope slide,adding a drop of water, and covering the slide witha coverslip.

13. Place the slide on a flat surface. Gently press downon the slide with your thumb. Use enough pressureto squash the nodule. Make sure that the coverslipdoes not slide.

14. Examine the slide under a microscope. Draw andlabel your observations in your lab report. Note thepower of magnification used.

15. Compare your wetmount preparationwith the preparedslide of Rhizobiumbacteria and thephotograph on theright. Cells withactive Rhizobiumbacteria should looklike the reddish cells in the photograph at right.

16. Clean up your materials, and wash yourhands before leaving the lab.

Analysis and Conclusions1. Which plant had the most nodules?2. How many nodules were found on the radish plants?3. How do legumes become inoculated with bacteria

in nature?4. What kind of relationship exists between the

legume plant and Rhizobium bacteria? How doesthis relationship benefit the legume plant? Howdoes this relationship benefit the bacteria?

5. If you were to try to grow legumes without rootnodules to use as experimental controls, why shouldyou plant the seeds in sterile soil?

Further InquiryPerform the experiment by using beans with and withoutRhizobium bacteria. Count the number of leaves on eachplant, and measure the mass of each whole plant as wellas the masses of roots, stems, and leaves separately.Predict which part of the plant will differ the most.

PART D

PART C


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