SECTION 2 How Species Interact with Each Other

Section 2 How Species Interact with Each Other 203

What’s the difference between lions in a zoo and lions in thewild? In the wild, lions are part of a community and a food web.In the African savanna, lions hunt zebras, fight with hyenas, andare fed upon by fleas and ticks. Interactions like these were partof the evolution of the lions that you see in zoos. Any species isbest understood by looking at all of the relationships the specieshas within its native communities.

An Organism’s NicheThe unique role of a species within an ecosystem is its(NICH). A niche includes the species’ physical home, the environ-mental factors necessary for the species’ survival, and all of thespecies’ interactions with other organisms. A niche is differentfrom a habitat. An organism’s habitat is a location. However, aniche is an organism’s pattern of use of its habitat.

A niche can also be thought of as the functional role, or job,of a particular species in an ecosystem. For example, Americanbison occupied the niche of large grazing herbivores on Americangrasslands. Kangaroos occupy a similar niche on Australiangrasslands. Herbivores often interact with carnivores, such aslions, if they both exist in the same habitat. Some parts of a lion’sniche are shown in Figure 9.

niche

Objectives� Explain the difference between

niche and habitat.� Give examples of parts of a niche.� Describe the five major types of

interactions between species.� Explain the difference between

parasitism and predation.� Explain how symbiotic relation-

ships may evolve.

Key Termsnichecompetitionpredationparasitismmutualismcommensalismsymbiosis

S E C T I O N 2

How Species Interact with Each Other

Figure 9 � Parts of a lion’s niche areshown here. Can you think of otherparts?

Copyright© by Holt, Rinehart and Winston. All rights reserved.

Ways in Which Species InteractInteractions between species are categorized at the level where onepopulation interacts with another. The five major types of speciesinteractions, summarized in Figure 10, are competition, predation,parasitism, mutualism, and commensalism. These categories arebased on whether each species causes benefit or harm to the otherspecies in a given relationship. Keep in mind that the benefit or harmis in terms of total effects over time. Also note that other types ofinteraction are possible. Many interactions between species are indi-rect, and some interactions do not fit a category clearly. Other typesof interactions seem possible but are rarely found. Therefore, manyinteractions are neither categorized nor well studied.

CompetitionFor most organisms, competition is part of daily life. Seed-eatingbirds compete with each other for seed at a bird feeder, andweeds compete for space in a sidewalk crack. is arelationship in which different individuals or populations attemptto use the same limited resource. Each individual has less accessto the resource and so is harmed by the competition.

Competition can occur both within and between species. Wehave learned that members of the same species must competewith each other because they require the same resources—theyoccupy the same niche. When members of different species com-pete, we say that their niches overlap, which means that eachspecies uses some of the same resources in a habitat.

Competition

204 Chapter 8 Understanding Populations

FIELD ACTIVITY FIELD ACTIVITY Observing Competition Youcan study competition betweenbird species at home or at school.Build a bird feeder using a plasticmilk jug, a metal pie pan, orsome other inexpensive material.Fill the feeder with unsalted breadcrumbs, sunflower seeds, or com-mercial birdseed.

Observe the birds that visit thefeeder. Sit quietly in the samespot, and make observations atthe same time each day for sev-eral days in a row.

In your EcoLog, keep a recordthat includes data about the kindsof birds that use the feeder, thekinds of seeds that the birds pre-fer, the factors that affect howmuch the birds eat, and the kindsof birds that are better competi-tors for the birdseed.

Figure 10 � Species Interactions

Types of Interactions Between Two Species

Interaction Species A Species B Description

Competition harmed harmed Each species negatively affects the other.

Predation and benefited harmed Species A feeds on species B.parasitism

Mutualism benefited benefited Each species is helpful to the other.

Commensalism benefited unaffected Species A benefits from spe-cies B, but B is unaffected.

MUTUALISM Yuccamoth pollinates andlays eggs on yuccaflowers; moth larvaespread yucca seeds

COMPETITION Fox and coyoteare predators of same prey

PREDATION Kit foxhunts and feeds onkangaroo rat

COMMENSALISMWren makes nest without affecting cactus


Indirect Competition Species can compete even if they nevercome into direct contact with each other. Suppose that one insectspecies feeds on a certain plant during the day and that anotherspecies feeds on the same plant during the night. Because they usethe same food source, the two species are indirect competitors.Similarly, two plant species that flower at the same time may com-pete for the same pollinators even if the plants do not compete inany other way. Humans rarely interact with the insects that eat ourfood crops, but those insects are still competing with us for food.

Adaptations to Competition When two species with similarniches are placed together in the same ecosystem, we might expectone species to be more successful than the other species. The better-adapted species would be able to use more of the niche. But in thecourse of evolution, adaptations that decrease competition will alsobe advantageous for species whose niches overlap.

One way competition can be reduced between species is bydividing up the niche in time or space. Niche restriction is wheneach species uses less of the niche than they are capable of using.Niche restriction is observed in closely related species that usethe same resources within a habitat. For example, two similarbarnacle species compete for space in the intertidal zone of rockyshorelines. One of the species, Chthamalus stellatus, is foundonly in the upper level of the zone when the other species ispresent. But when the other species is removed from the area,C. stellatus is found at deeper levels, as shown in Figure 11. Inthe presence of competition, the actual niche used by a speciesmay be smaller than the potential niche. Ecologists haveobserved various other ways of dividing up a niche amonggroups of similar species.


Chthamalus stellatus

Semibalanus balanoides

HIGH TIDE

LOW TIDE

Chthamalusniche size whencompetitionis absent

Chthamalus nichesize when

competitionis present

Figure 11 � The barnacle speciesChthamalus stellatus uses less of itspotential niche when competing forspace with a similar barnacle species,Semibalanus balanoides.

EcofactA Wide Niche Coyotes live in awide range of habitats and are will-ing to get close to human settle-ments. Coyotes are also known toeat a wide variety of animals andplants, including food that humansthrow away. Thus, coyotes have abroad and varied niche. Coyotestake advantage of those parts oftheir niche that are easiest to useand that present less competition.


Predator-Prey Adaptations

� Patterns of black and red, orange,or yellow are common warning signs.

Predation An organism that feeds on another organism is called a preda-tor, and the organism that is fed upon is the prey. This kind

of interaction is called Examples of predationinclude snakes eating mice, bats eating insects, or

whales consuming krill. Figure 12 shows a preda-tory bird with its captured prey.

Predation is not as simple to understand as it seems. We may think of predators asmeat-eating animals, but there can be lessobvious kinds of predators. In complex food webs, a predator may also be the prey of another species. Most organisms haveevolved some mechanisms to avoid or defend

against predators.Some predators eat only specific types of

prey. For example, the Canadian lynx feeds mostlyon snowshoe hares during the winter. In this kind of

predation.


Most organisms are vulnerable topredation, so there is strong selec-tive pressure for adaptations thatserve as defenses against predators.

Many animals are camouflaged—disguised so that they are hard tosee even when they are in view.Visual camouflage is very obviousto us, because vision is the domi-nant sense in humans. Many preda-tors also have keen vision. Ananimal’s camouflage usually dis-guises its recognizable features. Theeyes are the most recognizable partof the animal, and hundreds ofspecies have black stripes acrosstheir eyes for disguise. Dark bandsof color, such as those on manysnakes, may also break up theapparent bulk of the animal’s body.

Some predators do not chasetheir prey but wait for the prey to

During the course of evolution,members of several well-protectedspecies have come to resemble eachother. For example, both bees andwasps often have black and yellowstripes. This is an example ofmimicry of one species by another.The advantage of mimicry is thatthe more individual organisms thathave the same pattern, the lesschance any one individual has ofbeing killed. Also, predators learn toavoid all animals that have similarwarning patterns.

come near enough to be caught.Praying mantises and frogs areexamples of these types of preda-tors. Such predators are usuallycamouflaged so that the prey doesnot notice them waiting to attack.

Animals, and more oftenplants, may contain toxic chemi-cals that harm or deter predators.Many animals that have chemicaldefenses have a striking coloration.This warning coloration alertspotential predators to stay awayand protects the prey species fromdamage. Patterns with black stripesand red, orange, or yellow arecommon in many species of bees,wasps, skunks, snakes, and poison-ous frogs.

Warning coloration works wellagainst predators that can learn andthat have good vision.

Figure 12 � This predatory bird hadto outrun its prey. Many organismsare adapted to avoid predation.


close relationship, the sizes of each population tend to increaseand decrease in linked patterns, as shown in Figure 13. How-ever, many predators will feed on whichever type of prey is easiest to capture.


Occasionally, a harmless speciesis a mimic of a species that haschemical protection. You haveprobably tried to get away frominsects that you thought werewasps or bees. In fact, some ofthem were probably flies. Severalspecies of harmless insects haveevolved to mimic wasps and bees.On the other hand, sometimes a

CRITICAL THINKING

1. Making Comparisons Foreach of these types of adaptations,give an additional example that youhave seen or heard of.

2. Determining Cause andEffect Write a paragraph to explainhow one of these adaptations mighthave evolved. WRITING SKILLS

predator may look like another, lessthreatening species. Some speciesof spiders may be mistaken for antsor other types of insects.

A simple defense against preda-tion is some type of protectivecovering. The quills of a porcupine,the spines of a cactus, and the shellof a turtle are all examples of pro-tective covering.

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Lynx

Snowshoe hare

Figure 13 � Populations of predatorsdepend on populations of prey, sochanges in one of these populationsmay be linked to changes in the other.This graph shows population estimatesover time for Canadian lynx and theirfavorite food, snowshoe hares.

� Both predators and prey mayexhibit adaptations such as cam-ouflage or mimicry. The spiderthat looks like an ant (left) is apredator of insects. The protec-tive quills of this porcupine(right) are a simple but effectiveway to repel predators.


ParasitismAn organism that lives in or on another organism and feeds onthe other organism is a parasite. The organism the parasite takesits nourishment from is known as the host. The relationshipbetween the parasite and its host is called Examplesof parasites are ticks, fleas, tapeworms, heartworms, bloodsuck-ing leeches, and mistletoe.

The photos of parasites in Figure 14 may make you feeluneasy, because parasites are somewhat like predators. The differ-ences between a parasite and a predator are that a parasitespends some of its life in or on the host, and that parasites do notusually kill their hosts. In fact, the parasite has an evolutionaryadvantage if it allows its host to live longer. However, the host isoften weakened or exposed to disease by the parasite.

MutualismMany species depend on another species for survival. In somecases, neither organism can survive alone. A close relationshipbetween two species in which each species provides a benefit tothe other is called Certain species of bacteria in yourintestines form a mutualistic relationship with you. These bacte-ria help break down food that you could not otherwise digest orproduce vitamins that your body cannot make. In return, yougive the bacteria a warm, food-rich habitat.

Another case of mutualism happens in the ant acacia trees ofCentral America, shown in Figure 15. Most acacia trees havespines that protect them against plant-eating animals, but the antacacias have an additional protection—an ant species that livesonly on these trees. The trees provide these ants shelter withinhollow thorns as well as food sources in sugary nectar glands andnutrient-rich leaf tips. In turn, the ants defend the tree againstherbivores and many other threats.

mutualism.

parasitism.


Figure 14 � Parasites such as ticks(left) and intestinal worms (right)could be harmful to you. You proba-bly try to avoid these parasites,almost as if they were predators. Inwhat ways are parasites like preda-tors?

Figure 15 � These acacia trees inCentral America have a mutualisticrelationship with these ants. Thetrees provide food and shelter to theants, and the ants defend the tree.


CommensalismA relationship in which one species benefits and the other species isneither harmed nor helped is called An example isthe relationship between sharks and a type of fish called remoras,which are shown in Figure 16. Remoras attach themselves tosharks and feed on scraps of food left over from the shark’s meals.Another example of commensalism is when birds nest in trees, butonly if the birds do not cause any harm to the tree. Even a seem-ingly harmless activity might have an effect on another species.

Symbiosis and CoevolutionA relationship in which two organisms live in close association iscalled Many types of species interactions are con-sidered symbiotic in some cases. Symbiosis is most often used todescribe a relationship in which at least one species benefits.

Over time, species in close relationships may coevolve. Thesespecies may evolve adaptations that reduce the harm or improvethe benefit of the relationship. Recall that harm and benefit aremeasured in total effects over time. For example, coevolution canbe seen in the relationships of flowering plants and their pollina-tors. Many types of flowers seem to match the feeding habits ofcertain species of insects or other animals that spread pollen.

symbiosis.

commensalism.


1. List as many parts as you can of the niche of anorganism of your choice.

2. Give examples of species that have the same habi-tat but not the same niche that a lion has.

3. Describe the five types of species interactions.

CRITICAL THINKING4. Making Comparisons Read the definition of para-

sites and predators, and then explain how parasitesdiffer from predators.

5. Analyzing Relationships Choose an example ofmutualism, and then describe the long process bywhich the relationship could have developed.

READING SKILLS

S E C T I O N 2 Review

Figure 16 � Remoras have a com-mensal relationship with sharks.Remoras attach themselves to sharksin order to eat scraps from thesharks’ meals, or to hitch a ride else-where. The remoras cause neitherbenefit nor harm to the sharks. Thereare many examples of freeloadersand scavengers in nature.

www.scilinks.orgTopic: CoevolutionSciLinks code: HE4014

BiologyConnection to

An Ecosystem in Your BodyOur health is affected by our rela-tionships with microorganisms inour digestive system, skin, blood,and other parts of our body. Forexample, live-culture yogurt isconsidered a healthy food becausethe kinds of bacteria it containsare beneficial to us. The bacteriaassist our digestion of dairy prod-ucts and also compete with othermicroorganisms, such as yeast,that might cause infections.


HighlightsC H A P T E R 8

1 How Populations Change in Size

2 How Species Interact withEach Other

Key Termspopulation, 197density, 198dispersion, 198growth rate, 198reproductive

potential, 199exponential growth,

199carrying capacity,

200

Main Ideas� Each population has specific properties, includ-ing size, density, and pattern of dispersion.

� Each population has a characteristic repro-ductive potential. This is the fastest possiblegrowth rate of the population.

� When a population has few limits to itsgrowth, it may have an exponential growthrate. Usually, population growth is limited byfactors such as disease and competition.

� Carrying capacity is the maximum popula-tion a habitat can support over time.

� A population that grows rapidly may be sub-ject to density-dependent regulation.

niche, 203competition, 204predation, 206parasitism, 208mutualism, 208commensalism, 209symbiosis, 209

� The niche of an organism is its pattern ofuse of its habitat and its interactions withother organisms.

� Interactions between species are categorizedbased on the relative benefit or harm that onespecies causes the other. The categories arecompetition, predation, parasitism, mutualism,and commensalism.

� Competition between species occurs whentheir niches overlap. The competition may bedirect or indirect.

� Pairs of species that have close relation-ships often evolve adaptations that eitherincrease the benefit of or reduce the harmfrom the relationship.

210 Chapter 8 HighlightsCopyright© by Holt, Rinehart and Winston. All rights reserved.

Review

Chapter 8 Review 211

Using Key TermsUse each of the following terms in a separatesentence.

1. reproductive potential2. carrying capacity3. competition4. symbiosis

For each pair of terms, explain how the meaningsof the terms differ.

5. niche and habitat6. predator and prey7. predation and parasitism8. mutualism and commensalism

Understanding Key Ideas9. In which of the following pairs do both

organisms belong to the same population?a. a rose and a carnationb. a zebra and a horsec. two residents of New York Cityd. two similar species of monkeys

10. A population of some species is most likelyto grow exponentiallya. if the species is already very common in

the area.b. when the species moves into a new area of

suitable habitat.c. when it uses the same habitat as a similar

species.d. if the population size is already large.

11. A population will most likely deplete theresources of its environment if the populationa. grows beyond carrying capacity.b. must share resources with many other

species.c. moves frequently from one habitat to another.d. has a low reproductive potential.

12. The growth rate of a population of geesewill probably increase within a year ifa. more birds die than are hatched.b. several females begin laying eggs at

younger ages than their mothers did.c. most females lay two eggs instead of three

during a nesting season.d. some birds get lost during migration.

13. Which of the following is an example ofcompetition between species?a. two species of insects feeding on the same

rare plantb. a bobcat hunting a mousec. a lichen, which is an alga and a fungus

living as a single organismd. a tick living on a dog

14. Which of the following statements aboutparasitism is true?a. The presence of a parasite does not affect

the host.b. Parasitism is a cooperative relationship

between two species.c. Parasites always kill their hosts.d. Parasitism is similar to predation.

15. Ants and acacia trees have a mutualistic rela-tionship becausea. they are both adapted to a humid climate.b. they are part of the same ecosystem.c. they benefit each other.d. the ants eat parts of the acacia tree.

16. Which of the following is an example of coevolution?a. flowers that can be pollinated by only one

species of insectb. rabbits that invade a new habitatc. wolves that compete with each other for

territoryd. bacteria that suddenly mutate in a lab

C H A P T E R 8

Review with a Partner To review the mainideas of the text, try summarizing with a partner.Take turns reading a passage, and then try to sum-marize aloud what you have read. Try not to lookback at the text. Then, discuss and review the textwith your partner to check your understanding.

STUDY TIP


ReviewShort Answer17. A tapeworm lives in the intestines of a cow

and feeds by absorbing food that the cow isdigesting. What kind of relationship is this?Explain your answer.

18. Explain how two species can compete for thesame resource even if they never come incontact with each other.

19. Snail kites are predatory birds that feed onlyon snails. The kites use their hooked, needle-like beaks to pull snails from their shells.Explain how these specialized beaks mighthave evolved in these birds.

20. What would happen to the population of snailkites mentioned in question 19 if the snails’habitat was destroyed? Explain your answer.

Interpreting GraphicsThe graph below shows the population of somereindeer that were introduced to an Alaskan islandin 1910. Use the graph to answer questions 21–23.21. Describe this population’s changes over time.22. What might have happened in 1937?23. How would you estimate this island’s carrying

capacity for reindeer? Explain your answer.

Concept Mapping24. Use the following terms to create a concept

map: symbiosis, predation, predator, prey,parasitism, parasite, host, mutualism, andcommensalism.

Critical Thinking25. Analyzing Relationships Read the explana-

tions of competition and predation. If onespecies becomes extinct, and then soon after,another species becomes extinct, was theirrelationship most likely competition or pre-dation? Explain your answer.

26. Evaluating Hypotheses Scientists do not allagree on the specific carrying capacity ofEarth for humans. Why might this carryingcapacity be difficult to determine?

27. Evaluating Conclusions A scientist finds noevidence that any of the species in a particu-lar community are competing and concludesthat competition never played a role in thedevelopment of this community. Could thisconclusion be valid? Write a paragraph toexplain your answer.

Cross-Disciplinary Connection28. Health Viruses are the cause of many infec-

tious diseases, such as common colds, flu,and chickenpox. Viruses can be passed fromone person to another in many differentways. Under what conditions do you thinkviral diseases will spread most rapidlybetween humans? What can be done to slow the spread of these viruses?

Portfolio Project29. Create a Niche Map Create a visual repre-

sentation of the niche of an organism ofyour choice. Research the organism’s habi-tat, behaviors, and interactions with otherspecies. If possible, observe the organism(without disturbing it) for a day or more.Create a piece of art to show all of theinteractions that this organism has withits environment.

WRITING SKILLS

READING SKILLS

C H A P T E R 8

212 Chapter 8 Review

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Chapter 8 Review 213

Read the passage below, and then answerthe questions that follow.

Excerpt from Charles Darwin, On the Originof Species, 1859.

I should premise that I use the term strugglefor existence in a large and metaphorical sense,including dependence of one being on another,and including (which is more important) notonly the life of the individual, but success inleaving progeny. Two canine animals in a timeof dearth, may truly be said to struggle witheach other which shall get food and live. But aplant of the edge of the desert is said to strug-gle for life against the drought, though moreproperly it should be said to be dependent onthe moisture. A plant which annually producesa thousand seeds, of which on average onlyone comes to maturity, may more truly be saidto struggle with the plants of the same andother kinds which already clothe theground . . . In these several senses, which passinto each other, I use for convenience sake thegeneral term of struggle for existence.

1. Which of the following statements bestdescribes the author’s main purpose inthis passage?a. to describe the process of reproductionb. to persuade the reader that all animals

struggle for existencec. to explain the meaning of the author’s

use of the phrase struggle for existenced. to argue that life in the desert depends

on moisture

2. Which of the following statements mostclosely matches what the author meansby the phrase struggle for existence?a. whenever plants or animals interact in

natureb. whenever plants or animals compete to

survive and to produce offspringc. when plants produce many more seeds

than are likely to growd. when animals compete for food during

difficult times

MATH SKILLS

Use the equation below to answer questions30–31.

30. Extending an Equation The equation givesthe change in a population over a givenamount of time (for example, an increase of100 individuals in one year). Use the twoparts on the right side of the equation towrite an inequality that would be true if thepopulation were increasing. Rewrite theinequality for a decreasing population.

31. Analyzing an Equation Suppose you arestudying the small town of Hill City, whichhad a population of 100 people in the firstyear of your study. One year later, 10 peoplehave died, and only 9 mothers have givenbirth. Yet the population has increased to101. How could this increase happen?

WRITING SKILLS

32. Communicating Main Ideas Why do popu-lation sizes not grow indefinitely?

33. Creative Writing Write a science fictionstory about life without competition.

34. Writing from Research Find information in encyclopedias or natural history referencesabout different kinds of mutualism. Sum-marize the similarities and differencesbetween the various relationships. Focus onthe ways in which each species benefits fromthe other species.

Now that you have read the chapter, take amoment to review your answers to the ReadingWarm-Up questions in your EcoLog. If necessary,revise your answers.

READING FOLLOW-UP


WHERE SHOULD THE WOLVES ROAM?

216 Chapter 8 Points of View

The gray wolf was exterminatedfrom much of the northwesternUnited States by the 1920s.Ranchers and federal agents killedthe animal to protect livestock. TheRocky Mountain gray wolf waslisted as an endangered species in1973. Then in the 1980s, the U.S.Fish and Wildlife Service began aplan to restore wolf populations inthe United States. The agencydecided to reintroduce wolves intocertain areas. Biologists looked forareas where wolves could havelarge habitats and enough food.Three areas were chosen, as shownin the figure below.

Between 1995 and 1996, 64wolves were released in Yellow-stone National Park and 34 wolveswere released in central Idaho. Theoriginal goal was to have breedingpopulations of at least 100 wolvesin each location by 2002. This goalhas now become reality.

The wolf reintroductionefforts remain controversial. Somepeople would prefer that thewolves become extinct. On theother side, some people think that

the government has not done enough to protect wolf popula-tions. Read the following pointsof view, and then analyze theissue for yourself.

Wolves Should Not BeReintroducedSome opponents of the reintro-duction plan argue that wolvesare not truly endangered. Biolo-gists estimate that hundreds ofwolves live in Minnesota, andthousands live in Alaska andCanada. Because there are largenumbers of wolves in the wild,some people feel that wolvesshould not receive special treat-ment as endangered species.

Many hunters also oppose theplan to reintroduce wolves. Bothhunters and wolves hunt for largegame animals such as deer, elk, ormoose. Hunters believe the wolvesmight create too much competitionfor the game animals. Some studiessuggest that populations of gameanimals will decrease if hunted byboth humans and wolves.

Hunters point out that huntingis an important part of theeconomies of the western states.Also, licensed hunting has becomepart of the way large parks andwildlife preserves are managed.Hunting is sometimes allowed bypark and game managers to controlwildlife populations. Hunting feesalso help fund wildlife managementefforts, such as habitat improve-ment and biological studies.

Ranchers are among the peoplewho most strongly oppose wolfreintroduction. Ranchers worrythat wolves will kill their live-stock. Ranchers argue that theycannot afford to lose their live-stock. Even though there is a pro-gram to pay ranchers for lostlivestock, the program will last

� The breeding range of thegray wolf (far left) has been lost inmost of the United States. The U.S.government has reintroduced wolvesinto parts of Montana, Wyoming,and Idaho (left).

� A wild wolf is a now rare sightin most of the United States. Effortsto reintroduce and protect wolfpopulations are controversial.


Chapter 8 Points of View 217

What Do You Think?Like many plans to protectendangered species, the planto reintroduce wolves causessome people to weigh theirown interests against theneeds of a single species. Doyou feel that the decision is asimple one? Can you think ofother ways to look at thisissue? Explain your answers.

only as long as wolves are clas-sified as an endangered species.When there are many wolvesagain in the target areas, the wolf will no longer have endan-gered status. Ranchers point outthat the payment program willdisappear when it would beneeded most.

Other groups that oppose wolfreintroduction include groups thatuse public lands for activities suchas logging or mining. Thesegroups worry that protection forthe wolves may lead to the pre-vention of other uses of land inthe target areas.

Wolves Should BeReintroducedA basic argument in favor of wolfreintroduction is that the federalgovernment must uphold the lawby trying to restore wolf popula-tions in the United States. But sup-porters of wild wolves give otherreasons too.

Many environmentalists andscientists believe that the reintro-duction plan could restore a bal-ance to the Yellowstone ecosystem.Predation by wolves would keepthe herds of elk, moose, and deerfrom growing too dense and over-grazing the land.

The argument that wolves helpcontrol wild herds is like theargument in favor of hunting.Some wolf supporters even saythat licensed hunting of wolvesshould be allowed. In this way,hunters might support the reintro-duction plans, and populations ofboth wolves and game animalscould be managed.

In response to ranchers’ con-cerns that wolves will attack theirlivestock, biologists say that this isnot likely to be a problem. There

is evidence that most wolves preferto hunt wild animals rather thandomestic animals. Wolves rarelyattack livestock when large herdsof wild game are nearby. In fact,from 1995 to 1997, fewer thanfive wolf attacks on livestock werereported in the United States.

Still, some supporters of rein-troduction have tried to addressthe concerns of ranchers. Onegroup raised money to pay ranch-ers for livestock killed by wolves.Other groups conduct studies andeducational programs or talkwith local landowners. Most wolfsupporters are trying to createreintroduction plans that willwork for both humans andwolves.

In response to fears that thewolves pose a danger to humans,supporters say this is also unlikely.There have been no verified attacks on humans by healthywolves in North America. Wolfexperts insist that wolves are shy

animals that prefer to stay awayfrom people.

Most wolf supporters admitthat there are only a few placeswhere wolves may live withoutcausing problems. Supporters ofthe plan believe that the targetareas are places where wolves cancarry out a natural role withoutcausing problems for humans.

� The International Wolf Center in Minnesota tries to educate the public about wolves. The center’s “Wolves and Humans” exhibit is shown here.


SECTION 2 How Species Interact with Each Other

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Transcript of SECTION 2 How Species Interact with Each Other