Evolution and Community Ecology 133

Species Interactions

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How many species do you interact with each day? At first, you might think the number is a low one. But if you really think about it, it is probably quite high. Did you eat any plants today? Accidentally step on any insects? Did you pet a dog or swat a fly? In nature, species interact all the time. Sometimes, they interact in ways that benefit both species. Other times, one species can harm the other. Species interactions form the structure of communities and ecosystems.

The niche and competition An organism’s niche is affected by both its tolerance and

competitive interactions.

Recall that an organism’s habitat describes the general place it lives. Habitats provide organisms with all of the resources they need to survive. Together, habitat and resource use define the concept of niche.

Defining the Niche An organism’s niche describes its use of resources and its functional role in a community. The niche includes not only the habitat where an organism lives, but also what food it eats, how and when it reproduces, and what other organisms it interacts with. The niche is therefore a kind of summary of everything an organism does and when and where it does it.

Figure 6 niche A big part of a spider’s role, or niche, is to prey on insects caught in its web.

• Discuss the factors that influence an organism’s niche.

• Compare and contrast predation, parasitism, and herbivory.

• Describe mutualism and commensalism.

Reading Strategy As you read, complete a vocabulary word map for each of the boldface, highlighted words in the lesson.

Vocabulary niche, tolerance, resource partitioning, predation, coevolution, parasitism, symbiosis, herbivory, mutualism, commensalism

Guiding Question: How do species interact in nature?

FOCuS Watch the ABC News video The “invisible Cloak” of the Cuttlefish, which demonstrates the amazing ability of the cuttlefish to blend in with its surround-ings. Use this video to launch a discussion of the role natural selection likely played in the development of cuttlefish camouflage.

5.2 LESSON PLAN PREVIEWReal world Students make analogies for niche concepts.Differentiated instruction Less proficient readers use symbols to clarify species interactions.inquiry Students research examples of mutualism and commensalism.

5.2 RESOURCESBellringer Video, The “Invisible Cloak” of the Cuttlefish • Lesson 5.2 Work-sheets • Lesson 5.2 Assessment • Chapter 5 Overview Presentation

Tolerance Where and how an organism lives is influenced by its toler-ance. Tolerance is the ability to survive and reproduce under changing environmental conditions. Some organisms, such as panda bears, have very restricted tolerance ranges and are called specialists. Organisms with wide tolerance ranges, such as rats, are able to live in a wide array of habitats or use a wide array of resources. These organisms are called generalists.

Specialist and generalist strategies each have advantages and dis-advantages. Specialists can be very successful in their niche by being extremely good at the things they do. However, they might not be able to adapt when conditions change. Generalists succeed by being able to live in many different places and variable weather conditions. However, they may not be as successful as specialists in a given situation.

Competition When multiple organisms seek the same limited resource, such as food, light, water, or space, they compete. Competing organisms do not always fight with one another directly and physically like the foxes in Figure 7a. Flowers in a field, for example, do not physi-cally fight to attract pollinators, but they are still in competition for that valuable resource. Competitive interactions can take place among members of the same species, called intraspecific competition, or among members of two or more different species, called interspecific competition. Interspecific competition can give rise to different types of outcomes.

▶ Competitive Exclusion Direct competition between species often results in a winner and a loser. If one species is a very effective competi-tor, it may exclude another species from resource use entirely. This out-come, called competitive exclusion, can happen when two or more species try to occupy the exact same niche. In 1986, ecologists with the National Oceanic and Atmospheric Administration began monitoring mussel and clam populations in Lake St. Clair. The lake connects Lakes Erie and Huron. They found there were twenty native mussel species in Lake St. Clair. By 1997, they were all gone—only the invasive zebra mussel occu-pied the lake’s waters. Figure 7b shows zebra mussels attached to a native clam on the shores of Lake Erie. When colonized by zebra mussels, shell-fish cannot open their shells. They eventually suffocate or starve.

Figure 7 Competition Competition can occur between members of (a) the same species or (b) different species. Apply Concepts What does it mean that zebra mussels have “out-competed” native mussels in many ways?

134 Lesson 2

(b)

(a)

ANSWERS

Figure 7 It means that zebra mussels have been better able to get and use resources than native mussels.

Species 1 is limited in itsroles and/or uses only asubset of resources.

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Fundamental Niche Realized Niche

Species 1 ful�lls all itsroles and uses all theresources it can.

Species 1Species 1

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Species 2

Species 3

Species 1 is limited in itsroles and/or uses only asubset of resources.

Resource 1

Reso

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Fundamental Niche Realized Niche

Species 1 ful�lls all itsroles and uses all theresources it can.

Species 1Species 1

Resource 1

Reso

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Species 2

Species 3

White-breasted nuthatch climbs down trunk looking for insects.

Pileated woodpecker digs deeply into wood to �nd large insects. Brown

creeper climbs up trunk looking for tiny insects.

Yellow-bellied sapsucker drills rows of holes and consumes sap and insects stuck in sap.

Evolution and Community Ecology 135

▶ Fundamental and Realized Niche The zebra mussel’s total exclusion of its competitors in Lake St. Clair is unusual. Usually, neither compet-ing species fully excludes the other. Instead, competing species tend to adjust to each other, minimizing competition. Individuals can do this by changing their behavior or using only a portion of the resources they are capable of using. In such cases, individuals do not fulfill their entire niche, as shown in Figure 8. The full niche of a species is called its fundamental niche. A niche restricted by competition is called a realized niche.

▶ Resource Partitioning Over time, competing species may evolve to occupy only their realized niches. In this way, they adapt to competition by using slightly different resources or their shared resources in differ-ent ways. If two bird species eat the same type of seeds, for example, one might come to specialize on larger seeds and the other to specialize on smaller seeds. Or one bird may become more active in the morning and the other more active in the evening. This process is called resource partitioning because the species parti-tion, or divide, the resource they use in common by specializing in different ways, as shown in Figure 9.

▶ Character Displacement Sometimes, resource partitioning can lead to the evolution of physical char-acteristics among the competing species that reflect their specialized role in the environment. Ecologists call this character displacement. Through increased differences, two species can reduce competition. For example, through natural selection, birds that special-ize in eating larger seeds may evolve larger bills that enable them to make the best use of the resource. Similarly, birds specializing in eating smaller seeds may evolve smaller bills. This is precisely what exten-sive research has revealed about the finches from the Galàpagos Islands that were first described by Charles Darwin.

ReadingCheckpoint

How does resource partitioning affect competition between species?

Figure 9 resource Partitioning When species compete, they tend to divide resources. Many types of birds—including the woodpeckers, creeper, and nuthatch shown here—feed on insects from tree trunks. By each specializing in particular insects on particular parts of the tree, the birds minimize competition.

Figure 8 Fundamental and realized Niche (a) Without competitors, an organism can use its entire fundamental niche. (b) Competitors, however, limit an organism to a realized niche. The realized niche represents only a portion of what an organism can do and what resources it can use.

ANSWERS

Reading Checkpoint Resource partitioning decreases competition between species.

Wolf and Moose Populations on Isle Royale, Michigan

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Data from Isle Royale Wolf Project

136 Lesson 2

Predation, Parasitism, and Herbivory

Predation, parasitism, and herbivory are interactions in which one species benefits, while the other is harmed.

When organisms interact, they can affect each other in different ways. Throughout this lesson, the symbols “+”, “–”, and “0” are used to indicate how each interaction affects the success of the organisms involved. A “0” symbolizes a relationship in which there is no effect or the effect is neu-tral. Competition is a (–/–) relationship, because there is a negative effect on both organisms as each takes resources the other could have used. Other types of interactions are beneficial for one participant, but harmful for the other (+/–). Three examples of this type of species interaction are predation, parasitism, and herbivory.

Predation Every living thing needs food, and for many animals that means eating other living organisms. Predation is the process by which an individual of one species, a predator, hunts, captures, kills, and con-sumes an individual of another species, the prey. Interactions between predators and prey influence community structure by helping determine the relative numbers of predators and prey.

▶ Population Cycles Predation can sometimes cause cycles in popula-tion sizes. An increase in the population size of prey creates more food for predators, which may survive and reproduce more effectively as a result. As the predator population rises, additional predation drives down the population of prey. Less prey can then cause some predators to starve, so that the predator population declines. This allows the prey population to begin rising again, restarting the cycle. Most natural systems involve so many factors that such cycles don’t last long, but in some cases, like the one shown in Figure 10, cycles can continue for a long time.

ReadingCheckpoint

Describe how predator and prey populations can affect each other.

Figure 10 Predator-Prey Cycles Predator-prey systems sometimes show paired cycles. In this system, the number of moose is affected by the number of wolves and vice versa. Notice, too, that the wolf population was also affected by a disease outbreak.

How do organisms affect one an-other’s survival and environment? Perspective After students have read about predation, have them work in pairs. Ask one member of the pair to describe a scenario involving a change in a predator population. For example, the predator popula-tion is struck by disease. His or her partner should then predict how this would impact the prey population. Have students trade roles and repeat the activity, this time focusing on a change in the prey population.

BIG QUESTION

ANSWERS

Reading Checkpoint An increase in a prey population can create more food for predators, which can then increase the predator population. As the predator population increases, the population of prey decreases. Decreases in the number of prey can result in a decline in the predator population.

Find OutMore

Find OutMore

Evolution and Community Ecology 137

▶ Predation and Evolution Individual predators that are better at capturing prey will likely be more successful than less skilled predators. Thus, natural selection often leads to the evolution of adaptations that enable predators to be better hunters. Prey face an even stronger selective pressure—the risk of immediate death. In response to these pressures, prey organisms have evolved an elaborate array of defenses against being eaten, as shown in Figure 11.

▶ Coevolution and Evolutionary “Arms Races” Some predator–prey relationships are examples of coevolution. Coevolution is the process by which two species evolve in response to changes in each other. A change in one species, therefore, is usually followed by a change in the other. A newt, for example, might evolve toxins that kill animals that prey on it. Some predators, however, might evolve immunity to the toxins. Then, in turn, the newts evolve stronger toxins. Ecologists refer to this kind of coevolution as an arms race. Each species develops stronger and stronger “weapons” in response to the other.

Ecologists think that the rough-skinned newt, found on the west coast of the United States, and its predator, the common garter snake, have been locked in an arms race for a long time. Shown in Figure 12, the newt is one of the most poisonous animals in nature. A single newt has enough poison to kill more than 100 people. However, the common garter snake can still eat them! In this arms race, the newt’s weapon is its toxin, and the predator’s weapon is resistance to the toxin. As the newts evolved stronger toxins, the snakes evolved stronger resistance. It is important to remember, however, that natural selection does not pro-duce adaptation for a purpose. The newts did not evolve toxins to kill the snakes, and the snakes did not evolve immunity to respond to the newts. Rather, these adaptations enabled the newts and snakes to better survive and reproduce in their environments.

Figure 11 Prey Defenses Natural selection to avoid predation has resulted in many dramatic adaptations. (a) Some prey hide from predators by camouflage, such as this leafy seadragon in the seaweed. (b) Other prey are brightly colored to warn predators that they are toxic or distasteful, such as this poisonous redtail coral snake. (c) Still others fool predators with mimicry—imitation of something else. This katydid does an excellent leaf impression.

Figure 12 Fighting an Arms race The rough-skinned newt has evolved incredible toxicity. Yet, the common garter snake can still make a meal out of it.

Have you heard about antibiotic resistance in the news? It’s the result of an evolutionary arms race: bacteria vs. antibiotics. Look on the Internet or ask a doctor about antibiotic resistance in your area.

(a) (b) (c)

ANSWERS

Find Out More Students’ responses should indicate that they have researched antibiotic resistance using reliable sources.

Figure 13 Parasitism Unfortunately for this western long-eared myotis bat, ear mites get everything they need from their host.

Figure 14 Herbivory Herbivores like this giraffe get all of their nutrients and energy from plants.

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Parasitism Parasitism is a relationship in which one organism, the parasite, depends on another, the host, for nourishment or some other benefit. In the process, the host is harmed. However, unlike predation, parasitism usually does not result in an organism’s immediate death. Examples of parasites include tapeworms that live in the digestive tract of their host and ticks that attach themselves to their host’s skin.

A close, long-term association between organisms—such as between a tapeworm and its host—is called symbiosis, liter-ally “living together.” Ecologists debate the exact definition of symbiosis. Some argue that symbiosis is any long-lasting relationship between species, regardless of distance. Further, some ecologists think that both organisms must benefit for a relationship to be considered symbiotic. Here, however, we define symbiosis as a long-lasting and physically close rela-tionship in which at least one organism benefits.

Herbivory The interaction in which an animal feeds on a plant is called herbivory. Insects that feed on plants are the most common type of herbivore. In most cases, her-bivory does not kill a plant directly but may affect its growth and reproduction. Because of natural selection, plants have evolved a wide array of defenses against the animals that feed on them. Many plants produce chemicals that are toxic or dis-tasteful to herbivores. Others have thorns, spines, or irritating hairs. In response, herbivores may evolve ways to overcome these defenses.

Figure 16 A Perfect Fit Darwin’s hawk moth (Xanthopan morganii) and the comet orchid are an example of species that coevolved through a mutualistic relationship.

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Mutualism and Commensalism Mutualism and commensalism are relationships in which

neither participant is harmed.

Not every interaction between species results in one being harmed. Instead, both species can benefit. Or, one species can benefit without affecting the other one at all.

Mutualism A relationship in which two or more species benefit is called mutualism (+/+). Many mutualisms are symbiotic. For exam-ple, plant roots and some fungi together form symbiotic associations called mycorrhizae, shown in Figure 15. In these relationships, the plant provides energy and protection to the fungus, while the fungus assists the plant in absorbing nutrients from the soil.

Not all mutualists live in close proximity. One of the most impor-tant mutualisms, pollination, involves free-living organisms that may encounter each other only once in their lifetimes. Bees, birds, bats, and other creatures transfer pollen (male sex cells) from one flower to another, fertilizing eggs that become embryos within seeds. Most pollinating animals visit flowers for their nectar, a reward the plant uses to attract them. The pollinators receive food, and the plants are pollinated and reproduce. Some plant-pollinator relationships are so close that they are examples of coevolving species.

One classic example of coevolution, shown in Figure 16, is that of orchid flowers and the African moths that pollinate them. The proboscis of the Darwin’s hawk moth is an impressive 20–35 cm (8–14 in.) long—just long enough to reach the nectar at the bottom of the comet orchid’s flower. Although not discovered and described until 1903, Darwin predicted the existence of the moth when he was shown the comet orchid in 1862. If such a flower exists, Darwin reasoned, so must an organism that pollinates it. There are many such seemingly perfect pairs of pollinators and plants in nature, each resulting from mutualistic, coevolutionary relationships.

ReadingCheckpoint

How do mycorrhizae fit the definition of a symbiotic relationship?

Figure 15 Mutualism Together, the hyphae of a fungus (white) and the roots of a strawberry tree (brown) form a mutualistic association known as a mycorrhiza. Both the fungus and the plant benefit from their association.

ANSWERS

reading Checkpoint Mycorrhizae are long-lasting, physically close rela-tionships between plants and fungi in which both organisms benefit.

Commensalism

Competition

Herbivory

Mutualism

Parasitism

Predation

Interaction E�ect on Species A E�ect on Species B

140 Lesson 2

21. Relate Cause and Effect Explain how competition

can affect an organism’s niche.2. Compare and Contrast How are predation,

parasitism, and herbivory similar? How are they different?

3. Apply Concepts The human digestive tract is filled with bacteria. The bacteria live in the body and get nutrients while helping to digest food. What kind of species interaction is this—mutualism or com-mensalism? Is it symbiotic? Explain.

4. Explore the BIGQUESTION Copy the chart below into your notebook. For each species interaction, indicate whether it has a positive (+), negative (–), or neutral (0) effect on each species.

Commensalism Commensalism, describes a relationship in which one species benefits and the other is unaffected (+/0). For instance, palo verde trees in the American Southwest’s Sonoran Desert create shade and leaf litter that allow the soil beneath them to hold moisture. The area around palo verde trees becomes cooler and moister than the surround-ing ground, making it easier for young plants to germinate and grow. Seedling cacti and other desert plants generally grow up directly beneath “nurse” trees such as palo verde, as shown in Figure 17.

FiguRE 17 Commensalism Plants grow in the shady, moist, and nutrient-rich area beneath the palo verde “nurse” tree in the Sonoran Desert.

ANSWERS

Lesson 2 Assessment

1. Competition can cause species to divide common resources and spe-cialize in using only part of their possible resources. This results in a species occupying only a smaller, realized niche, not its entire funda-mental niche.

2. They all result in benefit to one species and harm to another. Predation involves one organ-ism hunting, killing, and eating another. In parasitism, a parasite depends on a host organism for some benefit, and harms the host in the process. In herbivory, the organism harmed is the plant that is eaten.

3. Mutualism; It is symbiotic because it is a long-lasting, physically close relationship in which both organ-isms benefit.

4. Commensalism (+/0); competition (–/–); herbivory (+/–); mutualism (+/+); parsitism (+/–); predation (+/–). (Note that answers may be reversed and still be correct.)