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Community 54

Figure 54.1 Which benefits from this interaction?

Key ConCepts in Motion At first glance, you might think the situation looks dire for this bluestreak cleaner 54.1 interactions are classified by whether they help, wrasse, which has ventured into the mouth of the giant moray eel, a voracious preda- harm, or have no effect on the tor in its (Figure 54.1). With one snap of its jaw, the eel could easily species involved crush the fish and it. However, the wrasse is not in danger of becoming this 54.2 Diversity and trophic eel’s dinner. The much larger remains still, with its mouth open, and allows characterize biological the smaller fish free passage as it picks out and eats tiny parasites living inside the eel’s communities mouth and on its skin. 54.3 influences species In this interaction, both benefit: The cleaner wrasse gains access to a diversity and composition supply of , and its moray eel client is freed of parasites that might weaken it or spread disease. There are many other examples of such mutually beneficial “cleaner” 54.4 Biogeographic factors affect community diversity and “client” relationships in marine , such as the cleaner shrimp and eel interaction shown below. However, other interactions between species are less benign 54.5 Pathogens alter community for one of the participants, and still other interactions can negatively affect the repro- structure locally and globally duction and survival of both species involved. In Chapter 53, you learned how individuals within a can affect other individuals of the same species. This chapter will examine ecological interactions between of different species. A group of populations of different spe- cies living in close enough proximity to interact is called a biological community.

When you see this blue icon, log in to MasteringBiology Get Ready for This Chapter and go to the Study Area for digital . Ecologists define the boundaries of a particular community supply on ; most terrestrial species use this but to fit their research questions: They might study the com- do not usually compete for it. munity of and other organisms living on a rotting log, the benthic community in Superior, or the Competitive Exclusion community of and in Sequoia National Park in What happens in a community when two species compete for California. limited resources? In 1934, Russian ecologist G. F. Gause stud- We begin this chapter by exploring the kinds of interac- ied this question using laboratory with two closely tions that occur between species in a community, such as related ciliate species, Paramecium aurelia and Paramecium the cleaner wrasse and eel in Figure 54.1. We’ll then consider caudatum (see Figure 28.17a). He cultured the species under several of the factors that are most significant in structuring stable conditions, adding a constant amount of food each day. a community—in determining how many species there are, When Gause grew the two species separately, each population which particular species are present, and the relative abun- increased rapidly in number and then leveled off at the appar- dance of these species. Finally, we’ll apply some of the prin- ent of the culture (see Figure 53.11a for an ciples of community ecology to the study of disease. illustration of the logistic growth of a Paramecium population). But when Gause grew the two species together, P. caudatum ConCept 54.1 became extinct in the culture. Gause inferred that P. aurelia had a competitive edge in obtaining food. More generally, his Community interactions results led him to conclude that two species competing for the are classified by whether they same limiting resources cannot coexist permanently in the help, harm, or have no effect same place. In the absence of disturbance, one species will use the resources more efficiently and reproduce more rapidly than on the species involved the other. Even a slight reproductive advantage will eventually Some key relationships in the of an are its inter- lead to local elimination of the inferior competitor, an outcome actions with individuals of other species in the community. called competitive exclusion. These interspecific interactions include , , herbivory, , , and commen- Ecological Niches and salism. In this section, we’ll define and describe each of these Competition for limited resources can cause interactions, grouping them according to whether they have evolutionary change in populations. One way to examine positive (+) or negative (-) effects on the survival and repro- how this occurs is to focus on an organism’s ecological duction of each of the two species engaged in the interaction. niche, the specific set of biotic and abiotic resources that For example, predation is a +/- interaction, with a positive an organism uses in its environment. The niche of a tropical effect on the predator population and a negative effect on the lizard, for instance, includes the range it prey population. Mutualism is a +/+ interaction because the tolerates, the size of branches on which it perches, the survival and of both species are increased in the of day when it is active, and the sizes and kinds of insects presence of the other. A 0 indicates that a species is not affected it eats. Such factors define the lizard’s niche, or ecological by the interaction in any known way. We’ll consider three role—how it fits into an . broad categories of ecological interactions: competition (-/-), We can use the niche concept to restate the principle of exploitation (+/-), and positive interactions (+/+ or +/0). competitive exclusion: Two species cannot coexist perma- Historically, most ecological research has focused on inter- nently in a community if their niches are identical. However, actions that have a negative effect on at least one species, such ecologically similar species can coexist in a community if one as competition and predation. As we’ll see, however, positive or more significant differences in their niches arise through interactions are ubiquitous and have major effects on commu- time. Evolution by natural selection can result in one of the nity structure. species using a different set of resources or similar resources at different of the day or year. The differentiation of niches that enables similar species to coexist in a community Competition is a -/- interaction that occurs when indi- is called resource partitioning (Figure 54.2). viduals of different species compete for a resource that limits As a result of competition, a species’ fundamental niche, the survival and reproduction of each species. Weeds grow- which is the niche potentially occupied by that species, is ing in a garden compete with garden for often different from its realized niche, the portion of its fun- and . Lynx and in the northern of Alaska damental niche that it actually occupies. Ecologists can iden- and Canada compete for prey such as snowshoe hares. In tify the fundamental niche of a species by testing the range contrast, some resources, such as , are rarely in short of conditions in which it grows and reproduces in the absence

chApter 54 Community Ecology 1213 Figure 54.2 Resource partitioning among Dominican Figure 54.3 Republic lizards. Seven species of lizards live in close proximity, inquiry Can a species’ niche be influenced and all feed on insects and other small . However, competition for food is reduced because each lizard species has a different preferred by interspecific competition? perch, thus occupying a distinct niche. Ecologist Joseph Connell studied two barnacle species— A. distichus perches A. insolitus usually perches Chthamalus stellatus and Balanus balanoides—that have a stratified on fence posts and on shady branches. distribution on rocks along the of Scotland. Chthamalus is usually other sunny surfaces. found higher on the rocks than Balanus. To determine whether the distribution of Chthamalus is the result of interspecific competition with Balanus, Connell removed Balanus from the rocks at several sites.

High Chthamalus A. ricordii Balanus Chthamalus realized niche

Balanus realized niche

A. insolitus Low tide A. aliniger A. christophei A. distichus Results Chthamalus spread into the A. cybotes formerly occupied by Balanus. High tide A. etheridgei

Chthamalus fundamental niche of competitors. They can also test whether a potential competi- tor limits a species’ realized niche by removing the competitor and seeing if the first species expands into the newly available Ocean Low tide . The classic experiment depicted in Figure 54.3 clearly showed that competition between two barnacle species kept Conclusion Interspecific competition makes the realized niche of one species from occupying part of its fundamental niche. Chthamalus much smaller than its fundamental niche. Species can partition their niches not just in space, as Data from J. H. Connell, The influence of interspecific competition and other factors lizards and barnacles do, but in time as well. The common on the distribution of the barnacle Chthamalus stellatus, Ecology 42:710–723 (1961). spiny (Acomys cahirinus) and the golden spiny mouse Instructors: A related experimental inquiry tutorial (A. russatus) live in rocky habitats of the Middle East and can be assigned in MasteringBiology. , sharing similar microhabitats and food sources. Where WHAt iF? Other observations showed that Balanus cannot survive high on they coexist, A. cahirinus is nocturnal (active at night), while the rocks because it dries out during low . How would Balanus’s realized niche compare with its fundamental niche? A. russatus is diurnal (active during the day). Surprisingly, laboratory research showed that A. russatus is naturally noctur- nal. To be active during the day, it must override its biological clock in the presence of A. cahirinus. When researchers in Israel removed all A. cahirinus individuals from a site in the species’ Closely related species whose populations are sometimes natural habitat, A. russatus individuals at that site became allopatric (geographically separate; see Concept 24.2) and nocturnal, consistent with the laboratory results. This change sometimes sympatric (geographically overlapping) pro- in behavior suggests vide more evidence for the importance of competition that competition exists in structuring communities. In some cases, the allopatric between the species and populations of such species are morphologically similar and that partitioning of their use similar resources. By contrast, sympatric populations, active time helps them which would potentially compete for resources, show dif- coexist. ferences in body and in the resources they use. This tendency for characteristics to diverge more in sym- patric than in allopatric populations of two species is called the golden spiny mouse character displacement. An example of character dis- (Acomys russatus) placement can be seen in two species of Galápagos finches,

1214 Unit eight Ecology Figure 54.4 Character displacement: indirect evidence of past competition. Allopatric populations of Geospiza fuliginosa SCIEnTIfIC SkIllS ExERCISE and Geospiza fortis on Los Hermanos and Daphne Islands have similar beak morphologies (top two graphs) and presumably eat similarly sized . However, where the two species are sympatric on Floreana and Making a Bar Graph San Cristóbal, G. fuliginosa has a shallower, smaller beak and G. fortis a deeper, larger one (bottom graph), that favor eating and a Scatter Plot different-sized seeds. Can a Native Predator Species Adapt Rapidly to an Introduced Prey Species? Cane toads (Bufo marinus) were G. fuliginosa G. fortis introduced to in 1935 in a failed attempt to control an insect pest. Since then, the toads have spread across northeastern Beak Australia, with a population of over 200 million today. Cane toads depth have glands that produce a toxin that is poisonous to snakes and other potential predators. In this exercise, you will graph and inter- pret data from a two-part experiment conducted to determine 60 Los Hermanos whether native Australian predators have developed to the cane toad toxin. 40 G. fuliginosa, 20 allopatric How the Experiment Was Done In part 1, researchers collected 12 red-bellied black snakes (Pseudechis porphyriacus) from areas 0 where cane toads had existed for 40–60 years and another 12 from areas free of cane toads. They recorded the percentage of snakes from 60 Daphne each area that ate either a freshly killed native (Limnodynastes 40 peronii, a species the snakes commonly eat) or a freshly killed cane G. fortis, 20 toad from which the toxin gland had been removed (making the allopatric toad nonpoisonous). In part 2, researchers collected snakes from areas 0 where cane toads had been present for 5–60 years. To assess how cane toad toxin affected the physiological activity of these snakes, they 60 Floreana, San Cristóbal Sympatric populations injected small amounts of the toxin into the snakes’ stomachs and 40 measured the snakes’ swimming speed in a small pool. 20 Data from the Experiment, Part 1 Percentages of individuals in each size class Percentages 0 % of Snakes From Each Area That Ate Each Type of Prey 8 10 12 14 16 Type Beak depth (mm) of Prey Area with Cane Toads Area with Offered Present for 40–60 Years No Cane Toads inteRpRet tHe DAtA If the beak length of G. fortis is typically 12% Native frog 100 100 longer than the beak depth, what is the predicted beak length of G. fortis Cane toad 0 50 individuals with the smallest beak depths observed on Floreana and San Cristóbal Islands? Data from the Experiment, Part 2 Number of Years 5 10 10 20 50 60 60 60 60 60 Geospiza fuliginosa and Geospiza fortis: Beak depths in these Cane Toads Had Been Present in the Area species are similar in allopatric populations but have diverged % Reduction in Snake 52 19 30 30 5 5 9 11 12 22 (Figure 54.4) considerably in sympatric populations . Swimming Speed

Data from B. L. Phillips and R. Shine, An induces rapid adaptive Exploitation change in a native predator: cane toads and black snakes in Australia, Proceedings of the Royal B 273:1545–1550 (2006). All nonphotosynthetic organisms must eat, and all - isms are at risk of being eaten. As a result, much of the drama INtERPREt tHE DAtA in involves exploitation, a general term for any +/- 1. Make a bar graph of the data in part 1. (For additional informa- tion about graphs, see the Scientific Skills Review in Appendix F interaction in which one species benefits by feeding on the and in the Study Area in Mastering .) other species, which is harmed by the interaction. Exploitative 2. What do the data represented in the graph suggest about the interactions include predation, herbivory, and parasitism. effects of cane toads on the predatory behavior of red-bellied black snakes in areas where the toads are and are not currently found? Predation 3. Suppose a novel that deactivates the cane toad toxin evolved in a snake population exposed to cane toads. If the Predation is a +/- interaction between species in which researchers repeated part 1 of this study, predict how the results one species, the predator, kills and eats the other, the prey. would change. Though the term predation generally elicits such images as 4. Identify the dependent and independent variables in part 2 a lion attacking and eating an antelope, it applies to a wide and make a scatter plot. What conclusion would you draw about whether exposure to cane toads is having a selective range of interactions. A rotifer (a tiny aquatic animal that is effect on the snakes? Explain. smaller than many unicellular ) that kills a by 5. Explain why a bar graph is appropriate for presenting the data eating it can also be considered a predator. Because eating and in part 1 and a scatter plot is appropriate for the data in part 2. avoiding being eaten are prerequisites to reproductive success, Instructors: A version of this Scientific Skills Exercise can be the adaptations of both predators and prey tend to be refined assigned in MasteringBiology. through natural selection. In the scientific skills exercise, you can interpret data on the impact of natural selection for a Just as predators possess adaptations for capturing prey, specific predator-prey interaction. potential prey have adaptations that help them avoid being Many important feeding adaptations of predators are eaten. In , these adaptations include behavioral obvious and familiar. Most predators have acute senses that defenses such as hiding, fleeing, and forming or schools. enable them to find and identify potential prey. Rattlesnakes Active self-defense is less common, though some large and other pit vipers, for example, find their prey with a pair mammals vigorously defend their young from predators. of heat-sensing organs located between their eyes and nostrils Animals also a variety of morphological and (see Figure 50.7b). have characteristically large eyes that physiological defensive adaptations. Mechanical or chemi- help them see prey at night. Many predators also have adap- cal defenses protect species such as porcupines and tations such as claws, fangs, or that help them catch (Figure 54.5a and b). Some animals, such as the European and subdue their food. Predators that pursue their prey are , can synthesize toxins; others accumu- generally fast and agile, whereas those that lie in ambush are late toxins passively from the plants they eat. Animals often disguised in their environments. with effective chemical defenses often exhibit bright

Figure 54.5 examples of defensive adaptations in animals.

(a) Mechanical (b) Chemical defense defense

▶ Porcupine ▶

(c) Aposematic (d) Cryptic coloration: coloration: warning coloration

▶ Poison ▶ Canyon dart frog tree frog

(e) Batesian : (f) Müllerian mimicry: A harmless species Two unpalatable mimics a harmful species mimic one. each other.

▲ Venomous green ▲ Yellow jacket parrot snake

◀ Nonvenomous hawkmoth larva ◀

MAKe ConneCtions Explain how natural selection could increase the resemblance of a harmless species to a distantly related harmful species. In addition to selection, what else could account for a harmless species resembling a closely related harmful species? (See Concept 22.2.)

1216 Unit eight Ecology aposematic coloration, or warning coloration, such as movement of more than a dozen marine animals, including that of poison dart (Figure 54.5c). Such coloration crabs, sea stars, sea snakes, fish, and stingrays. This octopus’s seems to be adaptive because predators often avoid brightly ability to mimic other animals enables it to approach prey— colored prey. Cryptic coloration, or camouflage, makes for example, imitating a crab to approach another crab and prey difficult to see (Figure 54.5d). eat it. The octopus also can defend itself from predators Some prey species are protected by their resemblance to through mimicry. When attacked by a damselfish, the octo- other species. For example, in , a palat- pus quickly mimics a banded sea snake, a known predator able or harmless species mimics an unpalatable or harmful of the damselfish. species to which it is not closely related. The larva of the Interview with Tracy langkilde: Studying the evolution of hawkmoth Hemeroplanes ornatus puffs up its head and thorax novel defensive adaptations in lizards (see the interview when disturbed, looking like the head of a small venomous before Chapter 52) snake (Figure 54.5e). In this case, the mimicry even involves Herbivory behavior; the larva weaves its head back and forth and hisses like a snake. Such cases of Batesian mimicry are thought to Ecologists use the term herbivory to refer to a exploitative result from natural selection, as individuals in the harmless (+/-) interaction in which an organism—an —eats species that happen to more closely resemble the harmful parts of a or alga, thereby harming it. While large mam- one are avoided by predators who have learned not to eat the malian such as cattle, sheep, and water buffalo harmful ones. Over time, closer and closer resemblance to may be most familiar, most herbivores are actually inverte- the harmful species evolves. In Müllerian mimicry, two or brates, such as grasshoppers, caterpillars, and beetles. In the more unpalatable species, such as the cuckoo bee and yellow ocean, herbivores include sea urchins, some tropical fishes, jacket, resemble each other (Figure 54.5f). Presumably, the and certain mammals (Figure 54.7). more unpalatable prey there are, the faster predators learn to Like predators, herbivores have many specialized adapta- avoid prey with that particular appearance. Mimicry has also tions. Many herbivorous insects have chemical sensors on evolved in many predators. The mimic octopus Thaumoctopus their feet that enable them to distinguish between plants mimicus (Figure 54.6) can take on the appearance and based on their toxicity or nutritional value. Some mammalian herbivores, such as goats, use their sense of smell to examine plants, rejecting some and eating others. They may also eat Figure 54.6 the mimic octopus. (a) After hiding six of its just a specific part of a plant, such as the . Many herbi- tentacles in a hole in the seafloor, the octopus waves its other two tentacles to mimic a sea snake. (b) Flattening its body and arranging vores also have specialized teeth or digestive systems adapted its arms to trail behind, the octopus mimics a flounder (a flat fish). for processing (see Concept 41.4). (c) It can mimic a stingray by flattening most of its tentacles alongside Unlike animals, plants cannot run away to avoid being its body while allowing one tentacle to extend behind it. eaten. Instead, a plant’s arsenal against herbivores may (a) Mimicking a sea feature chemical toxins or structures such as spines and snake

Figure 54.7 An herbivorous marine mammal. This West Indian manatee (Trichechus manatus) in Florida is grazing on , an introduced plant species.

(b) Mimicking a flounder

(c) Mimicking a stingray

chApter 54 Community Ecology 1217 thorns. Among the plant compounds that serve as chemical Mutualism defenses are the poison strychnine, produced by the tropical Mutualism is an interspecific interaction that benefits vine Strychnos toxifera; nicotine, from the tobacco plant; and both species (+/+). Mutualisms are common in nature, as tannins, from a variety of plant species. Compounds that are illustrated by examples seen in previous chapters, including not toxic to but may be distasteful to many herbi- cellulose digestion by in the digestive sys- vores are responsible for the familiar flavors of cinnamon, tems of and ruminant mammals, animals that pol- cloves, and peppermint. Certain plants produce chemicals linate flowers or disperse seeds, exchange between that cause abnormal development in some insects that fungi and plant roots in mycorrhizae, and eat them. For more examples of how plants defend them- by unicellular in . In some mutualisms, such as selves, see Make Connections Figure 39.27, “Levels of Plant the acacia- example shown in Figure 54.8, each species Defenses Against Herbivores.” depends on the other for their survival and reproduction. In other mutualisms, however, both species can survive Parasitism on their own. Parasitism is a +/- exploitative interaction in which one Typically, both partners in a mutualism incur costs as organism, the parasite, derives its nourishment from well as benefits. In mycorrhizae, for example, the plant another organism, its , which is harmed in the process. Parasites that live within the body of their host, such as tape- worms, are called endoparasites; parasites that feed on the Figure 54.8 Mutualism between acacia trees and . external surface of a host, such as ticks and lice, are called ectoparasites. In one particular type of parasitism, para- sitoid insects—usually small —lay eggs on or in living hosts. The larvae then feed on the body of the host, eventu- ally killing it. Some ecologists have estimated that at least one-third of all species on are parasites. Many parasites have complex life cycles involving multi- ple hosts. The blood fluke, which currently infects approxi- mately 200 million people around the , requires two hosts at different times in its development: humans and freshwater snails (see Figure 33.11). Some parasites change the behavior of their current host in ways that increase the likelihood that the parasite will reach its next host. For instance, that are parasitized by acanthocepha- lan (spiny-headed) worms leave protective cover and move (a) Certain species of acacia trees in Central and have into the open, where they are more likely to be eaten by the hollow thorns (not shown) that house stinging ants of the birds that are the second host in the worm’s life cycle. Pseudomyrmex. feed on nectar produced by the tree and on -rich swellings (yellow in the photograph) at the tips of leaflets. Parasites can significantly affect the survival, reproduc- tion, and density of their host population, either directly or indirectly. For example, ticks that feed as ectoparasites on moose can weaken their hosts by withdrawing blood and causing hair breakage and loss. In their weakened condition, the moose have a greater chance of dying from cold stress or predation by wolves.

Positive Interactions While nature abounds with dramatic and gory examples of exploitative interactions, ecological communities are also heavily influenced by positive interactions, a term that refers to a +/+ or +/0 interaction in which at least one spe- cies benefits and neither is harmed. Positive interactions include mutualism and . As we’ll see, positive (b) The acacia benefits because the pugnacious ants, which attack any- interactions can affect the diversity of species found in an thing that touches the tree, remove fungal , small herbivores, ecological community. and debris. They also clip vegetation that grows close to the acacia.

1218 Unit eight Ecology often transfers to the , while the fun- Figure 54.10 Facilitation by black rush (Juncus gerardii) gus transfers limiting nutrients, such as , to the in new england salt . Black rush increases the number of plant species that can live in the upper middle zone of the . plant. Each partner benefits, but each partner also experi- ences a cost: It transfers materials that it could have used to support its own growth and . For an interaction to be considered a mutualism, the benefits to each partner 8 must exceed the costs. 6 Commensalism 4 An interaction between species that benefits one of the spe- cies but neither harms nor helps the other (+/0) is called 2 . Like mutualism, are

commensalism Number of plant species common in nature. For instance, many wildflower species 0 that grow optimally in low levels are only found in (a) with Juncus With Juncus Without Juncus shaded, floor environments. Such shade-tolerant (foreground) (b) “specialists” depend entirely on the trees that tower above them—the trees provide their dim habitat. Yet the survival and reproduction of the trees are not affected by these ecological interactions: Their effects can change over time. wildflowers. Thus, these species are involved in a +/0 In this case, an interaction whose effects are typically +/0 interaction in which the wildflowers benefit and the trees (commensalism) may at times become +/+ (mutualism). are not affected. Positive interactions can have significant influence on In another example of a commensalism, cattle egrets feed the structure of ecological communities. For instance, the on insects flushed out of the grass by grazing bison, cattle, black rush Juncus gerardii makes the soil more hospitable for horses, and other herbivores (Figure 54.9). Because the birds other plant species in some areas of New England salt marshes increase their feeding rates when following the herbivores, they (Figure 54.10a). Juncus helps prevent salt buildup in the soil clearly benefit from the association. Much of the time, the by shading the soil surface, which reduces evaporation. Juncus herbivores are not affected by the birds. At times, however, also prevents the salt marsh from becoming oxygen the herbivores too may derive some benefit; for example, the depleted as it transports oxygen to its belowground tissues. birds may remove and eat ticks and other ectoparasites from In one study, when Juncus was removed from areas in the their skin, or they may warn the herbivores of a predator’s upper middle , those areas supported 50% approach. This example illustrates another key point about fewer plant species (Figure 54.10b). Like positive interactions, competition and exploitation Figure 54.9 Commensalism between cattle egrets (predation, herbivory, and parasitism) also can strongly and African buffalo. affect the structure of ecological communities, as examples throughout the rest of this chapter will show.

ConCept CHeCK 54.1 1. explain how competition, predation, and mutualism differ in their effects on the interacting populations of two species. 2. According to the principle of competitive exclusion, what outcome is expected when two species with identical niches compete for a resource? Why? 3. MAKe ConneCtions Figure 24.14 illustrates how a hybrid zone can change over time. imagine that two finch species colonize a new island and are capable of hybridizing (mating and producing viable offspring). the island contains two plant species, one with large seeds and one with small seeds, growing in isolated habitats. if the two finch species specialize in eating different plant species, would reproductive barriers be reinforced, weak- ened, or unchanged in this hybrid zone? explain. For suggested answers, see Appendix A.

chApter 54 Community Ecology 1219 the four types of trees in community 1, but unless you looked ConCept 54.2 carefully, you might see only the abundant species A in the Diversity and trophic structure second forest. Most observers would intuitively describe com- characterize biological communities munity 1 as the more diverse of the two communities. Ecologists use many tools to compare the diversity of com- Ecological communities can be characterized by certain gen- munities across time and space. They often calculate indexes eral attributes, including how diverse they are and the feeding of diversity based on and relative . relationships of their species. In some cases, as you’ll read, a One widely used index is Shannon diversity (H): few species exert strong control on a community’s structure, H = -( pA ln pA + pB ln pB + pC ln pC + c) particularly on the composition, relative abundance, and diversity of its species. where A, B, C . . . are the species in the community, p is the relative abundance of each species, and ln is the natural logarithm; the ln of each value of p can be determined using the “ln” key on a scientific calculator. A higher value of H The species diversity of a community—the variety of dif- indicates a more diverse community. Let’s use this equation ferent kinds of organisms that make up the community—has to calculate the Shannon of the two com- two components. One is species richness, the number of munities in Figure 54.11. For community 1, p = 0.25 for different species in the community. The other is the relative each species, so abundance of the different species, the proportion each H = -4(0.25 ln 0.25) = 1.39. species represents of all individuals in the community. Imagine two small forest communities, each with 100 For community 2, individuals distributed among four tree species (A, B, C, H = -[0.8 ln 0.8 + 2(0.05 ln 0.05) + 0.1 ln 0.1] = 0.71. and D) as follows: These calculations confirm our intuitive description of Community 1: 25A, 25B, 25C, 25D community 1 as more diverse. Community 2: 80A, 5B, 5C, 10D Determining the number and relative The species richness is the same for both communities because abundance of species in a community can two samples, they both contain four species of trees, but the relative abun- be challenging. Because most species in a one species dance is very different (Figure 54.11). You would easily notice community are relatively rare, it may be hard to obtain a sample size large enough to be representative. It can also be dif- ficult to identify some of the species in Figure 54.11 Which forest is more diverse? Ecologists the community. If an unknown organism would say that community 1 has greater species diversity, a measure that includes both species richness and relative abundance. cannot be identified on the basis of mor- phology alone, it is useful to compare all or ABCD part of its genome to a reference database of DNA sequences from known organisms. For example, although the two samples of red algae shown here might appear to be two different species, comparing their sequences of a short standardized sec- tion of DNA—a DNA “barcode”—to a reference database shows that they Community 1 belong to the same species. Researchers A: 25% B: 25% C: 25% D: 25% are increasingly using DNA sequencing for species as it becomes cheaper and as DNA sequences from more organisms are added to comparative databases. It can also be difficult to census the highly mobile or less visible members of communities, such as microorganisms, deep-sea creatures, and nocturnal species. The small size of microorganisms makes them particularly difficult to sample, Community 2 so ecologists now commonly use molecular tools to help A: 80% B: 5% C: 5% D: 10% determine microbial diversity (Figure 54.12).

1220 Unit eight Ecology Figure 54.12 Research Method Determining Microbial Diversity Using Molecular Tools

Application Ecologists are increasingly using molecular tech- niques to determine microbial diversity and richness in environmental samples. One such technique produces a DNA profile for microbial Figure 54.13 study plots taxa based on sequence variations in the DNA that encodes the small at the Cedar Creek ecosystem subunit of ribosomal RNA. Noah Fierer and Rob Jackson, then of Duke science Reserve, site of long-term University, used this method to compare the diversity of soil in 98 habitats across North and South America to help identify environ- experiments on manipulating plant mental variables associated with high bacterial diversity. diversity. technique Researchers first extract and purify DNA from the microbial community in each sample. They use the polymerase chain reaction (PCR; see Figure 20.8) to amplify the ribosomal DNA and Diversity and Community Stability label it with a fluorescent dye. Restriction In addition to measuring species diversity, ecologists manipu- then cut the amplified, labeled DNA into fragments of different lengths, late diversity in experimental communities in nature and which are separated by gel electrophoresis. in the laboratory. Many experiments examine the potential (A gel is shown here; see also Figures 20.6 benefits of diversity, including increased and and 20.7.) The number and abundance of these fragments characterize the DNA pro- stability of biological communities. file of the sample. Based on their analysis, Researchers at the Cedar Creek Ecosystem Science Fierer and Jackson calculated the Shannon Reserve, in Minnesota, have been manipulating plant diversity (H) of each sample. They then looked for a correlation between H and diversity in experimental communities for more than two several environmental variables, including decades (Figure 54.13). Higher-diversity communities gen- vegetation type, mean annual temperature erally are more productive and are better able to withstand and rainfall, and soil acidity. and recover from environmental stresses, such as . More diverse communities are also more stable year to year Results The diversity of bacterial communities was related almost exclusively to soil pH, with the Shannon diversity being highest in in their productivity. In one decade-long experiment, for neutral soils and lowest in acidic soils. Amazonian forests, which instance, researchers at Cedar Creek created 168 plots, each have extremely high plant and animal diversity, had the most acidic containing 1, 2, 4, 8, or 16 perennial species. The soils and the lowest bacterial diversity of the samples tested. most diverse plots consistently produced more (the total mass of all organisms in a habitat) than the single- species plots each year. 3.6 Higher-diversity communities are often more resistant to invasive species, which are organisms that become estab- 3.4 lished outside their native range. working in Long

) Island Sound, off the coast of Connecticut, created communi- H 3.2 ties of different levels of diversity consisting of sessile marine , including tunicates (see Figure 34.5). They then 3.0 examined how vulnerable these experimental communities were to invasion by an exotic tunicate. They found that the 2.8 exotic tunicate was four times more likely to survive in lower-

Shannon diversity ( diversity communities than in higher-diversity ones. The 2.6 researchers concluded that relatively diverse communities captured more of the resources available in the system, leav- 2.4 ing fewer resources for the invader and decreasing its survival.

2.2 Trophic Structure 3456789 Soil pH In addition to species diversity, the structure and dynamics of a community also depend on the feeding relationships between organisms—the trophic structure of the community. The Data from N. Fierer and R. B. Jackson, The diversity and of soil bacterial communities, Proceedings of the National Academy of Sciences USA transfer of food upward from its source in plants and 103:626–631 (2006). other (primary producers) through herbivores (primary consumers) to (secondary, tertiary, and

chApter 54 Community Ecology 1221 Figure 54.14 examples of terrestrial and marine food Figure 54.15 An Antarctic marine . Arrows follow chains. The arrows trace the transfer of food through the trophic levels the transfer of food from the producers () up through of a community when organisms feed on one another. Decomposers, the trophic levels. For simplicity, this diagram omits decomposers. At which “feed” on the remains of organisms from all trophic levels, various times over the last two centuries, humans have also played a are not shown here. role in the Antarctic food web as consumers of fish, krill, and .

Quaternary consumers whales

Carnivore Smaller Baleen toothed Tertiary whales consumers whales

Carnivore Carnivore Elephant seals Crabeater Leopard Secondary seals seals consumers

Carnivore Carnivore

Fishes Squids Primary Birds consumers

Herbivore

Carnivorous Primary producers

Copepods Plant Phytoplankton Krill A terrestrial A marine food chain visuAl sKills Suppose the abundance of carnivores that eat zooplankton increased greatly. Use this diagram to infer how that might affect phytoplankton abundance. Phyto- plankton quaternary consumers) and eventually to decomposers is referred to as a food chain (Figure 54.14). The position an visuAl sKills For each organism in this food web, indicate the number of other kinds of organism that it eats. Which two organisms are both predator organism occupies in a food chain is called its . and prey for each other?

Food Webs figure Walkthrough A food chain is not an isolated unit, separate from other feed- ing relationships in a community. Instead, a group of food zooplankton, are another important link in these food webs, chains are linked together to form a food web. Ecologists as they are in turn eaten by seals and toothed whales. diagram the trophic relationships of a community using How are food chains linked into food webs? A given spe- arrows that link species according to who eats whom. In cies may weave into the web at more than one trophic level. an Antarctic pelagic community, for example, the primary In the food web shown in Figure 54.15, krill feed on phyto- producers are phytoplankton, which serve as food for the plankton as well as on other grazing zooplankton, such as dominant grazing zooplankton, especially krill and cope- . Such “nonexclusive” consumers are also found in pods, both of which are crustaceans (Figure 54.15). These terrestrial communities. For instance, foxes are zooplankton species are in turn eaten by various carnivores, whose diet includes berries and other plant materials, her- including other plankton, , seals, fishes, and baleen bivores such as mice, and other predators, such as weasels. whales. Squids, which are carnivores that feed on fish and Humans are among the most versatile of omnivores.

1222 Unit eight Ecology Figure 54.16 partial food web for . Figure 54.17 test of the energetic hypothesis for the The sea nettle (Chrysaora quinquecirrha) and the juvenile striped bass restriction of food chain length. Researchers manipulated the (Morone saxatilis) are the main predators of fish larvae (the bay anchovy productivity of experimental tree-hole communities in Queensland, ( Anchoa mitchilli ), along with several other species). Note that sea nettles Australia, by providing litter input at three levels. Reducing are secondary consumers when they eat zooplankton, but tertiary energy input reduced food chain length, a result consistent with consumers when they eat fish larvae, which are themselves secondary the energetic hypothesis. consumers of zooplankton. 5 4 3 2 1 Sea Juvenile nettle striped bass links Number of trophic 0 1 1 High (control): Medium: 10 Low: 100 natural rate of natural rate natural rate litter fall Fish larvae Productivity

Fish eggs longer in habitats characterized by higher photosynthetic Zooplankton production, since the amount of energy stored in primary producers is greater than in habitats with lower photosyn- thetic production. Ecologists tested the energetic hypothesis in experiments that mimicked the “tree-hole” communities found in tropical Complicated food webs can be simplified in two ways for forests. Many trees have small branch scars that rot, form- easier study. First, species with similar trophic relationships ing holes in the tree trunk. These tree holes hold water and in a given community can be grouped into broad functional provide a habitat for tiny communities consisting of micro- groups. In Figure 54.15, more than 100 phytoplankton spe- organisms and insects that feed on leaf litter, as well as preda- cies are grouped as the primary producers in the food web. tory insects. Figure 54.17 shows the results of experiments A second way to simplify a food web is to isolate a portion of in which researchers manipulated productivity by varying the web that interacts very little with the rest of the community. the amount of leaf litter in an experiment using artificial tree Figure 54.16 illustrates a partial food web for sea nettles (a type holes (water-filled pots placed around the trees); previous of cnidarian) and juvenile striped bass in the Chesapeake Bay studies had shown that the communities that colonized these estuary on the Atlantic coast of the United States. pots were similar to those in natural tree holes. As predicted by the energetic hypothesis, the pots with the most leaf litter, Limits on Food Chain Length and hence the greatest total food supply at the producer level, supported the longest food chains. Each food chain within a food web is usually only a few links Another factor that may limit food chain length is that long. In the Antarctic web of Figure 54.15, there are rarely carnivores in a food chain tend to be larger at higher trophic more than seven links from the producers to any top-level levels. The size of a carnivore puts an upper limit on the size predator, and most chains in this web have fewer links. In of food it can take into its mouth. And except in a few cases, fact, most food webs studied to date have chains consisting large carnivores cannot live on very small food items because of five or fewer links. they cannot obtain enough food in a given time to meet their Why do food chains tend to be relatively short? The most metabolic needs. Among the exceptions are baleen whales, common explanation, known as the energetic hypothesis, huge filter feeders with adaptations that enable them to con- suggests that the length of a food chain is limited by the sume enormous quantities of krill and other small organisms inefficiency of energy transfer along the chain. On average, (see Figure 41.5). only about 10% of the energy stored in the organic of each trophic level is converted to organic matter at the next trophic level (see Concept 55.3). Thus, a producer level con- Species with a large Impact sisting of 100 kg of plant material can support about 10 kg of Certain species have an especially large impact on the struc- herbivore biomass and 1 kg of carnivore biomass. The ener- ture of entire communities because they are highly abundant getic hypothesis predicts that food chains should be relatively or play a pivotal role in community dynamics. The impact of

chApter 54 Community Ecology 1223 these species occurs through trophic interactions and their avoiding herbivory or the impact of disease. The latter idea influence on the physical environment. could explain the high biomass attained in some environ- Dominant species in a community are the species that ments by invasive species such as kudzu (see Figure 56.8). are the most abundant or that collectively have the highest Such species may not face the herbivores or parasites that biomass. There can be different explanations for why particu- would otherwise hold their populations in check. lar species become dominant. One hypothesis suggests that The impact of a dominant species can be discovered dominant species are competitively superior in exploiting when it is removed from the community. For example, the limited resources such as space, water, or nutrients. Another American chestnut was a dominant tree in deciduous forests hypothesis is that dominant species are most successful at of eastern before 1910, making up more than 40% of mature trees. Then humans accidentally introduced Figure 54.18 the fungal disease chestnut blight to New York via nurs- inquiry Is a ? ery stock imported from . Between 1910 and 1950, this fungus killed almost all of the chestnut trees in eastern North experiment In rocky intertidal communities of western North America. In this case, removing the dominant species had a America, the relatively uncommon sea star Pisaster ochraceus preys on relatively small impact on some species but large effects on such as Mytilus californianus, a dominant species and strong competitor for space. others. Oaks, hickories, beeches, and red maples that were already present in the forest increased in abundance and replaced the chestnuts. No mammals or birds seemed to have been harmed by the loss of the chestnut, but seven species of moths and butterflies that fed on the tree became extinct. In contrast to dominant species, keystone species are not usually abundant in a community. They exert strong control on community structure not by numerical might but by their pivotal ecological roles. Figure 54.18 highlights the importance of a keystone species, a sea star, in maintaining the diversity of an intertidal community. Still other organisms exert their influence on a commu- nity not through trophic interactions but by changing their physical environment. Species that dramatically alter their environment are called ecosystem engineers or, to avoid Robert Paine, of the University of Washington, removed Pisaster from an area in the intertidal zone and examined the effect on species implying conscious intent, “.” A familiar richness. is the (Figure 54.19). The effects of ecosystem engineers on other species can be positive or Results In the absence of Pisaster, species richness declined as mussels monopolized the rock face and eliminated most other inver- negative, depending on the needs of the other species. tebrates and algae. In a control area where Pisaster was not removed, species richness changed very little. HHMI Video: Some Animals Are More Equal than Others: keystone Species and Trophic Cascades

20 Figure 54.19 as ecosystem engineers. By felling 15 With Pisaster (control) trees, building , and creating , beavers can transform large areas of forest into flooded .

present 10

5 Without Pisaster (experimental) Number of species 0 1963 ’64 ’65 ’66 ’67 ’68 ’69 ’70 ’71 ’72 ’73 Year

Conclusion Pisaster acts as a keystone species, exerting an influence on the community that is not reflected in its abundance.

Data from R. T. Paine, Food web and species diversity, American Naturalist 100:65–75 (1966).

WHAt iF? Suppose that an invasive fungus killed most individuals of Mytilus at these sites. Predict how species richness would be affected if Pisaster were then removed.

1224 Unit eight Ecology Bottom-Up and Top-Down Controls Figure 54.20 Results of biomanipulation in a lake with top-down control of community . Decreasing the The ways in which adjacent trophic levels affect one another abundance of fish that ate zooplankton results in a decrease in the can be useful for describing community organization. Let’s biomass of algae, improving . consider the three possible relationships between plants Top-down Control Polluted State Restored State (V for vegetation) and herbivores (H): Fish Abundant Rare V S H V d H V 4 H The arrows indicate that a change in the biomass of one trophic level causes a change in the other trophic level. V S H means Zooplankton Rare Abundant that an increase in vegetation will increase the numbers or biomass of herbivores, but not vice versa. In this situation, her- Algae Abundant Rare bivores are limited by vegetation, but vegetation is not limited by herbivory. In contrast, V d H means that an increase in herbivore biomass will decrease the abundance of vegetation, but not vice versa. A double-headed arrow indicates that each and industrial wastewater until 1976. After controls trophic level is sensitive to changes in the biomass of the other. reduced these inputs, the water quality of the lake began to Two models of community organization are common: improve. By 1986, however, massive blooms of the bottom-up model and the top-down model. The V S H started to occur in the lake. These blooms coincided with an linkage suggests a bottom-up model, which postulates a increase in the population of roach, a fish species that eats unidirectional influence from lower to higher trophic levels. zooplankton, which otherwise keep the cyanobacteria and In this case, the presence or absence of nutrients algae in check. To reverse these changes, ecologists removed (N) controls plant (V) numbers, which control herbivore (H) nearly a million kilograms of fish from the lake between 1989 numbers, which in turn control predator (P) numbers. The and 1993, reducing roach abundance by about 80%. At the simplified bottom-up model is thus N S V S H S P. To same time, they added a fourth trophic level by stocking the change the community structure of a bottom-up community, lake with pike perch, a predatory fish Lake Vesijärvi, you need to alter biomass at the lower trophic levels, allowing that eats roach. The water became Finland those changes to propagate up through the food web. If you clear, and the last cyanobacte- add mineral nutrients to stimulate plant growth, then the rial bloom was in 1989. higher trophic levels should also increase in biomass. If you Ecologists continue to change predator abundance, however, the effect should not monitor the lake for evi- extend down to the lower trophic levels. dence of cyanobacterial In contrast, the top-down model postulates the oppo- blooms and low oxygen avail- site: Predation mainly controls community organization ability, but the lake has remained because predators limit herbivores, herbivores limit plants, clear, even though roach removal ended in 1993. and plants limit nutrient levels through nutrient uptake. As these examples show, communities vary in their degree The simplified top-down model, N d V d H d P, is also of bottom-up and top-down control. To manage agricultural called the model. In a lake community with , parks, reservoirs, and , we need to under- four trophic levels, the model predicts that removing the top stand each particular community’s dynamics. carnivores will increase the abundance of primary carnivores, in turn decreasing the number of herbivores, increasing phy- ConCept CHeCK 54.2 toplankton abundance, and decreasing concentrations of 1. What two components contribute to species diversity? mineral nutrients. The effects thus move down the trophic explain how two communities with the same number structure as alternating +/- effects. of species can differ in species diversity. Ecologists have applied the top-down model to improve 2. how is a food chain different from a food web? water quality in with a high abundance of algae. This 3. WHAt iF? consider a grassland with five trophic levels: approach, called biomanipulation, attempts to prevent grasses, mice, snakes, , and . if you released additional bobcats into the grassland, how algal blooms by altering the density of higher-level consumers. would grass biomass change if the bottom-up model In lakes with three trophic levels, removing fish should applied? if the top-down model applied? improve water quality by increasing zooplankton density, 4. MAKe ConneCtions rising atmospheric cO2 levels thereby decreasing algal populations (Figure 54.20). In lakes lead to ocean acidification (see Figure 3.12) and higher ocean , both of which can reduce krill abun- with four trophic levels, adding top predators should have dance. predict how a drop in krill abundance might affect the same effect. other organisms in the food web shown in Figure 54.15. Ecologists in Finland used biomanipulation to help purify Which organisms are particularly at risk? explain. Lake Vesijärvi, a large lake that was polluted with city sewage For suggested answers, see Appendix A.

chApter 54 Community Ecology 1225 some grassland require regular burning to maintain ConCept 54.3 their structure and species composition. Many and Disturbance influences species ponds are disturbed by seasonal flooding and drying. A high diversity and composition level of disturbance is generally the result of frequent and intense disturbance, while low disturbance levels can result Decades ago, most ecologists favored the traditional view from either a low frequency or low intensity of disturbance. that biological communities are at equilibrium, a more or less The intermediate disturbance hypothesis states that stable balance, unless seriously disturbed by human activities. moderate levels of disturbance foster greater species diver- The “” view focused on competition as a key sity than do high or low levels of disturbance. High levels factor determining community composition and maintain- of disturbance reduce diversity by creating environmental ing stability in communities. Stability in this context refers to stresses that exceed the tolerances of many species or by a community’s tendency to reach and maintain a relatively disturbing the community so often that slow-growing or constant composition of species. slow-colonizing species are excluded. At the other extreme, One of the earliest proponents of this view, F. E. Clements, low levels of disturbance can reduce species diversity by of the Carnegie Institution of Washington, argued in the early allowing competitively dominant species to exclude less com- 1900s that the community of plants at a site had only one petitive ones. Meanwhile, intermediate levels of disturbance stable equilibrium, a controlled solely by can foster greater species diversity by opening up habitats . According to Clements, biotic interactions caused the for occupation by less competitive species. Such intermedi- species in the community to as an integrated unit— ate disturbance levels rarely create conditions so severe that in effect, as a . His argument was based on the they exceed the environmental tolerances or recovery rates of observation that certain species of plants are consistently potential community members. found together, such as the oaks, maples, birches, and beeches The intermediate disturbance hypothesis is supported in deciduous forests of the northeastern United States. by many terrestrial and aquatic studies. In one study, ecolo- Other ecologists questioned whether most communi- gists in New Zealand compared the richness of invertebrates ties were at equilibrium or functioned as integrated units. living in the beds of streams exposed to different frequen- A. G. Tansley, of Oxford University, challenged the concept cies and intensities of flooding (Figure 54.21). When floods of a climax community, arguing that differences in soils, occurred either very frequently or rarely, rich- topography, and other factors created many potential com- ness was low. Frequent floods made it difficult for some munities that were stable within a region. H. A. Gleason, of species to become established in the streambed, while rare the University of , saw communities not as superor- floods resulted in species being displaced by superior com- ganisms but as chance assemblages of species found together petitors. Invertebrate richness peaked in streams that had because they happen to have similar abiotic requirements— an intermediate frequency or intensity of flooding, as pre- for example, for temperature, rainfall, and . Gleason dicted by the hypothesis. and other ecologists also realized that disturbance keeps Although moderate levels of disturbance appear to many communities from reaching a state of equilibrium in maximize species diversity in some cases, small and large species diversity or composition. A disturbance is an event, such as a storm, fire, flood, , or human activity, that Figure 54.21 testing the intermediate disturbance changes a community by removing organisms from it or hypothesis. Researchers identified the taxa (species or genera) altering resource availability. of invertebrates at two locations in each of 27 New Zealand streams. They assessed the intensity of flooding at each location using an index This emphasis on change in communities has led to of streambed disturbance. The number of invertebrate taxa peaked the formulation of the nonequilibrium model, which where the intensity of flooding was at intermediate levels. describes most communities as constantly changing after dis- 35 turbance. Even relatively stable communities can be rapidly transformed into nonequilibrium communities. Let’s exam- 30 ine some of the ways that disturbances influence community structure and composition. 25

20

Characterizing Disturbance Number of taxa 15 The types of disturbances and their frequency and severity vary among communities. Storms disturb almost all com- 10 0.9 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 2.0 munities, even those in the through the action of Index of disturbance intensity (log scale) waves. Fire is a significant disturbance; in fact, and

1226 Unit eight Ecology disturbances also can have important effects on community many burned areas in the park were largely covered with structure. Small-scale disturbances can create patches of new vegetation, suggesting that the species in this commu- different habitats across a , which help maintain nity are adapted to rapid recovery after fire (Figure 54.22b). diversity in a community. Large-scale disturbances are also In fact, large-scale have periodically swept through a natural part of many communities. Much of Yellowstone the lodgepole pine forests of Yellowstone and other north- National Park, for example, is dominated by lodgepole ern areas for thousands of years. In contrast, more south- pine, a tree species that requires the rejuvenating influence erly pine forests were historically affected by frequent but of periodic fires. Lodgepole pine cones remain closed until low-intensity fires. In these forests, a century of human exposed to intense heat. When a forest fire burns the trees, intervention to suppress small fires has allowed an unnatu- the cones open and the seeds are released. The new genera- ral buildup of fuels in some places and elevated the risk of tion of lodgepole pines can then thrive on nutrients released large, severe fires to which the species are not adapted. from the burned trees and in the that is no longer Studies of the Yellowstone forest community and many blocked by taller trees. others indicate that they are nonequilibrium communi- In the summer of 1988, extensive areas of Yellowstone ties, changing continually because of natural disturbances burned during a severe drought (Figure 54.22a). By 1989, and the internal processes of growth and reproduction. Mounting evidence suggests that nonequilibrium condi-

Figure 54.22 Recovery following a large-scale disturbance. tions are in fact the norm for most communities. The 1988 Yellowstone National Park fires burned large areas of forests dominated by lodgepole pines. Changes in the composition and structure of terrestrial communities are most apparent after a severe disturbance, such as a volcanic eruption or a , strips away all the existing vegetation. The disturbed area may be colonized by a variety of species, which are gradually replaced by other species, which are in turn replaced by still other species—a process called ecological succession. When this process begins in a virtually lifeless area where soil has not yet formed, such as on a new volcanic island or on the rubble (moraine) left by a retreating glacier, it is called . During primary succession, the only life-forms initially present are often and protists. and (a) Soon after fire. While all trees in the foreground of this photograph , which grow from windblown spores, are commonly were killed by the fire, unburned trees can be seen in other locations. the first macroscopic photosynthesizers to colonize such areas. Soil develops gradually as rocks and organic matter accumulates from the decomposed remains of the early colonizers. Once soil is present, the lichens and mosses are usually overgrown by grasses, shrubs, and trees that sprout from seeds blown in from nearby areas or carried in by animals. Eventually, an area is colonized by plants that become the community’s dominant form of vegetation. Producing such a community through primary succession may take hundreds or thousands of years. Early-arriving species and later-arriving ones may be linked by one of three key processes. The early arrivals may facilitate the appearance of the later species by making the environ- ment more favorable—for example, by increasing the fertility of the soil. Alternatively, the early species may inhibit estab- (b) One year after fire. The community has begun to recover. Herbaceous lishment of the later species, so that successful colonization plants, different from those in the former forest, cover the ground. by later species occurs in spite of, rather than because of, the Interview with Monica Turner: Studying the consequences activities of the early species. Finally, the early species may be of fire and other disturbances on completely independent of the later species, which tolerate

chApter 54 Community Ecology 1227 Figure 54.23 Glacial retreat and primary succession at Glacier Bay, Alaska. The different shades of blue on the map show retreat of the glacier since 1760, based on historical descriptions.

1941

1907

1 Pioneer stage 1860 2 Dryas stage

Glacier Bay

Alaska

1760 0 5 10 15 Kilometers

4 Spruce stage 3 Alder stage conditions created early in succession but are neither helped transitions in the vegeta- Alder root with a cluster nor hindered by early species. tion. Because the bare soil of nodules containing -fixing bacteria Ecologists have conducted some of the most extensive after glacial retreat is low in research on primary succession at Glacier Bay in southeast- nitrogen content, almost ern Alaska, where have retreated more than 100 km all the pioneer plant species since 1760 (Figure 54.23). By studying the communities at begin succession with poor different distances from the mouth of the bay, ecologists growth and yellow can examine different stages in succession. 1 The exposed due to limited nitrogen glacial moraine is colonized first by pioneering species that supply. The exceptions are include liverworts, mosses, fireweed, scattered Dryas (a mat- Dryas and alder, whose roots forming ), and . 2 After about three decades, host symbiotic bacteria that Dryas dominates the . 3 A few decades fix atmospheric nitrogen later, the area is invaded by alder, which forms dense (see photograph on this up to 9 m tall. 4 In the next two centuries, these alder stands page and Figure 37.12). Soil are overgrown first by Sitka spruce and later by western nitrogen content increases quickly during the alder stage hemlock and mountain hemlock. In areas of poor drainage, of succession and keeps increasing during the spruce stage the forest floor of this spruce-hemlock forest is invaded by (Figure 54.24). By altering soil properties, pioneer plant sphagnum , which holds water and acidifies the soil, species can facilitate colonization by new plant species eventually killing the trees. Thus, by about 300 years after during succession. glacial retreat, the vegetation consists of sphagnum bogs on In contrast to primary succession, secondary succession the poorly drained flat areas and spruce-hemlock forest on occurs when an existing community has been cleared by a the well-drained slopes. disturbance that leaves the soil intact, as in Yellowstone fol- Succession on glacial moraines is related to changes in lowing the 1988 fires (see Figure 54.22). Following the dis- soil nutrients and other environmental factors caused by turbance, the area may return to something like its original

1228 Unit eight Ecology Figure 54.24 Changes in soil nitrogen Figure 54.25 Disturbance of the ocean floor by trawling. content during succession at Glacier Bay. These photos show the seafloor off northwestern Australia before and after deep-sea trawlers have passed through. 60 ◀ Before trawling 50 ) 2 40

30

20 Soil nitrogen (g/m Soil nitrogen

10

0 Pioneer Dryas Alder Spruce 1 2 3 4 Successional stage

MAKe ConneCtions Figure 37.12 illustrates two types of atmospheric by prokaryotes. At the earliest stages of primary succession, before any plants are present at a site, which type of nitrogen fixation would occur, and why?

After ▶ state. For instance, in a forested area that has been cleared for trawling farming and later abandoned, the earliest plants to recolonize are often herbaceous species that grow from windblown or animal-borne seeds. If the area has not been burned or heavily grazed, woody shrubs may in time replace most of the herba- ConCept CHeCK 54.3 ceous species, and forest trees may eventually replace most of 1. Why do high and low levels of disturbance usually reduce the shrubs. species diversity? Why does an intermediate level of disturbance promote species diversity? 2. During succession, how might the early species facilitate Human Disturbance the arrival of other species? Ecological succession is a response to disturbance of the envi- 3. WHAt iF? Most prairies experience regular fires, typi- ronment, and the strongest disturbances today are human cally every few years. if these disturbances were relatively modest, how would the species diversity of a prairie activities. Agricultural development has disrupted what likely be affected if no burning occurred for 100 years? were once the vast of the North American prairie. explain your answer. Tropical rain forests are quickly disappearing as a result For suggested answers, see Appendix A. of clear-cutting for lumber, cattle grazing, and farmland. Centuries of overgrazing and agricultural disturbance have contributed to famine in parts of Africa by turning seasonal ConCept 54.4 grasslands into vast barren areas. Human actions have disturbed marine ecosystems as well as Biogeographic factors affect terrestrial ones. The effects of ocean trawling, in which boats community diversity drag weighted nets across the seafloor, are similar to those of clear-cutting a forest or plowing a . The trawls scrape So far, we have examined relatively small-scale or local fac- and scour corals and other life on the seafloor (Figure 54.25). tors that influence the diversity of communities, including In a typical year, ships trawl an area about the size of South the effects of species interactions, dominant species, and America, 150 times larger than the area of forests that are many types of disturbances. Large-scale biogeographic fac- clear-cut annually. tors also contribute to the tremendous range of diversity Because disturbance by human activities is often severe, it observed in biological communities. The contributions of reduces species diversity in many communities. In Chapter 56, two biogeographic factors in particular—the latitude of a we’ll take a closer look at how human-caused disturbance is community and the area it occupies—have been investigated affecting the diversity of life. for more than a century.

chApter 54 Community Ecology 1229 latitudinal Gradients Figure 54.26 energy, water, and species richness. Vertebrate species richness in North America increases with potential In the 1850s, both and Alfred Wallace , expressed as rainfall equivalents (mm/yr). pointed out that plant and animal life was generally more abundant and diverse in the than in other parts 200 of the globe. Since that time, many researchers have con- firmed this observation. One study found that a 6.6-hectare (1 ha = 10,000 m2) plot in tropical Malaysia contained 100 711 tree species, while a 2-ha plot of deciduous forest in Michigan typically contained just 10 to 15 tree species. 50 Many groups of animals show similar latitudinal gradients.

For instance, there are more than 200 species of ants in (log scale) Brazil but only 7 in Alaska.

Two key factors that can affect latitudinal gradients species richness Vertebrate of species richness are evolutionary history and climate. Over the course of evolution, a series of events may lead to increased species richness in a community (see 10 Concept 24.2). Tropical communities are generally older 0 500 1,000 1,500 2,000 than temperate or polar communities, which have repeat- Potential evapotranspiration (mm/yr) edly “started over” after major disturbances such as glacia- tions. As a result, species diversity may be highest in the tropics simply because there has been more time for specia- The first, and still widely used, mathematical description tion to occur in tropical communities than in temperate or of the species-area relationship was proposed a century ago: polar communities. z Climate is another key factor thought to affect latitudi- S = cA nal gradients of richness and diversity. In terrestrial com- where S is the number of species found in a habitat, c is a munities, the two main climatic factors correlated with constant, and A is the area of the habitat. The exponent z tells diversity are sunlight and , both of which you how many more species should be found in a habitat as occur at high levels in the tropics. These factors can be its area increases. In a log-log plot of S versus A, z is the slope considered together by measuring a community’s rate of of the line through the data points. A value of z = 1 would evapotranspiration, the evaporation of water from soil indicate a linear relationship between species number and and plants. Evapotranspiration, a function of solar radia- area, meaning that ten times as many species would be found tion, temperature, and water availability, is much higher in a habitat that has ten times the area. in hot areas with abundant rainfall than in areas with low In the 1960s, Robert MacArthur and E. O. Wilson tested temperatures or low precipitation. Potential evapotranspiration, the predictions of the species-area relationship by examining a measure of potential water loss that assumes that water is the number of animals and plants on different island chains. readily available, is determined by the amount of solar radi- As one example, in the Sunda Islands of Malaysia, they found ation and temperature and is highest in where both that the number of bird species increased with island size, are plentiful. The species richness of plants and animals with a value of z = 0.4 (Figure 54.27). These and other stud- correlates with both measures, as shown for vertebrates and ies have shown that z is usually between 0.2 and 0.4. potential evapotranspiration in Figure 54.26. Although the slopes of different species-area curves vary, the basic concept of diversity increasing with increasing area Area Effects applies in many situations, from surveys of ant diversity in New Guinea to studies of plant species richness on islands of In 1807, naturalist and explorer different sizes. described one of the first patterns of species richness to be recognized, the species-area curve: All other factors being equal, the larger the geographic area of a community, the Island Equilibrium Model more species it has. One explanation for this relationship Because of their isolation and limited size, islands provide is that larger areas offer a greater diversity of habitats and excellent opportunities for studying the biogeographic microhabitats. In , developing species- factors that affect the species diversity of communities. area curves for key taxa in a community helps ecologists By “islands,” we mean not only oceanic islands, but also habi- predict how the loss of a given area of habitat will affect the tat islands on land, such as lakes, mountain peaks separated by community’s diversity. lowlands, or habitat fragments—any patch surrounded by an

1230 Unit eight Ecology Figure 54.27 species richness and island area. The number Figure 54.28 MacArthur and Wilson’s island equilibrium of bird species on the Sunda Islands of Malaysia increases with model. The equilibrium number of species on an island represents a island size. The slope of the best-fit line through the data points balance between the of new species (red curve) and the (the parameter z) is about 0.4. of species already there (black curve). The black triangle shows the predicted equilibrium number of species. 1,000

I m m n o ig ti r c a n 100 t ti io x n E

10 Number of bird species (log scale) Number of bird 1 1 10 102 103 104 105 Rate of immigration or extinction Area of island (mi2) (log scale) Equilibrium number Number of species on island

WHAt iF? Suppose rising sea levels substantially decreased the size of the environment not suitable for the “island” species. While study- island. How would that affect (a) the population sizes of species already on the ing the species-area relationship, MacArthur and Wilson also island, (b) the extinction curve shown above, and (c) the predicted equilibrium number of species? developed a method for predicting the species diversity of islands (Figure 54.28). In their approach, the number of spe- cies on an island represents a balance between the immigra- rate of species extinction. The number of species at this equi- tion of new species to the island and the extinction of species librium point is correlated with the island’s size and distance already there. from the mainland. Like any ecological equilibrium, this spe- In Figure 54.28, note that the immigration rate decreases cies equilibrium is dynamic: Although the number of species as the number of species on the island gets larger, while the ultimately should stabilize at a constant level, immigration extinction rate increases. To see why this is so, consider a and extinction continue and hence the exact species compo- newly formed oceanic island that receives colonizing spe- sition on the island may change over time. cies from a distant mainland. At any given time, an island’s In 1967, Dan Simberloff, then a graduate student with immigration and extinction rates are affected by the number E. O. Wilson at Harvard University, tested the island equilib- of species already present. As the number of species already rium model in an experiment on six small mangrove islands on the island increases, the immigration rate of new species in the Florida Keys (Figure 54.29). Simberloff and Wilson decreases, because any individual reaching the island is less first painstakingly identified and counted all of the arthro- likely to represent a species that is not already present. At the pod species on each island. As predicted by the model, they same time, as more species inhabit an island, extinction rates found more species on islands that were larger and closer to on the island increase because of the greater likelihood of the mainland. They then fumigated four of the islands with competitive exclusion. methyl bromide to kill all of the arthropods. After about a year, Two physical features of the island further affect immigra- the species richness on these islands increased to tion and extinction rates: its size and its distance from the mainland. Small islands generally have lower immigration rates because potential colonizers are less likely to reach a small Figure 54.29 island than a large one. Small islands also have higher extinc- Mangrove tion rates because they generally contain fewer resources, island. The islands have less diverse habitats, and have smaller population sizes. that Simberloff and Wilson studied Distance from the mainland is also important; a closer island were small, each generally has a higher immigration rate and a lower extinction consisting of one rate than one farther away. Arriving colonists help sustain the or a few mangrove presence of a species on a near island and prevent its extinction. trees. MacArthur and Wilson’s model is called the island equilibrium model because an equilibrium will eventually be reached where the rate of species immigration equals the

chApter 54 Community Ecology 1231 Figure 54.30 testing the island equilibrium model. The disease-causing microorganisms, , viroids, or prions. graph shows results for one of the islands studied. The number of (Viroids and prions are infectious RNA and pro- arthropod species increased over time; by 240 days, it had reached levels teins, respectively; see Concept 19.3.) As scientists have similar to those found on the island prior to the start of the experiment. recently come to appreciate, pathogens have universal effects in structuring ecological communities. 40 Number of species on island before fumigation Pathogens produce especially clear effects when they are introduced into new habitats, as in the case of the fungus that 30 causes chestnut blight (see Concept 54.2). A pathogen can be particularly virulent in a new habitat because new host popula- 20 tions have not had a chance to become resistant to the patho- gen through natural selection. The invasive chestnut blight fungus had far stronger effects on the American chestnut, for 10

Number of species present instance, than it had on Asian chestnut species in the fungus’s native habitat. Humans are similarly vulnerable to the effects of

40 80 120 160 200 240 280 320 360 emerging diseases spread by our increasingly global economy. Days since fumigation Pathogens and Community Structure near their pre-fumigation values (Figure 54.30). The island The ecological importance of disease can be highlighted by how closest to the mainland recovered first, while the island far- pathogens have affected communities. White-band thest from the mainland was the slowest to recover. The num- disease, caused by an unknown pathogen, has resulted in dra- ber of arthropod species on the remaining two islands—which matic changes in the structure and composition of Caribbean were not fumigated and hence served as controls—remained reefs. The disease kills corals by causing their to slough approximately constant during the study. off in a band from the base to the tip of the branches. Because Over long periods, disturbances such as storms, adaptive of the disease, staghorn coral (Acropora cervicornis) has virtually evolutionary changes, and speciation generally alter the disappeared from the Caribbean since the 1980s. Populations species composition and community structure on islands. of elkhorn coral (Acropora palmata) have also been decimated. Nonetheless, the island equilibrium model is widely applied Such corals provide key habitat for lobsters as well as snappers in ecology. Conservation in particular use it when and other fish species. When the corals die, they are quickly designing habitat reserves or establishing a starting point for overgrown by algae. Surgeonfish and other herbivores that feed predicting the effects of habitat loss on species diversity. on algae come to dominate the fish community. Eventually, the corals topple because of damage from storms and other dis- Animation: Exploring Island Biogeography turbances. The complex, three-dimensional structure of the reef disappears, and diversity plummets. Pathogens also influence community structure in terres- ConCept CHeCK 54.4 trial ecosystems. Consider sudden oak death (SOD), a recently 1. Describe two hypotheses that explain why species diver- discovered disease that is caused by the protist Phytophthora sity is greater in tropical regions than in temperate and polar regions. ramorum (see Concept 28.6). SOD was first described in 2. Describe how an island’s size and distance from the main- California in 1995, when hikers noticed trees dying around land affect the island’s species richness. . By 2014, it had spread more than 1,000 km, 3. WHAt iF? Based on MacArthur and Wilson’s island from the central California coast to southern Oregon, and it equilibrium model, how would you expect the richness of had killed more than a million oaks and other trees. The loss birds on islands to compare with the richness of snakes and lizards? explain. of the oaks has led to the decreased abundance of at least five For suggested answers, see Appendix A. bird species, including the and the oak tit- mouse, that rely on oaks for food and habitat. Although there is currently no cure for SOD, scientists recently sequenced ConCept the genome of P. ramorum in hopes of finding a way to fight 54.5 the pathogen. Pathogens alter community structure Human activities are transporting pathogens around locally and globally the world at unprecedented rates. Genetic analyses using DNA sequencing suggest that P. ramorum likely came to Now that we have examined several important factors that North America from through the horticulture trade. structure biological communities, we’ll finish the chapter by Similarly, the pathogens that cause human diseases are examining community interactions involving pathogens, spread by our global economy. H1N1, the that causes

1232 Unit eight Ecology “swine flu” in humans, was first detected in Veracruz, hosts for a pathogen provides information that may be used Mexico, in early 2009. It quickly spread around the world to control the hosts most responsible for spreading diseases. when infected individuals flew on airplanes to other coun- Ecologists also use their knowledge of community interac- tries. By 2011, this flu outbreak had a confirmed death toll of tions to track the spread of zoonotic diseases. One example, more than 18,000 people. The actual number may have been avian flu, is caused by highly contagious viruses transmitted significantly higher since many people with flu-like symp- through the saliva and feces of birds (see Concept 19.3). Most toms who died were not tested for H1N1. of these viruses affect wild birds mildly, but they often cause stronger symptoms in domesticated birds, the most common Community Ecology and Zoonotic source of human . Since 2003, one particular viral Diseases strain, called H5N1, has killed hundreds of millions of poul- try and more than 300 people. In 2015, for example, H5N1 Three-quarters of emerging human diseases and many of the was one of three strains of avian flu that spread across poul- most devastating established diseases are caused by zoonotic try farms in the United States, killing more than 40 million pathogens—those that are transferred to humans from birds. While this outbreak was devastating to the poultry other animals, either through direct contact with an infected industry, it did not result in any human cases. animal or by means of an intermediate species, called a Control programs that quarantine domestic birds or moni- . The vectors that spread zoonotic diseases are often tor their transport may be ineffective if avian flu spreads parasites, including ticks, lice, and mosquitoes. naturally through the movements of wild birds. From 2003 Identifying the community of hosts and vectors for a to 2006, the H5N1 strain spread rapidly from southeast Asia pathogen can help prevent illnesses such as Lyme disease, into Europe and Africa. The most likely place for infected which is spread by ticks. For years, scientists thought that wild birds to enter the Americas is Alaska, the entry point for the primary host for the Lyme pathogen was the white- ducks, geese, and shorebirds that migrate across the Bering footed mouse because mice are heavily parasitized by young Sea from Asia every year. Ecologists are studying the spread ticks. When researchers vaccinated mice against Lyme dis- of the virus by trapping and testing migrating and resident ease and released them into the wild, however, the number birds in Alaska (Figure 54.32). of infected ticks hardly changed. Further investigation in While our emphasis here has been on community ecology, New York revealed that two inconspicuous shrew species pathogens are also greatly influenced by changes in the phys- were the source for more than half the infected ticks col- ical environment. To control pathogens and the diseases they lected in the field (Figure 54.31). Identifying the dominant cause, scientists need an ecosystem perspective—an intimate knowledge of how the pathogens interact with other species Figure 54.31 unexpected hosts of the lyme disease and with all aspects of their environment. Ecosystems are the pathogen. A combination of ecological data and genetic analyses subject of Chapter 55. enabled scientists to show that more than half of ticks carrying the Lyme pathogen became infected by feeding on the northern short- tailed shrew (Blarina brevicauda) or the masked shrew (Sorex cinereus). Figure 54.32 tracking avian ◀ Collecting ticks from a flu. A graduate white-footed mouse student bands a young gyrfalcon as 35 part of a project to monitor the spread 30 of avian flu. 25

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15 that fed on host 10 Percent of infected ticks Percent 5 ConCept CHeCK 54.5 0 Mouse Chipmunk Short-tailed Masked Other 1. What are pathogens? shrew shrew 2. WHAt iF? rabies, a viral disease in mammals, is not cur- Host species rently found in the British isles. if you were in charge of disease control there, what practical approaches might you employ to keep the rabies virus from reaching these MAKe ConneCtions Concept 23.1 describes genetic variation between islands? populations. How might genetic variation between shrew populations in different locations affect the number of infected ticks? For suggested answers, see Appendix A.

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