Community ecology 54 Figure 54.1 Which species benefits from this interaction? Key ConCepts Communities in Motion At first glance, you might think the situation looks dire for this bluestreak cleaner 54.1 Community 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 coral reef habitat (Figure 54.1). With one snap of its jaw, the eel could easily species involved crush the fish and swallow it. However, the wrasse is not in danger of becoming this 54.2 Diversity and trophic structure eel’s dinner. The much larger animal 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 Disturbance influences species In this interaction, both organisms benefit: The cleaner wrasse gains access to a diversity and composition supply of food, 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 habitats, 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 population can affect other individuals of the same species. This chapter will examine ecological interactions between populations 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 resources. Ecologists define the boundaries of a particular community supply on land; most terrestrial species use this resource but to fit their research questions: They might study the com- do not usually compete for it. munity of decomposers and other organisms living on a rotting log, the benthic community in Lake Superior, or the Competitive Exclusion community of trees and shrubs 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 experiments 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 carrying capacity of the culture (see Figure 53.11a for an ciples of community ecology to the study of human disease. illustration of the logistic growth of a Paramecium population). But when Gause grew the two species together, P. caudatum became extinct in the culture. Gause inferred that P. aurelia ConCept 54.1 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 life of an organism 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 competition, predation, herbivory, parasitism, mutualism, and commen- Ecological Niches and Natural Selection salism. In this section, we’ll define and describe each of these evolution 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 tree lizard, for instance, includes the temperature range it prey population. Mutualism is a +/+ interaction because the tolerates, the size of branches on which it perches, the time survival and reproduction 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 ecosystem. 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 times 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 plants for soil nutrients often different from its realized niche, the portion of its fun- and water. Lynx and foxes in the northern forests 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 oxygen, 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 Anolis lizards live in close proximity, inquiry Can a species’ niche be influenced and all feed on insects and other small arthropods. However, competition for food is reduced because each lizard species has a different preferred by interspecific competition? perch, thus occupying a distinct niche. experiment 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 coast 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 tide Chthamalus A. ricordii Balanus Chthamalus realized niche Balanus realized niche A. insolitus Ocean Low tide A. aliniger A. christophei A. distichus Results Chthamalus spread into the A. cybotes region 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 space. 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 mouse (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. Africa, 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 tides. 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.