III.3 Predation and Community Organization
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III.3 Predation and Community Organization Robert D. Holt OUTLINE 1. Introduction 2. Predators can enhance species coexistence 3. Predators can sometimes hamper prey species coexistence 4. The impact of predator and prey behavior on community organization 5. Predators can initiate trophic cascades 6. The diverse effects of predator diversity 7. Conclusions Acts of predation are among the most dramatic events one can see in nature, but the impact of predation on ecological communities goes well beyond the effect of direct mortality on the prey species itself. Because species are embedded in complex food webs, predation on one species can lead to chains of indirect interactions affecting many other species. Predation can sometimes enhance diversity, for instance, if it is differentially inflicted on dominant or abundant competitors . This can free up space or resources, thus permitting inferior or scarce competitors that are better able to withstand predation to persist. However, indiscriminate predation can instead shift the relative competitive rankings of species without enhancing coexistence. Prey species that are highly productive can sometimes sustain predators at levels where less productive prey species are vulnerable to exclusion. There are many complexities in predator–prey interactions that have implications for community organization, including behavioral games between predators and prey and interactions among predators themselves, altering their net effects on their prey. A deeper understanding of all these dimensions of predator– prey interactions is essential for developing wiser policies of conservation and resource management in our rapidly changing world. GLOSSARY apparent competition. An indirect interaction between prey species where a given prey species experiences more intense predation because of the presence of the alternative prey as a result of changes in either predator abundance or predator behavior. community. The assemblage of species that are found together at one place at one time that can potentially interact. community module. A small number of species involved in a clearly defined pattern of interactions, such as two consumers competing for a shared resource, or two prey species interacting indirectly via their impacts on a shared predator. community organization. A term that broadly encompasses the number of species found in a community, their relative abundances, and their pattern of interconnections via competition, exploitation, and mutualism. indirect interactions. When there are three or more species, a given pair of species may influence each other via changing the abundance, activity, or traits of other species (one to many). keystone predator. A predator that strongly interacts with its prey and facilitates their coexistence. natural enemy. A species that utilizes another species (the ‘‘victim’’) as a resource and harms that other species in so doing. Natural enemies include ‘‘true’’ predators, parasitoids, pathogens, and herbivores. predator. A natural enemy that kills its victim in order to utilize resources contained in that victim. switching. A behavioral response by predators to relative prey abundance such that common prey are disproportionately attacked. trophic cascade. A chain reaction in a community across trophic levels in which predation on one species relaxes consumption by that species of its own resource population. 1. INTRODUCTION Few things in the world thrill the nature lover as much as the sight of a predator in action or repose—a lion lazing in the sun, a killer whale gamboling in the waves, a diamondback rattler coiled menacingly under a desert shrub, a spider spinning a silken coffin around its quivering moth prey. Predators provide intellectual thrills too, for some of the most dramatic and intellectually rich stories in ecology involve elucidating the impact of predators on communities. Before recounting some of these tales, which I will use to illustrate principles governing how predators influence communities , it is useful to clarify some terms and to provide a sense of the overall complexity of this issue. Community organization denotes the number of species that co-occur and their relative abundances, along with those processes that control these structural features , including in particular interspecific interactions such as predation, competition, and facilitation. Predators as sources of mortality directly reduce prey and, if sufficiently severe, can cause extinction. Via such direct impacts, predation can profoundly influence community organization. Yet direct mortality (albeit dramatic and compelling) is just one of the many causal pathways by which predators govern community organization . Individual prey can respond adaptively to avoid predation, for instance by changing habitats, feeding rates, or even morphological traits. These responses may reduce predation, but at the cost of hampering resource acquisition, competitive ability, dispersal , or stress tolerance. Because predators and...