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II.6 Competition and Coexistence in Animal Communities Priyanga Amarasekare OUTLINE 1. Introduction 2. Basic principles of competitive coexistence 3. Coexistence under a single limiting factor 4. Coexistence under multiple limiting factors 5. Multiple coexistence mechanisms 6. Summary and conclusions Competition is the most ubiquitous of species interactions. It occurs any time a resource that is essential to growth and reproduction (e.g., food, shelter, nesting sites) occurs in short supply. The acquisition of the resource by one individual simultaneously deprives others of access to it, and this deprivation has a negative effect on both the fitness of individuals and the per capita growth rates of populations. Competition is thus an interaction that has mutually negative effects on its participants. Coexistence results when populations of several species that utilize the same limiting resources manage to persist within the same locality. This chapter focuses on mechanisms that allow competitive coexistence in animal communities. Animals have two characteristics that determine the kinds of resources they can use and the mechanisms by which they can tolerate or avoid competition for these resources. First, animals are heterotrophs and have to ingest other organisms to obtain the energy required for growth and reproduction; competition thus involves biotic resources. Second, most animals are mobile and hence able to avoid or reduce competitive effects through dispersal. GLOSSARY density dependence. Dependence of the per capita growth rate on the abundance or density of the organism in question. exploitative competition. Individuals have indirect negative effects on other individuals by acquiring a resource and thus depriving others of access to it. functional response. The relationship between per capita resource consumption and resource abundance . interference competition. Individuals have direct negative effects on other individuals by preventing access to the resource via aggressive behaviors such as territoriality, larval competition, overgrowth, or undercutting. per capita growth rate. Per-individual rate of increase as a result of reproduction, mortality, emigration, and immigration. stable coexistence. Competing species maintain positive abundances in the long term and are able to recover from perturbations that cause them to deviate from their long-term or steady-state abundances . 1. INTRODUCTION A thorough understanding of the mechanisms of coexistence requires a thorough understanding of the mechanism of competition. Because animals rely on biotic resources which themselves grow and reproduce, the appropriate theoretical framework is one in which the resource dynamics are considered explicitly. Tilman’s resource competition theory, although motivated by plant competition, provides such a framework for animal communities as well. When two or more species are limited by the same resource, the species that can maintain a positive per capita growth rate at the lowest resource level will exclude all other species. This is called the R* rule in exploitative competition (Tilman, 1982). Coexistence mechanisms are the processes that counteract the R* rule. They do so by increasing the strength of intraspecific competition relative to that of interspecific competition. The exact means by which this is achieved is obvious in some cases and quite subtle in others. There are several basic principles that underlie all coexistence mechanisms, and a clear grasp of these principles is necessary to understand the more subtle coexistence mechanisms. 2. BASIC PRINCIPLES OF COMPETITIVE COEXISTENCE First, stable coexistence requires species to exhibit ecological differences. These differences are typically thought of as the species’ niches. Following Chesson (2000), a species’ niche has four dimensions: resources, natural enemies, space, and time. Species could differ in terms of (1) which resources or natural enemies they are limited by, (2) when they use the resource or encounter the natural enemy, or (3) where they use the resource or encounter the natural enemy. Niche differences are essential to coexistence because they allow species to depress their own per capita growth rates more than they do the growth rates of their competitors (Chesson, 2000). Elucidating exactly how this occurs is often difficult, but such an understanding is vital for a mechanistic understanding of coexistence. A useful starting point is the idea of a negative feedback loop. Such a loop can cause a species’ per capita growth rate to decrease when the population size is large and to increase when the population size is small. Negative feedback processes arise naturally when individuals compete with conspecifics (other individuals of the same species) for a limiting resource. Coexistence requires that this self-limiting negative feedback be stronger than the negative effect that the species has on the per capita growth rate of another species it competes with; i.e...


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