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Coexistence theory is a framework to understand how competitor traits can maintain species diversity and stave-off competitive exclusion even among similar species living in similar environments. Coexistence theory explains the stable coexistence of species as an interaction between two opposing forces: fitness differences between species, which should drive the best-adapted species to exclude others within a particular ecological niche, and stabilizing mechanisms, which maintains diversity via niche differentiation. For many species to be stabilized in a community, population growth must be negative density-dependent, i.e. all participating species have a tendency to increase in density as their populations decline. In such communities, any species that becomes rare will experience positive growth, pushing its population to recover and making local extinction unlikely. As the population of one species declines, individuals of that species tend to compete predominantly with individuals of other species. Thus, the tendency of a population to recover as it declines in density reflects reduced interspecific (between-species) competition relative to intraspecific (within-species) competition, the signature of niche differentiation (see Lotka-Volterra competition). ==Types of coexistence mechanisms== Two qualitatively different processes can help species to coexist: a reduction in average fitness between species or an increase in niche differentation between species. These two factors have been termed equalizing and stabilizing mechnaisms, respectively. #''Equalizing mechanisms'' reduce fitness differences between species, or relative competitive ability in the absence of niche differentiation. As its name implies, it works by making similar species more equal in their competitive ability. For example, when multiple species compete for the same resource, competitive ability is determined by the minimum level of resources a species needs to maintain itself (known as an R *, or equilibrium resource density). Thus, the species with the lowest R * is the best competitor and excludes all other species in the absence of any niche differentiation. Any factor that reduces R *s between species (like increased harvest of the dominant competitor) is classified as an equalizing mechanism. For species to coexist, fitness differences must be overcome by stabilizing mechanisms. #''Stabilizing mechanisms'' promote coexistence by concentrating intraspecific competition relative to interspecific competition. There are large number of named stabilizing mechanisms including classical hypotheses of species coexistence. Resource partitioning, whereby interspecific competition is reduced because species compete primarily through differest resources, is a stabilizing mechanism. Similarly, if species are differently affected by environmental variation (e.g., soil type, rainfall timing, etc.), this can create a stabilizing mechanism (see the storage effect). Stabilizing mechansims increase the low-density grwoth rate of all species. A general way of measuring the effect of stabilizing mechanisms is by calculating the growth rate of species ''i'' in a community as〔 〕 where: : * is the long-term average growth rate of the species ''i'' when at low density. Because species are limited from growing indefinitely, viable populations have an average long-term growth rate of zero. Therefore, species at low-density can increase in abundance when their long-term average growth rate is positive. : * is a species-specific factor that reflects how quickly species ''i'' responds to a change in competition. For example, species with faster generation times may respond more quickly to a change in resource density than longer lived species. In an extreme scenario, if ants and elephants were to compete for the same resources, elephant population sizes would change much more slowly to changes in resource density than would ant populations. : * is the difference between the fitness of species ''i'' when compared to the average fitness of the community excluding species ''i''. In the absence of any stabilizing mechanisms, species ''i'' will only have a positive growth rate if its fitness is above its average competitor, i.e. where this value is greater than zero. : * measures the effect of all stabilizing mechanisms acting within this community. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「coexistence theory」の詳細全文を読む スポンサード リンク
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