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A new hypothesis, based on differences in the rates at which populations of competing species approach competitive equilibrium (reduction or exclusion of some species), is proposed to explain patterns of species diversity. The hypothesis assumes that most communities exist in a state of nonequilibrium where competitive equilibrium is prevented by periodic population reductions and environmental fluctuations. When competitive equilibrium is prevented, a dynamic balance may be established between the rate of competitive displacement and the frequency of population reduction, which results in a stable level of diversity. Under conditions of infrequent reductions, an increase in the population growth rates of competitors generally results in decreased diversity. This model clarifies an underlying pattern of variation in diversity and points out the common elements of previous hypotheses. Rather than arguing that either competition, predation, or productivity control diversity, it demonstrates that all of these may contribute to the same basic mechanism. In doing so, it not only explains the correlations of the other hypotheses with patterns of diversity, but also explains the exceptions that these hypotheses could not explain. This hypothesis may be applied to variations of diversity both on a latitudinal gradient and within specific regions.