The Maintenance of Species Diversity by Disturbance

Hypotheses that relate disturbance to the production and maintenance of species diversity are reviewed. The hypotheses have been classified traditionally by the effect of the disturbance on the community: Those that involve selective mortality, which maintains the species diversity of a community in equilibrium, and those that invoke causing random, localized, mass mortality, which prevent the community from reaching and equilibrium. Regardless of this difference, most hypotheses predict that the greatest number of species will occur at intermediate leves of disturbance. We develop graphical models for equilibrium and nonequilibrium situations, which we show to differ only in respect to temporal and spatial scale. We maintain that the usual outcome-the greatest number of species at intermediate levels of disturbance-can be explained by assuming that there are trade-offs in species-specific abilities that place constraints on immigration to, and extinction in, patches. Changes in immigration and extinction, the process that govern patch diversity, depend on species-specific abilities to fend off competitors or to endure disturbances. Members of a species can resist competitors or resist disturbance, but they cannot excel at both. If this trade-off does not exist, then the highest level of species diversity will not occur at intermediate levels of disturbance. Furthermore, we show that if this assumption is relaxed or if interactions between the frequency and magnitude of disturbances are considered, other unexpected outcomes are posible.