Document: JEA-3-92-20

Allee effects can promote species coexistence in a spatial model.

FERDY, J.* and J.MOLOFSKY

University of Vermont, Burlington, VT 05405 USA ¹

Abstract:
Many biological interactions can cause the reproductive success of an individual to increase with local density (Allee effect). In plants, for example, individuals might receive more pollinator visits when growing in a densely populated patch than when isolated from any other conspecific. Here we investigated the consequences of local positive density dependence on the coexistence of two species. We defined a pseudo spatial metapopulation model, where individuals experienced both competition for resources and positive density dependence within patches, and where patches were coupled through seed migration. Using moment equations, we derived differential equations for the density of both species as well as for the spatial variance and covariance of these densities. We found that positive density dependence allowed the maintenance of high spatial variance in density. This is because, in spite of local competition, when positive density dependence occurs, individuals cannot efficiently reproduce unless they are aggregated. Positive density dependence also maintained strongly negative spatial covariance, such that at equilibrium the two species were spatially segregated. Increased segregation in space promoted the coexistence of these species. We therefore show that in a spatial model, the species composition of a community might not be determined only by the competitive ability of species, but also by components of their reproduction regime that can cause positive density dependence. Such components are to be expected in most animal-pollinated plants.

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This abstract is being presented at: 10:30 AM in session:
Oral Session #39: Theoretical Ecology.