Dispersal, Variability, and Transient Niches: Species Coexistence in a Uniformly Variable Environment

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Abstract
Observations of communities of sessile organisms, notably plants and territorial fishes, have revealed species guilds which are hard to reconcile with the competitive-exclusion principle, which forbids the persistence of multiple competitors for exactly the same resource. Several mechanisms have been proposed to account for such cases. We show in particular that random environmental variation, with no long-term spatial inhomogeneity (i.e., a uniformly variable environment), can promote species'' coexistence in models of lottery competition for establishment sites. We explain this as a result of relatively intensive competition in patches favoring the currently more numerous species, which occurs in multiple-patch uniformly-variable-environment models with dispersal. Time dispersal (nonsynchronous establishment of offspring of an adult cohort) has a similar and complementary effect. Simulations show that the persistence of a competitively inferior species may involve rare environmental patches that have fortuitous runs of conditions favorable to that species. We infer that the spatial distributions of rarer species in sessile organism guilds may be highly clumped.