Species Diversity Gradients: Synthesis of the Roles of Predation, Competition, and Temporal Heterogeneity

We suggest that the "predation" and "competition" hypotheses of community organization and species diversity are complementary. Maintenance of high diversity by competition appears to be relatively more important at higher trophic levels, while maintenance of high diversity by predation seems relatively more important at lower trophic levels. Further, predation is probably the dominant organizing interaction in trophically complex communities, while competition is probably the dominant organizing interaction in trophically simple communities. These hypotheses are supported on a local scale by experimental studies in the rocky intertidal communities of New England and the West Coast. A probable consequence of its greater temporal heterogeneity (i.e., a less stable, less predictable, and more stressful environment) is that the East Coast is trophically more simple and has an increased incidence of competitive exclusion. As a result, diversity is lower on the East Coast compared with the West Coast. A similar interpretation is possible for differences in diversity along other gradients of temporal heterogeneity such as the shallow to deep-sea soft-sediment communities. In structurally simple environments, competition reduces diversity through competitive exclusion. On the other hand, predation first increases and then decreases diversity in spatially simple environments, presumably because refuges are few and hence overexploitation of a resource is more probable. In structurally complex environments, competition may increase diversity through increased habitat specialization. Such environments undoubtedly have more refuges and reduce predator foraging efficiency, both of which may allow the coexistence of more species. Predator-mediated escapes by primary producers from herbivores may explain the apparent importance of interspecific competition in certain primary producer associations.