Natural Variability and the Manifold Mechanisms of Ecological Communities

In response to Roughgarden's provocative essay, the editor of The American Naturalist has solicited articles that present the personal perspectives of some community ecologists and their sets of operating assumptions. Ecology is growing and changing rapidly, as is shown by this dispute over fundamentals, and my concern will be the importance of alternative and complementary forces in communities, in contrast to a singular emphasis on competition. I will emphasize density-vague population dynamics; independent and noncompetitive coexistence of species; weak and inconsequential competition; variable and diffuse species interactions; maintenance (by the weather, disturbance, and natural enemies) of populations well below densities that deplete resources to any important extent; and mutualistic and commensalistic relations between species. My purpose is to illustrate the commonness of communities that are quite different from those assumed by orthodox competition theory. I certainly do not deny that competition occurs in nature or that it is in some cases a dominant process. Competitively dominated communities have been amply reviewed elsewhere. Strong and persistent competition, however, does not necessarily square a set of species with orthodox competition and niche theory. Effects of competition are varied, and mathematical deductions about them need critical testing, both empirical and theoretical. Though interspecific competition, when and where it occurs, may affect aspects of population dynamics, shape niches, cause coevolution, or even result in character displacement of some species in some circumstances, it need not have ary of these effects. What are the possibilities for diffuse competition (Connell 1975)? What might nonequilibrium communities be like (Caswell 1982; Wiens and Rotenberry 1980)? What are the influences of habitat patchiness and high stochasticity (Levin 1976; Chesson 1978; Murdoch 1979)? My paper is in two parts. Part 1 deals with several logical and philosophical points. Part 2 is a description of some case studies that find factors other than interspecific competition to be of great influence in ecological communities. The dispute illustrated by this symposium was actually begun long ago (British Ecolog-