Competition for Food and Its Community-Level Implications

Abstract

Because the bivalve molluscs of estuaries and lagoons are sedentary yet lack permanent attach-ment to the substratum, they can be manipulated in an even wider diversity of experimental treatments than their invertebrate counterparts on rocky intertidal shores. Manipulations of local density of suspension-feeding bivalves in soft sediments have revealed an almost universal pattern of reduced individual growth with increasing local density. Despite reductions in growth rate, mortality rate is generally invariant with increasing density except where density-dependent predation is operating. Several lines of evidence suggest that food limitation is the cause of the reduction in growth at higher local densities. Consequently, the outcome of competition for food among suspen-sion-feeding bivalves in marine soft sediments is a reduction in growth rates but no competitive exclusion. A series of experiments assessing the role of interspecific competition in contributing to the observed segregation of species along the elevation gradient on an intertidal flat demonstrated that competitive exclusion does not occur and thus does not cause the observed habitat segregation. The absence of competitive exclusion may be general when food rather than space is limiting because food is a renewable resource : even less able competitors will get access to some food, whereas loss of living space is absolute and implies mortality. Current theory on the community-level conse-quences of competition should better reflect this dichotomy between the outcomes of interference and exploitation competition. This analysis is intended to illustrate how a study of soft-sediment ecology can be framed in a broader ecological context so as to contribute directly to construction and testing of general theory in community ecology. Future studies in marine soft sediments would have greater impact on the field of ecology if their motivation were broadened to address ecological issues intrinsic to all communities and to include meaningful attempts to synthesize system-specific results.