Experimental Studies of Microcrustacean Competition, Community Composition and Efficiency of Resource Utilization

Studies of perturbed laboratory microcosm communities composed of algae and competing herbivorous microcrustaceans were conducted to assess the role(s) of various species compositions and patterns of resource consumption on the efficiency of resource utilization. Equilibrium microcosm communities, consisting of four competing species, three cladocerans, and an amphipod, were perturbed by different intensities of highly size—specific fish predation that resulted in altered species compositions (two or three additional species) and new equilibrium densities. Analyses of food particle consumption patterns, spatial distributions, and life history characteristics under the different predation intensities indicated the importance of microhabitat and food specializations in permitting coexistence of some species but exclusion of others. Age— and size—specific competition for limiting small (3—6 μm) particles was especially important among young animals at low predation levels, whereas under high predation, competition for larger particles among adults was more important. Analyses of particle uptake efficiencies in the different communities revealed very uneven use of available resources when the presence of efficient competitors for some foods produced developmental bottlenecks among the young of competing specis. Resource utilization efficiencies were much less variable at equilibrium when predation reduced competition on the young. The potential general importance of age— and/or size—specific competition in reducing the efficiency of resource utilization in communities of organisms other than terrestrial homeotherms is suggested and contrasted to the current community models of competition.