KAPLAN, P.; University of Michigan: Testing the role of competition as a community organizer using limiting similarity

Biotic interactions, such as competition, are the chief structurers of modern communities. A uniformitarian view would suggest the same for paleocommunities, but paleontologists have made little headway in demonstrating such a relationship through deep time. The main difficulty in meaningfully quantifying paleobiotic interactions is the measurement of ecological processes on truly ecological timescales. Competitive interactions leading to competitive exclusion of species, for example, can hardly be expected to appear in the fossil record, as such exclusions normally take place within a geologic “instant.” A more profitable tack might be the examination of competitive interactions that led to coexistence among species. In such a case, the particular equilibrium of coexistence might yield clues as to the ecological processes that gave rise to the observed community. Specifically, some ecological aspect of the coexisting species should have adjusted in each so as to minimize overall interspecific competition at equilibrium. If a number of species are competing, then each must minimize its competition with both of its nearest ecological neighbors. Such a scenario should lead to equal spacing of species along some resource axis. As an example, a brachiopod community from the Ordovician Richmond Group of Indiana is subjected to tests of limiting similarity. Bootstrap analyses of body size suggest that the niche spacing among these species was surprisingly (p < 0.03) consistent, and that the total niche overlap was significantly (p < 0.03) minimized. A number of abductive inferences provide substantial information about ecological, physiological, and depositional processes. In addition, they suggest that biotic interactions, such as competition, have been important in structuring communities throughout the last 450 million years.