Curvilinearities in the Competition Equations: An Experiment with Ranid Tadpoles

Abstract

The existence and significance of higher-order and cross-product terms in the traditional Lotka-Volterra competition equations, were examined. The effects of conspecific and heterospecific density on parameters of development were investigated in laboratory experiments with 2 frog spp. Rana sylvatica and R. pipiens. The wide variety of complex density effects observed, included positive and negative interactions. Data presented in combination with those of Wilbur (1972), Neill (1974) and Ayala et al. (1973) strongly support the conclusion that hills and valleys in the competitive landscape are highly significant and that linear approximations of competition coefficients are probably insufficient and misleading. The usefulness of community matrix theory based on simple linear approximations is dubious. Competition coefficients are themselves density dependent and the structure of density functions may differ even among closely related species. Studies of the underlying biological mechanisms of species interactions may serve as the only legitimate basis of constructing potentially useful models of density functions and interspecific competition.