Abstract
Experiments were performed to determine the existence and significance of higher-order and cross-product terms in the traditional Lotka-Volterra competition equations. The effects of conspecific and heterospecific density on parameters of development were investigated in laboratory experiments with two species of frogs, Rana sylvatica and R. pipiens. A wide variety of complex density effects were observed, and they included positive as well as negative interactions. The data presented here, 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. We also conclude that the competition coefficients are themselves density dependent and that the structure of density functions may differ even among closely related species. We suggest that the studies of the underlying biological mechanisms of species interactions serve as the only legitimate basis of constructing potentially useful models of density functions and interspecific competition.
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