Density-Independent Mortality and Interspecific Competition: A Test of Pianka's Niche Overlap Hypothesis

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Abstract
A review of the relationship between nonselective density-independent mortality (DIM) and the maximum overlap of competitors in the Lotka-Volterra model fails to support Pianka''s niche-overlap hypothesis. An analysis of a simple model of exploitative competition reveals that higher DIM may increase the niche separation required for competing species to coexist. This is the opposite of the relationship proposed by Pianka. Confusion regarding this issue may have arisen from a failure to define the intensity of competition. Several possible meanings of this term are presented and discussed in relation to the evolution of interference competition. The nature of correlations between intrinsic rate of increase and competitive ability is especially important in determining whether DIM will ever make coexistence of competitors more likely.

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