Overlap, Similarity, and Competition Coefficients

Several problems arise if niche overlaps are equated with species similarities or interspecific competition coefficients. Niche overlaps based on food types in the diet rely on proportional utilization rates of the different food types. But because proportional utilizations reflect consumer—environment interactions, measures of similarity based on such proportions may reflect similarities between environments rather than similarities between species pairs. Competition coefficients, as derived by MacArthur, weight renewable resources on the basis of their productivity–not their relative abundance. Use of proportional utilization rates intrinsically incorporates relative resource abundance and ignores differences in resource productivity. Thus, equating overlaps based on proportional utilizations with competition coefficients will overestimate the contribution of abundant resources, which will be actually less limiting if they are very productive. These problems are illustrated with the MacArthur—Levin's measure of niche overlap, but the same problems will arise with other measures of overlap. On the other hand, measures of species' similarities which are independent of resource abundance may reflect evolutionary divergence of consumers' resource utilization patterns due to past competitive pressures.