Use of overlap indices as competition coefficients: Tests with field data

Abstract

The possibility of using overlap indices as competition coefficients was investigated by applying this substitution to field data on the food of a predator community. (1) The maximum value of the carrying capacity for each predator was estimated and compared to the carrying capacity predicted from the Lotka-Volterra competition equations with overlap indices used as competition coefficients. Carrying capacities were sometimes larger when computed with the latter method. The contradiction was not resolved with the use of more complex overlap indices leading to the conclusion that overlap indices have limited use as competition coefficients if a realistic description of the dynamics of populations is required. (2) The outcome of pairwise competition between all combinations of predators is considered. If overlap indices are used as competition coefficients, the theory of competitive exclusion tends to predict that foxes, stoats, and long-eared owls should be excluded rather than they should exclude other predators. The opposite applies to cats and buzzards. Both these results are apparently contradictions as all species were permanent members of the community studied. The pattern, however, can be explained if it is assumed that the carrying capacity of the first group has been underestimated and/or that of the second group overestimated. This could well be the case if the first group has 'food refugia' and the second is not able to fully exploit its prey populations. This is only a hypothesis suggested by the observed patterns. Even if they are not numerically equivalent to competition coefficients, overlap indices can thus be of use as a tool for hypothesis generation. © 1986.