Empirical relationships between predator and prey size among terrestrial vertebrate predators

In an effort to make complex food web relations more tractable, published data on the food habits of terrestrial vertebrate predators were analyzed for patterns in the use of prey by size. Regressions of prey weight on predator weight were run and provided descriptions of both the relationship between mean prey weight and predator weight and the variation in prey weights taken by the predators. Separate models proved necessary for three trophic specializations: insectivores, piscivores and carnivores. Insectivores were found to take proportionately much smaller prey than carnivores. Mean prey weight tends to decrease slightly relative to predator weight among larger insectivores, while the ratio of prey weight to predator weight tends to increase with carnivore size. On average, insectivores also take a relatively wider range, of prey sizes than carnivores. In all respects, piscivores were intermediate to insectivores and carnivores. These models produce log-normal approximations to the frequency distributions (by weight) of prey sizes in the ration of predators from knowledge of the predators's body mass. Combined with allometric models of the bionergetics and productivity of animals, these relations are used to predict, that: (1) the daily kill rate declines with predator weight and (2) the upper limit to predator biomass is independent of predator weight.