Predator body size variation may be a signature of spatial niche differences

Background/Question/Methods

Coexisting species often differ substantially in body size. A classic explanation for this pattern is that body size variation is a signature of resource partitioning. If larger species utilize larger resources and smaller species utilize smaller resources, differently sized species avoid resource competition. Though elegant, the adequacy of this explanation for predators is questionable. While there is a positive relationship between predator body size and average prey body size, a predator’s resource niche changes asymmetrically with body size. In particular, though larger predators include larger prey in their diet, they also tend to eat the full range of prey eaten by smaller predators. Thus, small predators share their entire resource niche with larger predators and resource niche differences alone cannot fully explain observed body size variation among coexisting predators. I propose that in the absence of sufficient resource niche differences, body size variation may instead be a signature of spatial niche differences, which lead to a competition-colonziation tradeoff and coexistence. To test this theory I use a nested metapopulation model where the within-patch dynamics are given by a standard Rosenzweig-MacArthur predator-prey model that incorporates allometric scaling relationships and where the between-patch dynamics are functions of the within-patch equilibrium population sizes.

Results/Conclusions

I demonstrate that, indeed, body size differences of coexisting predators may be a signature of spatial niche differences rather than resource niche differences. Furthermore, I show that there are limits to both the similarity and dissimilarity of coexisting predators in terms of body size. While analogous limits to similarity have been found using metapopulation models of competing species that do not include resource dynamics, the explicit consideration of the prey species introduces a novel limit to dissimilarity. I show that the relationship between a predator’s dispersal rate and body size significantly affects how similar or dissimilar coexisting predators can be. Overall, I show that the effect of body size on spatial niche differences may help explain observed variation in body size of coexisting predators.