Background/Question/Methods The outcome of competition is generally thought to be asymmetric, with larger bodied species dominating smaller species. We used mathematical models that incorporated allometric relationships between body size and metabolic rate, and body size and prey consumption, to investigate when competition should be asymmetric. Our models consisted of two predator species exploitatively competing for prey. Prey were assumed to grow logistically in the absence of predation, with growth rates and carrying capacities dependent on prey size. Consumption of the prey by predators scaled either in proportion to metabolic demands (generally metabolic demands are found to be proportional to body size raised to the 0.75 power) or as a function of metabolic demands and predator-prey body size ratios. Results/Conclusions In both scenarios, we found that the carrying capacities of predators in the absence of competition scaled with body size to the -0.75 power (in accord with experimental results). We also found that body size influenced the strength of competition; larger species were always better competitors. In the first scenario, when consumption was proportional to metabolic demands, smaller species could always offset their competitive disadvantage through their ability to maintain higher densities. Our results from this scenario suggest that coexistence of two dissimilar sized species is not possible, regardless of magnitude of differences in the sizes of predators and prey. In the second scenario, when consumption scaled in proportion to metabolic demands and predator-prey body size ratios, coexistence was possible but depended on both sizes, of competing predators and of their prey. Our results also suggest that predators can not coexist if they or their prey are similar in size. Comparing the results from the two scenarios suggests that coexistence of predators requires size based differences in the ability to handle and consume their prey.
