A test of ratio-dependent predation
Ratio-dependent predation theory proposes that predator-prey interactions are determined by the ratio of prey to predator density rather than prey or predator density alone. We tested this theory with a factorial experiment that included four prey densities and two predator densities in outdoor mesocosms constructed to resemble freshwater ponds. Twenty, 50, 80, or 110 squirrel tree frogs (Hyla squirella) served as prey and one, or two giant water bugs (Belostoma flumineum) served as the predator. We used a model selection approach to determine if prey density, predator density, or the ratio of prey to predators explained the most variation in prey mortality rates.
Results indicated that ratio-dependent predation accounted for prey mortality rates better than prey-dependent and predator-dependent models. Per-capita tadpole mortality increased with increasing prey-to-predator ratios. We found an 86% increase in per-capita mortality rate between the lowest and highest ratios. Intermediate ratios resulted in intermediate per-capita mortality rates. The addition of another Belostoma slowed tadpole consumption indicating that predators “shared” available prey. Behaviors like aggression or mating likely slowed predator foraging rate resulting in lower tadpole mortality. These results suggest that the ratio of prey to predators likely determines how predators function to structure prey populations and communities.