Intraguild predation (IGP) is a widespread interaction that occurs when a top predator consumes an intermediate predator but also competes with the intermediate predator for shared prey. Simple three-species mathematical models suggest that IGP should be rare in natural communities; however, recent theoretical work suggests that there may be a number of mechanisms promoting the coexistence of predators in IGP systems including cannibalism and alternative prey. Shared prey density is also likely to be an important factor influencing the intensity of IGP. Here, we present the results of a series of experiments designed to examine how the density of shared prey influences both trophic and behavioral interactions between top and intermediate predators. The experimental system included Sympetrum semicinctum and Pachydiplax longipennis dragonfly larvae (shared prey), Anax junius dragonfly larvae (intermediate predator) and Dytiscus sp. beetle larvae (top predator). We manipulated the density of shared prey across a range of mesocosm sizes and also conducted laboratory experiments to quantify behavioral interactions between predators as a function of shared prey density. We expected to observe an inverse relationship between the frequency of IGP and shared prey density across all spatial scales.

Results/Conclusions

At the largest spatial scale there was a significant effect of shared prey density on intermediate predator mortality, but the pattern of IGP was not in the expected direction. IGP was observed most frequently at intermediate rather than at low shared prey density. A laboratory experiment was conducted to evaluate the hypothesis that encounter rates between predators were highest at the intermediate shared prey density and led to IGP. Results from this experiment provided little evidence that behavioral interactions between predators change as a function of shared prey density. Furthermore, in the laboratory experiment IGP occurred most frequently at low shared prey density (i.e. in the expected direction). The discrepancy between these results can be partially explained by the spatial scale of the experimental venues. In a subsequent experiment investigating these interactions across multiple spatial scales, we observed IGP most frequently at the smallest spatial scale. Given the considerable spatio-temporal variation in the abundance of prey in nature, these results highlight the importance of prey density as a factor influencing the occurrence of IGP.