The direction and intensity of interactions such as cannibalism and intra-guild predation are strongly influenced by the size structure of individuals within and among populations. These size-structured interactions can lead to complex indirect effects and may ultimately impact the relative abundance of species within communities. Previous studies have examined how size structure within predators or prey influence predator-prey interactions, but very few have simultaneously examined the impacts of size structure in both predators and prey. These effects were explored in a field experiment with larval odonates. The experiment employed a 2x2x2 factorial treatment design manipulating the presence/absence of two size classes of the predator (Anax junius) and one size class of the prey (Pachydiplax longipennis). The presence of a second smaller size class of the prey was held constant across all treatments.

Results/Conclusions

There were significant main effects of both size classes of the predator on small prey survival (P < 0.001), but cannibalism did not have a significant negative effect on the survival of small prey (P > 0.7). The survival of large prey was significantly reduced by the presence of large predators but not by small predators. While the size, as measured by head width, of small predators and large prey was similar, small predators consumed 65% more small prey than did large prey. While classical community ecology theory has generally considered a species to be a single homogenous entity with all individuals occupying the same trophic level, these results demonstrate that differences in body size in both predators and prey can significantly influence the outcome of predator-prey interactions and have the potential to influence coexistence between competing predators.