Differences in prey preference among top predators affects the size structure of prey communities

Background/Question/Methods

Changing the identity of a top predator can have profound effects on predator-prey interactions and ultimately, community structure. A well-studied example of this phenomenon is the transition from invertebrate top predators to predatory fish in freshwater habitats along a permanence gradient.  Although we know a great deal about the broad-scale differences in community structure between  these habitats,  much less is known about the interactions among invertebrate top predators in fishless ponds. In a mesocosm experiment, we examined interactions between Dytiscid water beetle larvae (Cybister sp.) and size structured assemblages of Anax junius dragonfly larvae and their resulting impacts on shared prey (Pachydiplax longipennis) survival. We also quantified prey preference of the two top predators using Chesson’s index. A field experiment in enclosures was also conducted to compare the effects of Cybister sp. and final instar A. junius on the survival of different size-structured assemblages of A. junius prey.

Results/Conclusions

In the mesocosm experiment, mortality of shared prey was strongly influenced by the degree of predation between the top predators. Cannibalism between large A. junius was rare and resulted in the greatest level of mortality for shared prey (p < 0.04). There were significant differences in prey preference between the two top predators with Cybister sp. consistently preferring larger bodied prey (p < 0.03). Although there were no significant differences in the survival of the prey community related to the identity of the top predator in the enclosure experiment, there were impacts on the mean size of the surviving prey. Prey assemblages in the presence of large A. junius top predators were significantly larger than in the presence of Cybister sp. top predators (p < 0.05). Taken together these results highlight the subtle differences in prey preference that exist between invertebrate top predators in fishless ponds and suggest that the size structure of prey communities may be heavily affected by the composition of the top predator assemblage.