Does prey availability affect predator-prey interactions in an assemblage of size structured top predators?

Background/Question/Methods

Intraguild predation (IGP), is a mixed competition – predation interaction where a predator and its prey also compete for a shared resource. Size structure plays an important role in IGP and can determine whether the interaction between predators is symmetric (top predators eats the intermediate predator and vice-versa) or asymmetric (top predator eats the intermediate predator). Size structure within species can also lead to cannibalism, which has been shown to promote the coexistence of predators in IGP systems. This experiment examined the occurrence of IGP between larval Cybister fimbriolatus and Anax junius at different developmental stages in the presence and absence of shared prey. Treatments included all pairwise combinations of two size classes of each predator in the presence and absence of shared prey. The experiment was conducted in mesocosms and survival of prey and the occurrence of IGP and cannibalism were the response variables.

Results/Conclusions

The presence of prey did not significantly reduce the occurrence of cannibalism or IGP. These interactions occurred with similar frequency across the two prey treatment levels. Both A. junius and C. fimbriolatus cannibalized smaller conspecifics, but cannibalism among C. fimbriolatus was more frequent.  Although IGP occurred in a number of replicates, cannibalism was found to more prevalent than IGP. While both A. junius and C. fimbriolatus are among the guild of top predators in fishless ponds, this experiment provides evidence that C. fimbriolatus may exert a stronger impact on the structure of the aquatic insect assemblage in these systems. Taken together these results suggest that both size structure and species identity are key factors in survival for these predators in fishless ponds.