Differences in behavior and prey preference influence size-structured predator-prey interactions among top predators

Background/Question/Methods

Size structure, both within and between species, can strongly influence the outcome of predator-prey interactions. In fishless pond ecosystems, large-bodied aquatic insects often represent a significant proportion of the top predator guild and many of these species undergo drastic changes in body size over ontogeny. Many of these predators are also generalists that select prey based on size rather than species identity. As a result there is significant potential for cannibalism and intraguild predation within assemblages of top predators in fishless ponds. We conducted several experiments to examine the direction and intensity of predator prey interactions between two important predators in fishless pond ecosystems, Cybister fimbriolatus and Anax junius. Predation trials conducted in the laboratory were used to examine behavioral interactions between pairs of C. fimbriolatus and A. junius across two size classes of each species. Mesocosm and field experiments were used to examine the impacts of top predator identity on prey survival and assess the relative intensity of cannibalism versus intraguild predation.

Results/Conclusions

Across several mesocosm experiments, C. fimbriolatus was found to be highly cannibalistic and occasionally consumed similar-sized conspecifics. This is in contrast to A. junius, which rarely consumed similar-sized conspecifics. These results may be partly explained by the high activity level of C. fimbriolatus relative to A. junius observed in predation trials.  Results from both mesocosm experiments and field experiments indicate that C. fimbriolatus prefers larger bodied prey relative to A. junius and this preference appears to be present at multiple developmental stages.  Numerous studies have classified A. junius and other Aeshna species as top predators in fishless ponds. However, the results presented here suggest that C. fimbriolatus may have a greater impacts on community structure in these ecosystems and those impacts may be extended over a relatively large proportion of the aquatic larval stage. Ontogenetic niche shifts have the potential to change the trophic position of predators. Understanding when these shifts occur across species can help ecologists better characterize how developmental stage impacts an individual's ecological role in a community.