Quantification of hunting mode and habitat domain of predators has led to insights into understanding patterns of predator coexistence, although data on habitat domain for most species is lacking.  Predaceous diving beetles exhibit high richness and abundance in aquatic habitats and are effective predators on many organisms, including other dytiscids.  We collected data from natural ponds to understand one aspect of habitat domain (i.e., microhabitat use) for common dytiscid larvae and identified associations of beetle abundance and their prey.  We used a laboratory experiment to evaluate consumption of common prey under different environments.  Aquatic insects were sampled twice (early, late June) from fishless ponds near Calgary, Alberta, Canada.  Within each pond, we sampled three locations that were equidistant from one another along the pond margin.  At each location, we took three samples that varied in depth and distance from the edge.  For our laboratory experiment, we used the two most common genera of dytiscid larvae (Rhantus, Graphoderus) and three common prey (corixids, chironomids, damselflies) based on field sampling.  Single predators were placed in 1 L containers that varied in depth (7, 11, 15, 20 cm).  We placed 10 prey of each type into each container and enumerated the number of prey killed after 4 hr. 

Results/Conclusions

Beetle abundance in ponds did not vary with depth or distance from the edge for either genus.  In addition to being correlated to each other within samples, Graphoderus and Rhantus were correlated significantly with corixid abundance during both sampling periods. Rhantus abundance also was correlated with chironomids during late June, whereas beetles and damselflies never were positively correlated. Graphoderus ate significantly more corixids compared to other prey in the lab, whereas Rhantus consumed corixids and chironomids at similarly high rates.  Other combinations yielded significantly lower predation rates (damselflies by both species, chironomids by Graphoderus).  Thus, prey that were more often found in association with a particular beetle in the field also were consumed more often by that beetle in the lab.  In the lab, overall prey consumption for both dytiscid genera was highest in the shallowest depths and lower across other depths.  Specific predation results appear to match with past behavioral analysis indicating differences in hunting mode and spatial use of habitats for these dytiscids.  Thus, our field and laboratory findings provide further evidence that differences in the behavior of these predators may lead to prey specialization, which may ultimately reduce competition and promote coexistence.