Fishing spiders, green sunfish, and a stream-dwelling water strider: male-female conflict and prey responses to single versus multiple predator environments

Many studies have experimentally addressed the eAects of a particular predator species on prey be- havior. In nature, however, prey frequently face multiple species of predators that often vary in their predatory mode and in their level of predation risk. Relatively few studies have considered prey responses under these complex conditions. In Kentucky, the stream-dwelling water strider (Aquarius remigis) coexists with many po- tentially dangerous predators, two of which are the green sunfish (Lepomis cyanellus) and the fishing spider (Dolomedes vittatus). Green sunfish occupy stream pools and attack water striders from below. In contrast, fish- ing spiders hunt along stream shorelines where they perch on overhanging vegetation or rocks and attack water striders near shore. We compared how A. remigis individuals respond to these two very diAerent predators in pools with one or both predators. The presence of sunfish in pools had strong eAects on male water strider behavior, including increased use of three types of refuge from sunfish (riCes, climbing out of the water, sitting on the water but at the edges of pools), decreased activity and a decreased number of aggressive males on the water. Spiders also influenced water strider behavior; male water striders avoided spiders by shifting away from the edges of pools. Comparisons of the eAects of the two predator species showed that in general, anti- predator responses by male water striders were stronger in pools with fish alone than in those with spiders alone. In the presence of both predators, male water strider behavior (microhabitat use and activity) was generally similar to behavior in the presence of fish alone. In contrast, female water striders showed no significant response to the presence of sunfish, and little response to the presence of spiders. This lack of response could be because females spent much of their time in refuges even in the absence of predators (apparently hiding from harassment by males). Both spiders and fish caused de- creases in water strider mating activity. The presence of fish reduced both the number of matings per pool (mating frequency), and mean mating durations. Spiders induced a decrease in mean mating duration, but not in mating frequency. The largest reductions in mating ac- tivity occurred in pools with both predators present. Pools with either spiders or fish alone suAered 15-20% water strider mortality during our experiment (versus no mortality in predator-free pools). Extant theory suggests that when prey face conflicting microhabitat responses to two predators (as in this study), the predators should have facilitative eAects on predation rates (i.e., prey that avoid one predator are often killed by the other and vice versa). Mortality rates in pools with both predators present, however, were not significantly diAerent from that predicted by a null model of multiple predator ef- fects. The lack of predator facilitation can be explained by the compensatory reductions in water strider activity and mating activity in the presence of both predators.