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 effects of a particular pred ator species on prey behavior. In nature, however, prey frequently face mul tiple species of predators that often vary in their predatory mode and in t heir level of predation risk. Relatively few studies have considered prey r esponses under these complex conditions. In Kentucky, the stream-dwelling w ater strider (Aquarius remigis) coexists With many potentially dangerous pr edators, two of which are the green sunfish (Lepomis cyanellus) and the fis hing spider (Dolomedes vittatus). Green sunfish occupy stream pools and att ack water striders from below. In contrast, fishing spiders hunt along stre am shorelines where they perch on overhanging vegetation or rocks and attac k water striders near shore. We compared how A. remigis individuals respond to these two very different predators in pools with one or both predators. The presence of sunfish in pools had strong effects on male water strider behavior, including increased use of three types of refuge from sunfish (ri ffles, 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 th e water. Spiders also influenced water strider behavior; male water strider s avoided spiders by shifting away from the edges of pools. Comparisons of the effects of the two predator species showed that in general, antipredato r responses by male water striders were stronger in pools with fish alone t han in those with spiders alone. In the presence of both predators, male wa ter strider behavior (microhabitat use and activity) was generally similar to behavior in the presence of fish alone. In contrast, female water stride rs showed no significant response to the presence of sunfish, and little re sponse to the presence of spiders. This lack of response could be because f emales spent much of their time in refuges even in the absence of predators (apparently hiding from harassment by males). Both spiders and fish caused decreases in water strider mating activity. The presence af fish reduced b oth the number of matings per pool (mating frequency), and mean mating dura tions. Spiders induced a decrease in mean mating duration, but not in matin g frequency. The largest reductions in mating activity occurred in pools wi th both predators present. pools with either spiders or fish alone suffered 25-20% water strider mortality during our experiment (versus no mortality in predator-free pools). Extant theory suggests that when prey face conflic ting microhabitat responses to two predators (as in this study), the predat ors should have facilitative effects on predation rates (i.e., prey that av oid one predator are often killed by the other and vice versa). Mortality r ates in pools with both predators present, however, were not significantly different from that predicted by a null model of multiple predator effects. The lack of predator facilitation can be explained by the compensatory red uctions in water strider activity and mating activity in the presence of bo th predators.