Predation risk and social interference as factors influencing habitat selection in two species of stream-dwelling waterstriders

We investigated how an aggressive species of waterstrider, Aquarius remigis, and potential predators, green sunfish (Lepomis cyanellus), affected the habitat use and mating behaviors of a less aggressive congeneric species, A. conformis. Although these species sometimes co-occur, A. remigis typically lives in small streams with few or no fish, whereas A. conformis are typically in medium- or large-sized streams with large populations of potentially predatory fish. We tested in separate experiments in seminatural streams: 1) the effect of fish on behaviors of A. conformis; 2) the effect of A. remigis on A. conformis, and 3) the habitat use of A. conformis when given a choice between pools with A. remigis or fish. The first experiment showed no effect of fish on either mating behaviors or microhabitat use of A. conformis. This is in surprising contrast to the strong effects of fish already documented in A. remigis. The second experiment showed that the mating activity of A. conformis was reduced when A. remigis were present; hence, A. conformis should avoid A. remigis. Finally, when A. conformis were presented with a choice between two pools, one containing A. remigis and the other containing fish, both single males and pairs of A. conformis chose the pools with fish. In contrast, the habitat use of single female A. conformis was not affected by either fish or A. remigis. Results from these experiments demonstrate that closely related species exhibit contrasting social and antipredator behaviors and that aggressive social behavior is an important determinant of habitat partitioning.