PREDATION RISK AND SOCIAL INTERFERENCE AS FACTORS INFLUENCING HABITATSELECTION IN 2 SPECIES OF STREAM-DWELLING WATERSTRIDERS

We investigated how an aggressive species of waterstrider, Aquarius re migis, and potential predators, green sunfish (Lepomis cyanellus), aff ected the habitat use and mating behaviors of a less aggressive congen eric species, A. conformis. Although these species sometimes co-occur, A. remigis typically lives in small streams with few or no fish, wher eas 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 behavior s 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 wit h 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 documente d in A. remigis. The second experiment showed that the mating activity of A. conformis was reduced when A. remigis were present; hence, A. c onformis should avoid A. remigis. Finally, when A. conformis were pres ented with a choice between two pools, one containing A. remigis and t he other containing fish, both single males and pairs of A. conformis chose the pools with fish. In contrast, the habitat use of single fema le A. conformis was not affected by either fish or A. remigis. Results from these experiments demonstrate that closely related species exhib it contrasting social and antipredator behaviors and that aggressive s ocial behavior is an important determinant of habitat partitioning.