Mechanisms of intra-and interspecific interference between larval stoneflies.

Behavior of focal individuals of two potentially competing sympatric stonefly species, Megarcys signata and Kogotus modestus (Perlodidae), was videotaped in flow-through plexiglass arenas placed in the East River, Gunnison County, Colorado. Focal individuals were observed alone and in pairs with conspecifics and allospecifics at four prey (Baetis bicaudatus, Baetidae, Ephemeroptera) densities to determine whether competitors and prey resource levels affected prey capture rates. Presence of conspecific or allospecific competitors reduced stonefly prey capture rates, especially for Kogotus, the smaller of the two species, due to a significant decline in predator-prey encounter rates with competitors present. This competitive effect was not observed at the lowest and highest prey densities due to very low or very high predator-prey encounter rates, respectively. Thus, interference affected feeding rates only at intermediate prey densities. Competitors had no effect on the probability of attacks per prey encounter or capture success per attack. Within each stonefly species the effects of intra-and interspecific interference on feeding rates were similar, even though behavioral responses by both stoneflies to interspecific encounters were more frequent than to encounters with conspecifics. Kogotus showed the highest levels of response to encounters with other stoneflies, maintaining those high levels of response to Megarcys over all prey densities. Further, male Kogotus, which are the smaller sex, responded more frequently to competitive interactions than did females. These data are consistent with the hypothesis that interspecific interference was asymmetrical with Megarcys, the larger species, being the superior competitor.