Is Predaceous Stonefly Behavior Affected by Competition?

Behavioral experiments were carried out in flow—through observation boxes in New York and Colorado streams to determine whether interactions between pairs of predaceous stoneflies were characterized by exploitative or interference competition and to determine the effect of prey density on such interactions. The presence of a competitor generally reduced the number of attacks and the time that predaceous stoneflies spent eating mayflies (Baetidae). Coexisting perlids (Acroneuria carolinensis and Agnetina capitata) in the New York stream and perlodids (Megarcys signata and Kogotus modestus) in the Colorado stream had mutual negative effects on each other's foraging behavior despite significant size differences between species (first larger athan second). A larger omnivorous stonefly (Pteronarcella badia, Pteronarcydae) also suppressed the feeding behavior of Megarcys, but a reciporcal effect was not observed. Most competitive encountes resulted in submissive or evasive behavior by one or both stoneflies ultimately leading to avoidance of further contact. Some stoneflies, however, were occassionally aggressive, either toward consecifics (Kogotus) or toward a smaller species or predator (Megarcys toward Kogotus). This behavior is consistent with the hypothesis that interference is a mechanism of competition among these stoneflies. The frequency of aggressive interactions increased for all stoneflies when prey density increased from 0 to 15 individuals per box but declined at 30 prey individuals per box. This observation suggests that the level of aggression is variable with availability of a food resource. As prey density increased, behavior associated wth predator—prey interactions (attacks, feeding) increased for some stoneflies, and other activities (dispersal, cleaning, stationary behavior) decreased, generally increasing the predator's probability of finding and consuming prey. Large species of stoneflies had shorter prey handling times than small species, and all stoneflies had very low rates of capture (successful captures 10—25% of total attacks. We postulate that direct interference competition from other stoneflies and predator—mediated dispersal of prey (unstable prey patches) reduce the benefit of aggregating in regions of high prey density.