Do Stonefly Predators Influence Benthic Distributions in Streams?

Abstract

Experimental manipulations were conducted within the substrate of a Wisconsin stream and a Colorado stream to measure the effect of stonefly predators on the distribution of benthic invertebrates. Screen cages containing free predators, predators restricted from foraging, or no predators, allowed prey migration but no predator migration over 3—d periods. The presence of Acroneuria lycorias (Perlidae) in the Wisconsin stream significantly depressed the establishment of prey populations within cage microhabitats. Mechanisms for reduction were consumption of prey by the stonefly, and predator—avoidance by prey using contact and non—contact cues. The presence of Mergarcys signata (Perlodidae) reduced prey colonization in the Colorado stream by the same mechanisms, but restricted predators produced less consistent effects. This results could be due to colonization of cages by prey that could not detect predators without contact. Pternarcella badia (Pteronarcidae), a large stonefly detritivore that takes occasional prey, did not affect colonization of Colorado stream cages by prey. This differential response by prey to two moprphologically similar, but functionally different, stonefly species suggests that predator avoidance was not purely tactile. Chemotactile and non—contact chemical cues are possible mechanisms by which prey differentiated these stoneflies. The presence of A. lycorias and M. signata in experimental cages significantly increased the attrition of mayfly prey, compared to that from cages with no stonefly or a restricted stonefly in each stream. This result suggests that predation and avoidance by prey of contact with foraging predators were responsible for the higher disappearance of mayflies from cages. Free P. badia had a similar effect, probably due to tactile avoidance of this large detritivorous stonefly by some prey in the Colorado stream.