A comparative study of the functional response of four species of carnivorous stoneflies

Abstract

SUMMARY1. Comparisons were made of the functional responses of mature larvae ofPerlodes microcephalus,Isoperla grammatica,Dinocras cephalotesandPerla bipunctata. Experiments were performed in stream tanks with natural substrata and glass bottoms, so that feeding could be observed above and below the substratum. There was one stonefly per tank and one of 10 prey densities between 20 and 200 larvae of eitherChironomussp. orBaetis rhodaniper tank. Consumed prey were replaced in a first set of experiments but not in a second set. Additional experiments assessed intraspecific interference between larvae of each predator species (two to five predators per tank).2. The number of prey eaten increased curvilinearly with prey density. The relationship was described by two models, a Type II instantaneous model and its integrated equivalent, for experiments with and without prey replacement, respectively. Handling time did not change significantly with prey density, and was the same for experiments with and without prey replacement. Estimates of attack rate were similar for the two models, but varied between prey type and predator species. Handling time varied considerably but was normally distributed for each prey type and predator species. Mean handling time varied forChironomusfrom 39 s forIsoperla, which rarely ate a whole larva, to 57 s forPerlodesand forBaetisfrom 116 s forPerlodesto 167 s forPerla. All predators were more efficient at capturingBaetis, but the longer handling time forBaetisensured that moreChironomuswere eaten. It was concluded that these variations in attack rate and handling time were related to activity and growth differences between the predator species, and that experiments with and without prey replacement could both be relevant to the field, depending on how the predators searched for food.3. In the interference experiments, mean handling time did not change with increasing predator density, but attack rate decreased curvilinearly, the decrease varying from negligible forIsoperlato marked forPerlodes. Prey capture decreased with decreasing attack rate. Therefore, interference reduced prey consumption, but this effect was negligible forIsoperlaand increasingly severe in the orderDinocras,PerlaandPerlodes.