Intraspecific interference in a tropical stream shredder guild

The structure of stream communities is typically thought to be driven by stochastic events such as floods, in contrast with communities in many other systems in which biotic interactions have a major role. However, it is possible that biotic interactions are important in some situations in streams, especially where resources are limited and physical influences are stable for substantial periods. Leaf litter – the main energy source and a distinct habitat in forest streams – constitutes a patchy resource where biotic interactions among and within consumer species are likely to occur. The intraspecific interference in four leaf-eating species (shredders), common in Australian tropical streams, was experimentally examined – Anisocentropus kirramus (Trichoptera : Calamoceratidae), Lectrides varians and Triplectides gonetalus (Trichoptera : Leptoceridae) and Atalophlebia sp. (Ephemeroptera : Leptophlebiidae). All four species showed some degree of intraspecific interference, indicated by lowered leaf breakdown rates when density increased. Breakdown rates per capita decreased exponentially for all species with increased density, with slight differences among species. These differences were more evident when body size was taken into account, with breakdown rates depressed at lower densities for the two species with larger body sizes, T. gonetalus and Atalophlebia sp. Overall breakdown rates did not always increase with higher densities, because they were compensated for by depressed individual breakdown rates. Our results indicate that intraspecific interference can be an important mechanism regulating leaf breakdown in streams.