THE EFFECT OF INVERTEBRATE PREDATORS ON LEAF LITTER PROCESSING IN AN ALPINE STREAM

The effect of the predators Megarcys signata (Plecoptera: Perlodidae) and Rhyacophila sp. (Trichoptera: Rhyacophilidae) on the abundance of macroinvertebrates and on the rates of leaf processing in artificial leaf packs was studied by using manipulative field experiments. Predators were confined within artificially constructed leaf packs in an alpine stream in Utah. Both predators signif- icantly reduced the rate of breakdown of leaves in the fall, but had no effect in late winter, when the most important shredder, Zapada cinctipes, emerges. Predation on shredders caused a reduction in breakdown rates and an increase in the residence time of the leaf litter in the stream. In experimental treatments where predators significantly reduced the numbers of shredders and decreased the rate of leaf processing, leaf pack half-life increased an average of 10.3 d. These experiments demonstrate that invertebrate shredders can contribute substantially to the rate of leaf processing in streams and suggest that one of the factors limiting the abundance of natural shredder populations is predation.