Pathogen Outbreaks Reveal Large-Scale Effects of Competition in Stream Communities

Abstract

The ability of small-scale experiments to predict dynamics and patterns observed at larger scales is an important issue in ecology. In this paper, we describe responses of benthic, trout stream communities to replicated, whole-ecosystem perturbations resulting from pathogen-induced reductions in populations of a dominant grazer, the caddisfly Glossosoma nigrior. Previous small-scale experiments suggested that Glossosoma had strong competitive effects on other grazers (through exploitation) and small-bodied filter-feeders (through interference). Glossosoma populations collapsed in a number of Michigan trout streams since the late 1980s and have been maintained at low levels by recurrent pathogen outbreaks. Here we address the effects on periphyton, grazers, and filter-feeders of parasite-induced Glossosoma reductions in six streams for which considerable pre- and post-collapse data are available, and we compare the results of this large-scale perturbation to responses predicted from a small-scale experiment that had been previously conducted in one of these streams. Periphyton and most grazers and filter-feeders showed marked increases in abundance following reduction in Glossosoma populations. Several grazers that were rare or absent prior to Glossosoma collapse have established sizable populations, suggesting that they had been excluded by competition with Glossosoma. The small-scale experiment successfully predicted the direction of response of most taxa to whole-stream reductions in Glossosoma abundance, but it tended to underestimate the extent and magnitude of Glossosoma’s effects in the community. In these systems, observations at larger spatial and temporal scales have been essential to developing a clearer understanding of the mechanisms structuring communities.