Competition and the Structure of a Benthic Stream Community

The mechanisms structuring benthic stream communities composed largely of highly mobile taxa are poorly known. I studied the role of competition in structuring the benthic community of hard substrates in a coldwater stream dominated by mobile insects that grazed upon periphyton and by relatively sessile filter-feeding insects. I expected competitive interactions among periphyton grazers to be significant and chronic because the community was not subject to frequent flow-related disturbances and grazer densities were high despite consistently low periphyton availability. Laboratory and field experiments were performed to determine: (1) the potential for, and relative strengths of, competition within and between two common, widely distributed grazers, the caddisfly Glossosoma nigrior and the mayfly Baetis tricaudatus, (2) the role of Glossosoma in affecting the or- ganization of the community, and (3) the mechanisms of competition. Strong intraspecific effects on Baetis survivorship were observed in laboratory enclosures within the range of natural densities, but interspecific effects were not detected. Glossosoma survivorship was not affected by competition with itself or with Baetis. Individuals of both species experienced significant intra- and interspecific effects on growth, but the relative strengths of these effects differed between species. The effects of Baetis on Glossosoma growth were stronger than the effects of Glossosoma on itself. In contrast, Baetis growth was much less affected by the presence of Glossosoma than by conspecifics. Both species strongly depressed periphyton abundance, indicating that observed competitive effects resulted, in part, from exploitation. Exclusion of Glossosoma from habitats in the field resulted in a marked increase in periphyton biomass throughout a 10 -mo study in which the movements of other taxa were not restricted. Most periphyton grazers responded to this increase in resource availability with increased densities in the Glossosoma exclusion treatment relative to the control. The size distributions of several chironomid grazers suggested that larval growth rates were greater in the absence of Glossosoma than with Glossosoma present. The larvae of two relatively small-bodied filter-feeders (Simulium, Rheotanytarsus) responded positively to the removal of Glossosoma, but larger bodied species (Hydropsyche, Brachycentrus) were not affected. There was not strong evidence for seasonal variation in the intensity of competition. Effects of Glossosoma on filter-feeders appeared to result from a form of interference competition, but there was no evidence for the operation of interference among grazers. Glossosoma had no effect on the colonization of habitats by grazers, and the feeding rates of Baetis and Glossosoma were not affected by the presence of conspecifics or heterospecifics. Contrasting patterns in periphyton abundance between the laboratory and field exper- iments suggest that, when their movements are unrestricted, Baetis departs periphyton patches at higher food densities than does Glossosoma to search for food elsewhere. Thus, strong competitive effects of Baetis on Glossosoma in the laboratory experiments resulted, in part, from limited Baetis dispersal in the enclosures. Baetis and Glossosoma appear to exhibit trade-offs in feeding efficiency and the costs of exploiting food patches that should help to ensure their coexistence on a spatially variable food resource. Results of this study suggest that the structure of benthic communities in coldwater streams composed largely of mobile taxa may often be strongly influenced by Glossosoma. A similar role has been observed for sessile, territorial grazers in other systems. Glosso- soma's dominance does not result from aggressive behavior, but from physiological, be- havioral, and life cycle attributes that allow it to regulate periphyton biomass throughout the year.