The Relationship Between Reparian Vegetation and the Trophic Structure of Aquatic Macroinvertebrate Communities Within the Greater Yellowstone Area

Abstract

This study investigated the effects of reduced vegetation cover on stream ecosystems within the Greater Yellowstone Area. The emphasis of the research concerned whether these effects express themselves within the aquatic macroinvertebrate community. Three general vegetation categories, each comprising three to four streams, were used to distinguish gradations of riparian vegetation: dense cover (control), moderate cover, and low cover. Within each site, five aquatic macroinvertebrate samples were collected from riffle areas with a Hess sampler. The organisms were identified to the lowest practicable taxonomic level and assigned to trophic groups on the basis of gut content analysis. Riparian and in-stream variables also were measured. Statistical analyses were performed to elicit significant differences in riparian, stream, and insect community indices among the streams and to relate the structure (i.e., density, diversity, trophic organization) of the insect assemblage to the riparian and stream variables. The analysis of the data yielded no statistically significant differences among the vegetation categories with respect to embeddedness, bank overhang, and several physicochemical indices of water quality. Similarly, macroinvertebrate densities, biomass, and species richness did not differ among the categories. The trophic group analysis, however, revealed that the abundance and biomass indices of the scraper community were significantly greater within the low cover sites than within the other streams. Conversely, the indices of the shredder community were significantly higher within the control streams than within the other sites. It is proposed that a shift from allochthonous to autochthonous food resources, corresponding to the different vegetation gradients, accounted for these observations. Of critical importance is the fact that the nominally shredder genera were not excluded from or even reduced in abundance within the low cover streams, but rather seemed to have adapted to the decline in allochthonous matter by shifting their mode of food acquisition from shredding to gathering. The implications of such trophic generalism for understanding the structure and function of aquatic macroinvertebrate communities are discussed.