Alternative Approaches for Evaluating the Effects of Environmental Gradients on Stream Invertebrates

Taxonomic optima, i.e., the environmental conditions at which peak abundance is expected, are widely used to analyze assemblage response to environmental gradients. Typically analyses consider how the mean optima for an assemblage change over a gradient. In this study we consider how other characteristics of the assemblage vary. We ask two questions. First, how does variability in assemblage optima change over an environmental gradient? Second, is there a trade-off between environmental generalists, i.e., genera having a wide range of occurrence, and specialists, i.e., genera having a narrow range of occurrence, over these gradients? We use data collected in the U.S. EPA Environmental Monitoring and Assessment Program for benthic invertebrate assemblage and stream characteristic information. Optima estimates for genera are derived from logistic regression models over environmental gradients in streams. Assemblage characteristics are based on the distribution of these optima for genera occurring at a site. These results suggest alternative approaches for evaluating the effects of environmental gradients on stream biota.