Evaluating the effects of environmental gradients on stream communities: the role of optima and environmental specialists.
Pollard, Amina*,1, Yuan, Lester1, 1 US Environmental Protection Agency, Washington, DC, USA
ABSTRACT- Community characteristics are commonly used to assess the condition of ecosystems. For example, taxonomic optima, i.e., the environmental conditions at which peak abundance is expected, are widely used to infer the environmental characteristics of an ecosystem. Typically analyses consider how the mean optima for an assemblage change over a gradient. One of the assumptions of this type of approach is that specialist taxa, i.e., genera having a narrow range of occurrence, comprise a moderate portion of the assemblage across a gradient. Then changes in the occurrence of these specialist taxa are used to discriminate between environmental conditions. In this study we consider how the occurrence of specialist benthic invertebrate genera changes over different environmental gradients in streams (metal and nutrient concentrations, pH, temperature, and percent fine substrate), and ask how this distribution influences our ability to infer environmental condition from assemblage characteristics. We use data collected in the US EPA Environmental Monitoring and Assessment Program (EMAP) for benthic invertebrate and stream characteristic information. Optima estimates for genera were derived from logistic regression models over environmental gradients in streams. The range of occurrence was used to categorize genera as generalists or specialists. Preliminary results for a temperature gradient indicate that there is a significant positive correlation between mean assemblage optima and temperature, and that environmental specialists are common in cold-water streams but less frequently encountered in higher temperature streams. In this case, the occurrence of environmental specialists suggests that we may be better at inferring cold-water conditions than warm-water conditions using benthic invertebrate assemblages. These results suggest alternative approaches for evaluating the effects of environmental gradients on stream biota.
Key words: community, stream, environmental specialist, environmental gradient
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