Riffle-pool formation creates a pattern of alternating habitat types in streams, potentially dividing populations of organisms restricted to either riffles or pools. Such subdivision could lead to small-scale variation in ecological patterns, such as foraging behavior, driven by riffle- and pool-scale variation in habitat or prey availability. I examined prey use by the benthic insectivore orangethroat darter (Etheostoma spectabile) in a midwestern (USA) riffle-pool stream. Etheostoma spectabile primarily occupies riffles, where it forages on benthic macroinvertebrates. I tested for effects of body size and variation among riffles on size and number of prey consumed, then examined selectivity for and against common prey items for the study reach as a whole, and for individual riffles. I also compared variation in diet breadth among riffles to patterns predicted by foraging theory. Number of total prey items and of three common prey items consumed varied significantly among riffles. Overall number of prey items consumed did not vary with darter body size, and number consumed varied with body size for only 1 prey taxon. Prey selection varied greatly among riffles, and for four of seven prey items was a function of habitat differences. Contrary to theory, diet breadth within riffles was not dependent on abundance of energetically favorable prey, largely due to a lack of selection for these prey items. These results indicate that variation among riffles can have a strong effect on prey use by E. spectabile, and that attempts to characterize foraging behavior over longer stream reaches may omit an important level of variation. For species restricted to small patches within larger habitats, ecological processes may be driven by local patch characteristics more than by larger scale phenomenon, or intrinsic factors such as body size.
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Vol. 168 • No. 1