The influence of spatial heterogeneity on the behavior and growth of two herbivorous stream insects

Environmental heterogeneity can affect the behavior of organisms, but the consequences of patchiness for organismal energetics (e.g., growth, fitness) are not well understood. This study demonstrates that spatial heterogeneity can affect the growth of aquatic stream insects in laboratory streams, and reveals the behavioral mechanisms for these effects. In a 2×2 factorial design, I experimentally manipulated resource distribution (homogeneous vs. patchy, with the same overall resource levels) and current velocity (fast vs. slow) to investigate the direct and interactive effects of these factors on the drift behavior and growth of two mobile stream grazers, the mayflies Baetis bicaudatis and Epeorus deceptivus. B. bicaudatis nymphs grew larger in environments with homogeneously distributed resources than in patchy environments, and both species grew larger in fast than slow current environments. Patterns of drift behavior over the course of the study corresponded to observed differences in growth. Both species grew to larger body size in treatments where they drifted more successfully among substrates (fast-current treatments) and where they entered the drift less frequently (fast current for both species, and homogeneous treatments for B. bicaudatis). Overall, these results demonstrate that patchiness can significantly influence both the behavior of aquatic insects and the size to which these insects grow. In the light of previously published relationships between nymphal mayfly body mass and fecundity, these results suggest that patchiness in streams may have important consequences for mayfly populations.