Habitat Selection by Stream-Dwelling Predatory Stoneflies

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Abstract
Patterns of substrate size preference of predatory stoneflies were measured in a western Colorado, USA, stream and associations were examined between substrates and other physical and biological variables. Predatory Megarcys signata (Perlodidae) were found disproportionately on large stones that were not displaced during spring runoff. Predator densities were not consistently correlated with any other physical or biological variables measured. Prey densities were neither correlated with other physical variables nor with their own resource levels, with the exception that shredders were positively correlated with their relatively rare detrital resource. Interestingly, five prey taxa were autocorrelated, indicating that they recolonized, at similar densities, the same stones replaced in the same spots in the stream. Association analysis among the five most abundant taxa of mayfly grazers revealed that stones favorable for one grazer taxon were often favorable for others, but different grazer species rarely occupied distinctly different stones. Patterns observed suggest that predators are persistent enough to avoid negative associations with prey, yet prey taxa are mobile enough to avoid positive associations with predators. Further, habitat selection by predatory stoneflies may have evolved as a response to pressure from their own predators rather than as an adaptation for efficient prey acquisition.