Shift between carnivory and omnivory in stream stonefly predators

Omnivory can have profound effects on the trophic dynamics of communities and ecosystems, as they may interact with multiple trophic levels simultaneously. Some species of large‐bodied stoneflies may be viewed as omnivores rather than true carnivores even at later nymphal stages. We evaluated the seasonal change in the diet of stonefly predators by analyzing their stable isotope ratio, gut contents, physiological activity, and food availability. A two‐source‐based mixing model based on stable isotope analysis revealed that stoneflies shifted their diet between carnivory in summer and omnivory in winter—despite the higher availability of animal prey in winter. The gut content analysis showed that swift prey (mayflies) were consumed in the summer, whereas sluggish prey (Chironomidae) were consumed in the winter. The physiological activity of stoneflies also declined markedly in winter. These results suggest that, in winter, stoneflies foraged on a mixture of Chironomidae and algae. It appears that omnivory in some stream consumers is related to the seasonal change in temperature‐dependent physiological activity, rather than prey availability.