Trophic Cascades, Detrital Dynamics, and Biotic Resistance: Roles of Crayfish in California Streams

Dominant species can have wide-ranging impacts on food-web dynamics and ecosystem processes. Here we examined the direct and indirect impacts of the non-native omnivore, Signal Crayfish, on stream dynamics. In central California where we worked, these crayfish were introduced approximately 100 years ago. We performed a large-scale density manipulation and a comparative study to investigate the impacts of crayfish on stream ecosystems. We found that crayfish cause a trophic cascade, driving a threefold decrease in benthic invertebrate densities and the same magnitude increase in algal accrual rates. Crayfish also had strong direct impacts on leaf detritus dynamics, explaining 70 percent of the variation in leaf litter breakdown rate. However, it appears that over time this strong negative direct effect is negated by an opposing indirect effect caused by crayfish predation on shredding benthic invertebrates, especially large caddisflies. In addition, we discovered that crayfish are significant predators of invading New Zealand mudsnails. Based on lab experiments that quantified the functional feeding response of crayfish to New Zealand mudsnails, there is the potential for crayfish to provide biotic resistance to these invaders. Crayfish can be a dominant species that control aquatic ecosystems via a combination of direct and indirect impacts.