Flood disturbance mediates the strength of stream trophic cascades caused by trout

Abstract

Trophic cascades are important features of aquatic ecosystems, but much variation in their strength remains unexplained. Disturbance could restrict cascade occurrence by limiting predator distribution, and increase cascade strength by lowering defended herbivore abundance. These possibilities were investigated in 20 New Zealand rivers varying both in flooding disturbance (measured by bed movement) and presence of non-native predatory trout. Trout were often, but not always, associated with indirectly increasing algal abundance compared to similar rivers with native galaxiid fishes. Floods explained variation in cascades by (1) resetting algal abundance such that algal minima were related to bed movement; (2) preventing occurrence by restricting trout abundance in the most disturbed rivers; and (3) increasing potential cascade strength by removing defended herbivores. These observations indicate strong nonlinear influences of disturbance on both trophic cascade occurrence and maximum strength which will be important to consider as climate change alters disturbance regimes.