One of the explanations of why introduced species can establish and pose serious threats to natives is that they enhance the likelihood of their invasion success by modifying physical environment. Alien omnivorous crayfish are known to exhibit sudden increase in their density, largely reducing the densities of macrophytes and small aquatic animals. Our previous mathematical work demonstrated that macrophyte declines by crayfish enhance their foraging efficiency on animal prey through reduction of refuges, which can create alternative stable states through a positive feedback to their own growth. To empirically test whether macrophyte reductions enhance crayfish growth, we conducted a mesocosm experiment using red swamp crayfish, an artificial macrophyte, larvae of dragonfly and chilonomids.

In accordance with our expectation, decreasing macrophyte density lowered survival rates of dragonfly and chilonomids, and enhanced growth rate of crayfish. Analysis of stable isotope ratios showed that dragonfly contributed crayfish as a main energy source. Our results suggest that macrophyte reductions by crayfish can be a potential driver of crayfish outbreaks and large reductions of animal prey. Introduction of macrophytes with a high resistance to crayfish is expected not only to protect endangered animals such as dragonfly but to reduce the density of crayfish by retarding their growth.