Community interactions affecting the relative abundances of native and invasive amphipods in the St. Lawrence River

The Eurasian amphipod Echinogammarus ischnus is reportedly replacing the common native amphipod Gammarus fasciatus in the Great Lakes – St. Lawrence River system. A potential mechanism for this replacement is competition mediated by resident species. Other Eurasian invaders, dreissenid mussels (Dreissena polymorpha and Dreissena bugensis), dominate rocky substrates throughout the system and might be promoting the rapid expansion of E. ischnus by providing habitat and refugia from predation. Using an in-situ predator-exclusion experiment, we tested the hypothesis that E. ischnus is better able than G. fasciatus to use Dreissena spp. colonies as refugia and thus is less susceptible to predators in the St. Lawrence River. Co-occurring E. ischnus and G. fasciatus showed similar increases in density in the presence of Dreissena spp., in spite of E. ischnus having evolutionary experience with Dreissena spp. Predators reduced the density of both amphipod species, but E. ischnus was more susceptible to predation on dreissenid substrates, which suggests that predation mediates the coexistence of G. fasciatus and E. ischnus in the river.