Relative benefit of the invasive Echinogammarus berilloni (Catta, 1878) over native gammarids under fish predation (Gasterosteus aculeatus Linnaeus, 1758)

Abstract

Running waters of Central Europe are progressively invaded by the southwest European amphipod Echinogammarus berilloni—as currently observed in the karstic upper River Lippe catchment (Paderborn Plateau, northwest Germany). We tested experimentally whether the succession of the invader is favored by lower predation intensity of three-spined sticklebacks (Gasterosteus aculeatus) compared to predation on the native gammarids Gammarus pulex and G. fossarum. Predation experiments were combined with two modifiers: species interaction (separate exposure of each species vs. joint exposure of each native species with the invader) and habitat structure (absence vs. availability of refuge structures as hideouts against predation). When the species were exposed separately to the predator and no refuges were offered, predation rates did not differ significantly. When refuges were present, predation success generally decreased, but was relatively lower on the invader E.berilloni than on the native species. When native and the invasive species were exposed jointly to predation, predation success decreased further in both species combinations. However, under both conditions, with and without refuge structures, survival of E. berilloni was significantly greater than in G. pulex. The findings corroborate our assumption that the invader E. berilloni profits from sympatric occurrence with native gammarid species, especially when hideout options are available. Whether this effect was caused by competition for refuges or simply by a relatively lower activity level of E. berilloni and smaller predator stimulus for the fish remains to be cleared. Without doubt, the difference in predation is a factor that contributes to the invader’s success.