Is Dikerogammarus villosus (Crustacea, Gammaridae) a 'killer shrimp' in the River Rhine system?

Communities and food web structures of aquatic ecosystems can be strongly affected by the establishment of alien macroinvertebrate species. In many European waters, the invasion of the Ponto-Caspian amphipod Dikerogammarus villosus has led to displacement of other macroinvertebrates. Predation by D. villosus is often assumed to be the key driver of the displacement based on results of laboratory studies, but this has not been verified in the field. Here, we report our investigation of the relevance of D. villosus predation in the River Rhine system using both stable isotope analyses of delta C-13 and delta N-15, and molecular analyses of D. villosus gut contents with group-specific primers aiming at macroinvertebrate prey taxa. Stable isotope analyses of D. villosus from ten sites showed mean delta N-15 values comparable to those of primary consumers. Overall, only approximately 1% of all tested primer/gut content combinations revealed DNA of the respective taxa. Both indicate minor importance of predation by D. villosus as a driver of the observed macroinvertebrate displacement. Conceivably, competitive strength due to opportunistic feeding, indicated by different niche widths between and a strong intraspecific variation of delta C-13 values of D. villosus within sites of our study, is much more important for its invasion success.