Fast Turnover in Dominance of Exotic Species in the Rhine River Determines Biodiversity and Ecosystem Function: An Affair Between Amphipods and Mussels

Abstract

A high turnover in macroinvertebrate composition took place between 1993 and 2001 in the Upper and Middle Rhine because of successful invasion of exotic invertebrates. At the end of the 1980s, the Ponto-Caspian crustacean Corophium curvispinum and the Asiatic clam Corbicula spp. invaded and colonized rapidly the Rhine. In the middle of the 1990s, another Ponto-Caspian crustacean, Dikerogammarus villosus, colonized predominantly the littoral riprap. Today, the predatory D. villosus dominates the macroinvertebrate fauna throughout the navigable sector of the Rhine. Before its invasion, C. curvispinum, Gammarus tigrinus and Echinogammarus ischnus were the co-occurring nonindigenous amphipods. Since 1996, all gammarid species declined, except D. villosus, which nowadays occurs in high densities of more than 3,000 ind m². D. villosus interfered into the competition for space between the two filter-feeders C. curvispinum and Dreissena polymorpha. The impact of these exotics on biodiversity is species-specific: Dreissena provides new substrate for other invertebrates, Corophium reduces available habitat areas by transferring coarse and hard surfaces to fine and muddy areas, and Dikerogammarus exerts a strong predatory effect, in particular on C. curvispinum. The exotic mass invaders C. curvispinum, D. polymorpha and Corbicula spp. strongly influence ecosystem functions by linking pelagic and benthic processes by their intense filter feeding activity and thereby enhance the capacity for self-purification of the river system.