Rapid range extension of the Ponto-Caspian amphipod Dikerogammarus villosus in France:: potential consequences

Non-indigenous species are increasingly recognized as altering local communities in newly colonized areas. In some north European freshwater systems, the Ponto-Caspian invasive crustacean Dikerogammarus villosus (Amphipoda) is implicated to have such an effect, with general monitoring of its progress and general impact required. The present study contributes to this monitoring. D. villosus was observed in 2003 in all the major French rivers prospected (i.e. Rhine, Meuse, Moselle, Saone, Rhone, Seine, and Loire), a European region previously overlooked for its colonization. This species was also detected in some tributaries of the rivers Saone and Seine, and in Geneva Lake. The dynamics of this colonisation, inferred from samples made at different dates, show a rapid expansion westward, from its first appearance in the Saone in 1997. The colonised rivers are connected with the more important French harbours, which may facilitate future invasion of new countries. In two sites, D. villosus rapidly became the dominant crustacean species after its appearance. A comparison of the amphipod fauna between 2003 and ancient surveys also shows that some native species tend to disappear in the East of France. These data suggest an ongoing homogenisation process of amphipod assemblages.