Invasive species in the Rhône River floodplain (France): replacement of Elodea canadensis Michaux by E. nuttallii St. John in two former river channels

Abstract

The colonisation of two invasive species, Elodea canadensis and Elodea nuttallii was studied in the cut-off channels of the Rhone floodplain (France). The characteristics of the stations where the two species occurred are reported. The two species colonise new areas most often through vegetative fragments transported by water currents and the connectivity of cut-off channels to the main river is probably the essential parameter of their colonisation by the two Elodea species. The replacement of E. canadensis by E. nuttallii was compared in two cut-off channels, one subject only to the natural dynamics of the main river (flood disturbances), and the second subject to restoration work. In the first case the replacement was low and the two species were still found in the channel but did not co-exist in the same area. In the second case the replacement was very rapid (less than one year) and resulted in the total exclusion of E. canadensis. Several hypotheses are given (interspecific competition or other factors) to explain the replacement process, and more studies are needed to elucidate the mechanism of this phenomenon.