An exotic macrophyte bed may facilitate the anchorage of exotic propagules during the first stage of invasion

Abstract

The success of invasive macrophytes is due to the ability of introduced fragments to survive, regenerate and colonise freshwaters. In this study, carried out in an experimental garden, we evaluated: (i) the ability of Myriophyllum aquaticum fragments with or without an apex, to survive and colonise and (ii) the effects of a macrophyte bed on the colonisation success of two non-native species (Ludwigia grandiflora, M. aquaticum). Fragments of M. aquaticum with apical stems had the highest survival rate (85 vs. 62%) and colonisation abilities (i.e. 9% of rate of anchorage with a macrophyte bed). The anchorage rate of L. grandiflora was stimulated by the presence of the non-native Elodea canadensis and the relative growth rate of M. aquaticum fragments was higher in the presence of a L. grandiflora bed. Facilitation may enhance the colonisation of some non-native species during the first stage of invasion (colonisation and regeneration). However, the presence of a macrophyte bed limits the chances of L. grandiflora rooting in the sediment. This result suggests that spatial competition may occur at the beginning of growth in shallow waters. Competitive or facilitative interactions between neighbouring species depend both on the introduced species and on macrophyte bed composition.