Reciprocal indirect facilitation between an invasive macrophyte and an invasive crayfish

Exotic invasive species are one of the major threats for aquatic ecosystems. However, the interactions between exotic plants and exotic herbivores have been little studied, despite their role in invasion success. Here, we tested how interactions between an invasive plant and an invasive herbivore could affect their own growth and their colonization abilities in freshwater ecosystems. Ludwigia grandiflora and Procambarus clarkii, two exotic invasive species which co -occur in some French wetlands, were used for our laboratory experiment and field survey. Although L. grandiflora was consumed by crayfish, its growth was not significantly affected by crayfish density. Indeed, the final living biomass of L. grandiflora was similar to those of controls, despite significant losses induced by crayfish. Furthermore, L. grandiflora became more fragmented in the presence of a high density of P. clarkii, which might increase its dispersal abilities. In addition, our field survey revealed that the abundance of crayfish was higher in a patch invaded by L. grandiflora than in an uninvaded patch. Thus, the outcome of the interaction between the invasive plant and the invasive crayfish, via its positive effect on the invasion dynamics of these two species, could have dramatic consequences for native communities of freshwater ecosystems.