Strategies of the invasive macrophyte Ludwigia grandiflora in its introduced range: Competition, facilitation or coexistence with native and exotic species?

The success of invasive species is due to their ability to displace other species by direct competition. Our hypothesis is that the strategy of the invasive L. grandiflora differs according to the growth form of this plant (submerged/emergent) and to its density, and the presence and the density of neighbouring species, during the first step of introduction phase. Moreover, we also suppose that the invasive species L grandiflora affects the European native aquatic macrophyte and that invasive species can facilitate the establishment, growth and spread of exotic species coming from the same biogeographical area (the Invasional Meltdown Hypothesis). We studied the relationships between three exotic species coming from South America (Ludwigia grandiflora, Egeria densa and Myriophyllum aquaticum) and two European macrophyte species (Ceratophyllum demersum, Mentha aquatica) in monocultures and in mixed cultures. The experiments were carried out in containers placed in a greenhouse for one month in spring 2011. We measured six morphological traits to test the intraspecific and interspecific interferences. In accordance with our hypothesis, the strategy of L. grandiflora differed between its emergent growth form and its submerged growth form, whereas the establishment and the growth of L grandiflora did not seem to be facilitated by other exotic species (i.e. E. densa and M. aquaticum). The interspecific effect between C. demersum or E. densa on submerged L. grandiflora was stronger in inhibiting plant growth than the intraspecific interferences of L grandiflora on itself. Mutual inhibition of root production and growth was observed between L grandiflora and M. aquatica. However, L. grandiflora seemed to have little impact on native species, which may coexist with L. grandiflora during the early stages of L grandiflora establishment in the introduction area. L grandiflora stimulated the growth and the vegetative reproduction of E. densa. L grandiflora facilitated the establishment of E.densa in accordance with the Invasional Meltdown Hypothesis. L grandiflora stimulated the root production and the growth of M. aquaticum at low densities and inhibited it at high densities. L grandiflora, the first introduced plant in France, could slightly facilitate the growth of E. densa. However, spatial heterogeneity or differential use of resources could explain the coexistence of L grandiflora and M. aquaticum in the same environment.. (C) 2013 Elsevier B.V. All rights reserved.