Impact of an aquatic invasive plant: the case of Ludwigia grandiflora.

Aquatic plants have been widely introduced around the world trough horticulture and the aquarium trade. Because different aquatic plants can have contrasting effects on water chemistry, habitat structure and food resources, they can have dramatic effects on many parts of aquatic ecosystems. Within the framework of the ALIEN IMPACT project we investigated the impact of Ludwigia grandiflora, a highly invasive aquatic weed in Belgium and other European countries, on different functional groups: native plants, invertebrates and pollinators. Up to 32 ponds were selected for the impact study on plants and invertebrates. Native plant / invertebrate richness, abundance and composition were compared between invaded and uninvaded sites in close vicinity. To study the impact on pollinators an experimental design was set up to estimate the pollinator-mediated effect of the floral abundance of L. grandiflora (difference in cover of the alien plant) on native potted Lythrum salicaria plants. Our results showed that L. grandiflora significantly reduced native plant species richness, with fewer species in heavily invaded plots. Uninvaded ponds supported a more distinct invertebrate community, including species (e.g. Ephemeroptera) that were rare or missing from invaded ponds. Results on pollinators showed that more insects were recorded on L. salicaria plants when the cover of L. grandiflora was low compared to the control plants (weak 'facilitation' effect). Overall, the impacts on the different functional groups were variable, but related to density of the invasive plant.