Iris Stiers and Ludwig Triest
Ecology and Biodiversity, Department of Biology, Vrije Universiteit Brussel, Belgium
Mechanical control methods are the most widely used measures to control alien aquatic plants in Europe and their success varies considerably between growth forms and the different habitats. Further, it remains unclear whether management practices targeting alien removal actually improve habitat quality for resident biota and will have the desired impacts or if management practices are only implemented with the hope that native species are returning. The objectives of this study were two-fold, namely: to assess the effect of (i) invasive amphibious macrophyte removal on native macrophyte recovery, and (ii) Ludwigia grandiflora removal on plant-pollinator interactions of co-flowering native plants. Cover and composition of native macrophytes were investigated in 2007 in 15 invaded sites in Belgium (by L. grandiflora, Hydrocotyle ranunculoides or Myriophylum aquaticum) and again in 2015 after manual/mechanical control in 2009. To study the plant-pollinator interactions, we set up an experiment to compare visitation rate and pollen transfer of native Lythrum salicaria and Alisma plantago-aquatica in the presence of L. grandiflora and after removal. Firstly, the native vegetation showed no general increase in cover after removal of the invasive macrophytes. Species richness did not increase indicating that additional restoration management might be needed. Secondly, L. salicaria showed a significant increase in visitation rate after removal of L. grandiflora, but no difference in pollen deposition. For A. plantago-aquatica there was an increase in conspecific pollen deposition after removal of the alien species. Removal of an invasive species seems to have a strong species-specific effect and depends on the overlap in the pollinator community.