Critical review of the literature on key invasive alien freshwater plants in Europe with special focus on their impact on the invaded ecosystems

Biological invasions of alien aquatic plants are a major threat to conservation of freshwater habi­tats, as well as a socio-economic problem. Introduced primarily by human activities, these alien plants compete with native species, reduce local biodiversity and alter structure and function of the aquatic ecosystems. This review examines the most relevant scientific literature on the major invasive alien aquatic plants (IAAPs) found in Europe ( Alternanthera philoxeroides , Azolla filiculoides , Cabomba caroliniana , Egeria densa , Elodea canadensis , E. nuttallii , Gymnocoronis spilanthoides , Hydrilla verticillata , Hydrocotyle ranunculoides , Lagarosiphon major , Lemna minuta , Ludwigia grandiflora , L. hexapetala , L. peploides subsp. montevidensis, Myriophyllum aquaticum , M. heterophyllum , Pistia stratiotes , Pontederia crassipes , Salvinia molesta ), with a special focus on impacts exerted by these species on invaded freshwater ecosystems. It includes both qualitative and quantitative analyses and evaluates the temporal trends of the scientific contributions considering the impact and management of these species in Europe and worldwide. Despite a recent increase in contributions on these IAAPs, the knowledge on their impacts in Europe remains mainly concentrated on a few species and rather fragmented and deficient on others. In fact, evident inequalities emerge among these IAAPs in both the number of dedicated contributions and in the treatment of each species, as some of them are currently excluded from the list of IAS of Union Concern. Moreover, the European level research on these IAAPs turned out to be scarcely influenced by the “listing” effect (i.e. inclusion of IAAPs in the Union list), showing little or no increase in the number of studies on impact or management. The studies found on impacts document that the selected IAAPs largely exert environmental impacts on invaded ecosystems by altering both the abiotic (water chemical and physical factors) and biotic (plant and animal communities) components. The impact mechanisms of these species vary (chemical, physical, structural, competitive, toxicity) and were classified according to the EICAT (Environmental Impact Classification for Alien Taxa) protocol. Overall, the review reveals significant gaps in knowledge about the environmental impacts of most of these IAAPs in Europe, despite some being included in the list of IAS of Union concern. To address these gaps and protect European freshwater ecosystems from biological invasions, more field studies supported by laboratory investigations are needed, followed by effective management interventions. In addition, it is considered necessary that impactful alien species with a wide distribution in Europe, but which are currently excluded from the EU list, be included as soon as possible. This would allow for coordinated management practices at the European level, which are essential for their containment.