Plant invaders in European and Mediterranean inland waters: profiles, distribution, and threats

The present brief and selective review focuses on a set of invasive alien aquatic plant species (notably the 21 listed by the European and Mediterranean Plant Protection Organisation, i.e., five A2 species, 11 invasive, 2 in the Observation List, 1 in the Alert List, 2 noteworthy), on definitions in use, on introduction pathways, and ecological effects, although the social and economic effects of freshwater invaders can be diverse and substantial. Legislation, literature, and scientific research dealing with plant invasions in inland waters often merge different types of invasive plant species and invaded habitats, as there are some overlappings between the concepts of macrophyte, aquatic plants, inland waters, wetlands, and riparian habitats. Freshwater ecosystems may well be the most endangered ecosystems in the world. Countless aquatic plants have been introduced in Europe and in the Mediterranean region, either deliberately because they were thought to be ornamental or otherwise desirable, or accidentally as releases from aquaria, dumping from water gardens, or contaminants. The escape of alien plants from managed environments is also frequent. Aquatic invaders have strong negative impacts on native biodiversity and many different impacts are reported in the literature for Europe and worldwide. Plant invasions in inland waters may be considered as one of a syndrome of factors that characterize degraded aquatic ecosystems. There are a number of risk analysis, impact assessment, ranking systems, decision trees, and prioritization methods available throughout Europe and the EPPO region, but the lack of a common framework for assessing the risks posed by invasive alien aquatic species is seen as a key gap. Successful management should include prevention, habitat restoration, dedicated risk assessment, prioritization, awareness raising, dedicated legislation, collaboration between different stakeholders, development of codes of conduct with the horticultural sector, and prohibiting the sale, purchase, and intentional release of some species in the wild. Actions to protect the European and Mediterranean inland waters from plants invasions are urgently needed and could provide overarching benefits.