A global synthesis of naturalised and invasive plants in aquatic habitats

Global databases have contributed to our understanding of alien, naturalised and invasive plant species distributions. Still, the role of species invasions in habitats, specifically in aquatic habitats, remains underexplored at the global scale. Accordingly, a comprehensive global synthesis of the status of plant invasions in aquatic habitats has been missing. Here, we focus on macroecological patterns of naturalised non-invasive and invasive plants in aquatic habitats using the recently built SynHab database. Amongst all the plant records compiled in SynHab, 592 are assigned to aquatic habitats, of which 183 are unique plant taxa (further termed ‘species’) belonging to 49 families. Of the total number of records, 462 refer to taxa with naturalised non-invasive occurrences and 130 to invasive occurrences. The species pool analysed here refers to 78 regions distributed across all botanical continents as defined by the World Geographical Scheme for Recording Plant Distributions. The number of naturalised non-invasive aquatic species is similar across different continents and biomes, but Tropical Asia had more and the Mediterranean zonobiome had fewer invasive species than expected. Tropical Asia, Temperate Asia and Africa have the highest proportions of naturalised species that have become invasive, while across continents, invasive proportions were highest for tropical and subtropical zonobiomes. New Zealand, Italy and California contained disproportionately more naturalised species than expected, given the area covered by aquatic habitat in those regions, whereas South Sudan, Papua New Guinea and Kyrgyzstan had disproportionately fewer species. In pairwise dissimilarity comparisons, all continents had distinct species compositions (from 0.73 to 0.92 of the Jaccard dissimilarity index) and so did zonobiomes (0.69 to 1.00). The high proportion of invasive species in Tropical Asia in comparison with terrestrial invasions in this region, indicates a greater susceptibility of warmer regions to aquatic plant invasions. This may be exacerbated by further naturalisations in the future, as data from temperate regions suggest a larger pool of available species.