Invasion by an exotic grass species homogenizes native freshwater plant communities

1. A growing body of evidence has shown that biological invasions cause shifts in species composition of communities in space and time. Although biological invasions are considered a major driver of biotic homogenisation worldwide, most previous studies were conducted at small spatial scales and over short time periods, which may have underestimated the impacts of exotic species on native communities.2. Using a unique dataset of aquatic plants sampled in 235 sites over 12 years (2007- 2010 and 2015- 2019) in a large reservoir (Itaipu Reservoir; 1350 km(2)), we analysed how the invasion of a non-native grass Urochloa arrecta affects the species richness, ecological uniqueness (i.e. local contribution to beta diversity- LCBD) and temporal beta-diversity of native plant communities.3. From 3934 surveyed plant communities, U. arrecta was recorded in 2888 samples and it was absent from 1046 samples. Overall, species richness and ecological uniqueness of native plant communities were markedly lower in sites invaded than non-invaded by U. arrecta. From 2007 to 2019, the ecological uniqueness of native plants was 60% lower in the invaded than non-invaded sites. Whereas in invaded sites species loss was the dominant process driving native communities over time, in non-invaded sites gain of new native species was the primary process underlying community trajectories. Moreover, comparing native plant com- munities before and after the invasion of U. arrecta, species richness, ecological uniqueness and species gains of native plant communities decreased, whereas species losses increased after the invasion of U. arrecta. Finally, the positive relationship between native biodiversity and precipitation was stronger in non invaded than invaded sites.4. Synthesis. Our findings provide comprehensive evidence that an invasive plant is decreasing the spatial and temporal beta-diversity of native plant communities through declining species richness, rather than simply correlating with them. This suggests that U. arrecta is driving native plants to become less diverse and homo- geneous after the invasion, both spatially and temporally. Our findings illustrate that at broad scales, aquatic plant communities may become increasingly homogeneous with the increasing number of biological invasion events taking place worldwide.