Alien plants tend to occur in species-poor communities

Invasive alien species can have severe negative impacts on natural ecosystems. These impacts may be particularly pronounced within ecological communities, where alien species can cause local extinctions. However, it is unclear whether individual alien plant species consistently occur in species-poor or species-rich communities across broad geographical scales and whether this pattern differs amongst habitat types. Using ~17,000 vegetation plots sampled across the Czech Republic, we calculated the median, range and skewness of the distribution in community species richness associated with 73 naturalised alien plant species. We compared the observed values with those obtained under a null expectation to test whether alien species occurred at random with respect to species richness in forest and grassland communities. We found that the relationship between the occurrence of alien species and the diversity of local plant communities was species-dependent and varied across habitats. Overall, however, alien species occurred in species-poor communities more often than expected by chance. These patterns were more pronounced in grasslands, where alien species also occurred in communities with a lower range of species richness than under random expectation. Our study represents one of the most comprehensive quantitative analyses relating alien plant invasion to resident community diversity at a broad geographical scale. This research also demonstrates that multi-species studies are needed to understand the processes of community assembly and to assess the impact of alien plant invasions on native diversity