Impact of an invader on species diversity is stronger in the non-native range than in the native range

We have yet to adequately quantify the impacts of invaders in their non-native ranges relative to their native ranges. Such biogeographical comparisons are crucial to better understand the role of invasive species in plant-plant interactions and in the evolution of community organization. Using Solidago canadensis, we conducted field surveys in the native range (North America) and non-native range (Europe) and correlated stem density and aboveground biomass of Solidago with associated plant species richness and diversity indices. We also conducted experiments to compare the competitive and allelopathic impact of Solidago on the growth of species from its native and non-native ranges. In the field, increasing stem density and biomass of Solidago correlated with more significant decreases in total species richness and two diversity indices in Europe than in North America. Solidago strongly suppressed both European species and North American species in competition experiments, but this competitive effect did not differ between species from the different ranges. However, root extracts from Solidago strongly suppressed the root growth of European species as a group, but not North American species as a group. Our results indicate that the biogeographic origin of species can have important effects on plant interactions and community organization.