A transcontinental biogeographic comparison of native and invasive dominants: Are invasives indeed doing something different than natives?

Background/Question/Methods

Despite ongoing intensive research on exotic plant invasion, we still have a rather poor understanding of whether the same species has different impacts in its native and invasive ranges. Some species that are rare in their native range may suddenly reach staggeringly high population densities and become dominant when they are introduced into a new range. In contrast, other species are capable of being dominant in both ranges. This implies that the fundamental difference might be in how resident communities in both ranges respond to their dominants and what their resilience is following disturbance. The present study aims to test this assumption by using a biogeographic comparison of three model (semi)grassland species (Cirsium arvense, Leucanthemum vulgare s. l., and Tanacetum vulgare) native to Europe and invasive in North America. In both ranges, we have established eighteen experimental sites in which the given species is either a native dominant or an alien dominant to explore how the plant community responds (in terms of plant species richness, diversity and composition) after a disturbance event. We designed each plot with a complete block of four randomized treatments: soil disturbance/no seed addition, soil disturbance/seed addition, no soil disturbance/no seed addition (control), and no soil disturbance/seed addition. Seed addition means that seed of six common native plant species growing at the experimental site were added to the plot to strengthen the natural level of propagule pressure from the surroundings.

Results/Conclusions

Here, we report the preliminary results from the first year of the project. In general, the studied plant communities seem to be more species-rich in the native range compared to their new range. Concerning the species composition, North American sites contain many more aliens than their native counterparts. The first year after the disturbance, we observed an obvious difference between the seeded and unseeded plots in both ranges. Weedy species and r-strategists (annuals) have predominantly colonized disturbed plots in Europe, whereas alien species and r-strategists (annuals) were found within the plots in North America. The introduced species studied herein recover after disturbance better than their native counterparts in terms of abundance and cover. Moreover, the introduced species are significantly higher in the new range than at home that might facilitate them to compete for light.