Biogeographical comparisons of species in their native and introduced ranges are important to elucidate whether invasion changes fundamental features of population regulation. Although it has been stated that many exotic species are more abundant in their introduced range, the evidence may reflect a bias towards the worst invasive species. Moreover, we know little about how the structure of “novel communities” incorporating exotics may differ from the structure of their communities of origin. We took advantage of the large unidirectional invasion of annual plant species of Spanish origin into California grasslands as a large natural experiment to understand plant community convergence after invasion. By conducting coordinated floristic surveys in California and Spanish grasslands in similar climatic and land-use conditions, we explored the following questions: Does the dominance (abundance, richness) of exotic species in these novel communities match their dominance in the communities of their native range? Does the assemblage of shared species differ among regions?
Results/Conclusions
In total, we recorded 139 species in Spain and 95 in California. In Spain, all species were native. In California, we found 52 native species and 46 exotics. All exotic species found in Californian grasslands are native to Spain. We found 26 species shared between the two regions: 15 naturalised and 11 invasive species in California.
For shared species, cover was significantly higher on average in California, where they are exotic, than in Spain. Interestingly, species categorized as invasive in California based on Calflora (https://www.calflora.org/) had significantly higher cover in the introduced range, while naturalized species had similar cover between regions. However, in Spain those groups (naturalized vs. invasive in California) did not show a significant difference in cover.
At the local scale, Spanish grasslands were significantly richer and more diverse than Californian grasslands. In both regions, species composition was heavily dominated by the species that are now present as exotic species in California; the richness of shared species was five times higher than the richness of non-shared species. Our analysis also shows that certain Californian grasslands are so rich in Spanish species that they are more similar in species composition to Spanish grasslands than some Spanish grasslands among themselves. This indicates that in less than two centuries there has been an unpreceded convergence in floristic composition caused by invasion. Our findings provide interesting insights into community assembly in novel ecosystems.