A number of physiological and functional traits are thought to be responsible for biological invasions. However, trait-based approaches fail to fully explain why many aggressive biological invaders do not dominate communities in their native ranges. For instance, Centaurea solstitialis is relatively rare in most of its native range in Southern Europe but is a noxious invader in North America. On the other hand, the closely related species C. calcitrapa is very common in Southern Europe, but after being introduced nearly two centuries ago to North America it remains very rare. Centaurea sulphurea closely related to both of these species but is rare both in its native and introduced range. To discern factors responsible for these differences among these three species we experimentally studied traits and competitive abilities using plants from several populations of each of the three species from Spain and California.

Results/Conclusions

Our results suggest that since its introduction in North America rapid evolutionary changes may have occurred for C. solstitialis but not for the other two species. American C. solstitialis populations have significantly higher germination rates and are larger in total biomass than European populations. In contrast populations from the native ranges of the two non-invasive species produce larger individuals.

Several factors could be responsible for these apparent rapid evolutionary changes. The EICA (Evolution of Increased Competitive Ability) hypothesis poses that escape from herbivores present in the native range allows resources once allocated to herbivore defense to be reallocated to other functions such as growth or reproduction. However, for this to explain why only one of the three species experienced such evolutionary changes herbivores are likely to have strongly different effects in the home ranges among the three species. Our future experiments will focus on the role of herbivory on the abundances of the three species and to explore the potential for American populations to under-perform when introduced back into European communities.