Evolution of increased biomass does not result in increase competitive ability during invasion

Background/Question/Methods

Species that invade novel habitats experience unique selection pressures and their traits may rapidly evolve to adapt to their new environment. When these species experience release from their natural enemies, they may have increased resources available to invest in competitive ability. The Evolution of Increased Competitive Ability hypothesis predicts such trait evolution may explain invasion success of some species. We measured evolutionary changes in an invasive species (Medicago polymorpha) in California grasslands by comparing traits in genotypes from the native and introduced ranges. Specifically, we asked: (1) Are morphological traits, such as height and biomass, different between genotypes from the native and introduced range? (2) If we find evidence of evolutionary change, does this influence the competitive effect or response of these genotypes? We grew 15 native range M. polymorpha genotypes and 19 invasive range M. polymorphagenotypes in the greenhouse with each of three herbaceous co-occurring competitors collected from California.

Results/Conclusions

We found that the invasive M. polymorpha genotypes were 17.4% heavier and almost 30% taller than genotypes from the native range suggesting evolutionary changes in size occurred during or after the invasion process. However, invasive and native range genotypes had remarkably similar competitive effects on all three competitors, and they had similar responses to competition from these species. These results suggest that although contemporary evolution in the invasive range resulted in increased Medicago size, this had no effect on their competitive ability. If evolution affects invasion success it is through means other than direct interactions. With this in mind, further tests are needed to explore not only evolution, but also how it affects interactions with other species.