Different traits predict competitive effect versus response by Bromus madritensis in its native and invaded ranges

Abstract

Community assembly and coexistence theories predict that both fitness and plant functional traits should influence competitive interactions between native and invasive species. The evolution of the increased competitive ability hypothesis predicts that species will grow larger (a measure of fitness) in their invaded than native range; hence we hypothesized that species might exert greater competitive effects in their invaded range, lessening the importance of functional traits for competitive outcomes. In a greenhouse experiment we compared traits and competitive interactions between Bromus madritensis (an annual grass) and resident species from its native range in Spain, and its invaded range in Southern California. As predicted, B. madritensis collected in California grew larger and had a greater competitive effect on resident species than B. madritensis collected in Spain. However, residents from California also suppressed the growth of B. madritensis more than species from its native range in Spain. Competitive interaction strengths were predicted by different suites of traits in the native versus invaded range of B. madritensis; surprisingly, however, size of the resident species (fitness), did not predict variation in competitive interactions. This study shows that different suites of traits may aid in identifying those native species likely to strongly compete with invaders, versus those that will be competitively suppressed by invaders, with important implications for the design of restoration efforts aimed at promoting native species growth and preventing invasion. More generally, our study shows that fitness differences may not be as important as traits when predicting competitive outcomes in this system.