Background/Question/Methods Many studies have sought defining characteristics (such as life history traits) that can differentiate between successful and unsuccessful invasion by exotic species. However, few generalizations have resulted due to the large variation in habitats and organisms involved. Recent studies suggest that phenotypic plasticity, production of variable phenotypes by the same genotype under different environmental conditions, may be an important mechanism influencing invasiveness. Plasticity may increase the ecological niche breadth of exotic species, increasing the probability of successful invasion. Under this conceptual framework, I hypothesized that successful invasive species will exhibit higher plasticity than unsuccessful exotic species. I predicted that successful invasive species will either maintain a higher fitness across environments than unsuccessful exotic species (i.e., Jack-of-All-Trades reaction norm), or exploit favorable conditions better (i.e., Master-of-Some reaction norm). I explored plasticity of eight traits in response to variation in soil moisture using six closely related, exotic plant species (Bromus spp.) that differ in their invasive status (i.e., invasive or non-invasive). I focused on how several vegetative and reproductive traits are influenced by differences in invasive status, environmental condition, invasive status– by – environmental condition interaction, and species nested within invasive status. I also examined the distribution of the coefficients of variation of traits of lines within each species to test for differences in overall plasticity.

Results/Conclusions   Across treatments, successful and unsuccessful invasive species differed in plasticity for many of the traits measured; however, neither group showed consistent patterns in trait values. There were significant invasive status-by-environment interactions in both vegetative and reproductive traits. Contrary to expectations, the unsuccessful exotic species exhibited a Jack-of-All-Trades reaction norm, while the successful invasive species exhibited a Master-of-Some pattern. This suggests that there is more variation in fitness in the successful invasive species than unsuccessful ones. My data support the idea that successful invasive species are more plastic than unsuccessful invasive species in seed production, number of tillers, and total biomass.