UC Riverside

A minority of exotic plant species undergo differentiation in vigor following introduction, leading to an explosion in population sizes and aggressive range expansion. Investigations into the mechanisms that determine successful invasion historically emphasized phenotypic traits in hopes of identifying ecological predictors and subsequent control mechanisms. Yet, it is now recognized that post-introductory evolution of invasiveness is common in many systems, frustrating efforts to identify ecological predictors. This suggests that evolutionary mechanisms ought to be given increased consideration. But this does not mean that regional differences in ecological interactions are unimportant. Many investigations demonstrate that invasive plant species experience facilitation in introduced relative to native range soils. My objective was to integrate these two promising fields of study in order to obtain a more holistic view of the mechanisms underlying invasion. Here I utilized seed and soils from native and introduced regions of the locally abundant grass species Bromus rubens L. (Pavlick and Anderson 2007, = B. madritensis ssp. rubens, Fortune et al. 2008), also known as Red brome. B. rubens is a winter annual common in the Mediterranean (native range) and Southwestern United States (introduced range). I examined the complexities of potential evolutionary and ecological factors leading to the invasion success of this species by concentrating on 1) patterns and promoters of regional differentiation, 2) the impacts of differentiation on competitive ability, and 3) the contribution of multiple ecological factors to plant-soil interactions. I found that introduced populations showed a strong signal for diversifying selection toward more aggressive growth. In a competitive environment introduced genotypes demonstrated greater reproductive fitness relative to native genotypes, regardless of competitor’s genotypic or region of origin. Finally, a plant-soil interaction growth assay suggested that increased resource availability coupled with decreased interactions with both antagonistic and beneficial soil fungi in introduced soils contributed to the invasion success in B. rubens. Together these patterns indicate that the occurrence of post-introductory evolution is of major importance to the development of invasive characters and increased competitive ability, and that ecological interactions among hosts and respective soil communities greatly contributed to the dynamics observed in this system.