Invasion in a Community Context: Genetic Variation in the Strength of Direct and Indirect Effects
Abstract Predicting invasion success requires understanding how abiotic and biotic factors in the local environment interact with the particular traits of potential invaders. Relative to studies of direct antagonistic effects, fewer studies have examined how positive species interactions, such as facilitation or mutualism, or indirect interactions in multispecies communities, can affect invasion success. We examined the effects of drought and mutualisms with rhizobia bacteria on the performance of a widely invasive legume, Medicago polymorpha. In a greenhouse experiment, we found that watering regime affected plant performance, but non-linear patterns in response to decreasing water were dependent on the specific plant genotype. In a second experiment, we found that the effects of drought on plant performance were dependent on the presence of rhizobia, particularly for genotypes collected from the invasive range. This suggests that indirect ecological effects may have important consequences for invasion success. We contextualize the strength of these direct and indirect effects by comparing this study to effect sizes in other studies of the same species. In this species, predicting invasion into a natural community context will require understanding multiple direct and indirect effects in the local environment, as well as their effects on the specific genetic composition of the invading population.
