Genetic variation in invasive species response to direct and indirect species interactions

Biotic resistance to invasion arises from strong species interactions that decrease the fitness and population growth rates of potential invaders. Strong, direct interactions such as predation and competition are typically thought to drive biotic resistance, but in diverse communities, indirect interactions among species may also affect biotic resistance. Further, genetic variation in traits of the invading species that affect species interactions may allow some genotypes to overcome biotic resistance. We investigated the direct and indirect effects of a native legume (Acmispon wrangelianus) and insect herbivores on the fitness of different genotypes of an invasive legume (Medicago polymorpha) in a California grassland. Insect herbivores decreased Medicago fitness, but only in the presence of Acmispon, suggesting that indirect interactions mediated through insects and Acmispon are important for deterring Medicago invasion. Some Medicago genotypes were less affected by interactions with other species, however. This genetic variance suggests that while biotic resistance reduced the reproductive success of most genotypes, a few genotypes were able to overcome these complex interactions. However, Medicago invasion was unsuccessful in all treatments at several of our sites, suggesting that factors beyond those manipulated here also play a key role at many sites. At sites where biotic resistance is important, spatial and temporal variation in community composition and the genetic composition of the invasion pool may explain the invasion success of Medicago into this community.