A central goal of community ecology and invasion biology is to understand the factors that make communities resistant to biological invasions. Strong species interactions that decrease the fitness of potential invaders, such as competition and predation, are proposed to increase biotic resistance to invasion. These species interactions may be especially important in diverse communities, in part because a combination of multiple direct and indirect effects may be necessary for biotic resistance. However, some invading genotypes may possess traits that make them less susceptible to direct or indirect biotic threats. Such intraspecific variation in traits mediating species interactions may result in variation in the strength of biotic resistance and allow invasion by some genotypes, while preventing invasion by other genotypes. We investigated the role of direct and indirect effects in biotic resistance and the effectiveness of biotic resistance against different genotypes of an invading plant. Specifically, we ask (1) Do competition and herbivory contribute to biotic resistance? (2) Do indirect effects contribute to biotic resistance? (3) Is there intraspecific variation in the ability of an invader to overcome biotic resistance?
Results/Conclusions
The presence of insect herbivores reduced the reproductive success of the invasive plant Medicago polymorpha, but the presence of the confamiliar native plant Lotus wrangelianus tended to increase Medicago success, suggesting that herbivores, but not a closely related plant competitor, contributed to biotic resistance. However, the effect of Lotus was dependent on the presence of insects. Lotus increased the fecundity of Medicago in the absence of insects, but not in the presence of insects, indicating an indirect ecological effect of the shared insect herbivore. We also found significant genetic variation in the response of Medicago to insect herbivores. Insects significantly reduced the invasion success of Medicago genotypes collected from the native range, but had little effect on Medicago genotypes collected from the invasive range. This variation suggests that biotic resistance may be more effective against some invading genotypes than others. Moreover, because Medicago genotypes collected from other invaded regions were less affected by biotic resistance than genotypes collected from the native range, this intraspecific variation in traits related to biotic interactions may contribute to invasion success into a wide variety of novel habitats.