Two prominent hypotheses in plant invasion literature, the Enemy Release Hypothesis (ERH) and the Evolution of Increased Ability hypothesis (EICA), attribute the proliferation and spread of exotic, invasive plants to a release from natural enemies. The ERH posits that plants in their introduced range experience a release from the herbivores and pathogens that limit them in their native range, which leads to a demographic release. If the native plants are still limited by natural enemies, then exotic plants may achieve higher success compared to native plants. The EICA hypothesis suggests that plants evolve after introduction to have greater growth and reproduction because they allocate less to defense. Natural enemies have further been hypothesized to accumulate over time since invasion, such that exotic plants would be expected to lose their demographic advantage and become less invasive over time. Classical biological control of plants is predicated on the idea that natural enemies strongly regulate population growth and spread. However, evidence is mounting that enemy release does not explain many plant invasions. Here, I ask what the state of our current knowledge is about the importance of enemy release in plant invasions and what the consequences are for the field of biological control.
Results/Conclusions
I examined evidence from the invasion ecology and biological control literature and find that empirical data are still greatly lacking to evaluate critically the ERH and EICA. However, as has been found previously, studies that compare the effects of natural enemies on native and introduced individuals or populations of the same plant species (i.e. conspecifics) generally find evidence for the ERH, whereas those that compare exotic and native species in the same community generally do not find evidence for ERH. In addition, post-introduction evolution because of a release from natural enemies appears to be rare (i.e. little support for EICA). I found relatively little evidence for natural accumulation of herbivores and pathogens over time leading to lower invasiveness. The biocontrol literature offers support for the role of natural enemies in limiting plant populations, but more post-release tests are necessary to definitively determine how well introductions of one or a few agents control invasive plants. Going forward, we will need to specify sub-hypotheses for ERH and EICA and rigorously test them with field studies.