Is Enemy Release Strong for Invasive Plants?

The Enemy Release Hypothesis (ERH) suggests that escape from natural enemies causes introduced plants to become invasive. We used meta-analysis to synthesize and investigate the strength of the ERH using both the biological control of invasive plants by individual introduced insects and the effects of individual native insect herbivores on native plants. Insects used in biological control studies, specified as both pre- and post-release, had significantly greater effects on invasive plants than native insects had on native plants, which supports the ERH. However, this effect was driven mostly by study outcomes from biocontrol releases, which had significantly greater effects on invasive plants than native insects had on native plants. Insects used in pre-release biocontrol studies did not have significantly stronger effects than native insects on native plants. This suggests that ERH may not be as common a cause of plant invasiveness as thought since, by that logic, release from natural enemies would cause any introduced plant species to become invasive. Among all studies, there was no significant difference in the strength of enemy release between the effects of monophagous versus polyphagous herbivores, terrestrial and aquatic plants, insect orders or feeding guilds and latitude of experiment. However, there were significant differences in the effects of insect herbivores between above and below ground feeders, plant growth forms, and plant organ affected.