Insights from community ecology into the role of enemy release in causing invasion success: the importance of native enemy effects

The enemy release hypothesis (ERH) predicts that the success of invasive species is caused by reduced enemy pressure in species’ introduced ranges. The ERH is a highly-cited explanation for invasion success, yet rigorous evidence is lacking for most species and ecosystems. Most evidence comes from observations of enemies in native and introduced ranges. These studies assess one aspect of the ERH—“enemy loss.” They do not provide a direct test of the ERH and overlook the assumption of “native enemy effects.” This is a critical limitation as enemy release will not occur if enemies do not affect species in their native ranges, even if enemy loss occurs. Biogeographical experiments, providing a direct test of the ERH, are largely restricted to terrestrial plants. We present a synthesis of community ecology and invasion biology studies, including a novel meta-analysis of native enemy effects, to assess the potential for release for species in different taxonomic groups and ecosystems. We suggest that species that are subject to strong enemy effects in their native range will have a high potential for enemy release. We found that native enemy effects were stronger in aquatic systems than in terrestrial systems. They were particularly weak for terrestrial plants; and strong for marine organisms, and freshwater plants. Studies are needed for species that have strong potential for release, such as for aquatic invasive species. Alternative explanations should be explored for invasive species that are not affected by enemies in their native range, and future studies should emphasize native enemy effects rather than only enemy loss.