Conclusion
In this paper, we have demonstrated that contradictions in evidence for the ERH exist both within and between studies (Table S1), but that these apparent inconsistencies can be the result of explicable factors and contexts, rather than unpredictable variation (Fig. 4; Box 2). We posit that greater acknowledgement of the factors and contexts of the ERH will reduce apparent contradictions, reconcile apparently contrasting enemy-related invasion hypotheses and increase our ability to determine when and how enemy release contributes to plant invasions (Figs. 3, 4; Table 2).
It is unlikely that the ERH is universally true or false, but rather that enemy release is important in some circumstances and not others. Our framework provides a way to predict what those circumstances will be (Fig. 1, Table 1). As few studies have explicitly considered exotic performance, we cannot reliably say whether the ERH is a rare or prevalent mechanism for successful invasion. Outlining the core set of factors and contexts that can lead to a wide array of observed patterns in exotic performance enables greater theoretical clarity in invasion ecology. This should improve the quality of ERH studies and comparisons between them, guiding future research into the role of enemy release in invasions.