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.