Testing the ERH with our mechanistic framework
Using our framework, robust tests of the ERH require either: (a) data about population-level exotic performance and data for at least one of the factors that influence exotic performance; or (b) data for at least two of the factors that influence exotic performance. Approach (a) would provide the most rigorous test of the ERH, but approach (b) is more feasible.
The ERH was designed to explain population-level performance, so metrics of performance are the gold standard (approach a). Performance measures must be coupled with data on one or more of the three factors to ensure that performance is linked with enemy release; exotic performance could increase for reasons unrelated to enemies (Felker-Quinn et al. 2013), so it is important to discount that. Measuring population-level performance is difficult though, so most tests of the ERH will likely use approach (b). Of the two requisite factors that need to be measured in approach (b), enemy diversity and enemy impact seem the most beneficial and feasible combination. Host adaptation is difficult to measure, and the degree of host adaptation from enemy release will be driven by changes in enemy diversity and impact (Fig. 1b), so it makes most sense to focus on those two factors.
Using approach (b), where both enemy diversity and impact are measured, is important because evidence for one factor alone does not robustly test the ERH. We demonstrate this in Figure 2. Two exotic populations (A & C) experience identical levels of enemy pressure, and both are under higher enemy pressure than natives (Fig. 2). However, this identical enemy pressure between populations A and C would not be detected by looking at enemy diversity alone (Figs. 2a vs . 2b), nor by looking at the impact of certain enemies alone (Figs. 2c vs . 2d). In contrast, exotic population B experiences lower enemy pressure than co-occurring native species, and lower enemy pressure than populations A and C. However, it would be assumed to be identical to population A if only diversity was studied, or identical to population C if only impact was studied. Studies that assess total enemy diversity andestimate their impact on host individuals are rare (Table S2). In particular, studies on exotic impact are underrepresented in the ERH literature (Prior et al. 2015). The combination of enemy diversity and the impact of those enemies (enemy pressure) determines relative release for exotics (Fig. 2) and the likelihood and strength of exotic host adaptation (Fig. 1b). Appropriately measuring total enemy pressure is therefore critical to testing the ERH.
We suggest that studies of performance under field conditions are particularly valuable. Field conditions allow an assessment of native and exotic vital rates in the context of their complete enemy suites. These enemy suites can be indirectly assessed through a metric such as total damage at a leaf- or plant-level (Table 1b), as directly determining total enemy diversity is likely difficult. Field studies therefore allow for the interaction of multiple ERH factors to be seen. Further, relevant trade-offs in invader performance (i.e., growth and defence) are often only expressed under interspecific competition (Cipollini et al. 2014; de Vries et al. 2019), and competition can change a plant’s response to enemies (Honor & Colautti 2020). More studies of enemy release should take place in intact communities, providing further insight into the ERH under more realistic field scenarios.