Current conception of the ERH: a phenomenological framework
The ERH was designed as a hypothesis to explain exotic species performance. Here, we take ‘performance’ to mean population growth (following Keane & Crawley 2002), which could be measured by demographic metrics such as population growth rate, or geographic metrics such as rate of spread (Table 1a). In Keane & Crawley’s (2002) seminal paper, the ERH was laid out as a three-step argument:
Step 1: Natural enemies are important regulators of species fitness;
Step 2: Exotic species experience reduced enemy pressure relative to native species;
Step 3: Exotics can capitalise on reduced enemy pressure, resulting in increased competitive ability and population growth.
Keane & Crawley (2002) highlight that the advantages in Steps 2 and 3 are inherently biogeographic (i.e. they relate to the exotic in its invaded vs home range), but they should also lead to advantages relative to the native community (see Box 1).
There are many reasons why performance of exotic species may increase in their invaded range (Díaz et al. 2023), not all of which are related to release from enemies (Buckley & Catford 2016). Therefore, support for the ERH requires evidence for both Step 2 (exotics experience reduced enemy pressure) and Step 3 (this reduced enemy pressure leads to increased performance). Whether this reduced enemy pressure is compared to the exotic species in its home range or to native species in the invaded range depends on the goals of the researcher (Box 1). If evidence for these two steps exists, then Step 1 is redundant, as the combination of Steps 2 and 3 implies that enemies were regulating species fitness.
The dominant interpretation of the ERH has focused on the release of enemy pressure itself (i.e. Step 2), with Steps 1 and 3 gaining less attention (Prior et al. 2015). Worryingly, evidence that enemies have been lost has been taken as evidence that the ERH is supported (Prior et al. 2015). Exotic performance is seldom assessed. For example, of the 85 biogeographic comparisons compiled by Jeschke & Heger (2018) in their review of evidence for the ERH (hi-knowledge.org), just 16 of 85 (19%) considered performance, with 7 of those 16 (44%) supporting the ERH. Further, performance is influenced not just by enemy richness or abundance, but by the effect of those enemies, and other biotic and abiotic factors (Chiuffo et al. 2022). With a small number of studies testing Step 3, and no clear framework to integrate the range of potential influences on performance, it is extremely difficult to determine why the ERH has support in some cases but not others.