Box 1: Biogeographic and community approaches to the ERH: exotic naturalisation versus exotic impact?
The key outcome of the enemy release hypothesis (ERH) is increased exotic performance in the invaded range (Fig. 1). But increased relative to what? There are two possible comparators: 1) Exotic performance can be compared to conspecifics in the home range – a biogeographiccomparison; or 2) Exotic performance can be compared to co-occurring natives in the invaded range – a community comparison. The original conception of the ERH (Keane & Crawley 2002) does not explicitly state which comparator is appropriate, or why they may differ. Later conceptions incorporate both. For example, Sarabeev et al. (2022) premise their discussion of the ERH around two predictions, one biogeographic and one community.
Potential differences between biogeographic and community comparisons depends on underlying assumptions. The ‘universal trade-off hypothesis’ (Tilman 2011) suggests that biogeographic and community comparisons should yield the same result. This hypothesis posits that there is a ubiquitous trade-off surface along which all species lie. Release from enemies in the invaded range could cause an exotic to move off this shared trade-off surface (Catford et al. 2018). The exotic should therefore gain a performance advantage relative to conspecifics in its home range (biogeographic), and also relative to natives in the invaded range (community). In contrast, the ‘evolutionary imbalance hypothesis’ (Fridley & Sax 2014) proposes that old, highly diverse regions with large population sizes produce species that should be superior competitors to species from younger, more depauperate regions. Under these circumstances, biogeographic and community comparisons may produce distinct findings. For example, an exotic from a species-poor region may experience enemy release and compare favourably with conspecifics in its home range, but perform poorly in a community context if it has invaded a species-rich area with highly competitive natives.
Empirical evidence suggests that biogeographic and community comparisons often yield different results (Colautti et al. 2004). Biogeographic (home vs invaded range) comparisons tend to support the ERH more than community (native vs exotic species) comparisons. However, very few studies employ biogeographic and community comparisons together. It is therefore unclear whether this is the result of a meaningful ecological pattern (e.g., that evolutionary imbalance is more likely than universal trade-offs), or due to different types of study employing different methods to test biogeographic and community comparisons. When comparisons are made using the same metric (e.g. ‘insect diversity’), results between biogeographic and community tests are similar (Meijer et al. 2016). We suggest that one reason for the inconsistencies between biogeographic and community comparisons is that they emphasise different aspects of exotic performance and use different metrics. Biogeographic comparisons emphasise successful establishment and naturalisation, and tend to focus on enemy richness and abundance. In contrast, community comparisons emphasise the impacts of exotic species, and typically focus on the relative performance of exotic and native species (Jeschke & Heger 2018).
Whether a biogeographic or community comparison should be used thus depends on the goals of the researcher:
If a researcher is interested in the likelihood of successful colonisation and naturalisation, a biogeographic comparison is appropriate. A key barrier to successful colonisation is initial survival. If a universal trade-off holds (Tilman 2011), exotics are more likely to cross this barrier if they lose enemies, or if enemies have lower impact compared to in their home range. A biogeographic comparison tests whether these losses have occurred.
If a researcher is interested in exotic species’ impact and dominance in their invaded range, a community comparison is appropriate. Whether an exotic becomes invasive (becomes dominants and has a negative impact on the co-occurring native species or the recipient ecosystem) can only be fully understood relative to co-occurring native species (Zhang & Jiang 2006). A community comparison is required to compare performance against natives.
When choosing a comparator, a researcher should consider both the goals of the study, and the underlying assumptions about whether the two comparators should differ. This may vary depending on the stage of invasion (see ‘Context i: Time since introduction’ in main text).