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).