New conception for the ERH: a mechanistic framework
Our framework emphasises exotic performance as the key outcome of the
ERH (Fig. 1a; Table 1a). By making exotic performance the outcome, we
re-emphasise the ERH as fundamentally a hypothesis to help predict and
explain exotic naturalisation and invasiveness (Box 1). This approach
reduces the likelihood that evidence for individual steps will be
misconstrued as providing evidence for the whole 3-step hypothesis.
As noted above, we argue that ERH-induced increases in exotic
performance are the product of three factors: enemy diversity, enemy
impact, and host adaptation (Fig. 1b; Table 1b). Our three-factor
framework does not directly map onto the three steps of Keane &
Crawley. The steps of Keane & Crawley are phenomenological, while our
framework is mechanistic. Broadly, we take their Step 3 (exotic
performance) as the overall outcome of the ERH at the top of our
framework (Fig. 1a). Keane & Crawley’s Steps 1 and 2 are observations
of how enemies affect species, and how they may affect native and exotic
species differently. The drivers underlying these observations are
captured by our three factors (Fig. 1b). Our framework adds further
nuance by highlighting seven contexts that influence the strength of
these three factors (Fig. 1c; Table 1c); our contexts have no analogue
in the steps of Keane & Crawley.
We believe our mechanistic framework complements and offers advantages
over a phenomenological one. Conceptualising the ERH phenomenologically
– as with the steps of Keane & Crawley – allows for a valuable
documentation of trends and could help prioritise exotic species
particularly worthy of further investigation. However, to effectively
make predictions, generalise and deal with context, a mechanistic
framework is required (Johnston et al. 2019).
In the sections below, we introduce the three factors in detail and
describe the data required for robustly testing the ERH. We highlight
how context modulates effects of the three factors and discuss the wider
benefits of a mechanistic framework for the ERH. As the ERH has been
especially studied in plants (Jeschke et al. 2012), examples are drawn
from plant invasions, though we posit that our framework applies to any
taxonomic group. Given our plant focus, ‘enemies’ refers to herbivores,
parasites and pathogens of plants in this paper. We use the community
comparison as our basis (Box 1), and so all discussion and figures focus
on the differences between exotic and native species in the invaded
range. Nevertheless, the framework could equally be applied to
biogeographic comparisons (comparing exotic species in their home vs
invaded ranges), and the principles that we discuss should hold for
either comparison. By synthesising the factors and contexts of the ERH
that influence exotic performance (Fig. 1b, c), we believe that our
framework will provide insight into why some exotic species benefit from
enemy release while others do not.