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.