Factor 3: Host Adaptation
Host adaptation is the directional investment in defence or growth (Fig. 1b, blue box; Table 1b). Enemy release can drive selection by reducing the need for costly defences (Cipollini et al. 2014). Selection can lead to an evolutionary advantage for exotics as they can invest in less expensive generalist defences or reduce the absolute quantity of defences, freeing up resources for growth instead (Blossey & Notzold 1995; Joshi & Vrieling 2005). Host adaptation can also reflect phenotypic changes over the lifetime of a plant, mediated by features such as the jasmonate pathway (Howe & Jander 2008). As the focus of our synthesis is the ERH, we only consider adaptation caused by changes in enemy impact and diversity (see arrows in Fig. 1b).
Host adaptation is typically tested in common gardens, comparing the growth, defence, or tolerance of home and invaded populations of exotic species (Rotter & Holeski 2018) (Table 1b). To support the ERH, these measurements also need to be accompanied by evidence of changes in enemy diversity or impact, as there are many reasons beyond enemy release why growth and defence may change in an invaded range (Felker-Quinn et al. 2013).