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).