Factor 1: Enemy Impact
Enemy impact is the negative per capita effect that an enemy has on a plant or plant population (Fig. 1b, orange box; Table 1b). This factor explicitly considers how a given enemy affects performance of individual plants or plant populations, including their likelihood of survival, growth and reproduction. This factor acknowledges that the same enemy species can have different effects on plants depending on whether they are growing in the species’ native or invaded range.
Enemy damage is typically used to assess impact (e.g. % of leaves eaten). However, it would ideally be measured in terms of individual plant performance (Chun et al. 2010) and population vital rates (survival, growth or reproduction), which ultimately determine exotic success (Ramula et al. 2008) (Table 1b). Although extent of enemy damage likely correlates with vital rates (e.g. more damage leads to a greater reduction in growth rate), the shapes of these relationships are largely unknown and reductions in one vital rate can be compensated for by increases in another (Livingstone et al. 2020). We accordingly emphasise that damage provides only an indirect measure of enemy impact. Enemy impact is arguably the most difficult of the three factors to reliably measure, which likely explains why only half the studies of the ERH provide direct estimates of enemy impact (Table S2). Future studies on enemy impact are important to comprehensively quantify the occurrence and strength of enemy release.