Factor 2: Enemy Diversity
Changes in enemy diversity accounts for the loss (or gain) of enemy species upon invasion, as well as changes in enemy abundances (Fig. 1b, green box; Table 1b). Higher reductions in enemy diversity have been shown to increase geographic extent of exotic plants in the invaded range (Mitchell & Power 2003; Hawkes 2007) (but see van Kleunen & Fischer 2009). Identifying which exotics have the potential to experience large reductions in enemy diversity could therefore inform predictions about invasion risk.
Enemy diversity can be measured with enemy abundance (the prevalence of a given enemy species, or the number of individuals of that enemy per host) and richness (the total number of enemy species per host) (Table 1b). Together, richness and abundance determine the total enemy load faced by an exotic plant. Both are important to measure where possible: different enemy species may target different tissues and cause different fitness costs, while enemy abundances will determine the extent of those fitness costs.