Are invaders moving targets? The generality and persistence of advantages in size, reproduction, and enemy release in invasive species with time since introduction

Successful plant invasions are often attributed to increased size, reproduction, or enemy release, but the generality and long-term persistence of these patterns remain debated. I addressed these questions both empirically and with meta-analysis of published data. Changes in biotic interactions were measured in a common garden experiment with native and exotic Senecio congeners representing a range of residence times in the UK. The introduced species had lower rates of attack by aphids and pathogens and more colonization by mycorrhizal fungi compared to natives. The native-exotic division was further supported by biogeographic comparisons. However, more annual variability in the degree of enemy release was observed in the historically introduced (>300 yrs) compared to the recently introduced (>10 yrs) exotic species. I used meta-analysis to determine the generality of these patterns based on comparisons of conspecifics in home vs. introduced ranges and native vs. exotic congeners. Plants in the introduced range were larger, reproduced more, and had lower levels of herbivore damage compared to the home range; pathogen attack varied widely. In congeneric comparisons, exotic and native plants did not differ in size or herbivory, but exotics did reproduce significantly less and had lower levels of pathogen damage. Time since introduction was a significant non-linear predictor of enemy release for both herbivores and pathogens, with initial release in exotics disappearing over time. The observed non-linear relationships suggest an initial period of rapid evolution decelerating over time and may define a window of opportunity for eradication efforts.