Since at least Darwin, ecologists have sought to understand and predict the fate of introduced species.  Broad evidence indicates that, on average, introduced populations escape many or most of the species of natural enemies from their native range.  However, as introduced populations are established for longer times, and as they spread geographically, they will be exposed to more species of enemies.  This may lead to the accumulation of enemy species, including both species introduced from the host’s native range, and species resident in the host’s introduced range.  Thus, a major question is: on what spatial and temporal scales does enemy release erode?  To answer this question, we assembled data for 226 plant species naturalized to North America from Europe and the Mediterranean (“introduced” and “native” range, respectively).  We used published data on the number of viral pathogens and rust, smut, and powdery mildew fungal pathogens reported to infect each plant in its introduced range, and in its native range.  For each plant species, we quantified geographic range size as the number of U.S. states inhabited, and determined minimum residence time in North America by analyzing sources including early published floras and historical writings (e.g. compilations of colonial seed catalogues). 

Results/Conclusions

Across all pathogen and plant species, the number of pathogens per host species increased simultaneously with host species’ minimum residence time and geographic range size.  While both were important, geographic range size had a stronger effect than minimum residence time.  The cumulative probability of a successful species introduction increases over time, and pathogens from their host’s native range also require introduction to its introduced range.  Therefore, we predicted that accumulation of these pathogen species would depend more on host minimum residence time than on geographic range size.  In fact, number of introduced pathogen species increased strongly with both host minimum residence time and geographic range size.  Additionally, most species are geographically restricted, and most pathogen transmission occurs within local communities.  Therefore, we predicted that accumulation of pathogen species resident in the plants’ introduced range would depend more on geographic range size than on minimum residence time.  Supporting this conjecture, number of resident pathogen species increased weakly with minimum residence time, and strongly with geographic range size.  These results highlight the strength of enemy release early in the invasion process, while suggesting that in longer-established and more widespread invasions, enemy accumulation may provide feedback, either negative or positive, to the invasion.