Most introduced plant species never become invasive, failing to establish viable populations in their new range.  Although many factors have been proposed that potentially contribute to invasion success in plants, it remains unclear why some species successfully establish and become dominant while others do not.  The most often cited explanation for the success of invasive plants worldwide is the enemy release hypothesis (ERH), which maintains that populations are kept in check by their co-evolved natural enemies in their native range, but are released from this regulation in the new range. Although there is evidence that introduced plant species often experience a decrease in damage by herbivores in the novel range, it is uncertain if this decrease affects plant productivity.  The purpose of this study was 1) to determine if the invasive plant Eugenia uniflora (Myrtaceae) experiences release from enemies in Florida and 2) if enemy release increases growth and leaf number for this species relative to co-occurring native congeners E. axillaris and E. foetida.  An insect herbivore exclusion experiment using insecticide was conducted in the field to assess differences in insect herbivore damage, leaf number, and height for all three species at different life stages. 

Results/Conclusions

The results show that herbivore exclusion resulted in a decrease in damage and subsequent increase in leaf output, but no significant increase in height for native Eugenia.  Eugenia uniflora did not exhibit any change in insect herbivore damage levels in response to herbivore exclusion, however, a significant increase in both leaf number and height were observed.  This increase in leaf number was significantly higher than that observed for the native species.  Although no change in damage was observed for E. uniflora, the increase in productivity may be attributed to removal of Myllocerus undatus, a newly introduced weevil, which has been found to feed only on E. uniflora.  The larvae are root feeders, and therefore are not quantified by the methods used in this study.  The more pronounced increase in leaf number under enemy removal for E. uniflora indicates that E. uniflora may possess a competitive advantage over the native species independent of enemy release.  This study demonstrates that as the rate of introduction of nonnative species continues, the novel interactions among them may have unexpected effects, including a reduction in the significance of enemy release.