The introduced invasive shrub Eugenia uniflora outperforms introduced non-invasive, common native, and rare native congeners in Florida

Background/Question/Methods

Invasive plant species succeed in a new range by opportunistically capturing resources and outcompeting native species, however, the mechanism for this competitive advantage remains unclear.  One particularly compelling explanation is the ability of an invader to outperform other species in characteristics affecting fitness, such as superior germination, growth, or survival rates.  Past studies testing whether invasive plant species display increased plant performance attributes have compared native or introduced non-invasive species to their invasive counterparts, while a system incorporating common native, rare native, introduced non-invasive, and introduced invasive congeners provides an opportunity for multiple controls.  We used such a system of Eugenia congeners in Florida to test differences in performance both in the field and in a common garden experiment.  Assuming that outperformance of native and introduced non-invasive species leads to invasion success, we predicted that the invasive introduced congener, Eugenia uniflora, would experience increased performance compared to its congeners in at least one of the attributes quantified, and that none of its congeners would outperform E. uniflora for any attribute.  In order to test these predictions, we sowed seeds at both experimental locations and then followed them for up to three years, quantifying seedling emergence, growth, survival, and herbivore damage. 

Results/Conclusions

Experiments in the field and common garden yielded similar results.  There were no differences in the rates of emergence for E. uniflora seedlings and its introduced congeners in certain years, but emergence rates for E. uniflora were consistently high over multiple years while those for its introduced congeners were more variable.  Emergence rates for native species, however, were consistently low.    In addition, although there are no statistical differences among Eugenia species in the amount of insect herbivore damage they sustain, E.  uniflora had consistently higher growth and survival rates over time when compared to all other Eugenia species.  Our results support our predictions, indicating that invasive E. uniflora may hold a competitive advantage by having consistently higher emergence, growth, and survival rates when compared to its non-invasive and native congeners.  Our study further suggests that increased performance over other species may contribute significantly to invasion success and that measurements of such traits may be useful in determining the likelihood of invasion by newly introduced woody plant species.