Lack of evidence for enemy release and its implications for population growth of Eugenia uniflora: Invasional conflict?

Background/Question/Methods

Many factors have been suggested to contribute to invasion success in plants, however, it remains unclear why certain species successfully establish and become dominant while others do not.  One particularly compelling explanation is the enemy release hypothesis (ERH), which maintains that plant populations are kept in check by natural enemies where they are native and lose this regulation when introduced to a novel community.  Unfortunately, results of studies testing this hypothesis have been mixed, and few have explicitly tested the effects of reduced herbivory on plant fitness.   In order to test the ERH and examine the effects of herbivores on plant populations, an insect herbivore exclusion experiment was conducted using a system of co-occurring native and introduced invasive Eugenia species in south Florida to determine if 1) the invasive shrub Eugenia uniflora (Myrtaceae) experiences enemy release in its new range and 2) herbivory influences population vital rates for E. uniflora and its native congeners.

Results/Conclusions

The results of this experiment do not support enemy release as a contributing factor for invasion success of E. uniflora, with this species exhibiting significantly higher levels of herbivore damage than its native congeners, E. foetida and E. axillaris.  However, this study demonstrates that insect herbivores may indeed play a role in regulation of plant populations.  Reduced herbivory was shown to positively affect growth and survival for both native and invasive Eugenia species, with population viability analysis indicating that herbivore exclusion leads to increased population growth over natural levels.   Because E. uniflora experiences significantly higher levels of insect herbivore damage than its native congeners, its fitness and population growth are more negatively impacted in the new range.  This may be explained by the fact that E. uniflora exhibits, with few exceptions, damage characteristic of a notching weevil, Myllocerus undatus, a recent import for south Florida from Sri Lanka.  E. uniflora is native to Brazil, so these species share no coevolutionary history.  The results of this study indicate that M. undatus may have a negative effect on E. uniflora, potentially benefitting the native species by reducing competitive effects.  In contrast to invasional meltdown, this is a potential example of another type of interaction, which we have termed “invasional conflict”. This interaction will undoubtedly become much more prevalent as more species are introduced, become established, and interact with one another, forming new biological associations.