Enemy release and plant invasion: Patterns of leaf damage and defensive allocation in Hawai‘i

Background/Question/Methods: Herbivores may influence invasive species success in two ways.  First, herbivores may directly reduce native plant fitness by preferentially consuming native species relative to invasive species.  Second, native species may allocate more resources to the production of chemical and structural defenses (at the expense of growth and reproduction) in response to enhanced herbivory rates.   

Results/Conclusions: We surveyed a broad taxonomic diversity of invasive and native species in Hawai’i and found that herbivory rates were higher among native species compared to invasive species, but only during the dry season.  However, leaf loss across native and invasive species was only 0.8 - 2%, which is much lower than the 10-15% typically found across dry and wet forests.  These low leaf loss rates suggest that herbivory is not a strong mechanism of invasive species success in Hawai’i.  The defense allocation data support this conclusion.  Natives allocated more resources to structural defenses while invaders allocated more to chemical defenses.  These results contradict two widely accepted hypotheses pertaining to ‘enemy release,’ which posit that invasives species succeed because they allocate fewer resources to defense and experience less herbivory pressure.