Invasional conflict: Do invasive insect herbivores mediate the effects of enemy release for their invasive plant hosts?

Abstract

AbstractAlthough 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 and 2) if enemy release increases growth and survival 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, height, and survival for all three species. The results contradict the ERH, showing that E. uniflora has a greater proportion of damaged leaves and experiences a greater reduction in this damage when herbivores are excluded from the system, relative to the two native congeners. A trend exhibited by the data also demonstrates that exclusion of enemies positively affects growth and survival for all three species, indicating that E. uniflora has not experienced enemy release in its new range. This may be explained by the fact that E. uniflora exhibits, with few exceptions, mostly damage characteristic of a notching weevil, Myllocerus undatus, a recent import from Sri Lanka. E. uniflora originates in 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 competition. 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.