Competitive impacts of an invasive nectar thief on a pollinator community

Mutualistic plant-animal interactions, such as pollination, can be disrupted by a range of competitive interactions between invasive and native floral visitors. As the most abundant tree species in undisturbed Hawaiian forests, 'Ohi'a lehua, Metrosideros polymorpha, is critical to the Hawaiian fauna and the energy flow through Hawaiian ecosystems. The invasive Western yellowjacket wasp, Vespula pensylvanica, is an adept and aggressive nectar thief of the partially self-incompatible and pollen limited M. polymorpha. A BACI (Before-After, Control-Impact) experimental design was utilized to compare the standing nectar crop, floral visitation patterns, and fruit-set of M. polymorpha within high Vespula abundance control sites to low Vespula abundance removal sites. The experimental removal of V. pensylvanica, when compared to control sites, resulted in a significant increase in the availability of nectar; the diversity, visitation rate, and efficacy of the pollinator community; and the fruit set of M. polymorpha. These results provide evidence that V. pensylvanica suppresses the pollinator community through both exploitative and interference competition and negatively impacts the reproduction of M. polymorpha. By elucidating the strength, mechanisms, and implications of these competitive interactions this research provides insight into the impact invaders have on ecosystem function and the impact competition has on the structure of pollinator communities.