Competitive interactions between the invasive European honey bee and native bumble bees

Biological invasions represent both an increasingly important applied problem and a tool for gaining insight into the structure of ecological communities. Although competitive interactions between invasive and native species are considered among the most important mechanisms driving invasion dynamics, such interactions are in general poorly understood. The European honey bee (Apis mellifera) is a widespread and economically important invader long suspected to competitively suppress many native bee species. Yet the extent to which this introduced species alters native communities remains controversial, reflecting ongoing debate over the importance of resource competition in regulating pollinator populations. I experimentally tested the effects of competition with Apis on colony foraging behavior and reproductive success of a native eusocial bee, Bombus occidentalis Greene, in coastal California. B. occidentalis colonies located near experimentally introduced Apis hives had lower mean rates of forager return and a lower ratio of foraging trips for pollen relative to nectar. Both male and female reproductive success of B. occidentalis were also reduced with greater proximity to introduced Apis hives. Reproductive success correlated significantly with measures of colony foraging behavior, most strongly with the relative allocation of foraging effort to pollen collection. This pattern suggests that B. occidentalis colonies exposed to competition with Apis experienced increased nectar scarcity and responded by reallocating foragers from pollen to nectar collection, resulting in lowered rates of larval production. These results provide evidence that Apis competitively suppresses a native social bee known to be an important pollinator, with the potential for cascading effects on native plant communities. This work also contributes to a greater understanding of the role competitive interactions play in pollinator communities, particularly for social bees.