Impact of introduced honeybees, Apis mellifera, upon native bee communities in the Bonin (Ogasawara) Islands

The Bonin (Ogasawara) Islands are oceanic islands located in the northwest Pacific, and have ten native (nine endemic) bee species, all of which are n onsocial. The European honeybee (Apis mellifera), which was introduced to t he islands for apiculture in the 1880s, became naturalized in a few islands shortly after introduction. To detect the impact of the honeybees upon nat ive bee diversity, we analyzed pollen harvest by honeybees and surveyed the relative abundance of honeybees and native bees on flowers on several isla nds. Both hived and feral honeybee colonies were active throughout the year , harvesting pollen of both native and alien flowers and from both entomoph ilous and anemophilous flowers. Honeybees strongly depended on the alien pl ants, especially during winter to spring when native melittophilous flowers were rare. From June to November, honeybees exhaustively utilized native f lowers, which had originally been utilized and pollinated by native bees. O n Chichi and Haha Islands, where human disturbance of forests has been seve re, both native and alien flowers were dominated by honeybees, and native b ees were rare or extinct even in well-conserved forests. In contrast, on An i Island and Haha's satellite islands where primary forests were well conse rved and honeybees were still uncommon or absent, native bees remained domi nant. These results suggest that competition for nectar and pollen of the n ative flowers between honeybees and native bees favors honeybees on the dis turbed islands, which are thoroughly invaded by alien nectariferous, someti mes aggressive, weedy plants.