Community effects of introduced honeybees (Apis mellifera) on native bee fauna and plant pollination

Introduced species can contribute to the decline of native species through competition, predation, or parasitism, but the frequency with which they do so is largely unknown. Introduced species are commonly blamed for the devastation of native communities; however, the evidence supporting these claims is generally correlative and post hoc. This study investigates the impacts of introduced honeybees (Apis mellifera) on species interactions and abundances in Australian Banksia woodlands. A priori predictions about the effects of honeybees on native bee fauna were tested through the experimental manipulation of honeybee populations. Honeybees had a significantly negative impact on native bee forager abundance and trap-nest use. This effect is apparently driven by exploitative competition for floral resources; honeybees are able to out-compete some native bee species by foraging at lower ambient temperatures. In subtropical Queensland, Australia, honeybees started foraging at 15 C, while Trigona spp. did not start foraging until the ambient air temperature reached 18 C. This difference allowed honeybees to pre-empt floral resources: >90% of Xanthorrhoea johnsonii (Xanthorrhoeaceae) pollen was harvested before Trigona spp. started to forage. Honeybees also influenced the pollination dynamics of the native plant community. Honeybees were effective pollinators of three plant species with generalized flowers. They were less efficient than native bee species at transferring pollen to these plant species, but they were also more abundant than any of the native bee species. Overall, honeybees were the most important agents of pollen transfer to these generalized flowers. Since generalized pollination systems are thought to persist only when all visitors are roughly equivalent in pollination ability, the presence of honeybees has long-term implications for the persistence and evolution of generalized plant species in this community. For the vertebrate-pollinated plant species examined, honeybees were effective pollinators of X. johnsonii, but ineffective or unimportant pollinators of Banksia aemula (Proteaceae). This suggests that honeybees may be relatively poor pollinators of plant species with highly specialized flowers. In summary, introduced honeybees have significant impacts on native bee communities and on plant-pollinator dynamics.