Clear indirect effects of honey bee abundance on
pollination
Honey bee abundance indirectly decreased pollination by reducing native
bee abundance in meadows and decreasing C. quamash visitation by
more effective native bee pollinators. These reductions are likely the
result of exploitative competition because both pollen and nectar
availability declined with increased honey bee abundance, as has been
shown in other systems (Cane & Tepedino 2016; Carneiro & Martins 2012;
Paton 1993), and resource competition can lead native bees to shift
visits to different meadows or plant species (Herbertsson et al.2016; Valido et al. 2019). Although resource competition seems a
more likely explanation, other competition avoidance behaviors,
including scent-cues (Stout & Goulson 2001), could also reduce native
bee abundance and C. quamash visits in response to increased
honey bee visits.
Past studies have demonstrated that honey bees compete with wild bees
for floral resources, but our study is unique in that we clearly
document mechanistic evidence of floral resource depletion. More
fundamentally, this study is among the first to partition direct and
indirect pathways through which introduced honey bees influence
pollination. By isolating the minimally negative direct effect of honey
bee visits, we can confidently conclude that indirect effects drive the
magnitude of the negative association between honey bee abundance and
pollination.