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.