Possible direct effects of honey bee abundance on pollination
If there is a direct negative effect of honey bee visits on C. quamash pollination, the mechanism is not obvious. We did not observe signs of stigma damage and, although excessive pollen receipt can lead to pollen tube competition (Aizen et al. 2014), increased honey bee visits were not associated with changes in pollen deposition. Visitors that remove nectar without pollinating can directly reduce fitness by forcing plants to allocate resources to refilling nectar instead of fertilizing ovules (Pyke 1991). However, C. quamashdoes not refill nectar in the populations we studied and artificial nectar removal did not affect seed set for unvisited plants (Table S6). Other possible direct negative effects of visitation include fungal infections (Antonovics 2005), ovary damage by nectar-foragers (Travesetet al . 1998), and floral abortion induced by nectar robbing (Carbonari et al . 2009), but these mechanisms are rarely documented. Thus, there might be direct negative effects of honey bee visitation, but indirect effects mediated by changes in the visitor community are more convincing.