Colony-level variation in bumble bee foraging and pollen collection across a human-altered landscape

Background/Question/Methods

In recent years, pollinators have exhibited population declines due in part to pathogens, habitat loss and climate change. Some bumble bee species, in particular, show well-documented population declines, with foraging specialists being most affected. Considering the high economic dependence on pollinator services and the range of wild and cultivated plant species potentially affected in turn by pollinator declines, a more thorough understanding of pollinator foraging behavior is a conservation biology research priority. As historical analyses of pollen diets among declining bee populations have found reduced pollen diversity in declining species’ diets, monitoring pollen load diversity may help predict future trends. To characterize the impact of human activities on pollinator foraging, yellow-faced bumble bees (Bombus vosnesenskii) were collected from sites along a gradient of natural and human-altered landscapes in Northern California. Bees were genetically sorted into colonies and pollen load composition was analyzed via microscopy for three or more bees for each of twelve colonies to compare within-colony and between-colony variation in foraging  and pollen collection. Analyses of pooled pollen loads from each site were used to determine overall trends in pollen collection relative to availability.

Results/Conclusions

Individuals captured from shared sites tended to have more similar pollen load composition than individuals captured throughout the study region; however, individuals of shared colony were not significantly more similar than individuals at a site. Pollen load diversity did not correlate with site-level floral richness or floral cover. Pooled pollen load analyses revealed a significant preference for the native plant species, Heteromeles arbutifolia, in greater amounts relative to its abundance. However, pollen preferences varied between colonies and between individuals located at different sites. Identification of key pollen sources and colony-level variation in pollen collection are hoped to promote effective restoration efforts in degraded habitats.