Conversion of natural habitat to agriculture can profoundly impact pollinator communities with corresponding changes in pollination function. A growing number of studies now document patterns of community change, but we have limited understanding of the mechanism that link changes in the landscape to changes in pollinator communities. We explored the effects of local habitat and landscape composition on bumble bee colony productivity and examined the importance of resource use and availability in the landscape as a mechanism underlying population persistence. We established colonies of Bombus vosnesenskii at 38 sites along a landuse gradient in central California. Surrounding landscape varied from primarily semi-natural to entirely cultivated land. Local habitats included organic farms, conventional farms, and riparian habitats. We recorded mass gain and offspring production for all colonies. We also assessed floral resource use availability in the surrounding landscape. We used quadrat samples from multiple sites throughout the study region to estimate floral density for each crop-field type and all semi-natural habitats (e.g., tomato field, chaparral). We combined floral density data with GIS landuse layers to model the distribution of floral resources surrounding each colony at different spatial scales and how resource availability changed during the season. To determine pollen use we collected pollen loads from returning workers at each site in May, June and July. We then used light microscopy and a reference collection to score the proportion of pollen from different plant species collected by colonies throughout the landscape.

Results/Conclusions

Worker number but not queen production declined significantly with isolation from semi-natural habitat. Connectivity of the landscape proved important. Workers from colonies in all site types collected resources from native plants and crops growing in complementary habitats.