Bee diversity is declining due to several stressors including land-use change, lower resource availability, and parasites and diseases. Further, land-use change alters plant-pollinator networks, with implications for pollination services. In urban landscapes, gardens act as important refuges for biodiversity conservation; however, local garden management (e.g. vegetation complexity, floral diversity and abundance, ground cover management) and landscape surroundings (e.g. abundance and richness of natural, open, urban, and agricultural land use types) may affect the diversity and abundance of bees as well as plant-pollinator interactions and networks. As a first step towards understanding shifts in plant-pollinator networks along a local and landscape gradient in urban agroecosystems, we examined pollen collected by Bombus vosnesenskii bumble bees and asked whether local garden management and landscape surroundings influence the abundance and richness of pollen collected by bees in gardens. We worked in 21 gardens in the California that vary in both local management and landscape surroundings during 2016. We sampled vegetation, ground cover, and landscape surroundings, and created a pollen library by collecting pollen from all flowering plants in gardens. We netted 10 bees in each site and extracted full body pollen from bees to determine the abundance and richness of pollen carried by bees.
Several local and one landscape feature of urban gardens influenced number of pollen grains and pollen morphotypes carried by bumble bees, and variation in pollen carried. The number of pollen grains carried by individual bees increased with urban cover within 2 km and declined with increases in floral abundance. Pollen abundance on bees was more consistent (i.e. lower coefficient of variation) in sites with higher herbaceous plant richness and higher floral abundance, but more variable in sites with high mulch cover. Number of pollen morphotypes carried by bees increased with urban cover and pollen morphotype richness was more consistent in sites with higher urban cover and lower mulch cover. Higher floral abundance led to lower amounts of pollen carried, but more consistency; thus providing flowers in urban gardens may provide more stable resource collection for bees. Mulch cover was associated with higher variability in both pollen abundance and richness carried. Mulch may negatively have impacted bee foraging or interactions with other species. Finally, urban cover was a positive driver of pollen abundance and richness, an unexpected result. Yet, in the summer, urban areas may have higher floral abundance than natural or open areas surrounding gardens.