Pollinator Plant Network Interactions of Bees (Hymenoptera: Anthophila) in an Urban Garden

Urban pollinator gardens can provide refugia and support diverse populations of native bees amid threats from habitat destruction, pesticides, and potential ecological pressures from the introduced honey bee (Apis mellifera (Linnaeus, 1748)). The University of California, Berkeley, maintained a native bee garden at the Oxford Tract research facility to study the biodiversity, phenology, and foraging habits of urban bees from 2003 to 2009. That garden was decommissioned, and a new garden was re-established in 2019. Using diversity observations from the early 2000s garden and non-lethal sampling techniques, we characterized plant-pollinator interactions between flowers and urban bees in the newer bee garden with a bipartite interaction network. Across 12 flower species, we observed two non-native pollinators, the honey bee (A. mellifera) and the alfalfa leafcutter (Megachile rotundata (Fabricius, 1793)), along with at least ten native bee species across three families (Apidae, Halictidae, Megachilidae). We found that, despite the garden being created for native bees, honey bees accounted for 84% of all pollination interactions. The most abundant native bees were sweat bees (Family: Halictidae). Generalist interactions dominated the network, as both honey and sweat bees foraged on most available flowers. Honey bees showed a significant positive correlation with floral abundance, visiting flowers with the highest number of inflorescences, whereas native bees did not show this preference. These results indicate that native bee garden stewardship could benefit from greater floral diversity, while avoiding the dominance of any single species with high floral abundance, thereby reducing the likelihood of direct competition with honey bees.