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

Abstract

Amongst ever increasing habitat loss and compounding environmental stressors, urban pollinator gardens can provide refugia and support native bees. However, gardens are rarely assessed for their effectiveness in providing adequate habitat. This study focuses on a recently established local campus pollinator garden in California. Using diversity observations from the early 2000s garden and non-lethal sampling techniques, we characterized visitation frequencies between flowers and urban bees. We generated a bipartite interaction network and found that despite the gardens relatively small size, the network showed greater specialization and modularity, as well as lower nestedness than expected by chance. Across 12 flower species, we observed two non-native pollinators, the western honey bee (Apis mellifera) and the alfalfa leafcutter (Megachile rotundata), along with at least nine native bee species across three families (Apidae, Halictidae, Megachilidae). Although this garden was designed for native bees, honey bees represented over 80% of visitation events and thus suggests that the garden needs adjustments to provide more forage for native bees. Honey bees occupied the most central and connected position in the network, followed by native sweat bees (Family: Halictidae) which both had generalist patterns. Whereas other bees showed evidence of specialization (Genus: Mellisodes). We found a significant positive relationship between inflorescence count and honey bee abundance but not with native bee abundance. To better steward native bees that possess a range of floral specializations, managers may consider increasing plant species diversity rather than relying on a few but highly abundant flowering plants.