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1. Habitat fragmentation is thought to threaten biodiversity, but the response of pollinators to fragmentation is still poorly understood, and research seldom includes pollinator nesting requirements. 2. We investigated pollinator community composition in a highly fragmented oak-savannah ecosystem in south-western British Columbia, Canada. We sampled pollinators in 19 fragments ranging from 0.3 to 31 ha and surrounded by a variety of land-use types, including forest, low-density suburban, and urban neighbourhoods. Pan-trapping and netting surveys captured 4464 bees, flies, and wasps in 138 species and 48 genera. 3. Contrary to expectations, overall species richness did not increase with fragment size. However, ground-nester abundance (not diversity) and cavity-nester diversity (not abundance) were higher in larger fragments, as expected. Floral richness and abundance did not foster pollinator diversity for either guild. Brood parasite responses were complex: host availability (ground-nesting bees) was generally important, richness and abundance increased in fragments with high surrounding road density, and abundance also increased in fragments with other habitat fragments nearby. 4. Responses of different nesting guilds to fragmentation may be related to their use of the landscape surrounding habitat fragments. Some common cavity nesters can nest in fences and gardens, but urban land use may be less hospitable for ground nesters. Brood parasites apparently respond both to host availability and landscape characteristics associated with movement among fragments. 5. Consideration of nesting requirements of pollinators provided some insight into their response to habitat loss in our analysis and should be considered in future studies of fragmentation impacts on pollinator biodiversity. |
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