Pollen load composition and bee diversity on native-planted green roofs: A comparison with urban and natural environments
Although frequently touted by developers, the habitat quality of green roofs for urban flora and fauna has not yet been well established. As important pollinators, urban bees in particular may benefit from the floral resources provided on green roofs, and native-planted green roofs may attract both generalist and specialist bees. Only a handful of studies have assessed bee diversity on green roofs in North America and even fewer have compared native-planted rooftop bee diversity to that of corresponding natural habitat. Furthermore, very few studies have investigated links between bees observed visiting green roof sites and specific rooftop resources, such as pollen, nectar, nesting material, and nesting sites. My primary objectives were to quantify wild bee diversity and species composition at green roofs, ground-level urban habitats, and natural habitats and to examine the range of pollen used by corbiculate bees at these sites. The green roofs were planted with native coastal barrens species, which were also present at the corresponding ground-level urban and natural coastal barrens sampling sites. Bees were net collected between July-August, 2014 using spatially and temporally controlled sampling methods. The assemblage of co-flowering species present at each site was recorded.
More bees were captured at barrens compared to rooftop sites, with urban ground-level sites displaying intermediate capture totals. Species accumulation curves indicated that bee communities were incompletely sampled at all sites; individual-based rarefaction curves revealed significantly greater bee species richness at barrens sites compared to green roofs; bee species richness at urban ground-level sites was not significantly different from either green roof or barrens sites. Similarly, species diversity was significantly higher at barrens sites compared to green roofs, according to both the Shannon-Weiner and Simpson's inverse diversity indices, while species diversity of urban ground-level sites was not differentiated from either barrens or green roof sites. In addition to these results, I will present findings on the breadth of pollen usage by corbiculate bees at the three site types in relation to the available floral resources at time of sampling. Ultimately, capture records alone cannot define the habitat quality of green roofs for wild bees; researchers must also evaluate the availability, diversity, and use of these resources. In addition to providing information about bee diversity and ecology on green roofs, this research contributes to our knowledge of the bee fauna of an understudied natural environment, the coastal barrens of Nova Scotia.