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Exotics on exotics: Pollen analysis of urban bees visiting Sedum on a green roof

MacIvor, J. Scott, Ruttan, Ally, Salehi, Baharak
Urban ecosystems 2015 v.18 no.2 pp. 419-430
Andrena, Apis, Bombus, Hylaeus, Lasioglossum, Megachile, Sedum, cities, ecosystems, flowers, foraging, green roofs, industry, introduced plants, nesting, pollen, vegetation types
Numerous bee species were collected from a single green roof over the blooming period of the dominant exotic plant type, Sedum, a succulent stonecrop widely used in the green roof industry. As green roofs become more common in cities, an understanding of the potential positive and negative impacts of widespread use of this exotic but useful plant is needed. In this study we sampled bees visiting a green roof in downtown Toronto and compared the proportion of Sedum pollen in the loads they were carrying back to nesting locations. It was found that smaller bees (e.g. Lasioglossum, Hylaeus) were significantly less common on the roof compared with medium (e.g. Apis, Megachile) and large-sized bees (e.g. Bombus, Andrena). The proportion of Sedum pollen in the pollen loads of foraging bees collected was high amongst all bees (average of 80.5 % of total pollen load), but significantly greater for exotic bees compared to native bees. Moreover, native bees had significantly greater numbers of non-Sedum pollen types comprising more than >20 % of their pollen loads, meaning bees could be visiting flowers at ground level and on the roof in the same foraging bout. As the number of green roofs in cities increase, the characteristics of their designs, including the vegetation type and diversity, could have a significant impact in shaping local urban bee communities.