Building height matters: nesting activity of bees and wasps on vegetated roofs

Abstract

Vegetated, “green” infrastructure, including terraces, balconies, and vegetated roofs and walls are increasingly common in urban landscapes, elevating habitat into novel contexts above ground. Highly mobile species, like bees and wasps, are often seen foraging on green infrastructure, but whether nesting opportunities are facilitated is not known. Cavity-nesting bees and wasps that provision brood in human-made trap nests were monitored over three years on 29 vegetated and non-vegetated roofs in Toronto, Canada. The study identified 27 species nesting on rooftops but found that building height was negatively correlated with the abundance of brood cells provisioned in trap nests, and positively correlated with the number of unfinished nests. A decline in green space area within a 600 m radius around each rooftop resulted in decreasing species richness and abundance. Although the introduced bee, Megachile rotundata (Fabricius) occupied more sites than any other bee or wasp (27.6%) and was the most abundant species, amounting to half (48.9%) of all brood reared, native bees were 73% of all bee species reared. The most abundant wasp was the native spider-collecting Trypoxylon collinum Smith (11.4%), but the introduced aphid-collecting Psenulus pallipes (Panzer) occurred at more sites (24.1%). For the pollination and pest controlling services they provide, bees and wasps should be considered in the design of vegetated roofs. Evidence here suggests that building height and surrounding green space at ground level impact bee and wasp diversity on vegetated roofs. Efforts supporting their populations using trap nests should target low- and mid-rise buildings (<5 building levels).

Keywords:

• urban
• biodiversity
• green roofs
• cavity-nesting
• trap nest
• Megachile

Acknowledgements

I thank all those who granted access to the different rooftops included in this study. Thanks to Dr Laurence Packer for comments, Bahar Salehi, Veronica Ladico, and Sheila Dumesh for help in opening trap nests, and the PCYU lab for discussion. Funding was obtained primarily from an NSERC-CGS research award to J.S. MacIvor, and additionally from the Canadian Pollination Initiative (NSERC-CANPOLIN) and an NSERC Discovery Grant award to his supervisor, Dr Laurence Packer. This is CANPOLIN article number 103.

Disclosure statement

No potential conflict of interest was reported by the author.

Additional information

Funding

NSERC-CGS [grant number CGS D 408565]; Canadian Pollination Initiative (NSERC-CANPOLIN); NSERC Discovery Grant.