City landscapes are more strongly three-dimensional and complex than surrounding natural landscapes. Increasingly, building rooftops are vegetated and promoted to improve local rainwater capture, urban cooling, and biodiversity. Studying how different wildlife interact with green roofs and other vertical green infrastructure (including vegetated walls, gardened terraces, balconies) may provide new insight into their function as ecological traps: habitat that appears suitable but are unable to support populations.
Horizontal landscape isolation from high quality habitat is correlated with less wild bees nesting and foraging activity, yet there has been no work to examine the effects of vertical isolation. This study investigates the effect of building height from ground level habitat, surrounding ground level green space (within 150m and 600m), and vegetation type, on the reproductive success of solitary bees and wasps. Species were sampled from May to October over three years (2011-2013) on 29 rooftops (one per roof) throughout the city of Toronto, Canada. Bees and wasps were sampled using trap nests, which are used as nesting substrate and so reproductive success can be determined by rearing the offspring to adulthood.
Twenty-seven species in 16 bee and wasp genera were recorded from 21 green roofs over the study period (eight sites were never colonized). Bees were three times more abundant on green roofs than wasps, but richness was greater among wasps (N=16; including 5 parasites) than bees (N=11). Reproductive success of bees and wasps declined significantly with increasing building height (t=3.240, p=0.004) and with decreasing green space within a 600 m radius (t=3.035, p=0.006). Reproductive success was greatest on intensive green roofs having more diverse plantings. The number of incomplete brood cells (some food and nesting material gathered but no egg laid) among all species significantly increased with building height (t=3.432, p=0.003).
The city of Toronto has a green roof by-law, construction standard, and incentive program aimed at increasing green roof cover on certain building types, including condominiums greater than six storeys. Since the impact of wind on buildings increases with height, bees and wasps might find it difficult to travel to and from nests on exposed green roofs. To reduce the potential for green roofs to act as ecological traps, more study of green roofs as sources (populations can increase) or sinks (populations do not increase) are needed. It is recommended green roofs that promote nesting activity of bees and wasps should be targeted on low- to mid-rise buildings.