Important pollinator populations grow more quickly in areas with urban and sub-urban gardens than flower-rich farmland. Allotment gardens have a greater diversity of nectar flowers compared with monocultures of crops in farmland, therefore support more populations of (for example) the buff-tailed bumblebee, Bombus terrestris terrestris. The close proximities of gardens in these urban parks allow pollinators to forage in a ‘matrix’ of gardens, each with a different floral make-up. But can roofs with built-in wild vegetation (green roofs) enhance biodiversity?
Green roofs are becoming increasingly common in cities as they absorb rainwater, regulate building temperatures (insulation), lower urban air temperatures and reduce the heat-island effect. These novel ecosystems also support generalist insect species, and scientists propose that they may also help conserve rare taxa, even vertebrates if connected to ground level habitats.
Williams and colleagues evaluated 6 hypotheses to test their effectiveness. They found that green roofs could aid rare species conservation if specific populations of species are targeted. For example, the Bay Checkerspot is an endangered species and only persists in a few fragmented populations, so populations would have to be relocated to an area within the butterfly’s range first before making use of the green roofs.
The study concluded that green roofs overall do provide important ecological and environmental benefits in the urban environment. It is also clear that green roofs support greater diversity than non-green roofs. However a policy shift towards replacing lost or declining habitats with poor-quality ones must be avoided. More research on the biodiversity of green roofs and the ecological interactions is required before policy action can maximise biodiversity gains.
Oberndorfer, E., Lundholm, J., Bass, B., Coffman, R. R., Doshi, H., Dunnett, N., … & Rowe, B. (2007). Green roofs as urban ecosystems: ecological structures, functions, and services. BioScience, 57(10), 823-833.
Williams, N. S., Lundholm, J., & MacIvor, J. S. (2014) Do green roofs help urban biodiversity conservation?. Journal of Applied Ecology. DOI: 10.1111/1365-2664.12333