Rooftops offer an ideal location for reintroducing habitat for native wildlife, and there's some good evidence that vegetated rooftops are in fact valuable to wildlife species in urban areas. The need for more biodiversity in urban areas is already great and will continue to grow, since wildlife habitat and the biodiversity it supports are in decline in many cities.
Vegetated roofs can actually compensate for the loss of native wildlife habitat when their substrates, or growing media, are deep enough to support grasslands, meadows, or native prairies that are typically composed of deep-rooted plants. The Chicago City Hall rooftop prairie planting, for instance, is known to support substantial plant biodiversity that is used by local and migrating bird species, spiders, butterflies, crickets, and more.
Green roofs with shallow substrates have also been found to support some biodiversity, but the amount is limited compared to roofs with deeper growth substrates.
Strategies that can be used in urban regions to increase biodiversity include zoning rooftops across metro areas to support wildlife habitat in the forms of wildlife corridors, patches, and edge habitats. But further development and promotion of habitat for local and migrating species is needed. Those interested in such efforts should consult local ecologists, landscape architects, city planners and government officials.