Introduction: Green Roofs and Biodiversity
Biodiverse green roof in Berlin, Germany (photo by Manfred Köhler).
Until recently, the design of green roofs was based almost entirely on engineering considerations. Much has been written about how extensive green roofs—those that are grown on a shallow substrate and require little irrigation or other maintenance—affect building performance, especially energy consumption and storm-water retention. Although advocates have said repeatedly that green roofs can be a source of habitat for wildlife, there have been few studies and little data to back up these claims.
In the past few years, however, researchers have turned their attention to the role that green roofs can play in the conservation of biodiversity in towns and cities, where natural habitats are few and far between. They have produced a small but growing body of evidence suggesting that green roofs can indeed provide living space for plants and animals, at least mobile species such as invertebrates and birds. Six pioneering papers on the biological value of extensive green roofs are included here.
Some of the longest-term data come from Germany, where the first wave of green roof construction came at the end of the 19th century. In his paper, Manfred Köhler describes his studies of green roofs constructed in Berlin in the mid-1980s, representing the second boom in German green roof creation. Köhler concludes that a relatively diverse flora is possible on extensive green roofs in inner cities as well as rural areas. He also suggests that plant diversity can be even higher if varied microclimates, especially sunny and shady areas, are created, initial plantings are enhanced, and a minimal amount of irrigation and maintenance is provided.
Stephan Brenneisen discusses his work in Basel, Switzerland, where green roofs have become an important part of the city's biodiversity strategy. Based on research conducted at a 90-year-old green roof in Zurich, in which native soil was used and which has become an orchid meadow with high conservation value, the use of natural soil as well as different substrate thicknesses is stipulated in the design criteria for green roofs in Basel and other Swiss cities. On the most biodiverse of the Basel green roofs studied, a dense combination of microhabitats supports 79 beetle and 40 spider species; 13 of the beetles and 7 of the spiders are endangered species.
The work in Basel has been the inspiration for the creation of innovative replacement rooftop habitat in London. The redevelopment of derelict "brownfield" sites, which have become critical habitat for many species since World War II, has resulted in a squeeze on the city's biodiversity, leading biologists to look to buildings as potential habitat. In his paper, Gary Grant reviews the various types of green roofs that have been constructed in London during the past 15 years. Among the most interesting are the customized "brown roofs" constructed from recycled crushed concrete and brick aggregate specifically for the black redstart, a rare and protected bird threatened by the development of their brownfield refuges. Gyongyver Kadas discusses the results of her surveys of invertebrate diversity on green roofs in London, focusing on three groups: spiders, beetles, and aculeate Hymenoptera (wasps, bees, and ants). She has found a higher abundance of invertebrates on rooftops than at brownfield sites, and at least 10% of the species from the target groups are nationally rare.
Nathalie Baumann presents preliminary data from a long-term study of green roofs as potential bird habitat in Switzerland. Her research suggests that green roofs may be able to provide not only food habitat but also breeding habitat for ground-nesting birds such as the endangered little ringed plover and northern lapwing.
As several of the papers in this issue show, green roof design is becoming more sophisticated. Ecologists have begun looking for alternatives to widely used sedum mats that incorporate microhabitats customized for particular species and/or more closely mimic natural habitats, with varied microtopography (including hollows and "clifflets"), scattered rocks, rubble, dead wood, and more diverse vegetation. In fact, there is an increasingly nuanced understanding of creating entire plant communities on rooftops. In his paper, Jeremy Lundholm suggests that green roof designers should look to natural analogs of these manmade environments, especially rock outcrop habitats such as cliffs, scree slopes, and limestone pavements. These rare habitats include suites of species adapted to shallow substrates and extreme temperature and moisture conditions—the same characteristics of extensive green roofs—and therefore can be useful natural models. The natural rock barren ecosystems also typically include varied microtopography, increasing the diversity of the vegetation and providing a greater range of habitats for invertebrates.
These papers point to other promising areas of research. For example, what role can green roofs play in regional landscape and ecological planning? Can they function as green corridors, linking fragmented habitats and facilitating wildlife movement and dispersal?
Although the data presented in these papers are for the most part preliminary, they suggest that if suitable niches are provided on green roofs, plants and animals will move in rapidly and establish communities. Customized green roofs can even provide habitat for declining and endangered species, suggesting that they have the potential to be an essential tool in species conservation.
Urban habitats are often seen as too disturbed, too degraded, and too depauperate to serve as reservoirs of biodiversity. Even ecologists have been slow to acknowledge that cities offer biological benefits. Green roofs may prove them wrong.