Urban form, biodiversity potential and ecosystem services
Introduction
More than half the world's population now lives in cities, compared with about 14% a century ago (United Nations, 2001). This increasing urbanisation radically modifies the ecology of landscapes. The effects include alteration of habitat, such as loss and fragmentation of natural vegetation, and the creation of novel habitat types (Davis, 1978, Niemelä, 1999a, Niemelä, 1999b, Wood and Pullin, 2000); the alteration of resource flows, including reduction in net primary production, increase in regional temperature, and degradation of air and water quality (Henry and Dicks, 1987, Rebele, 1994, Donovan et al., 2005, Bonan, 2000); the alteration of disturbance regimes, with many habitats experiencing more frequent disruption (Rebele, 1994); the alteration of species composition, species diversity, and proportions of aliens (Davis, 1978, Ruszczyk and de Araujo, 1992, Rebele, 1994, Roy et al., 1999, Hardy and Dennis, 1999, McKinney, 2002).
One approach to reduce the impact of increasing urbanisation is to minimise the spatial extent of urban areas by developing more compact city forms. There has been much recent debate over the “compact city” paradigm, with its aims of centralising services and reducing urban land take (Jenks et al., 1996, Williams et al., 2000, Jenks and Dempsey, 2005). Such developments reduce urban sprawl, and significant long-term social and ecological benefits have been claimed (Burton, 2000, Jenks and Burgess, 2000). However, whilst the focus has been on the benefits of reducing urban area, we know much less about how urban densification changes the ecosystem characteristics of the urban areas themselves. For example, is it possible to build dense, compact cities that maintain areas of natural habitat and provide useful levels of ecosystem services, such as carbon sequestration and storm-water interception? The net ecological effect of moving towards high-density urban forms clearly depends on the balance of the benefits of reduced land take against the changes in ecosystem function of the higher density urban areas.
In the UK, current policy is to build new developments at high net density, partly as a response to increasing urban populations and social and demographic pressures resulting in a reduction in average household size (ODPM, 2002). One possible ramification of increased urban density might be deterioration in ecosystem service provision in urban areas and declines in both urban biodiversity and the quality of life of the urban human population. As ecosystem services such as carbon sequestration, storm-water interception, climate regulation and biodiversity potential are influenced by the availability and type of vegetated ground cover, increased densification, if it brings with it a reduction in the proportion of such cover and changes in its spatial configuration, may have undesirable effects on these services (Arnold and Gibbons, 1996, McPherson, 1998, Simpson, 1998, Xiao et al., 1998, Weng, 2001, Whitford et al., 2001). This would be a particular concern for at least three reasons. First, many of the ecosystem services provided by urban green spaces carry with them significant economic implications, both locally and regionally (McPherson, 1992, Chee, 2004, Farber et al., 2006). These include implications for house prices, the costs of lighting, cooling and heating of buildings, and the ease of attracting businesses and employees (e.g. Luttik, 2000, Tyrväinen and Miettinen, 2000, Morancho, 2003, CABE Space, 2004). Second, in the face of intensive agriculture in the wider landscape, in some regions urban green spaces now act as important havens for native plant and animal populations (Mörtberg and Wallentinus, 2000, Gregory and Baillie, 1998, Mason, 2000, Gaston et al., 2005). Third, there is growing evidence that local green spaces contribute to both the physical and mental well-being of people living in urban areas (Hartig et al., 1991, Chiesura, 2004, Takano et al., 2002, de Vries et al., 2003), and that the pattern of provision of such spaces is an important issue for social equity (Whitford et al., 2001, Pauleit et al., 2005).
In this paper we investigate how urban form affects the ecological performance of the urban environment through an evaluation of the relationships between urban form and measures of environmental quality and biodiversity potential, over 15 sites distributed across five UK cities. This work forms part of a much broader consortium project to assess multiple dimensions of the sustainability of a variety of urban forms using these study areas (Jones, 2002; http://www.city-form.com).
Section snippets
Data
In each of five UK cities, Edinburgh, Glasgow, Leicester, Oxford and Sheffield, three study sites were selected, each containing ca. 2000 households (Fig. 1). Sites were selected on the basis that each city should contain a city centre site (Inner), an outer suburban site (Outer) and a site situated between the centre and suburbs (Middle). This is similar to the urban gradient approach widely used in urban ecology (see, for example, McDonnell and Pickett, 1990, Hahs and McDonnell, 2006). It was
Comparison of ecosystem performance among 15 study sites
The Inner, Middle and Outer site categorisations were intended to reflect variations in urban form within cities rather than between them, and to provide an overall continuum in urban form across all the cities, and thus formal comparisons between these groups were not conducted (almost all variables did indeed exhibit continuous variation across the 15 study sites; see below). Thus, whilst cover of green space and gardens generally increased from Inner to Outer sites, the proportion of this
Discussion
In this study across five cities in the UK, we have shown that high-density urban developments were generally associated with poor environmental performance, as measured by green space patch size and the levels of provision of key environmental services. More densely urbanised areas had less coverage by green space and gardens, smaller habitat patch sizes, greater predicted run-off, higher predicted maximum temperatures and lower predicted carbon sequestration (and hence tree cover) (Table 1,
Acknowledgements
This work was supported by EPSRC grant GR/S20529/1 to the CityForm consortium. Ordnance Survey kindly provided MasterMap data under license to CityForm. We are grateful to I. Fishburn and C. Gascoigne for assistance, and to two anonymous referees for helpful suggestions to improve the paper.
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