Introduction

This section introduces some of the methods used in urban ecology to analyse urban habitats, map those habitats, study invasive and introduced species, examine nutrient cycles and biogeochemical fluxes, and to analyse urban metabolism. The section does not, and cannot, provide an overview of all the ecological methods likely to be needed in understanding urban ecosystems, but reference is made to the authoritative sources where those generic methods can be found. Urban habitats are extremely varied and offer a wide variety of opportunities for vegetation to become established, for the protection of natural vegetation and the establishment and maintenance of gardens and other planted landscapes. This diversity has been subdivided in terms of vegetation types, biotic urban ecosystem characteristics and urban land use types. Ian Douglas re-examines these and compares them with the European EUNIS habitat classification. In Britain, Phase 1 surveys provide a standardized system for classifying and mapping wildlife habitats, providing, relatively rapidly, a record of the semi-natural vegetation and wildlife habitat over large areas, including urban areas. The habitat classification is based on vegetation, with topographic and substrate features included where appropriate, and is augmented by as full a species list as was possible in and at the time of survey. The mapping is based on field visits and is usually plotted at 1:10,000. Surveys also need to identify potential patches and corridors in built-up areas and may require more detailed mapping. Urban land-cover and land-use mapping by means of satellite remote sensing has been investigated since the early 1970s. Increasingly more powerful data and more sophisticated evaluation methods have been introduced. As soon as reliable results on vegetation cover or sealing degrees could be obtained for small reporting units like city blocks, remote-sensing techniques became interesting for operational use in the framework of city planning. In 1997 the European Union tested Russian high-resolution photographic satellite data (KVR 1000) for land-use purposes. Land-use classes at a certain (but not on the highest) level could be obtained for statistical blocks both in Athens and in Berlin. Automated classification of urban vegetation using remote sensing and geographical information systems can subdivide trees, grass and lawn effectively. False colour IKONOS images are useful for establishing urban land cover types, urban vegetation health and can be used for detailed evaluation of water use by urban vegetation. Peter Jarvis assesses the validity of various types of mapping, including habitat and biotype mapping, selective surveys and tree cover mapping. He notes that throughout the UK, habitat surveys have also provided information for the many local

and regional biodiversity action plans that were produced in the latter part of the 1990s and the first few years of the twenty-first century. There remain a number of limitations concerning the acquisition, storage, cartographic representation, analytical modelling, interpretation and use of spatial data appropriate to planning for a green urban environment. Nevertheless a great deal of progress has been made, particularly since 1980. Urban areas contain large numbers of deliberately and inadvertently introduced plant and animal species. Long records exist of some plants that escaped from botanic gardens or were dumped into ponds after domestic use. Activities as seemingly benign as the planting of exotic trees and shrubs in parks and along byways disrupt the distribution of natural components of biodiversity. These activities combine to decrease habitat area and disturb the equilibrium between extinction and immigration amongst the remaining natural habitats. Domestic pets have escaped and become feral. Animals such as mink have been released on the fringes of cities. These introductions and invasions change urban ecosystems, but climate change is adding more. Globally, many species are already being affected. Ian Douglas shows how pest distributions are beginning to change and diseases of plants and animals are spreading, not merely because of alterations to the climate, but through the combined effect of the whole range of human impacts on the environment and ecosystems. Many pathogens of terrestrial and marine taxa are sensitive to temperature, rainfall and humidity, creating synergisms that could affect biodiversity. Climatic warming may increase pathogen development and survival rates, disease transmission and host susceptibility. Many animals have shown adaptations to the urban environment. In Manchester in the nineteenth century, insects adapted, with the melanic form of the peppered moth gradually replacing other forms after 1848. By 1895 black peppered moths accounted for 98 per cent of the total peppered moth population. Urbanization typically leads to a turnover in species composition such that species that cope well with urban conditions replacing those that occurred in the pre-urban habitat. Many species do not live in cities and do not breed close to highways, and indeed the birds of urbanized areas are highly similar: the same few species become common everywhere, while the area’s original species variety is lost. Studies of the songs of the great tit (Parus major), a successful urban-dwelling species, in the centre of ten major European cities, including London, Prague, Paris and Amsterdam, compared to those of great tits in nearby forest sites showed that for songs important for mate attractions and territory defense, the urban songs were shorter and sung faster than the forest songs. The urban songs also showed an upshift in frequency that is consistent with the need to compete with low-frequency environmental noise, such as traffic noise. Urban biogeochemistry should not be regarded as distinct from that of other ecosystems because the physical and chemical laws that govern biogeochemical reactions are universal. However, constant physicochemical laws do not enable a simple transfer of biogeochemical models to urban ecosystems because the drivers of biogeochemical reactions are under human control. Although human control is complex, the drivers are linked to three classes of human activity: engineering, urban demographic trends and household-scale actions. A biogeochemical analysis has to examine the combination of the natural circulation of chemicals with the deliberate shifting and storing of materials in the city, the emissions of gaseous, liquid and solid wastes and the accumulation of materials in temporary sinks such as landfills. Nancy Grimm emphasizes that a major challenge in scaling urban biogeochemistry to the globe is the heterogeneity of cities. Many variables interact to determine the biogeochemical fluxes to and from an individual city. There are significant biogeochemical differences between cities in developed and developing countries due to the differences in municipal services (e.g. sewage and solid waste removal) and available resources for technology (e.g. wastewater treatment, recycling). Ultimately, the challenge

is to integrate human choices and ecosystem dynamics into a seamless, transdisciplinary model of biogeochemical cycling in urban ecosystems. Such a model will inform the nature and location of significant habitat changes that will influence urban wildlife and human activity in built-up areas. Urban ecosystems have a unique metabolism that we can characterize and measure in terms of stocks and flows of materials and energy that move between ecosystems and socio-economic systems. Cities cycle and transform raw materials, fuel and water into the urban built environment, and into human biomass, consumable products and waste. Studies of the mass balance of cities have provided important insights about the role that urban dwellers play in the cycling of chemicals. They are beginning to gain information on the mechanisms by which other factors – demographics, human activities and wastewater infrastructure – affect nutrient cycling in urban ecosystems. Such analyses also incorporate the impact of urban consumption on other ecosystems, both in terms of the urban ecological footprint and in terms of land use competition and soil and water resource degradation. Huang and Lee note that most current research on socio-economic metabolism ignores energy flow because of the difficulty of comparing materials and energy with the same units. Aggregating material flows according to their mass neglects the relative contribution associated with the values of materials with different qualitative contents. They use energy and transformity introduced by H.T. Odum to show how they obtained an overview of the socio-economic metabolism of Taiwan material flows and energy flows by incorporating an emergy synthesis. These specific analyses could be augmented by an assessment of greenspace quality. Good quality greenspace plays a vital role in enhancing the quality of urban life. In many cities the quality of greenspace declined during the second half of the twentieth century. After 2000 greater emphasis on the role of open air recreation in improving health and the role of green areas in adapting to climate change led to signs of improvement in some countries. Qualitative assessment of greenspace improvement is usually made by assessing the views of users and managers, but those who live close by, but do not use the greenspaces, are seldom surveyed and brought into focus groups. More quantitative assessments of greenspace quality are made using indicators of various functions, such as biodiversity indicators for the nature conservation function or user statistics (e.g. number of persons playing football per week) for the recreational functions. There is little debate about the best way to examine quality of multiple functions and multiple benefit provisions. Correcting this is a task for the future.