Introduction

To understand the ecological character of urban areas, a good knowledge of the biophysical character of the urban area is required, including the climate, hydrology, geomorphology, soils and special character of ecological processes in the most-modified environment. The chapters in this part set out how urban activities have modified the impacts of the natural circulation of air, water and materials, and so created the potential for a great diversity of habitats. It also examines the way ecological processes, such as succession, occur in the urban environment, and assesses the contributions that can be made to those processes by recombinant and creative ecology. Urban development radically alters the nature of the ground surface. Buildings of all types profoundly influence the conversion of solar radiation. They also affect wind flow at ground level, reducing wind velocity, causing changes in wind direction, greater turbulence, and localized acceleration. Built-up areas also differ from the countryside in terms of their thermal regime and in levels of, and periodic changes in, relatively humidity and water vapor content. In the urban energy budget, heat from combustion processes can play a significant role, especially in winter in high latitudes. Being areas of concentrated emissions of pollutants and fine particles, cities modify the receipt of sunlight, create conditions for fog (smog) and produce condensation nuclei for rain formation. Sue Grimmond demonstrates how urban climates are due to the surface-atmosphere exchanges of energy, mass, and momentum. Understanding these exchanges, and the effects of a particular urban setting on their spatial and temporal dynamics, are key to understanding urban climates at the scale of the city, neighborhood, or individual street or property level, and to predicting and mitigating negative effects. Urban temperatures tend to be warmer, especially at night around the city center than at the edges of the built-up area. Some of the radiation received during the day is stored in buildings and released at night. Tim Oke points out that the convoluted configuration of the urban building materials exposes a much larger surface area for exchange than a flat site and because they are often dry (due to their ability to shed not store water) the heat they absorb is used efficiently to warm the material rather than to evaporate water. In addition heat from combustion intensifies during peak traffic periods, and also comes from domestic heating on winter evenings. The heat island generates its own wind system, with wind flow converging on the city centre and then rising, assisting the development on convectional clouds under calm conditions, and subsequently flowing outwards and descending at the city edge. Parks and other

greenspaces modify the intensity of the urban heat island, a phenomenon found in large cities in all latitudes, but being most marked under calm weather conditions, such as those that prevail for much of the year in the sub-tropics and mid latitudes. Urban areas can develop their own rainfall through the formation of convectional clouds as a result of the urban heat island effect. Well-documented in Europe and North America, the effect was also reported by the world’s first space-based rain radar aboard NASA’s Tropical Rainfall Measuring Mission (TRMM) satellite. Mean monthly rainfall rates within 30-60 kilometers downwind of the cities were, on average, about 28 percent greater than the upwind region. In some cities, the downwind area exhibited increases as high as 51 percent. Marshall Shepherd and his co-authors show how the urban environment alters aspects of regional hydroclimates, particularly precipitation and related convective processes. Although at times the human influence has been exaggerated, studies have established the scientific basis for how human activity in urban environments shapes their pattern of convection, precipitation, and lightning. Higher urban rainfalls, which may increase further under climate change, have implications for urban drainage design, but because of the large impermeable areas, they may not have much implication for soil-plant-water relations. Urban areas have a dual hydrologic system: the modified natural drainage system, including canals and river diversions, and the artificial water supply and waste water disposal system. Some parts of the original drainage system may be buried and some of the buried streams may become interconnected with the sewer system. This introduces complexities for runoff management, especially when large roofed and paved areas feed into combined sewers that are designed to overflow into rivers after heavy rains. For aquatic life, much depends on the management of the discharges from the artificial system. Just one overflow of an old combined sewer system can create a depletion of oxygen that lasts long enough to greatly reduce fish stocks. Ian Douglas shows that climate change is possibly already causing more urban flooding from surface water and sewer overflows which suggests a need for creative conservation, stream daylighting and sustainable urban drainage systems to create multi-functional spaces that can support wildlife, provide recreation and, for a few days every few years, provide stormwater runoff storage. Good urban planning requires a sound understanding of the ground on which cities are built. Some are built on glacial deposits that are liable to move when excessively wet. Others overlie cavernous limestone terrain that poses foundation problems and can be subject to sinkhole collapse. Some sit upon shrink-swell clays that can dry out and cause subsidence. Almost everywhere in hilly terrain, urban construction can potentially trigger landslides, but the potential for such mass movements all too often goes unrecognized. Removal of the original forest vegetation and exposure of bare soil or weathered rock leads to erosion and sediment production. The sediment is washed into rivers and aggravates flood problems, often causing damage to water supply intakes, while its deposition can disrupt both urban and rural activities. Ian Douglas demonstrates that urban development also creates landforms, whether they are the result of deliberate encroachment by filling on to floodplains or shorelines, or just the accumulation of material on the surface by the dumping of construction debris, domestic waste and landfill operations. All such changes to landforms have to be made with an understanding of the geomorphic processes that will affect them. They all have implications for urban plant and animal life. Urban soils are highly varied in character, not only because they reflect the original soil associations of the rural landscape, but because a whole variety of materials have been added to the soil as a result of urban change. Often land has been bulldozed and filled or excavated. Soils have been compacted, and their organic matter has been lost. Soil parent materials now include debris from construction and demolition, fragments of concrete, sand, and bricks. Peter

Marcotullio shows how soils in highly built-up areas have high nitrogen and phosphate levels, due to excreta from domestic animals, and runoff from car-washing. Many roads soils in higher latitudes are affected by de-icing salt. They can also have high lead concentrations, resulting from the former use of lead in petrol. Soil pH can be high in areas of acid rain. Many soils on old brownfield sites are contaminated with heavy metals and complex compounds derived from hydrocarbons. Succession is a dynamic and continuous process, often occurring gradually over time. Urbanization and its associated activities have a profound impact on natural succession, with the end result that little natural succession occurs in most metropolitan areas. However, Wayne Zipperer notes that from a species performance perspective, the increase in CO

resulting from fossil fuel burning, coupled with the increase in temperature from the urban heat island effect, has a significant effect on species productivity. This increase in productivity is further augmented by nitrogen deposition (wet and dry) in the urban landscape. These effects may offset the decrease in productivity likely to be caused by O

depends on management. For example, a widespread practice in urban forests is to clean out the understory by raking leaves, branches, seeds, and seedlings on the forest floor. Such a loss of the understory may have negative consequences for many wildlife species. Likewise, the extensive use of ornamental invasive species and ‘weed-free’ lawn areas has similar impacts. Recombinant ecology is a concept that acknowledges the dynamic reconfiguration of urban ecologies through the ongoing relationships between people, plants, and animals. Recombinant design is the interweaving of ecological, urban, architectural, and social systems to produce metadisciplinary results not possible in each system’s parent organization. It involves exploration of the practical implications of exotic species in urban floras and faunas in relation to natural communities. Colin Meurk uses examples from New Zealand to show that recombinant ecosystems can be a vehicle for landscape legibility (or eco-revelation) as much as pure indigenous communities may have once been. The purpose and value of recombinant ecosystems lies in their intrinsic ecological interest, but also they can be seen as a means of maintaining biodiversity through managed coexistence or ‘reconciliation’. The notions of ‘creative ecology’ and ‘creative conservation’ have encouraged reassessment of the purpose and practice of wildlife-resource management in the built environment. They apply ideas of people-made nature and ecosystem modification to create improved urban space, such as wildflower meadows. Creative conservation and urban design have been used to generate empirical recommendations for enhancing biodiversity in urban gardens. The Greenwich Peninsula Ecology Park in London is frequently cited as an example of such creative conservation. Grant Luscombe and Richard Scott use examples from their LANDLIFE projects to show that creative conservation focuses on using common core species of native origin that occur widely across the country. It is an opportunity to put back simple habitats suited to soil type, for people to enjoy: the buttercup meadow, the poppy field, and the cowslip bank. Such wildflower spectacles are supposed to be common, yet few have seen them, and fewer are there to be seen. These chapters show how the climatic, geologic, and ecological aspects of the urban environment, as modified by human action, create opportunities for organisms to colonize, adapt and create habitats. Our work in cities can lead to new combinations of plants and animals in the very varied habitats to be discussed in Part 3.