III. The urban biophysical environment
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 of environments. 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.
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. In Chapter 10, 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. Matthias Roth in Chapter 11 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 center and then rising, assisting the development of 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 subtropics and mid latitudes.
Today, at night urban areas are characterized by night brightness, glare, and the loss of visibility of the night sky. Light at night, brighter than full moonlight, is now common and it spills into important areas of habitat in urban regions, altering animal behavior and affecting urban ecosystems. In Chapter 12, Margaret Grose and Theresa Jones examine the biological impact of night lighting and its impacts on urban ecology. They synthesize current ideas on the impacts of light on evolutionary change in biota due to urban light at night, and emphasize the relationship of light at night and urban ecology with regard to urban design and planning. They show how small-scale areas of light and dark patches, such as those created by the presence of irregularly-spaced streetlights, and their associated heterogeneity potentially impact the behavior of urban animals. Artificial light at night emits blue light that disrupts nocturnal melatonin production, compromising a key regulator of biological rhythm and protector of the cells of all life on Earth.
Changes in the type of lighting, particularly the widespread use of LED lighting for streets and public spaces, are likely to have impacts on street trees and other plants. The global move to LED lighting has seen an unprecedented shift to brighter, whiter cities. Even greenspaces within cities are increasingly experiencing the edge impacts of light at night, or of being spatially isolated as major patches of dark in a matrix of highly lit environments. This area of urban ecological impacts is beginning to be more widely explored, but incorporating nocturnal light considerations into urban greenspace planning and urban ecosystem management remains a task for the future.
Water management in towns and cities has become an increasingly urgent problem in the last decade, with urban paved areas extending rapidly, the volume of water discharged from urban piped water systems increasing, and both intense local storms and major river basin extreme rainfalls appearing to increase in frequency with climate change. Understanding and managing the dual hydrologic systems of urban areas (the modified natural drainage system, including canals and river diversions, and the artificial water supply and wastewater disposal system) is of great importance for human health and safety and for urban ecosystems. 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 depletion of oxygen that lasts long enough to greatly reduce fish stocks. Ian Douglas (Chapter 13) 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 causes damage to water supply intakes, while its deposition can disrupt both urban and rural activities. In Chapter 14, Ian Douglas demonstrates that urban development also creates landforms, whether they are the result of deliberate encroachment by filling along 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. However, vegetation plays and important role in stabilizing the urban landscape, especially in protecting hillslopes and reducing sand drift. Geomorphology and ecology are part of the same landscape system and always ought to be considered together in urban planning and design.
The importance of green solutions for managing coastal areas is emphasized by Larissa Naylor and her colleagues in Chapter 15. The potent cocktail of rapid urbanization and risk associated with climate change directly affects the geomorphologic and ecological functioning and water quality of urbanized coasts where human activities are reducing the extent, quality, and functioning of natural coastal landforms and the biotic systems they support. To avoid further damage and to help restore degraded coastal features many countries are now developing techniques to reintroduce natural elements into artificial structures at the coast and in estuaries. Nature-based solutions are viable alternatives that can mediate habitat loss, improve resilience, and contribute to the multi-functionality of urbanized coastal environments. These interventions are designed to improve both the geomorphic and ecological functioning of coastal landforms and improve ecosystem services including social—ecological resilience.
The influence of climate change is becoming pervasive and in Chapter 16 Camilo Ordonez and his co-authors examine how the impacts of such changes on urban nature may be assessed with special reference to urban forests. Infrastructural and human elements are critical factors in understanding the vulnerability of natural systems in urban areas. Trees in cities frequently face a unique array of stressors compared to woodland trees, making them more susceptible to further disturbance. However, there is a high degree of uncertainty around the cumulative effects of these existing urban stressors with the effects of a changing climate. Approaches to the climate change vulnerability assessment of urban forests, and of urban ecosystems in general, involve participatory methods that include the people involved in the use and management of urban greenspaces as well as broader climate change adaptation work. Urban ecological understanding in the context of socio-ecological systems is an essential component of dealing with climate change in towns and cities.
Healthy soils in cities are essential to the functioning of healthy urban ecosystems. 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 has been added to the soil as a result of urban change. Often land has been bulldozed and filled-in 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. Alan Yeakley (Chapter 17) shows how soils filter pollutants: without sufficient contact with soil, runoff waters will contain higher amounts of chemical contaminants including metals, hydrocarbons, excess nutrients, pharmaceuticals, and personal care products. Soils provide substrate for vegetation and the faunal communities that inhabit vegetation (see Chapter 32); with inadequate amounts and quality of soil, urban areas are left susceptible to invasive vegetation and a general poverty of plant communities. Many soils on old brownfield sites are contaminated with heavy metals and complex compounds derived from hydrocarbons.
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 highly varied habitats to be discussed in Part IV.