ABSTRACT
The city as a system has been a central metaphor for urban management since the 1950s (Marcotullio et al. 2004). The systems approach is essentially a formalized method of determining the role of components within the overall operation of a system (Exline et al. 1982). A system can be defined as ‘a structured set of objects (components), or a structured set of attributes, or a structured set of objects and attributes combined together’ (Dury 1981: 4). The components of a system are physical objects, the attributes non-physical but rationally definable characteristics. In cities we are not dealing with isolated systems cut off from external influences, but with open systems involving the transfer of both energy and matter with their surroundings. All biological systems have this characteristic. They require energy to operate and nutrients in order to exist. They have outputs of waste products and materials that are carried away (Douglas 1983) or which accumulate with, or at the edge of, cities in dumps or landfills that sometimes create problems for future generations. Cities may be seen as various types of systems. As economic systems, on the one hand, cities can be interpreted in terms of flows of money, goods, services and materials. As ecosystems, on the other hand, cities can be viewed in terms of flows of energy, water and chemical elements, or alternatively as a habitat for organisms, including human beings (Douglas 1983). The urban systems approach to urban planning focuses on the articulation of various components of a city and the flows and processes between them (Marcotullio et al. 2004). The systems approach leads to more dynamic, adaptive thinking about the future of cities and to more awareness of possible consequences, especially for the environment and human and ecosystem health, of planning decisions and the character of specific urban developments. Ecological studies of terrestrial urban systems have involved a variety ways of examining cities in contrasting ways, some examining ecology within cities, others the overall ecology of cities. They may take land use planning versus biological perspectives, or disciplinary versus interdisciplinary, or biogeochemical compared to organismal. Increasingly the characteristic spatial diversity and patchwork nature of land cover in cities has been seen as particularly important in the study of urban systems. This characteristic heterogeneity of the urban surface and diversity of soils and substrates, including both natural, usually displaced, materials and people-made substances,
including wastes, creates a host of habitats and ecological niches which contribute to the high level of urban biodiversity. However, since about 1990, there has been a much more conscious effort to understand the true complexity of urban systems through the integration of the organic, biogeochemical and energetic approaches to the city, emphasizing throughout the need for understanding the social dimensions of urban ecology and integrating humans into the study of urban ecosystems (Grimm et al. 2000). The work of the Long Term Ecological Research (LTER) projects in Baltimore and Phoenix in the USA (Redman 1999; Redman et al. 2004) has used the watershed approach to analyse urban systems, because it embodies both the integrated dynamics of the overall topographic unit, and encompasses the heterogeneity, both natural and resulting from human action, within it. Like any watershed or river basin that has been modified by human activity, an urban area is part of the coupled human and natural system (CHANS) (Liu et al. 2007). In CHANS, people and nature interact reciprocally across diverse organizational levels. They form complex webs of interaction that are embedded in each other. Thus by 2010 we have full recognition of the global environmental significance of urban ecology in its widest sense. Urban ecosystems can be defined at different scales, but they depend upon and impact upon the remainder of the world. From the litter in the world’s oceans from cruise ships carrying urbanites on holiday and tankers bringing oil and gas to heat and power buildings and movement in cities, to the atmospheric pollutant fallout embedded in the world’s icecaps, the evidence of the effects of urban life is scattered around the globe.
