Environment and Planning II Planning and Design IW), volume 26, pages 603 630 Modeling the urban ecosystem: a conceptual framework M Aibcrtt Department of Urban Design and Planning, University of Washington, Box 355740, Seattle, WA 98195, USA; e-mail: maibcrttou.wasliington.edu Received 14 October 1998; in revised form 22 March 1999 Abstract. In this paper I build on current research in urban and ecological simulation modeling to develop n conceptual framework for modeling the urban ecosystem. Although important progress has been made in various areas of urban modeling, operational urban models are still primitive in terms of their ability to represent ecological processes. On the other hand, environmental models designed to assess the ecological impact of an urban region are limited in their ability to represent human systems, I present here a strategy to integrate these two lines of research into an urban ecological model (UEM). This model addresses the human dimension of the Pugct Sound regional integrated simulation model (PRISM)—a multidisciplinary initiative at the University of Washington aimed at developing a dynamic and integrated understanding of the environmental and human systems in the Pugct Sound. UEM simulates the environmental pressures associated with human activities under alternative demographic, economic, policy, and environmental scenarios. The specific objectives of UEM are to: quantify the major sources of human-induced environmental stresses (such as land-cover changes and nutrient discharges); determine the spatial and temporal variability of human stressors in relation to changes in the biophysical structure; relate the biophysical impacts of these stressors to the variability and spatial heterogeneity in land uses, human activities, and management practices; and predict the changes in stressors in relation to changes in human factors. 1 Introduction Planning agencies worldwide are increasingly challenged by the need to assess the environmental implications of alternative urban growth patterns—and policies to control them—in a comprehensive manner. Urban growth leads to rapid conversion of land and puts increasing pressure on local and global ecosystems. It causes changes in water and energy fluxes. Natural habitats are reduced and fragmented, exotic organisms arc introduced, and nutrient cycles are severely modified. Although impacts of urban development often seem local, they cause environmental changes at larger scales. Assessments of urban growth that are timely and accurate, and developed in a transparent manner, are crucial to achieve sound decisions. However, operational urban models designed to analyze or predict the development of urban areas are still primitive in their ability to represent ecological processes and urban ecosystem dynamics. Though important progress has been made in various areas of urban modeling (Wegener, 1994; 1995), only a few scholars have attempted to integrate the environmental dimension. The majority of these models are designed to answer a set of fundamental but limited planning questions relevant to housing (Anas, 1995; Anas and Arnott, 1991; Kain and Apgar, 1985), land use (Landis, 1992; 1995; Prastacos, 1986; Waddell, 1998), transportation (Boyce, 1986; Kim, 1989) and in some cases the inter­ actions among them (de la Barra, 1989; Echenique et al, 1990; Mackett, 1990; Putman, 1983; 1991; Wegener, 1983). On the other hand, the environmental models designed to assess the ecological impact of an urban region are limited in their ability to represent human systems. These models represent people as static scenarios of land uses and economic activities and predict human-induced disturbances from aggregated measures of economic develop­ ment and urban growth. Only with the increasing attention paid to the role of human 606 M Alberti activities in global environmental change has the need emerged to represent more explicitly human systems in environmental models. Whereas integrated assessment modeling can be traced back to the late 1960s (Forrester, 1969; Meadows et al, 1972), the first generation of operational integrated models has emerged only in the mid-1980s. During the last decade, integrated assessment modeling has been proposed as a new approach to link biophysical and socioeconomic systems in assessing climate change (Dowlatabadi, 1995). At present more than thirty integrated assessment models (IAMs) have been developed (Alcamo, 1994; Dowlatabadi, 1995; Rotmans et al, 1995). The focus of current IAMs is global; however, a new generation of spatially explicit regional integrated models is now emerging (Maxwell and Costanza, 1995). These models have started to treat human decisions explicitly but are still too limited in the repre­ sentation of human behavior and the heterogeneity of urban land uses (Alberti, 1998). Recent progress in the study of complex systems (Schneider and Kay, 1994) and the evolution of computer modeling capabilities (Brail, 1990) have made possible a more explicit treatment of the link between human and ecological systems. The development of GIS has provided the capability to integrate spatial processes. However, the greatest challenge for integrating urban and environmental modeling will be in interfacing the various disciplines involved. Urban subsystems have been studied for several decades but progress in urban-ecological modeling has been limited because of the difficulty in integrating the natural and social sciences. A recent National Science Foundation workshop on urban processes pointed out that ecologists, social scientists, and urban planners will need to work together to make their data, models, and findings compatible with one another and to identify systematically where fruitful clusters of multidisci- plinary research problems can be developed (Brown, 1997). Such an approach can offer a new perspective on modeling urban systems. In this paper I build on research in urban and ecological simulation modeling to develop an integrated urban-ecological modeling framework. This framework is part of a current effort to develop an urban-ecological model (UEM) at the University of Washington as part of the Puget Sound regional integrated simulation model (PRISM). UEM simulates the environmental impacts associated with human activities under alter­ native demographic, economic, policy, and environmental scenarios. Its objectives are to: (1) Quantify the major sources of human-induced environmental stresses (such as land- cover changes and nutrient discharges); (2) Determine the spatial and temporal variability of human stressors in relation to changes in the biophysical structure; (3) Relate the biophysical impacts of these stressors to the variability and spatial hetero­ geneity in land uses, human activities, and management practices; and (4) Predict the changes in stressors in relation to changes in human factors. The development of an integrated urban-ecological framework has both scientific and policy relevance. It provides a basis for developing integrated knowledge of the processes and mechanisms that govern urban ecosystem dynamics. It also creates the basis for modeling urban systems and provides planners with a powerful tool to simulate the ecological impacts of urban development patterns. 2 The urban ecosystem Early efforts to understand the interactions between urban development and environ­ mental change led to the conceptual model of cities as urban ecosystems (Boyden et al, 1981; Douglas, 1983; Duvigneaud, 1974; Odum, 1963; 1997; Stearns and Montag, 1974). Ecologists have described the city as a heterotrophic ecosystem highly dependent on large inputs of energy and materials and a vast capacity to absorb emissions and waste (Boyden et al, 1981; Duvigneaud, 1974; Odum, 1963). Wolman (1965) applied an 'urban Modeling the urhan ecosystem 607 metabolism1 approach to quantify the Hows of energy and materials into and out of a hypothetical American city. Systems ccologists provided formal equations to describe the energy balance and the cycling of materials (Douglas, 1983; Oclum, 1983). Although these efforts have never been translated into operational simulation models, they have laid out the basis for urban-ecological research. Urban scholars were rightly skeptical about the attempts to integrate biological and socioeconomic concepts into system dynamics models. None of these models represented explicitly the processes by which humans affect or are affected by the urban environment. At best, human behavior was reduced to a few differential equations. These models simplified the interactions of natural and social systems so much that they could provide little useful insight for planners and decisionmakers. Since then, however, urban and ecological research has made important progress with respect to understanding how urban ecosystems operate and how they differ from natural ecosystems. Urban-ecological interactions are complex. Urban ecosystems consist of several interlinked subsystems -social, economic, institutional, and environmental each representing a complex system of its own and affecting all the others at various structural and functional levels. Urban development is a major determinant of eco­ system structure and influences significantly the functioning of natural ecosystems through (a) the conversion of land and transformation of the landscape; (b) the use of natural resources; and (c) the release of emissions and waste. The earth's ecosystems also provide (d) important services to the human population in urban