The Urban Ecosystem

Abstract

Cities are complex ecological systems dominated by humans. The human elements make them different from natural ecosystems in many ways. From an ecological perspective, urban ecosystems differ from natural ones in several respects: in thei: r climate, soil, hydrology, species composition, population dynamics, and flows of energy and matter (Rebele 1994, Collins et al. 2000, Pickett et al. 2001). Humans create distinctive ecological patterns, processes, disturbances, and subtle effects (McDonnel et al. 1993). Planners must consider all these factors in order to effectively plan cities that will be ecologically resilient. Managing these systems requires an understanding of the mechanisms that link human and ecological processes and control their dynamics and evolution. Because change is an inherent property of ecological systems, the capacity of urban ecosystems to respond and adapt to these changes is an important factor in making cities sustainable over the long term (Alberti and Marzluff 2004). Ecology has provided increasing evidence that humans are dramatically changing Earth's ecosystems by increasing landscape heterogeneity and transforming their energy and material cycles (Vitousek et al. 1997). We know a great deal about the processes through which human activities affect the material and energy budgets that cause heterogeneity. For example, we appropriate natural resources, convert land surfaces, modify land forms, burn fossil fuels, and build artificial drainage networks. Human action has transformed 30% to 50% of the world's land surface and humans use more than half of the accessible fresh water. More nitrogen is now fixed synthetically than naturally in terrestrial ecosystems (Vitousek et al. 1986). According to the most recent global ecosystem assessment, humans have changed ecosystems more rapidly during the past 50 years than in any other time in human history, and as a consequence have irreversibly modified biodiversity (Figure 1.1, Turner et al. 1990, MEA 2005).