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variability has important consequences for the relationship between human populations and the environment, because places (and thus the populations in those places) become urban through the transformation of the land into a built environment, and, as urban places evolve, the subsequent changes in the built environment may well have forward-linking influences on human behavior. In concert with Redman (Chapter 7) and Seto (Chapter 8), we posit a fundamental ecological relationship between urban places and human behavior: humans transform the environment and are then transformed by the new environment.

Evolutionary ecological processes such as this cannot be captured by the standard distinctions of urban and rural or urban and nonurban. The concept of urbanness, as we use it here, implies that the rural/urban distinction is a continuum, rather than a dichotomy. Humans increasingly live not simply in urban, but in highly differentiated urban, settings. This means that there is an ever-increasing variability in population-environment interactions because a majority of people live in an environment that is substantially and differentially transformed by human invention and intervention, with the exact nature of that transformation varying considerably from place to place. The ecological perspective suggests that this variability will be associated with variability in human behavior—in how people organize their lives, interact with each other, and utilize the resources of nature. Although we could extend this idea to the entire range of human settlements, in this chapter we focus more specifically on the variability within an urban area. As dramatic as human behavioral differences may be between rural and urban areas, there are nonetheless very large intraurban differences in demographic processes (Weeks et al., 2004), and it is probable that these differences will become more important over time as increasing fractions of the world’s population come to live in urban places.

Our conceptual framework is thus based on the ecological principle that human behavior is shaped by a combination of the natural, built, and social environments in which people live. Most social science literature that describes the nature and character of urban populations focuses almost exclusively on the measurement of the social environment, often drawing on census data to describe this milieu. But variations in the social environment depend in part on variability in the built environment. For example, high population density—an index that is often used as a measure of urbanness—can be achieved with some kinds of physical structures, but not others. In order to put a lot of people in a relatively small amount of space, buildings must be very high above ground or very far below ground or both. The technology to build such structures is relatively new in human history and the expense of doing so limits its uses, adding to the spatial variability in the application of the techniques that permit very high density.

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