| The urban ecosystem is investigated through both a macroscopic analysis of global data and a more microscopic analysis of the flows of energy and material through individual urban systems. First, the long standing observation that cities in a given region follow a rank-size, or power law, distribution of population size holds only for the largest cities in a region. The majority of cities, i.e. the smallest 90% or so, are lognormally distributed. This robust pattern is recreated by a simple model of human migration and population growth, suggesting that at the broadest scales, the size distribution of cities may be the result of basic human demographic behavior. Second, the spatial arrangement of cities in large, homogeneous regions is generally indistinguishable from random, although the few deviations from random do suggest that cities are structured in a hierarchic fashion around the largest cities in a region. Finally, I review what is known about the flow of materials and energy through the world's largest cities, focusing on atmospheric processes and modelling efforts. While little is known about how urban ecosystems flux resources, I show that the study of urban metabolism is a fruitful research agenda that will be useful in addressing problems pertaining to urbanization, globalization, and conservation. |
