Cities as ecosystems

In many ways a city is like an ecosystem, with its own structure, dynamic and metabolism. Just as a forest's air and soil are shaped by its plants and animals, so are changes in the urban environment rooted in the patterns of urban activities. Urban density, mobility and lifestyles directly affect demands for space and the flow of resources.

What a city consumes and produces 

The concentration of people and activities in a limited space places a high demand on local natural habitats. No city sustains itself within its own boundaries. Energy, water and other resources are brought into the urban system, where they are processed or consumed. Residuals of these processes are then disposed of, often outside the city.