The city, in fact, can be thought of as a natural system on at least three different levels, each of which can give us insight into what a city is. At the most obvious level, although we don't normally think in these terms, a city is an ecosystem, much as a salt marsh or a forest is. A city operates in pretty much the same way as any other ecosystem, with its own particular collection of flora and fauna. At a somewhat deeper level, a natural ecosystem like a forest is a powerful metaphor to aid in understanding how cities work. Both systems grow and evolve, and both require a larger environment to supply them with materials and to act as a receptacle for waste.
Ecosystems are the parts of nature most strikingly analogous to cities.
An ecosystem is composed of all the plants and animals that live in a place, together with their physical surroundings. There are three important features of all ecosystems that we should have in mind before we talk about cities. First, energy flows through ecosystems. The second important feature of ecosystems is that their materials tend to move in cycles. Finally, ecosystems are characterized by the existence of niches.