Today, the most common human habitat is cities. Far more people in the developed world now live in urban rather than rural areas and there's a rapid push towards urbanization in developing nations too. But cities displace existing ecosystems and remove habitat for other species. Unplanned or poorly planned growth leads to urban sprawl, increased air and water pollution, loss of farmland and other problems. In the US between 1982 and 1992, 8.3 million acres of farm and range land and 5.4 million acres of forests were swallowed by sprawling cities.
Some cities are now so large and built-up that they are actually changing the local climate.
For example, in Tokyo, warm, humid air from all the buildings, cars and factories rises to form clouds over the city during the day. In the evening, these clouds cause rain and even thunderstorms. These are questions posed by urban ecologists - scientists who study how organisms, including human beings, function and interact in urban settings. It's an emerging field - only 0.4 per cent of ecology papers published last year were on urban ecology. But it's a critical one as more and more people move into ever-expanding cities.