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Science Matters by David Suzuki Human beings, like all animals, need habitat in which to live. 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 U.S. 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. Because of this urban "heat island" effect, between 1979 and 1995, summer rainfall in Tokyo increased by 20 per cent. Are such problems inevitable? Are our cities doomed to continued sprawl, pollution and isolation from the natural world? Or are there ways to reduce the impact of cities on the ecosystems they displace, and create healthy, vibrant communities? How does the natural world adapt to these human-dominated environments and what can we learn from them? 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. While natural ecosystems are driven by energy from sunlight, rainwater and biological or chemical interactions, human-dominated ecosystems like cities are extremely energy intensive and need to be subsidized with energy in the form of raw materials, finished products, fossil fuels and food brought in from elsewhere. The amount of energy needed to provide all the goods and services for a city is tremendous. According to Bill Rees of the University of British Columbia, a city like Vancouver consumes resources that require a productive land area that is 180 times larger than the city itself. So while Vancouver covers some 114 square kilometres, providing all the needs for the city requires a productive land area of 20,000 square kilometres. Urban ecosystems are also unique in that a large number of the species living in them are often imported. Research in Phoenix, Arizona, for example, found that 95 per cent of plant species and 25 per cent of all bird species living in the city were not native to the region. These exotic species often displace native species, thereby reducing biodiversity. The exotics also tend to be "generalist" species like rats, starlings and cockroaches that can readily adapt to the cities' changing conditions. So will our cities all end up as energy gluttons dominated by rats, starlings and cockroaches? Not necessarily. Better urban design and more efficient use of resources can reduce the environmental impact of cities. Some of these changes are starting to happen already. Toronto, for example, is planning to remove the expressway that currently cuts the city off from Lake Ontario. Other cities are also making changes to reduce sprawl and increase green space. That's a good start, but urban ecologists have their work cut out for them because the trend towards urbanization is not likely to subside any time soon. Cities can be vibrant centres of culture and art, but to make them truly liveable, we have to understand how they work. To discuss this topic with others, visit the discussion forum at www.davidsuzuki.org.

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