I'd like to suggest that cities, megacities and metropoles are not inherently detrimental to the environment. Instead it is presently our conceptions and implementations of cities that are the problem. In fact, several aspects of city design may minimize the collective impact of its inhabitants.

The scale of cities affords them efficiencies not approachable by hamlets and townships. Thus far a successful public transporation system would seem to require a large population to support it. Cities also lend themselves to a spoke-and-hub distribution system, which reduces the amount of transportation required to move goods. "Dilution is not the solution to pollution" is a well-known axiom amongst environmentalists, and cities certainly produce large quantities of waste and pollution. However, the concentration of some of these waste streams, such as sweage, may facilitate their processing.

There still exists, of course, the numerous flaws in conception and execution. While much wildlife fares better in contiguous open space than a fragmented area of the same size, which would indicate that cities may be a better choice than numerous towns in a region, cities tend to be constructed in (often sensitive) riparian realms. Cities create their own micro-climates. They generally consist of large swathes of impermeable surfaces that redirect rainfall, and exacerbate flooding. Many city surfaces also reflect light differently than a natural environment, increasing the ambient temperature.

Finally, I should point out that many of the advantages cities currently hold would become moot were we to revisit many of our assumptions and assume a benign set of technologies. That is, technologies which particpiate or leverage natural systems, instead of attempting to thwart, exploit, or divest themselves from them. If the manner in which you live has no negative impact, it doesn't matter where you live.