550 cites will have populations of more than 1 million by 2015. 58% of the known human pathogens are zoonotic - they can jump between humans and animals. 371 people have been diagnosed with avian influenza as of March 2008, including 235 deaths. 5,000 western lowland gorillas have died from Ebola virus over the past several year. Visit Wildlife Conservation Society's State of the Wild website (or buy the book) to learn more about the state of the wild. You can also watch video presentations of the recent event in New York City.
It is by no means obvious that increasing urbanization, and the rise of megalopolises, directly causes a decline in wilderness and wildlife.
In our geopolitical world, both are happening at once. But, in principle, if everyone lived in one hyper-city, the rest of the world could be split between farm, fishery, and (larger than today) nature preserves.
See also the late Professor Isaac Asimov's vision of the galactic empire's capital city/planet, Trantor. The whole planet is roofed over as a hyper-city, except for the emperor's garden.
Also, sustainable production of wilderness is about 10 times more economically valuable than non-sustainable agriculture and animal husbandry, according to Dr. Timothy Jessup, based on his years of work in Kalimantan for the UN and the Ford Foundation.
It's not a question of human population growth versus the environment.
The right question is management of human population and its growth.
You can have 10 billion people on this planet, have a mostly healthy population, and still have the planet (for the most part) unpolluted. We would have to get off plastics and fossil fuels, and re-engineer our distribution systems.
I don't have the big answers to those questions, of course. But thousands of engineers and entrepreneurs are working on those issues.
I'm just pointing out that environmental destruction is not an inevitable result of human population growth, but it is the result if population growth is unmanaged or improperly managed.
Also, I'd like to point out that we are struggling with this issue on a planetary scale for the very first time in human evolution. I think other human populations have had to deal with it in localized geographic areas (such as towns polluted by the mining and oil industries, etc, and possibly even prehistoric scenarios where a tribe or village of humans over-exploited their local resources).
As a late baby boomer, I an in a generation that has had to make that shift. When I was a child, the planet seemed overwhelmingly big and the idea that humans could degrade the entire planet seemed unimaginable. The myth we were fed was "man vs nature" - that "conquering" nature was a good thing, and the rise of plastics and other industrial materials, as well as the processes that created all these plastics were hailed as symbols of man's great progress. Cities of smokestacks were seen as symbols of success. Muscle cars, those gas-guzzling hunks of American metal, were a worldwide status symbol (and still are for many people).
Now, within one generation only, we are having to grapple with the reality that those very things we thought were progress are destroying our own habitat, and causing diseases in our own people.
Within one generation, the planet went from being overwhelmingly huge and unlimited to being a small fragile biosphere that we are in serious danger of contaminating to the point where it starts killing our own species off.
So, it's not just a problem of re-engineering (something humans are good at), but a question of changing very basic viewpoints and socially inculcated ideas about what constitutes health and progress for humans (something which humans seem to have harder time doing).
Even in this, the 21st century, I've encountered other people, who are educated and clear-minded folks, who still don't believe that it's possible for humans to have the level of impact on the planet that the environmentalists claim.
This touches on denialism, which is a very pernicious problem. When people simply do not want the discomfort of changing their beliefs to fit new realities, they are willing to endure, in some cases, quite dire consequences rather than change their thinking.
The thought seems to be that labeling the results of human population growth mismanagement as simply "unpredictable catastrophes," even if that means cleaning up the dead after disasters and the dislocation of whole communities of people, is preferable to changing set beliefs acquired from childhood.
I think the real battle is getting people to see the need for changing their viewpoints regarding humanity's relation to the planetary environment.