This New Scientist article, which summarizes the meeting of the American Ecological Society of America, gives a rather exaggerated, yet realistic analogy.
When one places some number of bacteria in a confined environment, and give them enough food, they reproduce without any end, finish all the food, and drown in their own waste. The moral of the analogy is that there is no built-in mechanisms in organisms to harness their urge to reproduce and conqueror. Humans are no exceptions.
This is another evidence to my hypothesis, that "greed" is retrospective most of the time. We only feel greedy, and regret our actions, after we feel the consequences. The article goes onto discuss, how the culture of consumerism, endorsed by the American government after the World War II to increase the internal demand, is feeding on top of the inherent greed of humans, and how this may lead to collapse of civilization.
It is easy to take an anti-American, or anti-Capitalist stance, but I think this misses the point. Epicurus (BC 341-270) was writing about the ills of advertisement and consumerism 2000 years ago. And the Germans are still trying to get rid of the dirt off of the East German buildings, deposited by years of worst air pollution in the world.
The point is clear, personal lifestyle changes are needed -and they may still not be sufficient. Sadly, this comes in a time, when China, the world's biggest economy, has to pump up internal demand. I fear the Communist China will have no option but endorsing consumerism. I remember a a BBC special on Chinese bourgeois' newly found love for 1960's old American Army Motorcyles -all fuel efficiency champions of course. I fear the New Scientist article may not be all that alarmist after all.