From Science Daily: Overpopulation is the worst environmental problem

http://http://www.sciencedaily.com/releases/2009/04/090418075752.htm

I would go so far as to say that overpopulation is the root of all environmental problems.
Without overcrowding and unsustainable, prolonged use of resources, our planet would be able to rebound from our impact. But when there is so much impact, the effect can't be healed.

Excerpt from the article:
"“Overpopulation is the only problem,” said Dr. Charles A. Hall, a systems ecologist. “If we had 100 million people on Earth — or better, 10 million — no others would be a problem.” (Current estimates put the planet’s population at more than six billion.)"