The world won't end, it will still be around after the humans are gone. And, even if 99% of all humans are wiped out in a pandemic, asteroid or major war, there will still be millions of survivors.
There was a story a few months ago about what would happen if there were no humans on the planet. I can't believe I remembered reading it and was able to find the links.
http://www.netscape.com/viewstory/20...jpg&frame=true
http://www.newscientist.com/channel/life/mg19225731.100