What would happen to the Earth if every human suddenly disappeared?
Alan Weisman has written a book about it, which I can't wait to read.
One of the things I often envision is the animals we've moved all over the world regenerating in new environments. Like the Japanese snow monkeys thriving in South Texas. All the inhabitants of zoos could potentially proliferate and create completely new ecosystems.
Weisman concludes that the planet will survive even if we annihilate ourselves but suggests we can make that demise less inevitable by having one child per family, to get the population back to 19th Century levels. Good luck! Human nature is going to interfere there, and we already know fundies kill for babies (and to keep women birthin' them).