I think nature will take care of most of the population long before man gets the chance to blow it all to shit.
That's what I've always figured: Nature will end mankind before mankind is able to end nature. We are living in a relatively stable period in time, aside from a few earthquakes, tidal waves, hurricanes, and tornadoes. The earth will always go through changes. If a species can survive, then it will. If it cannot survive, it will perish. Just look at the dinosaurs. Only certain species survived from that time period. Who would have guessed that the cockroach would outlive the mighty Tyrannosaurus Rex?
Hopefully, the human cockroaches won't be the ones to survive if some drastic change were to occur. I would doubt it, though. I think the ones who will survive are going to be the ones who are in touch with nature, and the natural world.