Hmmm....
Well, Amp Logic says that, any aliens intelligent enough to actually travel to Earth and pose a threat to us must certainly be intelligent enough to realize that we've also got enough nukes to take out all of us, them, and the rest of the entire ball of dirt. Back an animal (human) into a corner and there probably won't be anything left to 'harvest' or fight over.
I believe that, at the core when the chips are down, we are a species that generally accepts the notion of "If you're taking us out, we're taking you with us." which, to an intelligent species would be enough saber rattling to reconsider.
But then I was a soldier, not a theoretical physicist.
