My first line of defense was obvious. Animals kill one another all the time. Why treat animals more ethically than they treat one another? (Ben Franklin tried this one long before me: during a fishing trip, he wondered, “If you eat one another, I don’t see why we may not eat you.” He admits, however, that the rationale didn’t occur to him until the fish were in the frying pan, smelling “admirably well.” The advantage of being a “reasonable creature,” Franklin remarks, is that you can find a reason for whatever you want to do.) To the “they do it, too” defense, the animal rightist has a devastating reply: do you really want to base your morality on the natural order? Murder and rape are natural, too. Besides, humans don’t need to kill other creatures in order to survive; animals do. (Though if my cat, Otis, is any guide, animals sometimes kill for sheer pleasure.)
And finally, though this is not a pleasant thought, when times were tough, dogs could have served as an emergency food supply. Thousands of years before refrigeration and with no crops to store, hunter-gatherers had no food reserves until the domestication of dogs. In tough times, dogs that were the least efficient hunters might have been sacrificed to save the group or the best hunting dogs. Once humans realized the usefulness of keeping dogs as an emergency food supply, it was not a huge jump to realize plants could be used in a similar way.
Ultimately, it's not the North America's mega-fauna that pose the most threat to zombies. In nature, there are a whole host of tiny creatures whose main purpose is to feed upon and break down the flesh of the dead: the decomposers. Zombies, with their rotting flesh, are obviously not immune to these decomposers (what do you think causes the rotting effect?), many of which are too small to see with the bare eye. Bacteria, fungi, molds, insects such as fly maggots or flesh-eating beetles, and other invertebrates, all make up nature's diminutive clean-up crew . And it can obliterate a dead body in surprisingly little time. The clumsy undead wouldn't have the dexterity to pick off these decomposers, even if they could see or feel them. It would just be a matter of time. Stripped off all soft tissue, including brains, the zombies would be reduced to hollowed-out skeletons.