There are two ways to approach this argument. One is to confront it head on--dissect the premises, figure out where there might be problems. The other is to charge McMahan with "bad attitudes"--this is hyperrational, overly managerial, scary stuff. The right attitude is ... what? We ought to be humble in the face of nature's goodness, and leave things alone--not entirely, but at least on a very basic level. We ought to value lions, and let them keep on eating zebras, even if we can't perfectly explain why that's for the best.
My gut feeling, to be honest, is the second--I would distrust the urge to give nature a moral makeover. Maybe this amounts to a religious feeling, really (but without any supernaturalism)-- the world is good, as far as fundamentals go, and predation is fundamental. But let's not leave it at that--all that compex reasoning cries out for close scrutiny.
To begin with, I have doubts about (2). It's true that our carnivorism causes misery, and lion carnivorism causes misery, but we breed our "prey" and zebras exist naturally. If lions stopped eating zebras, there would still be zebras. Instead of the weaker ones being eaten, they'd live longer and possibly suffer other problems. Unless the idea is that animals are going to be citizens of our nations, there isn't going to be free veterinary care or food stamps for ailing herbivores. If there would be no reduction in suffering from eliminating lions, but only in killing, it's unclear that there would be any net benefit.
But that's assuming a lot. Maybe there would be a reduction in suffering, if all animals were herbivores. If so, then we ought to focus on (7)--whether there's a value to there being lions that makes up for the suffering they inflict. No individual lion cares about the ongoing existence of the species, so the value of the species is not a value to the individual members of the species. There's value to us, since we enjoy lions, but that seems potentially canceled out by the suffering they cause. So what's the good of lions? It seems to me there is impersonal good here--goodness that's not goodness to anyone. It's valuable for there to be lions because of the specific qualities and capacities that come of there being lions.
Right--that's not crystal clear. That's why I started with the point about humility.
UPDATE: Jerry Coyne asks today:
But what on earth does religion have to contribute to an exploration of morality? I maintain that there is not a single ethical insight contributed by religion that could not be be better contributed by secular morality—and without the taint of the supernatural.It depends what you mean by "religion." In the end, it seems like the right response to McMahan has to appeal to humility, maybe even reverence, some basic sense of nature being good as is, even if we can't exactly say why. You don't have to believe in the supernatural to have those attitudes, but they are certainly cultivated in places of worship, and they're not attitudes that really have any place in secular, rational, morality (typified by McMahan). So "not a single ethical insight" is too strong, even if it's true that secular, rational, morality has much more to offer than traditional religion.