Vegetarians will probably agree with Rutgers philosophy professor Jeff McMahan's premise in his lengthy New York Times essay that meat eating is an inherently harmful and cruel act. As vegetarian advocates often point out, eating animals causes them pain and suffering in a case where it's far from necessary. But even hard-core vegans might balk at McMahan's argument that not only should humans stop eating animals, but we should do what we can to keep carnivorous animals from eating animals. Even, he says, if that means "arrang[ing] the gradual extinction of carnivorous species, replacing them with new herbivorous ones."

We should start by withdrawing our own participation in the mass orgy of preying and feeding upon the weak. ... But ought we to go further?  Suppose that we could arrange the gradual extinction of carnivorous species, replacing them with new herbivorous ones.  Or suppose that we could intervene genetically, so that currently carnivorous species would gradually evolve into herbivorous ones, thereby fulfilling Isaiah’s prophecy.  If we could bring about the end of predation by one or the other of these means at little cost to ourselves, ought we to do it?
McMahan's argument is clearly meant to explore complex philosophical questions and not to recommend a specific course of action. He spends more time addressing caveats--we'd have to understand how it would effect local eco-systems, we'd have to bring about these extinctions humanely, overcrowding would be a serious risk--than actually explaining his argument. But if you're willing to suspend disbelief for a moment, the ideas are interesting to consider. McMahan sees humanity taking an entirely new approach to its engagement with the animal world:
Many thousands of animal species either have been or are being driven to extinction as a side effect of our activities.  Knowing this, we have thus far been largely unwilling even to moderate our rapacity to mitigate these effects.  If, however, we were to become more amenable to exercising restraint, it is conceivable that we could do so in a selective manner, favoring the survival of some species over others.  The question might then arise whether to modify our activities in ways that would favor the survival of herbivorous rather than carnivorous species. ... Rather than continuing to collide with the natural world with reckless indifference, we should prepare ourselves now to be able to act wisely and deliberately when the range of our choices eventually expands.
After a lengthy defense against the many (quite compelling) possible counter-arguments to his idea, McMahan concludes, "I am therefore inclined to embrace the heretical conclusion that we have reason to desire the extinction of all carnivorous species, and I await the usual fate of heretics when this article is opened to comment."