To give up eating animals is to give up on these places as human habitat, unless of course we are willing to make complete our dependence on a highly industrialized national food chain. That food chain would in turn be even more dependent than it already is on fossil fuels and chemical fertilizer, since food would need to travel even further and fertility - in the form of manures - would be in short supply. Indeed, it is doubtful you can build a genuinely sustainable agriculture without animals to cycle nutrients and support local food production. If our concern is for the health of nature - rather than, say, the internal consistency of our moral code or the condition of our souls - then eating animals may sometimes be the most ethical thing to do."
-Michael Pollan, The Omnivore's Dilemma