I once saw a cartoon which depicted vegetarianism in an unflattering light: it showed a closeup of the front of a combine harvester, before which fled an array of inoffensive woodland creatures, yelling things like “Where’s mama?!?” and “I don’t know, just run!”
Edit: found it. It’s from Saturday Morning Breakfast Cereal:
Agriculture is not without its damage to the environment, and to animals. A field of wheat or corn is home to mice, rats, birds, rabbits, various insects, and a host of other creatures, at least some of which are inevitably ground up along with the harvest. This recent article on an Australian web site suggests that the many small lives we grind up to harvest a crop in an area of land outweigh the lives of the cows which would graze that land were it devoted to pasture. (If you’re interested, this article wanders into the notion a little further.)
The presentation of the issue is somewhat simplistic: it assumes that only a few cows are raised in the hypothetical pasture (i.e., it’s not a feedlot, the American standard); that “pasture” is equivalent to unspoiled natural land; that all the wildlife in the field are killed by the plow; etc. I think it’s a valid notion, but the solution to this issue is not to have everyone eat nothing but red meat. The problem lies more with how we produce our food, and what methods we’ve adopted to produce that food cheaply, and less with exactly what food we are producing. For example, we can certainly develop methods to raise and harvest crops more sustainably and with less “collateral damage”.
I don’t think we’re really able to exist, at all, without causing some damage to the world. It’s in our nature as consumers of energy — it’s got to come from somewhere. However, we can choose to minimize the amount of damage we cause, and try to choose the least damaging places to cause it.