@Sustainability of meat
Warning, /rant is in effect.
Farming 101-Crop Rotation.
Lets say I have 4 plots of land. I use 4 crops.
1-X-Depletes Nitrates, replenishes Phosphorous
2-Y-Depletes Phosphorous, replenishes Potassium
3-Z-Depletes Potassium, replenishes Nitrates
4-"Fallow"-Replenishes all 3. Empty field, compost field, growing clover, growing grass, growing alphalfa, etc.
First thing I want to point out is that many farmers (specifically, industrial farms, "Big Agriculture", etc) stopped utilizing the natural balance of crop rotation years ago. Many farmers grow one crop exclusively, year after year, and replenish with fertilizers, natural or artificial. But, as a result, soil is depleted of nutrients, this also damages soil pH and actually gimps the production of whatever crop is in the ground. So ensuring that farmers actually utilize crop rotation would increase production of non-meat food products.
Second, see that 4th plot of land that says "Fallow" up there? Guess where your livestock is supposed to graze? The fallow land. In fact, having animals graze on that accelerates the replenishment of nutrients. AND, such fallow crops like clover, grass, and alphalfa are typically better for livestock to eat than grain.
So to sum up the two points, you're feeding the livestock essentially for free (so long as your livestock population doesn't exceed the amount that your fallow land can feed, and minus all the other concerns such as water and vet bills and the like), and replenishing the soil faster for your other three crops. This is how livestock can be sustainable, for sheep, goats, chicken, pigs and boar, and certain breeds of cattle.
Your logical question then is, why the heck are we feeding grain and corn to livestock instead of feeding them for free?
Money. Stuffing the livestock with grains and corn bulks them up faster. For example, Cattle used to take a minimum of 4-6 years to get to market, now it takes typically 2 years. This is great for the farmers, as they can make significantly more money. It's not so great for the land, for the price of grain or corn. I'm not saying the farmers are greedy, but in the current economic paradigm, this is what they have to do to survive.
There is a breed of cattle which are perfectly designed for feeding on fallow land, and living free range. Highland Hill Cattle, most breeds of Bison and Buffalo, some breeds of Ox. Problem? They take WAY too long to get to market for a farmer to make a reliable paycheck from.
Second issue with using fallow land to feed a sustainably sized group of livestock? Why do that when the farmer can put their land to full production of a crop that will net them more money, every year, thanks to current industrial farming methods? Again, money is the issue.
Last major issue is demand. We do eat considerable amounts of meat in a year. A reduction of a few servings per week (as in, cutting out 3 or more servings a week) as well as better portion control (some restaurants use 2 chicken breasts in a dish, cut back to 1) could bring demand into line where a more sustainable model could support it. The problem with curbing demand like this is the open hostility that even the most polite suggestion seems to be met with. See spoiler for more ranty rantness for this specific aspect of the topic:
"Hey man, you should eat more veggies"
Stereotype Response 1: "Why, you saying I'm fat?"
Stereotype Response 2: "Why, we don't have to be vegetarians or vegans."
Stereotype Response 3: "Why, don't I have the right to eat what I want?"
Stereotype Response 4: "Why, so we can all be hippies and join PETA and sing Kumbaya?"
The suggestion to eat less meat and more vegetable matter is treated as some horrible suggestion to quit meat eating altogether. The common public views vegetarians/vegans as horrible people (hipsters, hippies, protesters), which simply is not true. These connotations that eating veggies makes you somehow less of a person for some reason need to be addressed. At the same point I will also draw attention to the stereotype of the angry red meat eating alpha-douche-jock who drives an uber polluting SUV. That needs to be addressed for pretty much the same reasons. Both stereotypes create an us-or-them environment, which hamstrings rational discussions on both sides of the arguement. Eating meat doesn't make someone a monster or a caveman, eating veggies doesn't make someone a hippie or tree-hugger or a member of PETA or anything else.
Yeah, such attitudes don't help.
Originally Posted by Xondoure
TL;DR being a vegetarian is much better for the planet, and is a moral choice each of us has to make unlike other species. Now, not judging what choices you do make (as I said, I eat meat) but it's hard to argue that the world wouldn't be at the very least a more sustainable place if more people cut down on meat and other animal products.
Moral choices are not necessarily natural choices, but that's okay, it doesn't make them necessarily right or wrong.
That said, it would do the world at large a great deal of good to see a reduction of meat eating, even as little as reducing it by 1 serving per week. There is also a new trend I've seen called "weekend vegetarian" or "weekend vegan" which would be great if it caught on just a bit more.