So, I took keen enough interest in what rephikul was saying that I decided to register here, and defend the justness of vegetarians in a way he (I feel I may presume) will respect.
First, I would like to start with the assumption that we all want what is best for mankind. I am making the statement that it is in the individuals' best interest to work together as a society.... If you would argue that historically it is more justifiable to be a raider than a farmer, then you are either sociopath or a hardcore us-vs.-them nationalist. The conversation would be over. If you feel humanities' future and betterment is best served through cooperation and teamwork, then we may continue.
If you were to peer inside the mind of the modern geologist/ Earth-Scientist academic, you would see a sea of urgency and pessimism involving this planet's future. Even ignoring the experts' growing sense of despair, it is quite easy to see that large portions of the world are either starving or lack sustainable levels of freshwater. If you are to combine that with growing population levels, increased strain on our oceans ecosystems, deforestation, and more and more excess carbon being pumped into our atmosphere, I think any forward thinking individual should fear a mass die-off. Why should an American fear a mass die-off, when it would be mostly people in other parts of the world doing the dying? Stability. If you think terrorism and war are widespread now, wait to see what happens when entire crops go bust around the world, and countries organic resources run dry.
Now, so far, I have not argued about vegetarianism, only environmentalism. I am clearly an environmentalist, and even if your forecast is less dire than mine, you should at least be cognizant that there are really smart people who see things as being even more dire than I do.
So now for the break down.
Land usage: Ultimately as a species, we should be as close to dirty elves as we can be. Forests are our natural buffer from mass extinction. The more trees, the cleaner air, the better for our crops, the better the weather patterns, the easier our survival. As a species we should aim to have every inch of land we don't need having trees growing on it. So what happens when we have cattle farms? Well, depending on the type of farm, one of two things. For a traditional/free-range nomadic herder arrangement, things don't look so bad. Sure, you may be able to grow some trees on that grass field, but not enough to make a difference. Other crops could be grown, but without other, harmful farming techniques, there probably wont be high enough of a yield to make a difference in a typical grassland. There are a few problems though. First, look at places that are somewhat densely populated that have been doing this for centuries, such as most of Spain. Unless you have huge plains, cattle will damage the quality of land gradually over the years, leaving room for heavy erosion, and a reduction in the quality of life. Also, producing meat this way is very expensive, and can not feed that many outside of the herders and their families. As a means of feeding millions, in other words, free range, traditional meat is highly inefficient. Now the American way of raising meat... that is where the real land inefficiency takes place. Acres and acres of land that could be perfectly good forest are used to grow corn. Did you know that you need to grow eight pounds worth of corn for every one pound of Beef that is consumed? In fact, if meat was eliminated completely, you could convert every farm into a more environmentally friendly but smaller yield organic farm, and still be able to grow quite a few forests where there is now only corn.
Water usage The amount of extra freshwater needed per pound of flesh created is a similar equation to the amount of extra grain needed, except its even worse. Not only do you have to water the many extra crops, but also you need to water your non-free-range-herd. Freshwater is a valuable resource that we often take for granted living in North East United States. Yes, I assume that is where each and every one of you live. Unfortunately in America's real bread basket water is not in such easy supply, and we should be trying to conserve as much as we can so we may feed ourselves for as long as possible. Back in the 20s-30s there was a dust bowl because of the rampant farming in areas in the middle of the country just didn't have enough water. Instead of learning our lesson, we rebuilt those farms, but with new technology. Now, like dwarves, we run on water stored in giant aquifers, that we just pump water out at need. These aquifers replenish a little every year, but nowhere near the level that we draw from them.
Yes. People are that short sighted. Water is precious.
Pollutants Life the first two points, this is also an issue of the exorbitant amount of extra farming we have to do in order to grow flesh instead of plants. In order to create high yields we use all sorts of fertilizers and pesticides that poison are water, and muck everything up. While they may be a necessary evil, the few pollutants we use, the healthier are ecosystem will be, and the more likely we can restore the planet before massive die-offs. I should also mention that cows put an unhealthy level of carbon into the air through their digestive systems. While these are not humble numbers, they pale in comparison to humanities use of fossil fuels as power, and our need to live in Suburbia. Note that agriculture releases more methane in the air then cars. Remember, the less forests we have, the greater of a problem this methane is.
Over-fishing Honestly, I know very little about ocean ecosystems. The only people I know who are screaming about the falling skies and the end of the world louder than your run of the mill Earth Scientist though, are Marine Biologists. Generally, I just take their word that we should stop eating most seafood. Also there are some studies I don't understand very well about farm fish being yucky.
Ultimately, you will not see me making any cases against game meat. Hunters, enjoy your venison, cause I have no argument against you. After you get used to a vegetarian diet, there is not much of a point in partaking in that hobby though. Hunting would, after-all, probably take away good dwarf fortress time. Also for all your "survival is the only objective point of existence" philosophies, I sort of doubt you, or any of us, are much of a jock.
You should note that I do not expect other people to be vegetarian. I am willing to make sacrifices because I am willing to do my part to make humans and their environment prosper for that much longer by making what is, for me, a small sacrifice.
If you are the type who needs meat to be healthy, or just must have to have meat once a month to be happy, all the power to you. In other ways I am irresponsible, and I hope you can carry my weight in those cases, and I will do my best to carry yours. As people, we need to be a team, or life will suck for everyone. The least I demand is not to be derided.
If you like to pretend that everybody can eat and live however they like without harming anything, I feel you you are overly optimistic. We trust different scientists, so our worldview is different.
If you think, one person can't make a difference, maybe you are right, but we are changing the culture and people are getting healthier and more environmentally conscious every day. And why be part of the problem?
If you acknowledge my points, but think, 'I get what I want,' or 'nothing really matters' you are kind of a thug, or at least a parasite.