Okay, I read through the responses and I must say that turning vegan could help reduce a lot of pollution and free up a lot of land, but it is not the only choice. Humans have consumed meat for many many years, and if done properly it does not have to tax the land.
One example is certain South American people eat a lot of guinea pig that they raise inside a small room, and they eat only a little grass. The ammount of meat yield from ammount of vegetation and room needed is amazing and conservative compared with cows. (though I think of them more as a pet than edible
... but to each their own)
Another great example (almost all over the world EXCEPT western countries) eating insects. They are plentiful take up few resources and have an amazing ammount of protein and little fat.
So being vegan could help, but so could trying to open non-vegans up to healthier, cheaper, and more enviromentally friendly alternative protein sources.
I am quite sure the earth can sustain an even larger population of humans than what is currently here, but it is often the selfish choices of a minority that causes problems. If people could use sustainable energy and used less land for paper and feeding animals to eat the earth could support more people... but humans are often greedy and don't want to consider changing their consumption habits...
Also upon the subject of organics, there are a few naturally occuring crops that resist pests and disease, but many of these plants have been lost because of the industrialization of producing food. (ie inbreeding of fewer ammount of plants causes many weaknesses in the genes) And most of the food consumed by westerners do not have very nice organic costs because of the ammount of resources needed and vurnerability to pests and disease.
Oh and finally I thought of something else. Often our land is not used effectively or wisely. There is a place in China where the land is VERY fertile, but instead of planting crops in a place where they could grow more easily and use the resources, such as water, that come into the place naturally, a computer company bought up most of this land and is essentially wasting it. The crops had to be put elsewhere most likely needing a lot of irrigation.
We just seem to be kind of foolish when it comes to our resources.... they are not used to their full potential.
Cows, Toilets wasting water, watering lawns, burning coal (which I grew up in northern illinois and I was shocked when I found out the US still burned this stuff!)... it adds up to needing more earths than we have. But eventually people will have no choice but to change these ways, hopefully governments will choose to change before this happens, though.