Eating lower on the food chain is also important. When you eat a diet that is high in meat, you must realize that it takes a lot of agriculture land to support all of those animals. Most of that energy is lost (as waste and heat).
For example, in Colorado, about 90% of our agriculture land is devoted to feeding livestock. If we ate lower on the food chain, much of that land could be returned to wildlife habitat.
Eating a diet high in meat has caused much more of our land to be converted to agriculture FOR livestock.
It takes about 12-15 kg of grains to yield 1 kg of beef; most beef consumed in the world is grain fed. Jeremy Rifkin estimated that we could reduce our land devoted to agriculture by less than half it's current area if we ate very little livestock. This would greatly reduce our depndence on irrigation/fertilizers/pesticides and so on.
Now, in some less developed countries, less land is devoted to directly feeding livestock. I worked in Nepal as a forester and did agriculture work. Land is at a premium and Nepalies know well that if they tried to graze their livestock on their agriculture land, they would have little to nothing to eat. Furthermore, their forests and wildlands are severly degraded due to overgrazing for the little livestock they do have.
Much wildland/rangeland in Colorado and the west show degerdation from grazing. For example, livestock are one of the main vectors of noxious weeds. Noxious weeds are right up there with concret and asphalt for major contributiers to loss of habitat.
Realize that one of the main things our tropical forest are being cleared for (apart from the wood) is a quick, cheap place to raise corn for cattle.
Remember, your average trophic level shows about a 90% reduction in energy converted.
I address some of the points below:
1)It does not take 12-15kg of grain to produce 1kg of beef. It takes no grain at all.
That would be nice if that was the way it happened, but it's simply not. The truth is that the vast majority of livestock is grain fed. Again, more than 80% of our agriculture land in the US is devoted to feeding livestock.
It CAN take 12-15kg of grain to produce 1kg of beef, but it can also take 12-15 kg of grain to produce a kilogram of soybeans.
2) By not eating meat you are forcing people to entirely abandon huge swathes of currently productive land.
Not true, since you will not "force" anyone to do anything. People will use the land the way it is most productive for them. It would never be a realistic scanario for ALL people to stop eating meat. The idea I'm talking about is to lessen the pressure on the meat industry which would lessen the reliance on grain-fed livestock. Much land for livestock could come out of agricultural production (mono-cropping) and be reclaimed for wildlife and wetlands. The remaining livestock could be fed through range lands (a practice that currently rarely happens).
That shortfall in production has to be made up elsewhere where the environment is conducive to cultivation. That basically entails clearing forests or draining swamps.
Again, not true because of the reduction in land needed for growing grains and soybeans for livestock.