Consider the great plains. The grain grown there is mostly for animal feed -- and that must stop. But growing grain _at all_ must stop also. The only ecologically sound use of the great plains is for pasture. Therefore the only food fit for human consumption that the great plains can produce is meat from animals capable of transforming the protein found in grass to protein available for human consumption.
There are areas of Australia which, for ecological reasons, should produce only goats, chickens, etc.
Vegetarianism can never be a possible diet for the bulk of the human species but remain an option only for scattered inividuals.
Carrol