From the ecological point of view, for human beings universally to become vegetarians is tantamount to a shift of trophic niche from omnivore with carnivorous preferences to herbivore. The shift is a downward one on the trophic pyramid, which in effect shortens those food chains terminating with man. It represents an increase in the efficiency of the conversion of solar energy from plant to human biomass, and thus, by bypassing animal intermediates, increases available food resources for human beings. The human population would probably, as past trends overwhelmingly suggest, expand in accordance with the potential thus afforded. The net result would be fewer nonhuman beings and more human beings, who, of course, have requirements of life far more elaborate than even those of domestic animals, requirements which would tax other "natural resources" (trees for shelter, minerals mined at the expense of topsoil and its vegetation, etc.) more than under present circumstances. A vegetarian human population is therefore probably ecologically catastrophic.
(J. Baird Callicott, "Animal Liberation: A Triangular Affair," Environmental Ethics 2 [winter 1980]: 311-38, at 335 [italics in original])