Aside from the limited availability of freshwater, there are indeed constraints on the amount of food that Earth can produce, just as Malthus argued more than 200 years ago. Even in the case of maximum efficiency, in which all the grains grown are dedicated to feeding humans (instead of livestock, which is an inefficient way to convert plant energy into food energy), there’s still a limit to how far the available quantities can stretch. “If everyone agreed to become vegetarian, leaving little or nothing for livestock, the present 1.4 billion hectares of arable land (3.5 billion acres) would support about 10 billion people,” Wilson wrote.

The 3.5 billion acres would produce approximately 2 billion tons of grains annually, he explained. That’s enough to feed 10 billion vegetarians, but would only feed 2.5 billion U.S. omnivores, because so much vegetation is dedicated to livestock and poultry in the United States.

So 10 billion people is the uppermost population limit where food is concerned.