in nature, natural predators control the population of a species together with available resources.

since man do not have natural predators, the only factor that control human population is resources.

important resources are land, food, fresh water and energy.
when these resource is very limited, there will be competition over it, resulting in wars and increase in price for the resource. this will control the human population. this is the situation we see in some countries in which there is no growth or sometimes negative growth of population.

i believe what we need now is not radical methods of population control. we need to change our method of producing goods to be more sustainable and more efficient so there is less or no damage to the environment.