While it seems way off base, John B. Calhoun’s work with rodent populations predicts this. If he is right, our populaton will be reduced by about 95% over the next 150 years.
Zero Population Growth has been fiendishly successful it seems.
Hmm. I remember that research. Given security and an abundance of food, the populations expanded until a certain population density was reached. Then, in spite of the fact that there was still ample food and adequate, if restricted space, a generation of “Beautiful Rats” occurred which did not reproduce at replacement rates, nor did their offspring, nor their 3rd generation. In 3 generations (rat generations), the populations in the experimental colonies dropped precipitously.
But what is really frightening about that study, is that the experimental rat colonies were closed populations. No new rats were introduced from outside the failing colonies. In the real world, population pressure will always push new individuals into the vacuum left by the falling populations of the original experimental colonies. Or in terms of human societies, there were no barbarian hordes ready to migrate into the former lands of the Roman Empire as the population dropped. There are today.