Nature's very good with overpopulation, ever played Foxes and Rabbits?
When there's lots of rabbit's there's plenty of food for the foxes and they start to increase. As they increase they eat more rabbits so the number of rabbits starts to diminish. Eventually there aren't enough rabbits to feed all the foxes and their population starts to drop. As the fox population drops less rabbits are being eaten and their numbers begin to increase again. And hey presto the whole process begins again.
Humans are the foxes and the environment the rabbits. While there is sufficient environment to support us we will continue to increase in numbers until their is nolonger enough environment to support us, when this happens our numbers will rapidly decrease allowing the environment to recover and the process will restart.