This is the board for you if you wish to discuss or explore conspiracies of any kind, historical or current.
Easter Island is an example of over population without any outside influences. The natives living on the island were isolated yet the population became larger than the island could support. Tribal wars broke out until the population was decimated. It was at some point after all the killing, starvation and death through lack of resources for fire (i.e. most of the trees had been cut down without new ones to growing fast enough to take their place) that the few people left over could make some sort of life with whatever resouces they had left.
This is the point I was trying to make. Picture the Earth as a macrocosm of Easter Island. The socio-economic state of the world is dependant on the resources and the resources (i.e. food, living space, arriable land (sp), etc.) are dependant on the size of the huiman population and the rate at which that population consumes both the renewable and non-renewable resources. Once those are depleted you can choose whatever political, socio-economic or religious reason you want for the catastrophy to follow.