One day a nuclear bomb could destroy a part significant of humanity and this is the mechanism for controlling the expansión massively as mr. Smith said..
The other form is the natural disasters some automatically made by nature and others by human like fukuchima or chernobil creating narural exclusion zones.
Natural exclusion zones, created by Nukes :D
But yes, I understand.
It is going to get very nasty when there is nowhere to grow or get food from.
Thanks.