Would that be "seasonal" migrations? Like some four-legged mammals? Man migrated to feed himself on what was available. "Depletion" may be just another way of saying the seasonal foods were no longer available.
There was that too...In fact, there still is in some places of the world.
I'm just saying that much of the conventional wisdom about the ecologically pure indians is a myth. Even they were cursed with human nature, and they were just as likely to exploit mother Geia as modern day humans. I'll bet that modern logging companies are far better stewards of the environment.