More recent scholarship suggests the Americas, before the arrival of european plagues, were far more densely populated, and the environment more shaped by native agriculture, than you might expect. A good summary of the ideas is Charles C. Mann's article "1491":

http://www.theatlantic.com/doc/200203/mann

One estimate is that in the first 130 years of contact, 95% of a prior American population of over 100 million died.

Those 130 years happen to be about the first half of the 'little ice age', and the natives often used large-scale burns to clear and prepare land. So while IBD mocks the idea that the depopulation of the Americas caused the little ice age, it's at least plausible.