[neffy]

I think you will find the numbers come from 1491 by Charles C. Mann, which is one of the books that kicked off a reexamination of the entire settlement by Europeans of North And South America, and which also goes into the reasons why the native population had little resistance to the European diseases. The climate change theories compete with Volcanic explanations, but there was certainly an impact, the great swarms of the now extinct passenger pigeon for example are believed to be due to the die off.

It wasn´t in some sense on purpose, smallpox blankets aside, the first contacts from Europe spread disease everywhere they went, mortality rates have been estimated at 90% or more. No coherent society survives that kind of event. Francisco de Orellana or somebody on his expedition probably carried the diseases that depopulated the Amazon, and led to it being empty when it was revisited..

And had that not happened. If the native population had been resistant to the new diseases, then the Europeans would have been thrown back, as the first US colonies seem to have been. The technological advantage was certainly there, but would have been impossible to sustain on a sea based invasion across an entire ocean at that time.