The must-read book for the context is 1491, by Charles C. Mann [1]. The main point is that pre-Columbian America was much more densely populated than previously thought, with the Native Americans managing a good deal of the ecosystem. European contact brought in diseases (mainly smallpox) that killed off the vast majority of the inhabitants, with momentous consequences for the ecosystem (e.g. the extreme proliferation of bison and passenger pigeon). But really read the book, it's very well written, based on the latest research, and quite enlightening.
[1]http://www.amazon.com/1491-Revelations-Americas-Before-Colum...