While I honestly don't recall precisely where I saw it, I came across an interesting hypothesis about the American bison a week or two ago.
In a nutshell, the hypothesis argues that Native Americans were the only significant predator for the American Bison for a long period prior to the European arrival. The hypothesis goes on to argue that the massive abundance of bison on the Great Plains when Europeans explorers and pioneers first came on the scene was an unusual biproduct of depleted Native American populations due to exposure to epidemic European infectious diseases.
The hypothesis doesn't gainsay the notion that the bison were then hunted to near extinction with European hunting technology. But, it does suggest that steady state status quo for the bison population was much smaller than the earliest reports would suggest.