The great plains are large enough to support a ton of ruminant life. I'm not sure there needed to be a collapse in North American human population but it probably didn't hurt. I believe I read somewhere that the longhorn population boomed in Texas during the Civil War. It was afterwards that the cowboy culture really took off thanks to the abundance of free ranging cattle.
The argument was that the plains were man made, that there were forests there before, and then suddenly there were no men left, only grass and buffalo.
