There are at least three others that I think are probably pretty important.
One is the fact that the United States government in the 1830s is busy removing Indians from the eastern United States. They relocate during the 1830s almost 90,000 eastern Indians into what is now Oklahoma and southern Kansas. Many of those groups were located right on the edge of the Great Plains. They want to hunt buffalo, and they do. And I think the intrusion of almost 90,000 Indian hunters, hunting families, onto the edge of the Great Plains in the 1830s, very clearly introduced a level of human stress on the buffalo populations that had a considerable impact.
Still another impact--although we don't know very much about this one, except that there is all kinds of interesting evidence about it--is the arrival of exotic diseases of cattle-like animals, from Europe and Asia. Those diseases include diseases that many of the herds of buffalo today still have.
Brucellosis, of course, is the one that we all know about. Brucellosis might not have arrived early enough in the 19th century to have affected buffalo populations on the Great Plains during the time of the great hunt. Some scholars--Mary Meagher of Yellowstone National Park, for example--think that brucellosis didn't arrive, probably, in North America until the early 1890s. The first evidence, she says, of buffalo with brucellosis, is in Yellowstone in 1916. So brucellosis may not have played a role in the 19th century.
But bovine tuberculosis, and anthrax, almost certainly did. Anthrax, we know, was present in the southwest as early as 1800. Now, anthrax, of course, can lie undisturbed in the soil, in the form of spores, sometimes for decades. And one of the interesting things about anthrax in Africa is that it's usually released into ungulate populations in times of drought, at which point it has usually a fairly dramatic effect on wildlife populations.
I've suspected for a while now that anthrax probably was in the soil on the Great Plains, introduced by immigrant oxen and cattle, across the immigrant trails in the 1830s and 40s, and that when the Little Ice Age ended in the series of droughts beginning in the 1840s, 50s and 60s, that anthrax probably was released into the buffalo herds, and may account for some rather strange accounts we have of inexplicable buffalo die-ups in the 1860s.
Charles Goodnight, for instance, the famed Texas ranger and cattle rancher, in Palovero Canyon, observed a die-up of buffalo in the Concho River valley, in 1867. He said buffalo lay dead in the Concho River valley in an area that was approximately 10 miles wide by 25 miles long. He said there were, he thought, probably more than a million animals dead. He had no idea why they had died, and his speculation was that they had eaten out all the grass in the Concho Valley and for some reason had refused to migrate where there was more grass, and therefore starved to death.
But I've thought for a long time that this may be evidence of buffalo contracting some kind of disease--if not anthrax, perhaps bovine tuberculosis, or something.