From The Comanche Empire, by Pekka Hämäläinen (Yale U. Press, 2008), pp. 294-297:
It has been estimated that full-time plains hunters needed a yearly average of 6.5 bison per person for food, shelter, and clothing, which means that the Comanches and their allies were killing approximately 175,000 buffalos a year for subsistence alone. Moreover, although first and foremost horse traders, Comanches also produced bison robes, meat, and tallow for the market. In the early nineteenth century, their commercial harvest probably rarely exceeded 25,000 animals, but their hunting practices seriously aggravated the damage. Like most Plains Indians, Comanches did their market hunting in winter, when the robes were the thickest and most valuable, and they preferred killing two- to five-year-old cows for their thin, easily processed skins. Since bison cows produce their first calves at the age of three or four and their gestation period usually extends from mid-July to early April, Comanches slaughtered disproportionate numbers of pregnant cows, thus impairing the herds’ reproductive capacity.
Making matters worse, Comanches’ commercial ambitions induced them to open their hunting grounds to outsiders. For much of the eighteenth century, Comanches had restricted outsiders’ access to their hunting ranges, but that environmental policy became increasingly difficult to maintain as their trading links multiplied. One by one, they disposed of the neutral buffer zones skirting Comanchería, inadvertently depriving the bison of their crucial sanctuaries. Particularly inauspicious in this respect was the 1835 Treaty of Camp Holmes, in which Comanches granted the Osages and the populous immigrant tribes of Indian Territory access to their lands in exchange for trading privileges. Discouraged by the poor lands of Indian Territory, Cherokees, Chickasaws, Choctaws, and Creeks—all numerous groups—embarked on active bison hunting, and many Delaware, Shawnee, and Kickapoo bands became specialized hunters. Together with the Osages, the removed Indians did most of their hunting in the prime bison range between the upper Canadian and Red rivers, in the heart of eastern Comanchería. By 1841 the region’s bison populations were thinning rapidly.
At the same time on Comanchería’s western edge, ciboleros, the New Mexican bison hunters who had won hunting privileges in Comanchería in the aftermath of the 1786 Spanish-Comanche treaty, made animal hunting expeditions to the Llano Estacado, harvesting an estimated 23,000 animals per season. Even more pressure fell on the bison herds with the peace of 1840 among Comanches, Kiowas, Naishans, Cheyennes, and Arapahoes, which unlocked northern Comanchería for Cheyenne and Arapaho hunters, who embarked on a large-scale robe trade at Bent’s Fort and probably harvested a large portion of them in Comanchería. In all, in the early 1840s tens of thousands of Comanchería’s bison died every year in the hands of people not living in the region.
The combined toll of Comanches’ and their allies’ subsistence and market hunting probably neared, and in some years exceeded, the sustainable yearly rate of killing of 280,000, placing Comanchería’s bison herds on a precarious balance. This balance was rendered even shakier by the Comanches’ burgeoning horse herding economy. Horses and bison have an 80 percent dietary overlap and very similar water requirements, which makes them ecologically incompatible species. Even more critically, both animals could survive the harsh winters of the plains only by retreating into river valleys, which provided reliable shelter against the cold, and cottonwood for emergency food. But suitable riverine habitats were becoming increasingly scarce. To meet the expansive grazing needs of their growing domestic herds, Comanches had turned more and more bottomland niches into herding range, gradually congesting Comanchería’s river valleys. By the mid-nineteenth century, huge winter camps and horse herds could be seen stretching for dozens of miles along key wintering sites, covering the prime foraging and watering spots, and forcing the bison to retreat to poorer areas.
Most such areas were at the headwaters of major rivers and far from Comanches’ principal hunting and wintering grounds, but when the bison gravitated toward these perpheral habitats, they were blocked there as well. Southern Comanchería near the Texas frontier was the home for massive herds of wild horses, which had virtually taken over the region’s river valleys and resources. On the western portion of the Llano Estacado, at the headwaters of the Canadian, Red, and Brazos rivers and their tributaries, the bison had to compete for grass, water, and shelter with thousands of sheep driven there each winter by New Mexican herders, pastores. Perhaps most disastrously, freighting along the Santa Fe Trail grew into a large-scale industry in the early 1840s. A typical trade caravan consisted of some two dozen freight wagons and several hundred oxen and mules, and each year hundreds of such caravans trekked back and forth along the Arkansas corridor, destroying vegetation, polluting springs, accelerating erosion, and driving out the bison from their last ecological niches in the valley. It is also possible that the traders’ livestock introduced anthrax, brucellosis, and other bovine diseases to the bison herds….
In 1845 a long and intense dry spell struck Comanchería. The rains resumed briefly around 1850, but the drought returned and lasted in varying degrees until the mid-1860s. As the rains failed or came only as drizzles, springs, ponds, and creeks dried up and rivers shrank to trickles….
Although an unexpected climatic swing brought on the bison crisis, the Comanches’ actions had contributed to the shortage. By monopolizing the river basins for their horses, by slaughtering vast numbers of bison for subsistence and for trade, and by opening their hunting grounds to outsiders, Comanches had critically undercut the viability of the bison population, rendering it vulnerable to ecological reversals.