Photo by RedGazelle123, Wikimedia Commons
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The National mammal of the United States (named by Presidential decree in 2016) is the American bison (Bison bison). This is a very curious selection since Americans did their best to drive the bison into extinction back in Nineteenth Century! Let’s play with some numbers to try to develop a picture of what the American bison was doing in its natural habitat. These numbers are, at best, ballpark estimates loaded with assumptions, but they may help us see something interesting about the bison.
First: the Great Plains of North America have an area of 1,125,000 square miles according to E. B. Robinson in Britannica. There are references that describe the area of the Great Plains sometimes as little as half of this estimate, but let’s run with the million plus square miles. While the American bison’s pre-European settlement range actually extended beyond the Great Plains (however large its area really is) even to the north and east into Pennsylvania and Western New York and to the west into Oregon and Northern California, I am going to use this area of the Great Plains as the bison’s “natural” range.
Second: estimates as to how many American bison existed in North America before the coming of Europeans vary from thirty million to sixty million individuals. I am going to use the sixty million estimate (from the National Park Service) for my bison ruminations.
Third: the Texas Parks Commission estimates that an average bison eats about twenty-four pounds of forage (mostly grasses) a day.
Bison dung. Photo by R. Bair, Flickr
Fourth: The National Park Service estimates that an average American bison produces 10 to 12 quarts of feces a day and “many” gallons of urine (weight of feces and exact volume of urine will depend on water consumption). For the sake of my calculations, I am going to estimate that 12 quarts of bison dung will weigh 20 pounds, and that “many” gallons could translate into four gallons of urine (weight: 32 pounds).
Fifth: the Pasture Map website estimates that there are two thousand pounds of dry forage in a managed field with ten inch tall grass. In an unmanaged, natural, prairie grassland, I am going to estimate that there is, on average, thirty percent less grass forage available due to an abundance of other types of non-grass plants in the prairie and also to a less robustly stimulated growing system that lacked fertilizers, pesticides and overall management. So, an acre of Great Plains prairie would have had a maximum of 1340 pounds of dry forage available for grazers. This amount of forage would fluctuate greatly over the year and in relationship to the growing season, so let’s reduce this maximum to 1100 pounds per acre to compensate for winter grass senescence.
Putting all this together: sixty million bison living on 1,125,000 square miles of area means that there were 53 American bison per square mile. If we change square miles to acres (640 acres per square mile) then that would be 0.083 bison per acre or one bison for every 12 acres all across the Great Plains of North America! THAT is a lot of bison!
Photo by H. Mulligan, Wikimedia Commons
If each bison ate, on average, 24 pounds of forage a day, then over one year each bison would eat 8760 pounds of dry forage. The 12 acres each bison is living on would generate 13,200 pounds of forage. So, each bison would consume 66% of the grass produced in the High Plains prairie ecosystem (about 8 acres worth)! At least the bison left some grasses over for the ten million or so elk and all of the deer and pronghorns (not to mention the prairie dogs!) that were also grazing (or, as the song says, “playing”) out on the “range”).
And, of course, what goes in must come out! Our bison on his (or her) 12 acres would transform the ingested grasses and drinking water into 7300 pounds of feces a year and 11,680 pounds of urine! This excreta is rich in nitrogen, phosphorus, calcium, sulphur and magnesium which is immediately available to plants and soil microorganisms. The National Park Service indicates that over three hundred species and over one thousand individual insects can live in just one American bison dung pat! Also, over a two week period a single bison pat produces three thousand flies (great food for all sorts of birds, bats, arachnids and reptiles!).
Dung beetles. Photo by A. Meintjes, Flickr
These discretely deposited bison dung pats would be rapidly spread out across an expansive area of soil not only by the action of trampling by the bison but also by the frenetic activity of armies and armies of dung beetles! This spreading of these vital dung nutrients would stimulate grass growth all across the bison’s range which in turn would provide fresh forage not only for bison but also for the multitudes of other grazers that also occupied this ecosystem. Further, as the bison trimmed down the grasses more sunlight could reach the soil surface stimulating the germination of new plants and their overall growth.
In a paper published last year in the journal Agriculture, Ecosystems & Environment (July, 2019), Hillenbrand et al. found that shortgrass prairie sites in South Dakota that were grazed by bison had increased fine litter cover, improved water infiltration rates, 200 to 300% increases in available forage, improved plant composition, decreased numbers of invasive plants, and decreases in bare soil patches! This “bison impact” on these prairie ecosystems was far from subtle!!
Now the image of sixty million America bison spread out on their individual 12 acres all across the 1,125,000 square miles of the Great Plains is compelling, but bison were actually bunched together in herds of staggeringly large sizes. Historical accounts (taken from J. H. Shaw’s paper in the journal Rangelands (October 1995)) describe a bison herd in the middle decades of the Nineteenth Century that was 45 mile long and 30 miles wide, and another that was 50 miles long and 25 miles wide. Still another herd was so extensive that it took men on horseback six days to pass through it, and there was another that was over 100 miles long! These herds contained many millions of individual bison that were constantly on the move to find fresh grazing areas.
On average these bison herds moved two miles a day and had intense impacts on the surrounding vegetation and landscape. The paths followed by these herds were ground into the very substrate of the prairies. These “buffalo traces” ran typically north and south across the Great Plains marking the seasonal migration patterns of the great herds. These traces followed the crests and firmer ground across the plains avoiding low areas that would be muddy and impassable in wet seasons or covered in great snow drifts in the winter. The traces that did run east to west were later used as wagon train trails for settlers heading west and then as the paths for the transcontinental railroads.
Bison skull pile outside of a processing factory in Detroit (1892). Wikimedia Commons.
We all know that fate of the American bison. Starting in the early decades of the Nineteenth Century humans began a systematic destruction of the great herds. The impetus for this slaughter was both economic (especially regarding the leather that could be made from the buffalo hides) and also political (destruction of the food supply for the native tribes of the plains).
Many of the tanneries that made leather from the buffalo hides were located in northern Pennsylvania in the middle of the state’s great hemlock forests. Hemlock bark was the source of the tannins used in the leather production process. The huge, ancient hemlocks of northern Pennsylvania were clear cut and the hemlocks themselves were nearly timbered into extinction to fuel the leather industry.
Great train loads of buffalo hides poured into the tanneries of Pennsylvania where they were preserved and softened by the concentrated tannins leeched out the remains the hemlocks. The four largest tanneries in Pennsylvania processed over 600,000 hides a year at their peak. The hides were made into strong, tough leathers from which horse harnesses, drive belts for factory machines, shoes, and more were made. The tanneries devoured the remains of both the hemlocks and the bison and persisted until both were almost completely gone. In 1910, the Central Leather Company of Pennsylvania was one of the ten largest companies in the United States. Today, like the great bison herds and the “endless” hemlock forest, Central Leather no longer exists.
By 1889 there were only 541 American bison left, and, fortunately the mass slaughter stopped. Today there are 500,000 bison in North America. Some 30,000 of these animals live on public lands but only 15,000 or so are wild and free ranging. The great bulk of the remaining bison are kept in private herds where they are culled and harvested for their meat. Most of the Great Plains is bison free, and we are just beginning to appreciate how much the soils and the natural flora and fauna suffer for the lack of this vital, keystone species.