The bison factor


Here I focus on the role that bison may have played in determining both the distribution and abundance of North American grizzly bears. Given that bison were concentrated on the Great Plains throughout the Holocene, the geographic focus of this hypothesized effect is in this mid-continental region. The extent and variability of bison populations is obviously relevant, and a topic that I perhaps belabor below, in part because I was motivated by simple curiosity about Holocene bison distributions and not satisfied with the blob maps that I was able to find in books and online.


The essence of my thesis is that bison were a critical food of grizzly bears especially on the Great Plains, and that bison carrion was at the center of a niche that differentiated grizzlies from their chief competitors for foods--black bears and humans (see the Black bear and Human factors). Because of this, I speculate that grizzly bear populations on the Great Plains waxed and waned in concert with bison populations, with effects on whether grizzly bears were able to penetrate the Great Plains barrier and colonize the deciduous forests of eastern North America, which, by and large, they didn't (see  The Holocene).


I start with evidence for the potential importance of bison as a source of food for grizzly bears, and then move on to closely examine the distribution of bison at middle latitudes of North America, as well as likely variations in this distribution and the numbers of bison within over the course of the Holocene. An important point, though. I don't deal with the demise of bison during the 1800s here, but instead deal with this catastrophe in the section on extirpations--not only of grizzlies, but also of bison.

The bison opportunity

The map immediately above left shows overlap between the distribution of grizzlies around 1800 and the core distribution of bison around roughly the same time. In addition, I've delineated a portion of the northern US Great Plains that contains the greatest concentration of bison traps (jumps, pounds, and arroyos) used by pre-equestrian First Peoples to hunt bison. I speculate that these traps functioned as transient ecocenters for grizzly bears by provisioning them with the carrion of bison that were left unbutchered by the involved people (see The human factor). I've also delineated the current distribution of grizzlies with a red line. The obvious point of this last delineation is that Yellowstone is the only region of all the vast area where bison and grizzlies once overlapped that still contains both of these species. In fact, Yellowstone is a highly valuable archive of a relationship (between grizzlies and bison) that is currently quite rare, but once exceedingly common.


The figure upper right is the fruit of many hours spent reading through ethnographical and ethnobotanical studies, various histories, and as many journals of early European explorers as I could find. The result is an approximate reckoning of the relative importance of various animals and plants to the diets of grizzly bears, black bears, and humans on the Great Plains. It is hard to derive an exact quantitative reckoning, but I've ranked all of the foods above as either having been commonly used, heavily used, or not used at all. Most of the foods are denoted by scientific Latin names; but I've denoted the main animals foods by their common names (elk and bison) and whether the acquisition was through scavenging or predation.


A couple of key points: At least by my reckoning, competition between grizzlies, black bears, and humans was pretty intense, involving diets that overlapped substantially. Moreover, most of these foods were concentrated in or near rich riverine areas and woody draws, which would have brought an intensifying spatial focus to the competition. bears posed little competition for breadroots (Psoralea esculenta), primarily because they are not built to efficiently excavate such things; and, most important, scavenging bison carrion was a uniquely open opportunity for grizzlies. They could easily dominate bison carcasses in the face of competition from black bears, and humans generally eschewed this food source, except in times of extreme dearth. Moreover, the area within which bison were available to grizzlies was extensive--comprising a near majority of grizzly bear range. More specific to the Great Plains, grizzlies didn't occupy the far eastern edge of core bison distribution and, in, the north, more-or-less aligned with the eastern edge of concentrated bison traps. As I elaborate below, the comparative absence of grizzlies form the eastern margin of core bison range was probably a result of low bison densities in the tall-grass prairies. Similarly, the comparative absence of grizzlies to the south was also probably due to low bison densities (at least at times) in the less-production southern grasslands. Both the tallgrass prairies and the southern grasslands were dominated by C4 grasses, which are known to be less palatable to bison compared to the C3 grasses that dominate farther north and west.

So, what do we know more directly about relations between bison and grizzlies? Fortunately, we have been able to study these relations in the only place where we still have the two species together--the Yellowstone region of Wyoming and Montana. Here, a population of several-hundred grizzlies coexists with a population of several-thousand bison, largely segregated between two herds. We studied the consumption of meat by grizzlies from bison and elk (the other common large herbivore) first by following radio-collared bears around and, second, by walking transects every year on bison winter ranges looking for the remains of winter-killed bison and elk along with any evidence of scavenging by bears. Gerry Green did much of this latter work.

Our results our summarized in the figure to the left. Relative to their numbers, bison were used much more heavily by grizzlies compared to elk. This was evident not only in the estimated amount of meat consumed from each species, but also by the percent of winter-killed bison exploited by grizzlies: 24% in the case of bison and only 6% in the case of elk.


The disproportional use and importance of bison to Yellowstone grizzlies can be easily explained in terms of biomass. Bison carcasses offer far more meat than do the carcasses of virtually any other large herbivore, with the possible exception of moose. Moreover, this meat is encased within a thick hide. The mass of meat and encumbrance of hide mean that a carcass will have that much more meat left on it by the time it is found by a scavenging grizzly bear, which matters when there are a lot of smaller-bodied scavengers out there competing for the same carrion--notably coyotes, wolves, and ravens. This enhanced availability is evident in the fact that it took Yellowstone scavengers, in toto, roughly 7 days to consume half of a bison carcass, but only about 1 day to consume half of an elk carcass. 

So, where were bison and in what numbers?

The quartet of maps above represents my efforts to summarize all of the information I could find regarding the distribution and relative abundance of bison during the last 1,450 years or so. My sources were archeological finds and observations by early European explorers, combined with a map by Francis Haines that encompassed historical European records from eastern North America. I've attempted to differentiate areas where bison were relatively more persistent and abundant (core range) from areas that were less so in both regards (peripheral range). I also show two different reckonings of grasslands (and sagebrush steppe grassland) in North America, combined with the estimated extent of those grasslands where C4 rather than C3 grasses were dominant (one reckoning from Dwight Brown, the other by Larry Tieszen). Compared to C3 grasses, C4 grasses are less nutritious for bovids such as bison. Moreover, there is good evidence that C4 grasses expanded from south to north at the end of the last Ice Age into the early part of the Holocene--to the detriment of bison.


Panel A at top left shows archeological sites with bison, differentiated by three different time periods spanning 1,450 to 200 years before the present. These points are superimposed on a fairly comprehensive representation of grasslands (some with abundant sagebrush), differentiating tallgrass prairies from shortgrass plains, and areas dominated by C4 grasses (the tallgrass prairies and southern plains) from areas dominated by C3 grasses. These data points pretty clearly suggest three main conclusions: first, that bison were most abundant in the northern plains, coincident with domination of C3 grasses; second, that bison were present in the Great Basin and Columbia Basin; and, third, that bison were probably rare in eastern North America during the last 1,500 years or so. The bison observations in panel A are repeated in Panel C, but with my guesstimate of core versus peripheral bison range delineated for a period of 1,500-500 years before the present. According to this, most of the southern plains in Texas was probably marginal for bison.


Panel B shows observations of bison made by European explorers who kept detailed notes of such things (red dots, as summarized by Richard Hart), along with observations of bison remains at late Paleoindian sites. The point of this being that these observations encompass a period during which the impacts of Europeans had already begun to manifest. I also show a reckoning of the relative abundance of bison, elk, and pronghorn on the tall-, mid-, and shortgrass prairies of the southern grasslands. These tallies were made by James Shaw & Martin Lee from European journals. The take-home message from this map is: first, bison were largely concentrated in the mid- and especially shortgrass prairies by the time of active European exploration; and, second, that bison were rare by then in most of Texas south of the Panhandle.


Again, as in Panel C, I translate the locations in Panel B (plus some) into a distribution map in Panel D that differentiates core from periphery. The added locations in Panel D are from archeological and other excavated sites dating between 700 and 100 years before present, which overlaps somewhat with the data in Panels A and C. But the emphasis here is on more recent locations that take us well into historical times. The delineation of extensive peripheral range to the east and south comes almost wholly from Francis Haines's map. The basic point remains, though, that core bison ranges was centered on the short- and midgrass plains, and that bison became less common as you headed south into central Texas from the Panhandle.


The implications for grizzly bears? Grizzlies were probably always more numerous in the northwestern plains compared to anywhere else, and rare along the edges of the tallgrass prairie as well as in the southern plains. In fact, as the distributional map at top suggests, grizzlies were probably rare in if not entirely absent from central Texas during all or most of the Holocene. There was one archeological find in the Hill Country of central Texas that was billed as being a grizzly, but a recent reexamination of the skull concluded that it was from a black bear.  

Changes in bison over time

These changes in numbers and sizes of bison support a speculative scenario regarding the plight of grizzlies on the Great Plains. First and foremost, grizzly bears were probably much rarer on the Great Plains during the middle Holocene than they were during the most recent two millennia. In fact, Chrissina Burke speculates that grizzly bears were largely absent from the Great Plains between 8,000 and 3,000 years ago (see The Holocene). This comparative dearth of bears sustained over a 5-6,000 year period was probably a major reason why grizzlies failed to establish themselves in eastern North America. Beginning 2,000 year ago, when bison (and presumably grizzly) populations took off, the eastward spread of grizzlies was then probably forestalled by the burgeoning numbers of people (see  The human factor). In fact, human-caused mortality together with competition for food from both people and black bears could have stopped grizzlies in their figurative tracks, and even driven them farther west in the Great Plains. As I speculate above, these effects were probably amplified on the southern plains, which throughout the Holocene have been less productive than the northwestern plains, and may even have been desert-like during the peak of the Holocene's sustained droughts.

A speculative scenario

I conclude this page with an examination of changes in not only numbers, but also body size of bison during the Holocene. To start with, it is perhaps worth noting that bison numbers on the Great Plains numbered around 21-88 million, which is a lot of bison. This is the best estimate I have been able to find, produced by Keith Weber using a GIS-based estimate of habitat productivity coupled with reckoning of drought effects and lags in population dynamics. Other earlier estimates placed bison numbers at around 15-65 million or more. So indicators of bison numbers that have been generated for earlier millennia probably need to be reckoned against this baseline.


The top graph at left shows various measures or indicators of bison body size over the last 14,000 years, derived from the work of a number of researchers including Patrick Lewis, Matthew Hill, Michael Wilson, and R. Dale Guthrie. The obvious trend is one of progressive diminution, by perhaps as much as 50%. These measures of size are referenced in the top horizontal bar to various distinctions that have been made by taxonomists in an effort to make sense of, not only these size changes, but also other variations over time in bison morphology. The conventional saw was that Pleistocene bison gave rise to Bison antiquus, which gave rise to B. occidentalis, which gave rise, finally, to our contemporary bison, B. bison. Current thinking is that it's more defensible to consider all of the post-Pleistocene bison to be of one species, but with enough variation over time to warrant calling the ever-shrinking morphs different subspecies.


The graph bottom left shows an indicator of bison numbers during the last 14,000 years (from Eric Scott), as well as an indicator of bison numbers for the last 1,200 years (from Judy Cooper). Scott also provides an estimate of relative abundance for the other large herbivores that went extinct at the end of the last Ice Age (the orange bars). These estimates of abundance (as well as size, above) are referenced to prominent periods of Holocene warmth and drought denoted by vertical orange bars. Darker colors correspond with periods of greater consensus among the various indicators of drought.


Overall numbers of megaherbivores (comprised solely of bison after the Holocene transition) appear to have been at sustained low levels between roughly 9,000 and 3,000 years ago--which is a 6,000 year period. This period corresponds with the hottest driest parts of the Holocene as well as with a sustained decrease in average body size of bison shown above. I've also noted two periods during which bison were probably altogether absent from the southern plains. There has been ample research suggesting that grassland productivity on the Great Plains is largely governed by drought, and that bison populations track productivity. Hence, a sustained reduction in bison numbers during the middle Holocene. But, then, roughly 2,000 years ago, bison numbers appear to have skyrocketed. This substantial increase handily corresponds with an amelioration in climate along with increases in grassland productivity. It's interesting to note that populations of First Peoples also appear to have increased dramatically during the same 2,000 year period, but with a decline beginning during the last millennium (see The human factor).


It is also interesting to note that the second period during which bison presumably disappeared from the southern plains coincides with population highs for bison and a benign climate. Some researchers have speculated that the driver was not drought, but rather the encroachment of woodlands and forest in the south as moisture increased.