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Yellowstone National Park is the only place on Earth where bison and grizzly bears coexist in significant numbers. Most people, inured to the on-going ecological holocaust of recent centuries, probably think this 2-million-acre area is huge. Yet the current joint distribution of bison and grizzlies comprises only around 2% of what we once had in this country; 1% of what we once had on this continent; and a truly miniscule fraction of what we once had in the Northern Hemisphere. Bison and brown or grizzly bears of some sort coexisted in close relationship for 10s of thousands of years in a 11,000-mile-long swath from Europe, across Siberia, through northwestern North America, to northeastern Mexico. What we have in Yellowstone Park is a truly infinitesimal remnant barely 60 miles across.
But this remnant is not inconsequential. Yellowstone’s populations of bison and grizzly bears have sustained a relic relationship between these two species that was probably critical to grizzly bears in much of their former North American range.
A Little About Yellowstone’s Grizzlies and Bison
I was fortunate enough to participate in and then oversee field investigations of grizzly bears for 15 years in the Yellowstone ecosystem. We documented exploitation of ungulates such as bison and elk by radio-collared grizzlies, and monitored transects along which we documented scavenging by bears during springtime. This work followed a similar decade-long pioneering effort by John and Frank Craighead. These studies, along with less intensive monitoring of spring scavenging between 1996 and the present, spanned a total of over 50 years and provided unparalleled insights into relations between bison and grizzly bears. From this we can glimpse the undoubtedly richer and more complex natural history of bears and bison in Europe, Asia, and North America, recognizing that our modern-day studies span only ¼ of 1% of the time and space shared by these two species.
One thing we know for sure. Bison have been and still are an important source of food—meat—for Yellowstone’s grizzly bears, far exceeding anything one might expect simply from numbers of bison in the ecosystem—by a factor of 2-½-fold and more. Bears obtain almost all this meat by scavenging rather than by predation. Bison are too big, too well-armed, too collectively aggressive, and typically too far from ambush cover for a bear to kill outright. But bison do tip over dead for all sorts of reasons—from starvation and disease during winter and spring, from complications of birthing, from injuries sustained during the rut, and less often from being killed by wolves. And when bison do die, they constitute a big package of meat that not only provides a large reward for any bear lucky enough to find it, but also increases odds that stinky carrion will persist long-enough for discovery by animals, such as bears, endowed with an acute sense of smell.
These fundamentals suggest that bison were always a comparatively important food for brown/grizzly bears wherever both species overlapped in time and space. In fact, availability of meat from bison probably dictated the terms of existence for grizzly bears for over 10 millennia in the mid-section of North America.
Ice Age Bison
The murky scientific waters of ever-evolving methods applied to ever-more data have, over time, clarified the evolutionary biogeography of bison and bears, along with the formative context of dynamic climates, ephemeral corridors, and shifting barriers of ice and vegetation. Disagreements and uncertainties remain, especially given the indirect measures necessary to reconstruct a long-gone past, but the broad outlines of a shared journey have emerged.
Steppe bison, Bison priscus—the progenitor of all subsequent bison—probably first arrived in North America around 150,000 years ago during the penultimate Illinoian Ice Age. Like all bison, steppe bison were predominantly grazers that followed a broad swath of grass-dominated steppe tundra from eastern Siberia across the Bering Land Bridge into what is now Alaska. This bridge between Eurasia and North America was exposed then, as it was during all Ice Ages, by lower sea levels resulting from the capture of ocean water in continent-spanning ice-sheets.
Bison of North America, including the giant Ice Age Bison latifrons farthest left, our comparatively small modern-day bison farthest right, and the ancestral steppe bison, Bison priscus, in the middle. Illustrations by Roman Uchytel.
Roughly 25,000 years later North America’s continental ice sheets were rapidly melting, creating an ephemeral grass and forb-dominated corridor from eastern Beringia south to mid-latitudes. It was during this early part of our penultimate interglacial period—the Sangamonian—that bison moved south into the grasslands that they have occupied ever since, and where they were then isolated by consolidating boreal forests from steppe bison in the north. By 120,000 year ago these southern bison had rapidly evolved into an enormous long-horned variant called Bison latifrons, presumably in response to the predatory pressures of a host of now-extinct large carnivores that included lions, saber-toothed tigers, scimitar-toothed cats, and giant short-faced bears. These giant Ice Age bison weighed 2-3-times more than modern-day bison.
Enter Ice Age Grizzlies
Grizzly bears didn’t arrive in North America until much later, also from Eurasia, and only with reemergence of the Bering Land Bridge early during the last, Wisconsinan, Ice Age. We don’t know exactly when grizzlies arrived, but probably around 70,000 years ago during a mini-maximum of glacial ice. Subsequent short-lived retreats of the ice sheets produced perhaps more than one ephemeral ice-free corridor from Beringia south to mid-latitudes that closed by between 55,000 and 30,000 years ago. But grizzlies slipped through, as evidenced by 32,000-year-old remains found near Edmonton, Alberta.
As a result, we had grizzly bears living among bison, along with a slew of other large predators, throughout the Last Glacial Maximum south of the continental ice sheets. Curiously, grizzlies in Beringia mixed it up with steppe bison for perhaps 35 millennia, up until around 35,000 years ago, and then disappeared or declined to very low densities for a 15,000-year period that coincided with a burgeoning of giant 700 to 2200-pound short-faced bears. Some paleontologists have implicated competition from or even predation by the formidable short-faced bears in the demise of Beringian grizzlies.
The Pleistocene bestiary of North America large carnivores, including grizzly bears top middle, and giant short-faced bears top left. All illustrations of extinct species by Sergio De la Rosa.
But by 20,000 year ago short-faced bears were largely gone from eastern Beringia, only to be replaced by another wave of migrant grizzly bears from Siberia, arriving hard on the heels of a second major wave of migrant bison. Both bison and grizzly bears remained bottled up in Beringia for another 9,000 years, with grizzlies probably scavenging bison whenever and wherever they could, up until yet another ice-free corridor opened between the rapidly-melting Laurentide and Cordilleran Ice-sheets around 13,000 years ago. Not long after, the Bering Land Bridge disappeared for the last time, isolating both bears and bison from conspecifics on the supercontinent of Eurasia, where brown bears persisted, but steppe bison went extinct.
The Cusp of the Holocene
As an upshot, North America is the only place on Earth where an undiluted bison lineage survived, notwithstanding a distant relative called wisent that occurred across much of Europe until being nearly driven to extinction by hunters during the 1800s and 1900s. Despite sometimes being called bison, wisent descended from an aurochs x bison hybrid, leaving yaks as the closest surviving relative of our North American bison.
The rapidly melting ice at the end of the last glacial epoch and beginning of our current Holocene era was associated with a wildly fluctuating climate, rapidly shifting vegetation, expansion of human populations armed with lethal stone-tipped weapons, and extinctions of virtually all the large carnivores and herbivores that had dominated Ice Age landscapes—including mammoths, mastodons, camels, horses, giant stag-deer and muskox, and lumbering ground sloths. Around 10,000 years ago the last and largest of the megafauna literally and figuratively left standing were bison, moose, and grizzly and polar bears. Although grizzlies remained much the same, albeit of different genetic lineages north and south of the melting ice sheets, between 22,000 and 15,000 years ago bison had yet again rapidly evolved into different forms, including a southern morph called Bison antiquus.
Lots of ink has been spilled disputing the taxonomic status of bison, but there is widespread agreement that bison looked different 14,000-10,000 years ago compared to before or after, and were still larger than our present-day bison. There is also widespread agreement that essentially all bison and grizzly bears in mid-latitudes of North America descended from genetically distinct animals that were part of the earliest waves of migrants south from Beringia. Grizzly bears of the last wave of migrants from Siberia never made it south of central Alberta, and northern bison of the direct Bison priscus lineage eventually went extinct, in some places as recently as 200-400 years ago.
The Rocky Altithermal
Bison of the mid-latitudes have been denizens primarily of the great North American grasslands ever since the end of the Ice Age. When you plot known locations of bison dating between roughly 500 and 10,000 years ago, virtually all occur in areas with abundant grass, which is not surprising given that modern bison are grazers. But all was not static during those 9,500 years.
For one, North America entered a sustained period of severe drought even as the continental ice sheets were still melting. This epic period of hot dry conditions that lasted between 9,000 and 4,500 years ago is known by various names, most commonly the Altithermal. Bison numbers were held at a nadir during this bleak period not only by drought-diminished forage, but also by the spread of less nutritious grasses north across the Great Plains. These grasses, which include species such as blue grama and buffalo grass, are adapted to warm dry conditions, but support significantly lower densities of bovids such as bison, in the past, and cattle now-a-days. By contrast, grass species typical of northern latitudes, such as wheat- and bluegrasses, support much higher densities of both wild and domesticated grazers. Several researchers have suggested that conditions were so tough on the southern Great Plains during the height of the Altithermal that bison disappeared there for several millennia.
Grizzlies were profoundly affected by these environmental changes. In fact, grizzly bears seem to have disappeared from most of the Great Plains for most of the Altithermal as well as for several millennia after. I have no doubt that this dearth of bears was a direct consequence of a dearth of bison, aggravated by competition with humans and black bears for shared foods concentrated along the west-east-trending river corridors of the Plains. People were also probably killing their share of grizzlies.
But, despite the epic droughts, grizzly bears probably hung on and even flourished in the Northern Plains, and may have even benefited from the activities of humans. Bison densities were almost certainly always highest in the north, simply because of more bounteous forage. Not by coincidence, most of the known pre-equestrian sites where humans engaged in mass kills of bison were also located in this region, including most buffalo jumps and corrals. Kills at these sites could be so large that some bison carcasses were left untouched by the involved people, while others were butchered only for the choicest cuts—“gourmet butchering.” As a result, ample meat was available for scavengers such as grizzlies who undoubtedly took full advantage of the opportunity. In fact, it is easy enough to imagine repeated vignettes where grizzlies lurked around kill sites negotiating access with the wary but heavily-laboring people.
European Bio-ethnic Cleansing
But all this changed with arrival of Europeans and their diseases in the late 1400s. Native peoples were decimated. Some scholars reckon that between 40 and 80% died from fast-spreading disease alone, well before any contact with Europeans themselves. This reduction in Native populations, along with related increases in grassy woodlands, seems to have opened the way for the spread of substantial numbers of bison into eastern North America for the first time in the last 9,000 years. European adventurers thus encountered bison—but not any grizzly bears—as far east as coastal Florida and South Carolina during the early 1600s.
But by 1800, a short 200-years later, the tide had irreversibly turned for bison and bears. Europeans embarked upon a systematic slaughter of both. By 1810 only a few enclaves of bison survived in the East. A decade later, they were completely gone from the East and pushed back west of the Mississippi River. By around 1800, Europeans, most notably Meriwether Lewis and William Clark, were recording their first encounters with grizzlies, including their first accounts of killing them.
The accounts thereafter become tediously terrible. Bison were served up on the altar of an ever-widening trade in hides and meat spawned by European demand, but with Native Americans fully complicit. Most bison in most places where they had existed in 1600 had probably been slaughtered by the time the Transcontinental Railroad split surviving bison into the “great” northern and southern herds around 1870. Within 25 years the 40-80 million bison that had once roamed the Great Plains had been reduced to a few hundred wild survivors in the remote enclave of what had become our first National Park.
Grizzly bears didn’t fare much better. They were gone from the central and southern Plains by around 1850, out-pacing even the slaughter of bison. Sixty years later grizzlies could only be found in scattered haunts restricted to the most rugged and remote parts of the mountain West. By 1950 they were extirpated from roughly 97% of the areas they had once occupied in 1800 and relegated to two main populations each of a few-hundred individuals. One of these populations hung on in Yellowstone along with our remnant wild bison.
Where does this leave the epic relationship between bison and grizzlies—and humans? For the last 2,500 years, up until roughly 1800 A.D., bison and grizzly bears coexisted in an area roughly 950,000 square miles in size, spanning most of the western part of what is now the United States, and encompassing roughly 60% of former grizzly bear range within this same area. The only place on Earth where this relationship persists is in the Yellowstone ecosystem, in an area comprising <2% of what we once had.
Historical (gray) and current (green) overlap of grizzly bears and bison in North America. Historical overlap is from around 1800 A.D. Current overlap is restricted almost wholly to Yellowstone National Park in the middle of the green high-lighted area.
And how are we treating this rare and highly vulnerable relic? Last year we killed roughly 1,400 wild bison as part of a plan to deliberately confine them to within the boundaries of Yellowstone National Park. This slaughter was justified by the presumed need to protect a handful of regional livestock producers from the near non-existent threat of disease transmitted from bison to cattle—a disease called brucellosis that was originally introduced by cattle. Simultaneously, we are poised to remove Endangered Species Act protections for Yellowstone’s grizzly bears, with the intent of turning them over to the tender mercies of State wildlife managers fully intent on instituting a trophy hunt.
And nowhere in the mountain of tedious planning documents spawned by the numerous involved management agencies will you find any recognition of, much less prioritization for, the precious remnant we have in Yellowstone of a relationship between bears and bison that once spanned continents and millennia. In short, a travesty.
There is much more that can be said—that needs to be said—about our current management of bison and the unique surviving relationship between bison and grizzly bears in the Yellowstone ecosystem. Stay tuned for more on this front in the next part of this two-part series…
For more information on the Evolutionary Biogeography of North American Grizzly Bears and Bison, see here and these papers:
Barnes, I., Matheus, P., Shapiro, B., Jensen, D., & Cooper, A. (2002). Dynamics of Pleistocene population extinctions in Beringian brown bears. Science, 295(5563), 2267-2270.
Davison, J., Ho, S. Y., Bray, S. C., Korsten, M., Tammeleht, E., Hindrikson, M., … & Cooper, A. (2011). Late-Quaternary biogeographic scenarios for the brown bear (Ursus arctos), a wild mammal model species. Quaternary Science Reviews, 30(3), 418-430.
Drummond, A. J., Rambaut, A., Shapiro, B. E. T. H., & Pybus, O. G. (2005). Bayesian coalescent inference of past population dynamics from molecular sequences. Molecular biology and evolution, 22(5), 1185-1192.
Froese, D., Stiller, M., Heintzman, P. D., Reyes, A. V., Zazula, G. D., Soares, A. E., … & MacPhee, R. D. (2017). Fossil and genomic evidence constrains the timing of bison arrival in North America. Proceedings of the National Academy of Sciences, 114(13), 3457-3462.
Heintzman, P. D., Froese, D., Ives, J. W., Soares, A. E., Zazula, G. D., Letts, B., … & Jass, C. N. (2016). Bison phylogeography constrains dispersal and viability of the Ice Free Corridor in western Canada. Proceedings of the National Academy of Sciences, 113(29), 8057-8063.
Marsolier-Kergoat, M. C., Palacio, P., Berthonaud, V., Maksud, F., Stafford, T., Bégouën, R., & Elalouf, J. M. (2015). Hunting the extinct Steppe Bison (Bison priscus) mitochondrial genome in the Trois-Freres paleolithic painted cave. PloS one, 10(6), e0128267.
Massilani, D., Guimaraes, S., Brugal, J. P., Bennett, E. A., Tokarska, M., Arbogast, R. M., … & Madelaine, S. (2016). Past climate changes, population dynamics and the origin of Bison in Europe. BMC biology, 14(1), 93.
Matheus, P., Burns, J., Weinstock, J., & Hofreiter, M. (2004). Pleistocene brown bears in the mid-continent of North America. Science, 306(5699), 1150-1150.
Shapiro, B., Drummond, A. J., Rambaut, A., Wilson, M. C., Matheus, P. E., Sher, A. V., … & Willerslev, E. (2004). Rise and fall of the Beringian steppe bison. Science, 306(5701), 1561-1565.