Layers of charcoal remains buried under Montana’s northern prairie show that pre-Columbian native hunters on the Great Plains once burned patches of grassland to encourage new growth. This provided a tempting feast for herds of bison, which the hunters then used to lure the bison inside to kill. And that, archaeologists say, means that even relatively small, mobile groups of hunter-gatherers could have a greater impact on the environment than they have hitherto assumed.
It’s not the fall, it’s the sudden stop at the end
For a group of hunters on foot like the ancestors of today’s Blackfeet people, one of the most efficient ways to take out large prey like the American bison is simply to chase a group of them off a cliff and then collect the remains. to harvest underneath. Various hunter-gatherer societies around the world have used versions of this tactic for thousands of years, leaving piles of animal bones (many with evidence of slaughter or cracking to get to the marrow) at the base of ancient cliffs. It takes planning and coordination between many fighters – and a fair amount of luck.
In the highlands of north-central Montana, in what is now the Blackfeet Reservation, pre-Columbian hunters built miles of cairns called drivelines, which hunters used to help guide buffalo herds from fertile grazing areas, called gathering basins, to the edge of a steep bluff overlooking a tributary of the Two Medicine River. At two different powertrain sites, archaeologists have found radiocarbon-dated bison bones between 900 and 1650 CE, with most of the killings occurring in the last 250 years of that period. (The sites are on a tributary that flows into the Two Medicine River from the north and another on another tributary that flows in from the south.)
And at the same time, there’s evidence that those same hunters burned patches of prairie to encourage the growth of fresh, tasty new grass in the collecting basins to attract herds of hungry bison. Southern Methodist University archaeologist Christopher Roos and his colleagues, in a project designed by John Murray of the Blackfeet Tribal Historic Preservation Office, studied layers of sediment uncovered in the riverbed walls of two tributaries of the Two Medicine River.
Each tributary would have had one of the drivetrains and collection basins in the drainage basin, so those layers of sediment record what happened in the collection basin and along the drivetrain. At each location, the team, including members of the Blackfeet Tribe, found between five and eight layers of charcoal residue, a clear sign of nearby prairie fires. These were radioactively dated between 1100 and 1650 CE – the heyday of the bison leaps.
Burning grass, smoking gun
The prairie sometimes burns naturally, as lightning strikes can cause wildfires. This is common in years when the Great Plains have seen enough rain for the grass to grow properly, providing plenty of fuel for combustion. Wildfires scorched these lands long before humans built powertrains to help them hunt bison, and wildfires are still a vital force on the modern prairie. But the layers of charcoal residue in the riverbed walls don’t look like the product of natural wildfire seasons.
Roos and his colleagues examined sediment layers dating back at least 500 years before the first massive bison killed at both sites, and they found no obvious layers of charcoal residue — just the usual traces of charcoal mixed with normal sediment layers. And in sediment deposited after 1650, the charcoal layers were also conspicuously absent. So there was a distinct class of fires that could lay down a substantial layer of charcoal, and this only happened in the years when hunters were using the powertrains, suggesting a connection, the archaeologists say.
The most likely explanation is that hunters deliberately burned the gathering pools several months before the hunting season – this probably meant a spring fire in preparation for fall hunts, or perhaps a fall fire in preparation for spring hunts. Fire leads to new growth, something that bison find hard to resist. On the modern prairie, ranchers practice controlled burns every year or every few years to promote healthy new grass for grazing livestock. It would have been a smart strategy for bison hunters; organizing a tempting grass buffet for the bison helped increase the likelihood that a herd would actually congregate in the gathering basin, making a successful hunt much more likely.
How should we define the Anthropocene?
So the prairie fires would have been precise, controlled burns in carefully chosen spots, not random burning of an entire landscape. Still, the fires had an impact on the prairie’s ecosystem.
“This kind of combustion in fescue pastures changes the composition of the grasses and forbs,” Roos told Ars. That doesn’t make the burned area any more or less diverse than an unburned area, he explained. “[But] what it does do is create patchiness in the grassland with different compositions, creating greater diversity between patches. This can be especially important for small animals.”
That means that even a hunter-gatherer society with a relatively low population density had a significant impact on the local ecosystem. (Early European estimates, from just after populations largely recovered from a smallpox epidemic of 1781-1782, put the average hunting group at about 150 people and a large winter camp at about 2,000 people.) In fact, that effect was significant enough to make its way into the geological record – which is an important part of how scientists now define the geological epoch called the Anthropocene, in which human activity is a major force shaping the Earth’s environment.
Several archaeological finds in recent years have suggested that the Anthropocene began earlier than we first suspected, dating not to the Industrial Revolution, but possibly to the earliest agricultural communities. This “ancient Anthropocene” model suggests that when those early farmers cleared forests for fields, and when they used fire to cook and heat at much greater concentrations than before, they released enough carbon into the atmosphere to have an impact. on early Holocene climates.
Of course, it’s unlikely that hunters on the Great Plains, which burn parts of the prairie to attract herds of bison, would have released enough carbon to have a real impact on the climate. But this is another sign that humans have left a lasting mark on the environment for much longer than we realized. William Ruddiman, who first proposed the Old Anthropocene hypothesis, has also suggested that a more flexible definition of “Anthropocene” may be more useful than pinning a fixed start date to a capital Anthropocene geological epoch. .
“My take on this is that our evidence of hunter-gatherer manipulation of their environment (including possibly carbon budgets and carbon cycle through fire use) gives more weight to the ‘vague’ and evolving definition of the Anthropocene, rather than a fixed geological unit of time,” Rose told Ars.
Lessons from the past
The telltale layers of charcoal also hint at a complex, nuanced collaboration between humans and climate to shape how fires hit the Great Plains several hundred years ago. Scientists who study wildfires often see climate and human activity as opposing forces. For example, people may burn regularly when climatic forces would lead to few fires, which in turn can help reduce the amount of fuel available to burn when an extreme burning season begins.
But according to Roos and his colleagues on the Great Plains, bison hunters’ fires amplified the effects of climate on the prairie’s natural fire cycle. Natural wildfires are more likely, and generally larger, in seasons when there is more grass to burn. And since rain makes the grass grow, wetter decades are ironically linked to more combustion.
Of the 13 charcoal deposits in the study, six were associated with periods when the Great Plains climate took a wetter turn (on a scale of decades). The other seven were associated with shorter wet periods or with a much smaller increase in rainfall. In the context of the bison jumps, it seems that the fighters who built and used the powertrains took the opportunity to burn their collecting basins when there was enough grass to burn. Thanks to human activity, a relatively small increase in average rainfall had a much greater impact on the frequency of fires in the area than they would naturally.
That understanding of how humans, even in relatively small groups, interacted with climate patterns to produce a pattern of fires that in turn shaped an ecosystem offers some lessons for grassland management today.
“I tend to think that the past offers general lessons about contemporary problems rather than specific lessons. A lesson from our work is that this landscape (fescue grasslands) can use quite intensive fire and that fire can be used to successfully manage grazing in this environment,” Roos told Ars. “This is in line with some of the arguments coming out of Oklahoma State University, where they advocate burning plasters to manage the grazing patterns of bison and livestock.”
Future studies could shed more light on how widespread the practice in North America was of burning gathering basins to attract herds of bison.
“Late Holocene bison jumps are found wider than just the northwestern plains. Was fire also used as part of those hunting strategies?” said Roos. “What impact did this combustion have on the burning regimes of the landscapes between clusters of powertrain complexes? I mention food webs in the paper and it would be great to know what the implications of this burning strategy were for other plant and animal species.”
PNAS2018. DOI: 10.10703/pnas.1805259115 (About DOIs).