Native Americans made widespread use of fire on the Great Plains, probably to alter bison grazing patterns. A study of these deliberately lit fires suggests we have been underestimating and misunderstanding the impact of fire regimes by nomadic peoples.
Fire may have been more important for cooking food and keeping people warm, but it was also used in hunting. Indeed, with increasing evidence Australian raptors spread fire to scare small animals from their hiding places, this use may have predated the others.
Dr Christopher Roos of Southern Methodist University, Dallas, has shown that Native Americans did something similar, but on an altogether different scale.
On the Great Plains, Native Americans used techniques such as stone structures to push bison towards cliffs, or bison jumps, where the prey would leap to their deaths. The abundance of resulting meat fed families for months.
Roos argues strategically placed fires produced abundant food six months later, thus luring bison close enough to jumps for these drive lines to take effect. Roos studied centuries of charcoal deposits near bison jumps, beginning around 1,000 years ago after a climatic shift increased rainfall in the region and greatly increased the number of bison and fire fuel.
Natural fires rise and fall with the climate since only long wet seasons produce enough fuel to make a large burn. Consequently, we can trace a record of the climate in the charcoal density of the soil.
In northern and western Australia, where traditional fire-stick farming is still practiced (or in the process of being revived), human-induced fires have a buffering effect. By burning relatively small patches of vegetation early in the season, indigenous Australians limit the available fuel, thus avoiding huge conflagrations when the bush is more dried out.
Anthropologists anticipated something similar elsewhere, but Roos has shown in Proceedings of the National Academy of Sciences that the areas around the bison jumps have an abundance of charcoal deposits – far more than in places where lightning was the prime cause.
After 1650, the dense charcoal layers cease, which Roos attributes to the devastating effects of smallpox on the local population.
Roos found that the deliberately lit fires were most common during wet decades. Not only does this show that humans were responding to climate effects in their burning but it demonstrates humans' amplification of fire in response to climate, rather than dampening it. Moreover, it reveals even sparse nomadic populations have a big effect on fire regimes, something previously thought to be limited to denser agricultural societies.
The changes these fires created on the landscape could force a rethink on when the Anthropocene began, that is, when humans first changed the landscape so much as to define a new geological era.