Social and Natural Systems in the Decline of North America's Megafauna

One of the most striking characteristics of humans is that we are adaptive generalists. Unlike most species, which are adapted to specific ecological niches, humans have radiated out to populate virtually every land-based ecosystem on the planet. We can do this because we build shelters, transport food and otherwise make arrangements for the necessities of life in those parts of the globe that would otherwise be inhospitable.

It is this capacity, the ability to transform situations to meet our needs, that lies at the root of the interconnection between social and natural systems. The most obvious example of this interconnection is climate change, where humans are pumping the carbon stored in the ground as fossil fuels into the atmosphere and, hence, fundamentally altering both atmospheric chemistry and the global climate.

But what lies at the root of this human capacity? The standard answer is technology. Through technology we transcend the limitations and constraints placed on other species. But a recent article by Christopher Johnson in Science (Science 20 November 2009: Vol. 326. no. 5956, pp. 1072 - 1073) detailing the process of magafaunal decline in North America casts doubt on this account. Here is the story in brief:

Twenty thousand years ago, North America had a more impressive array of big mammals than Africa does today; by 10,000 years ago, 34 genera of these mammals were gone, including the 10 species that weighed more than a ton. Many other drastic changes occurred in this interval, all of which have been advocated as possible causes of megafaunal extinction. The climate flipped from cold to warm, then back to cold in a 1000-year chill (the Younger Dryas), before rapidly rewarming. There were more, larger fires, and the structure and species composition of vegetation changed drastically. People arrived, and the Clovis culture—with a characteristic style of beautifully crafted stone spear points—flourished for less than 1000 years. Some scientists have argued that an extraterrestrial object struck Earth ~13,000 years ago, triggering the Younger Dryas, starting fires, killing the megafauna, and putting an end to the Clovis culture. ....

What about people? It has long been argued that Clovis artifacts signal the first arrival of people in North America south of the boreal ice sheets and that the Clovis people were specialized big-mammal hunters who caused a crash of megafaunal populations from prehuman abundance to extinction within a few hundred years. This “blitzkrieg” scenario is supported by the fact that terminal dates on megafaunal fossils range from 13,300 to 12,900 years ago, which coincides almost exactly with the Clovis period. But the new data show that the megafaunal decline had begun more than a thousand years earlier. If people were responsible for that decline, they must have been pre-Clovis settlers. The existence of such people has been controversial, but archaeological evidence is slowly coming to light and is consistent with their arrival around the beginning of the megafaunal decline. It is beginning to look as if the greater part of that decline was driven by hunters who were neither numerous nor highly specialized for big-game hunting. Clovis technology may have been a feature of the endgame, possibly reflecting an intensified hunting strategy that developed once megafauna had become rare, possibly wary, and harder to hunt. ...

Before 14,800 years ago, the environment around the site studied by Gill et al. was an open savanna or parkland, probably with scattered spruce and rare broad-leaved trees growing over a short grass-dominated pasture, and almost no fi re. As the megafauna declined, woody biomass increased, mainly by growth of broad-leaved trees that had presumably been suppressed by the large herbivores. The result was a transitory spruce/broadleaf woodland, the like of which does not exist today. Big fires broke out ~14,000 years ago, and for the next few thousand years, major fires returned every few centuries. These changes were widespread: Fire increased throughout North America ~14,000 years ago, and the transitory “no-analog” woodland extended over a vast area.

In short, we begin with an ecosystem dominated by open savanna and numerous species of megafauna. Humans arrive, and apparently without the aid of significant technology, kill off the majority of the megafauna thus setting loose a cascade of ecological changes that ultimately result in the replacement of the savanna by a "no-analog" woodland. Thus we have a clear, early example of the interconnection of social and natural systems, but one that seems not to implicate technology as the fundamental driver.