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[naturenews] from [nature.com]

[naturenews]
Published online 2 November 2009 | Nature | doi:10.1038/news.2009.1046
Corrected online: 2 November 2009
News
Native American culture sowed seeds of its own collapse
Floods brought the Nazca to their knees — but they crippled themselves by over-farming first.
By Lizzie Buchen

{{An irrigation canal once used by the Nazca, who may have helped bring about their own demise by clearing trees to make way for farmland.}
David Beresford-Jones}

The mysterious Peruvian culture that preceded the Incas had a significant hand in its own catastrophic collapse, new research suggests.

The Nazca people are thought to be responsible for the enormous drawing or geoglyphs etched into the deserts of southern Peru, known as the Nazca lines. Around 500 AD, archaeological evidence indicates that the then-flourishing society came to a sudden and bloody demise.

A leading hypothesis for this precipitous collapse proposes that massive floods destroyed the society's agricultural system, causing the society to fragment and feud over abruptly scant resources. But archaeologist David Beresford-Jones of the McDonald Institute for Archaeological Research at the University of Cambridge and his colleagues found that — although such flooding did occur — the Nazca brought on their own demise by logging trees to make way for farmland. Their findings appear in Latin American Antiquity¹.

"Dramatic climactic events are always used to explain culture change in the Andes," says Beresford-Jones. "But this is not satisfying based on what we know about human culture. It paints a picture of culture sitting there, not changing, hit by events over which they have no control. But Native Americans did not always live in harmony with their environment."

Cultural forensics

Ice-core records suggested that severe storms — a mega El Niño — hit the Peruvian Andes around the time the Nazca's fall began², but this had not been corroborated in the coastal valleys where the Nazca once lived. Beresford-Jones and his colleagues, focusing on the lower Ica valley, solidified this evidence when they discovered a flood layer that sat directly on top of a Nazca rubbish dump. The authors then recreated the flood using a computer simulation, demonstrating that a flood that left such a layer could have caused the damages to the Nazca canal system known to have occurred around 500 AD.

"But that's not the end of the story," says Beresford-Jones. "The landscape was only exposed to the effect of the El Niño because of what the Nazca were doing to their river valleys."

{{“Native Americans did not always live in harmony with their environment.”}
David Beresford-Jones
McDonald Institute for Archaeological Research}

Preserved tree trunks are scattered across the now-deserted lower Ica valley, about 200 km south of Lima, indicating a significant landscape change. To investigate this further, team member Alex Chepstow-Lusty of the French Institute of Andean Studies in Lima analyzed the pollen that had been blown to the edges of the basin by strong winds. For much of the older portion of the record, the pollen came from riparian trees, like huarango, which once created woodland oases that lined the rivers in the otherwise desert landscape.

But as Chepstow-Lusty moved forward in time through the pollen record, he found a gradual decrease in huarango pollen and a concomitant increase in pollen from agricultural sources, like cotton and maize, indicating that the Nazca were cutting down woodland to make room for farms. The records show that agricultural plants dramatically disappeared and were replaced by weeds; eventually the weeds died and the land became the lifeless desert it is today.

Beresford-Jones says that when the Nazca cut down the trees they destroyed the root system that had been anchoring the landscape.

"When the El Niño came it cut into the floodplain because it was no longer supported by woodland. That caused erosion and made the irrigation system useless," he explains. "Storms like this should have just replenished the water table and wouldn't have hurt them, but [the Nazca] exposed their own land."

Beyond the lower Ica

"The study is original in lots of ways," says Warwick Bray, a retired archaeologist formerly at University College London. "They've brought to bear pollen analysis, geomorphology and archaeology all together in one programme. There have been hints of [vegetation changes] all along, but this is the first time all these techniques have come ogether for southern Peru."

But Bray points out that the findings don't necessarily apply to the collapse of the entire Nazca culture. "They were spread over several southern coastal valleys, each with slightly different topography and geological possibilities," he says. "This is a marvellous study of a small region, but how much we can extrapolate across the whole range of the Nazca no one knows."

Beresford-Jones agrees that there are considerable variations among the valleys. "But the cultural and ecological changes we record in the lower Ica Valley seem to correlate with the wider social changes recorded by archaeology," he says. "It seems reasonable that our lower Ica Valley results include some lessons for those wider changes."

The findings don't bode well for today's southern Peruvian valleys, says Beresford-Jones, where people are removing the last remaining riparian forests for charcoal.

"Populations have exploded, resulting in tremendous pressure upon water resources, agricultural production and the fragile biomes, all of which increases vulnerability to climatic perturbations such as El Niño," he warns. "History repeats itself."

Corrected:The original version of this story misidentified David Beresford-Jones as performing the pollen analysis.
References
Beresford-Jones, D.G. et al. Latin American Antiquity 20, 303–332 (2009).
Thompson, L.G. et al. Science 229, 971–973 (1985). | Article | PubMed