The fate of an ancient and enigmatic South American civilisation which carved a strange network of ritual lines and drawings into the landscape before its sudden collapse has been pieced together by British scientists.

Some 1,500 years after the disappearance of the Nazca culture of what is now southern Peru, its demise has been attributed to a combination of a freak extreme weather event and its own despoliation of the environment, which left it unable to survive.

The Nazca civilisation, which thrived in the Ica and Rio Grande de Nazca river valleys between the birth of Christ and about AD500, is best known for creating the mysterious Nazca Lines. The lines are geoglyphs, or drawings on the ground; some are simple lines or geometric shapes, others depict monkeys, hummingbirds, llamas and sharks when viewed from above. They are thought to be routes of religious processions.

Some observers, such as the Swiss author Erich von Däniken, have suggested the lines are evidence of visits by aliens. Flights over the Nazca Lines remain popular with tourists.

The Nazca culture is also known from beautiful ceramics and elaborate textiles, and is known to have disappeared relatively quickly between 500 and AD600.

“There are rearrangements of settlement, there is evidence from human bodies that life expectancy was falling and infant mortality rising, and the orthodox understanding is that something awful happened,” said David Beresford-Jones, of the McDonald Institute for Archaeological Research at the University of Cambridge. “The major ceremonial site at Cahuachi was also abandoned.”

The standard explanation for the Nazca’s collapse is that the region was struck by an extreme El Niño event — the intermittent climate oscillation of the southern hemisphere that brings higher temperatures and increased rainfall.

Dr Beresford-Jones’s new research, however, has established that this was only part of the story. In a study published in the journal Latin American Antiquity, his team has used the effects of the 1998 El Niño event — the most intense of modern times — to model the impact of similar extreme weather at the end of the Nazca age.

The 1998 El Niño flooded the modern city of Ica to a depth of two metres, and a computer reconstruction shows that the ancient El Niño would have had still more catastrophic effects on the lower Ica valley, one of the two centres of the Nazca civilisation.

The Nazca might have survived this potentially devastating event, however, had it not been for their past behaviour, the research suggests. The lower Ica valley, though now a desert, was at the time heavily wooded, with a tree known as the huarango, which can live for more than 1,000 years.

As well as providing the Nazca with wood for fuel and construction and seeds to supplement their diet, the huarango trees played an important ecological role. Their deep root systems held the soil together, protecting it against water and wind erosion.

The trees were also a defence against flash floods. Analysis of ancient pollen by co-author Alex Chepstow-Lusty of the French Institute of Andean Studies has shown that huarango trees declined in the years before the collapse of the Nazca, to be replaced by pollen from crops such as maize and cotton. This suggests that the forest was cleared to make way for agriculture.This forest clearance seems to have removed natural defences that would have protected the Nazca civilisation against the severe El Niño that coincided with their collapse, Dr Beresford-Jones said.

“This catastrophe was preceded by human-induced changes, particularly chopping down the woodland,” he added. “In time, gradual woodland clearance crossed an ecological threshold, sharply defined in such desert environments, exposing the landscape to the region’s extraordinary desert winds and the effects of the El Niño floods. The climate wasn’t enough to induce collapse on its own. The Nazca partly wrought their own demise.”

Deforestation is also widely acknowledged as a factor in the demise of the Easter Island civilisation, and in the fall of the Anasazi people of the southwestern United States.

Oliver Whaley, of the Royal Botanical Gardens Kew, another author of the paper, said: “The mistakes of prehistory offer us an important lesson for our management of fragile arid areas in the present.”

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