Archaeologist Alex Chepstow-Lusty of the French Institute of Andean Studies in Lima and colleagues used pollen in the soil to reconstruct the horticultural history of the Ica Valley, which hosted the Nazca.

Once a fertile riverine oasis, the valley is now almost totally barren. It was previously dominated by huarango trees, massive slow-growing relatives of the mesquite that can live for more than 1,000 years, providing food and wood for fuel and construction, trapping water from the mists that blow in from the Pacific, and stabilising the soil.

The researchers found that as the Nazca civilisation grew, the level of huarango pollen declined and cotton pollen increased. Then, in around 400 AD, cotton was replaced by maize, squash, beans and other food crops, and huarango pollen dropped off sharply. The consequences of this were felt around 100 years later, when a major El Niño event brought heavy rains to the Andes and, consequently, floods to the Ica Valley. After 500 AD, only pollen from plants adapted to salty and arid conditions could be found.

According to Chepstow-Lusty, the Nazca may have survived the floods had the forests been left intact, but with this vital defence removed, they were extremely vulnerable.

January 2010