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Satellite imagery may provide a missing puzzle piece in Easter Island saga

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Hundreds of monumental stone heads dot the coastline of the remote Pacific island of Rapa Nui, or Easter Island. Settled by a small group of Polynesian seafarers about 900 years ago, it’s a fascinating place that has been the subject of fierce debate about how complex societies can sometimes ruinously fail.

Some experts, such as geographer Jared Diamond in his 2005 book, “Collapse,” used Easter Island as a cautionary example of how the exploitation of limited resources can result in catastrophic population decline, ecological devastation and the destruction of a culture through infighting.

Other researchers suggest the exact opposite — that Easter Island is a tale of a how an isolated people created a sustainable system, allowing a small but stable population to thrive for centuries until first contact with European colonial powers in the early 18th century.

Now, research involving remote-sensing data and machine learning to map evidence of island farming offers a fresh clue that may help unravel the mysterious demise of the island’s original civilization. The new finding suggests that the island was not densely populated, making ecological collapse a less likely scenario.

“There’s all these bits of evidence that have been collected over the past 15 or 20 years that kind of start to throw a wrench” in the collapse story, said Dr. Dylan Davis, lead author of the study that published Friday in the journal Science Advances.

“And this is what this paper builds on.”

Easter Island inhabitants used rock gardens

Rapa Nui, today part of Chile, is more than 2,000 kilometers (1,242 miles) from the nearest inhabited island of Pitcairn and about 3,700 kilometers (2,300 miles) from the South American mainland, according to the study.

Davis, a postdoctoral research fellow at the Columbia Climate School’s Lamont-Doherty Earth Observatory, and his team focused on farming practices to understand how large a population the island could have supported. At 163 square kilometers (63 square miles), it’s a little smaller than Washington, DC.

An ancient farming technique known as rock mulching used scattered and pulverized rocks to make the land more productive by adding and sealing in nutrients and moisture. - Terry Hunt

An ancient farming technique known as rock mulching used scattered and pulverized rocks to make the land more productive by adding and sealing in nutrients and moisture. - Terry Hunt

The rock gardens had covered up to 21.1 square kilometers (8.1 square miles) and could have sustained up to 17,000 people, previous research suggested. That February 2013 finding bolstered the idea that people drained Rapa Nui’s limited resources.

Archaeologists have identified the remains of rock gardens on which islanders would have grown sweet potatoes and other crops. The scattered and pulverized rocks make the land more productive by adding and sealing in nutrients and moisture and protecting young plants from winds — an ancient farming technique also known as rock mulching.

However, in the new study, Davis and his colleagues found that the maximum number of people on Rapa Nui was nearly 4,000, less than one-quarter of that higher estimate.

The team determined the substantially smaller population count by using a machine learning model trained to identify rock gardens from high-resolution shortwave infrared and near-infrared data gathered by satellite.

“What we use in this paper is called shortwave infrared imagery,” Davis said, “and it’s really good at picking up on very subtle differences in moisture content and mineralogical changes in the soil.”

The new study determined ancient Rapa Nui's population by using a machine learning model trained to identify rock gardens from high-resolution shortwave infrared and near-infrared data gathered by satellite. - Courtesy Dylan Davis

The new study determined ancient Rapa Nui's population by using a machine learning model trained to identify rock gardens from high-resolution shortwave infrared and near-infrared data gathered by satellite. - Courtesy Dylan Davis

The researchers found that rock gardens, identifiable by patterns of vegetation and soil composition, covered about three-quarters of 1 square kilometer (0.4 square mile), and rock garden cultivation alone would only have supported around 2,000 people. When combined with estimates for the availability of fishing and other marine foods, the team believes Rapa Nui could have sustained a population of 3,901 people.

Davis said the team manually verified the model, which was 83% accurate. “This is good at the moment because of the data that’s available,” he said. “If there were any obvious errors, we removed them.”

Another limitation of the approach was the possibility that rock gardening features could have been destroyed over the centuries.

What really happened on Easter Island?

Thegn Ladefoged, a professor of archaeology at New Zealand’s University of Auckland who conducted the similar study published in February 2013 that resulted in the higher population estimate, said that the latest research provides “new insights into the carrying capacity of ancient Rapa Nui and possible population estimates.”

“Their analysis of newly acquired high-resolution shortwave infrared remotely sensed data found that the total area of rock gardening was 5 to 20 times lower than previous estimates,” Ladefoged said via email. “This finding was the result of integrating new remotely sensed data, data not available when we did our original study.” He wasn’t involved in the new research.

“I concur with the authors that a pre-colonial ecocide on Rapa Nui did not occur, and the population did not experience a collapse.”

However, Christopher Stevenson, a professor of anthropology at the School of World Studies at Virginia Commonwealth University, said that the machine learning methodology was “far from clear and not well evaluated.”

“The authors make an effort to say how their approach is so much better than past work without actually demonstrating how they handle the complexities of the data set,” Stevenson said via email.

Researchers focused on farming practices to understand how large a population the island could have supported. Rapa Nui could have sustained less than 4,000 people, with rock garden cultivation alone accounting for about half of the population, the study found. - Carl Lipo

Researchers focused on farming practices to understand how large a population the island could have supported. Rapa Nui could have sustained less than 4,000 people, with rock garden cultivation alone accounting for about half of the population, the study found. - Carl Lipo

The view that the island was once home to a population of several thousand people stems from the assumption it would have taken large numbers to build and move the 800-plus huge stone statues or moai erected across the island.

However, a January 2022 study suggested that it might not have required as much muscle power as previously thought. What’s more, while it was initially thought the islanders cut down trees in part to move the carved heads, a January 2017 study suggested that the native palm vegetation was burned to make the soil more fertile.

“Ultimately, we don’t have evidence that thousands and thousands of people lived there. In fact, when Europeans first make contact with Rapa Nui people, they only report seeing maybe 3,000 or 4,000 people and report that people were in good spirits,” Davis said.

“And the real population collapse happens after that, which is probably due to exposure to disease.”

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