(CNN) 鈥� Hundreds of monumental stone heads dot the coastline of the remote Pacific island of Rapa Nui, or Easter Island. Settled by聽a small group of Polynesian seafarers about 900 years ago, it鈥檚 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, 鈥淐ollapse,鈥� used Easter Island as a cautionary example of how the exploitation of limited resources can result in catastrophic population decline, ecological devastation聽and the destruction of a culture through infighting.
Other researchers suggest the exact opposite 鈥� 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鈥檚 original civilization. The new finding suggests that the island was not densely populated, making ecological collapse a less likely scenario.
鈥淭here鈥檚 all these bits of evidence that have been collected over the past 15 or 20 years that kind of start to throw a wrench鈥� in the collapse story,聽said Dr.聽Dylan Davis, lead author of the study that published Friday in the journal .
鈥淎nd this is what this paper builds on.鈥�
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鈥檚 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鈥檚 a little smaller than Washington, DC.
The rock gardens had covered up to 21.1 square kilometers (8.1 square miles) and could have sustained up to 17,000 people, . That February 2013 finding bolstered the idea that people drained Rapa Nui鈥檚 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 鈥� 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聽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.
鈥淲hat we use in this paper is called shortwave infrared imagery,鈥� Davis said, 鈥渁nd it鈥檚 really good at picking up on very subtle differences in moisture content and mineralogical changes in the soil.鈥�
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. 鈥淭his is good at the moment because of the data that鈥檚 available,鈥� he said. 鈥淚f there were any obvious errors, we removed them.鈥�
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鈥檚 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 鈥渘ew insights into the carrying capacity of ancient Rapa Nui and possible population estimates.鈥�
鈥淭heir 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,鈥� Ladefoged said via email. 鈥淭his finding was the result of integrating new remotely sensed data, data not available when we did our original study.鈥� He wasn鈥檛 involved in the new research.
鈥淚 concur with the authors that a pre-colonial ecocide on Rapa Nui did not occur, and the population did not experience a collapse.鈥�
However, Christopher Stevenson, a professor of anthropology at the School of World Studies at Virginia Commonwealth University, said that the machine learning methodology was 鈥渇ar from clear and not well evaluated.鈥�
鈥淭he 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,鈥� Stevenson said via email.
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 聽suggested that it might not have required as much muscle power as previously thought. What鈥檚 more, while it was initially thought the islanders cut down trees in part to move the carved heads, a suggested that the native palm vegetation was burned to make the soil more fertile.
鈥淯ltimately, we don鈥檛 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 and report that people were in good spirits,鈥� Davis said.
鈥淎nd the real population collapse happens after that, which is probably due to exposure to disease.鈥�
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