Hundreds of monumental stone heads adorn the coastline of the remote Pacific island of Rapa Nui, or Easter Island. It's a fascinating place, settled about 900 years ago by a small group of Polynesian sailors. It has been the subject of fierce debate about how complex societies can sometimes fail ruinously.
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 quite the opposite: Easter Island is a story of 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 using remote sensing data and machine learning to map evidence of island agriculture offers a new clue that could 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 are all these bits of evidence that have been collected over the last 15 to 20 years that throw a spanner in the works" in the collapse story, said Dr. Dylan Davis, lead author of the study published Friday in the journal Science. Progress.
"And this is what this article builds on."
Residents of Easter Island used rock gardens
Rapa Nui, now part of Chile, is located more than 2,000 kilometers from the nearest inhabited island of Pitcairn and about 3,700 kilometers from the South American mainland, according to the study.
Davis, a postdoctoral researcher at the Columbia Climate School's Lamont-Doherty Earth Observatory, and his team focused on agricultural practices to understand how large a population the island could have supported. At 163 square kilometers (63 sq mi), it is slightly smaller than Washington, DC.
The rock gardens covered as much as 21.1 square kilometers (8.1 sq mi) and could have housed up to 17,000 people, previous research suggested. That February 2013 finding reinforced the idea that people had exhausted Rapa Nui's limited resources.
Archaeologists have identified the remains of rock gardens where islanders would have grown sweet potatoes and other crops. The scattered and pulverized rocks make the land more productive by adding and trapping nutrients and moisture and protecting young plants from wind - an age-old agricultural technique also known as rock mulching.
In the new study, however, Davis and his colleagues found that the maximum number of people on Rapa Nui was almost 4,000, less than a quarter of that higher estimate.
The team determined the significantly smaller number of residents using a machine learning model trained to identify rock gardens based on high-resolution shortwave infrared and near-infrared data collected by satellite.
"What we're using in this paper is called shortwave infrared imaging," Davis said, "and it's very good at picking up very subtle differences in moisture content and mineralogical changes in the soil."
The researchers found that rock gardens, identifiable by patterns of vegetation and soil composition, covered about three-quarters of 1 square kilometer (0.4 sq mi), and that rock garden cultivation alone would have supported only about 2,000 people. Combined with estimates of the availability of fish and other seafood, 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 right now because of the data that is available," he said. "If there were any obvious errors, we removed them."
Another limitation of the approach was the possibility that rock garden features could have been destroyed over the centuries.
What really happened on Easter Island?
Thegn Ladefoged, professor of archeology at New Zealand's University of Auckland, who conducted a similar study in February 2013 that resulted in the higher population estimate, said the latest research "provides new insights into the carrying capacity of ancient Rapa Nui and the possible population estimate ." estimates."
"Their analysis of newly acquired high-resolution shortwave infrared data showed that the total area of rock gardening was five to 20 times smaller than previous estimates," Ladefoged said via email. "This finding was the result of integrating new remotely sensed data, data that was not available when we did our original study." He was not involved in the new investigation.
"I agree with the authors that there was no pre-colonial ecocide on Rapa Nui and that the population did not collapse."
However, Christopher Stevenson, professor of anthropology at Virginia Commonwealth University's School of World Studies, said the machine learning methodology was "far from clear and not well evaluated."
"The authors go out of their way to say that their approach is so much better than previous work, without actually demonstrating how they handle the complexity of the dataset," Stevenson said via email.
The view that the island was once home to a population of several thousand people stems from the assumption that large numbers of people would have been needed to build and move the more than 800 enormous stone statues or moai that dot the island built.
However, a January 2022 study suggested it may not require as much muscle power as previously thought. Although it was initially thought that the islanders had partially cut down trees to relocate the carved heads, a January 2017 study additionally suggested that native palm vegetation was burned to make the soil more fertile.
"Ultimately, we have no evidence that thousands and thousands of people lived there. When Europeans first make contact with Rapa Nui people, they only report seeing maybe 3,000 or 4,000 people and report that the 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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