Easter Island (Rapa Nui) is often used as an example of how overexploitation of limited resources resulted in a catastrophic population collapse. A vital component of this narrative is that the rapid rise and fall of pre-contact Rapanui population growth rates was driven by the construction and overexploitation of once extensive rock gardens. However, the extent of island-wide rock gardening, while key for understanding food systems and demography, must be better understood. New research by Binghamton University and Columbia University archaeologists shows that the extent of this agricultural infrastructure is substantially less than previously claimed and likely could not have supported the large population sizes that have been assumed.
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Map of Easter Island and its location in the southeastern Pacific. Image credit: Davis et al., doi: 10.1126/sciadv.ado1459.
In their research, Binghamton University’s Professor Carl Lipo and colleagues used modern technology to more accurately gauge the number of rock gardens on Easter Island an its pre-contact food production.
“The volcanic island was formed from eruptions a million years ago, giving the rain ample time to wash away the potassium, phosphorus and nitrogen plants need to grow,” Professor Lipo said.
“The salt-laden ocean spray further hurts soil fertility.”
“The soils on Rapa Nui were never particularly productive. When people reached the island, they had to deal with those constraints.”
“Their first method was slash-and-burn agriculture, which involved cutting down the island’s trees.”
“This temporarily returned nutrients to the soil, but once the trees ran out, the islanders resorted to other methods: composting plant waste and rock mulch.”
“The fertility benefits of composting aren’t enough to support a culture’s food supply. Rock mulch did — but it was a highly labor-intensive process.”
“Islanders broke off parts of the exposed bedrock and then turned these chunks of stones into the soil, which both restored nutrients and protected it from further weathering.”
Rock mulch has been traditionally used in other places, such as among the Maori people in New Zealand, Indigenous peoples in the American Southwest and in the Netherlands.
“We do it ourselves with non-organic fertilizer; essentially, we use machines to crush rock into tiny pieces, which is effective because it exposes a lot of surface area,” Professor Lipo said.
“The people in Rapa Nui are doing it by hand, literally breaking up rocks and sticking them in dirt.”
“While the gardens could grow dry-land taro and yams, the primary crop was sweet potatoes of dozens of different varieties. Not every rock pile is an ancient garden, however,” said Binghamton University’s Dr. Robert DiNapoli.
But how many gardens were on Easter Island? When they first encountered the island, Europeans reported that 10% of the land was covered by gardens. Researchers have previously relied on satellite imagery to map rock gardens, but that resulted in misidentifications — including, for example, roads.
Rapa Nui is one of Earth’s most remote human-populated locations. It is over 2,000 km from the nearest inhabited island (Pitcairn) and over 3,700 km from the South American mainland. The island is small (164 km2) and has relatively limited soil productivity and freshwater sources. Image credit: Bjørn Christian Tørrissen / CC BY-SA 3.0.
The study authors used shortwave infrared (SWIR) satellite imagery and machine learning to generate a more accurate estimate: around 180 acres were covered by mulching stones, much less than previously believed.
“Primarily used for geological mapping, SWIR imagery can differentiate between mineral compositions and moisture content,” said Dr. Dylan Davis, a postdoctoral researcher at Columbia University.
“The distinct mineralogical characteristics and moisture patches allow the rock gardens to stand out better from their surroundings.”
Using the updated estimate of the number of gardens, the researchers calculated that approximately 3,000 people lived on Easter Island at the time of European contact.
The earliest European accounts indicate a population of 3,000 to 4,000 people, which tracks with the artifacts discovered on the island.
“What we’re actually seeing here is that the island couldn’t sustain that many people to begin with based on ecological constraints,” Dr. Davis said.
“People actually modified their landscapes to increase the amount of what they could intensively cultivate and that number was still very small.”
“This isn’t an example of ecological catastrophe but of how people survived despite really limited natural resources in a fairly sustainable way for a long time.”
“The misconceptions about the island’s population size come from its large and striking moai statues, and the assumption that it would take large groups of people to erect such statues,” Professor Lipo said.
“Ecologists also tend to use Easter Island as a model for how population size can lead to ecological catastrophe.”
“We can’t use Easter Island as an example that’s convenient for stories.”
“We need to understand the island in its own context because what it’s really telling us is something very different than what people believe.”
The findings appear today in the journal Science Advances.
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Dylan S. Davis et al. 2024. Island-wide characterization of agricultural production challenges the demographic collapse hypothesis for Rapa Nui (Easter Island). Sci. Adv 10 (25): eado1459; doi: 10.1126/sciadv.ado1459
