Easter Island (Rapa Nui) is often held up as an example where overexploitation of limited resources led to catastrophic population declines. A key element of this story is that the rapid rise and fall in pre-contact Rapa Nui population growth rates was caused by the construction and overexploitation of once-extensive rock gardens. However, the extent of rock gardens across the island, important for understanding food systems and demographic dynamics, needs to be better understood. New research by archaeologists from Binghamton University and Columbia University shows that the extent of this agricultural infrastructure was significantly smaller than previously claimed, and likely could not have supported the large populations assumed.
Map of Easter Island and its location in the southeastern Pacific Ocean. Image courtesy of Davis. others., doi: 10.1126/sciadv.ado1459.
In their study, Professor Carl Lipo of Binghamton University and his colleagues used modern techniques to more accurately estimate the number of rock gardens on Easter Island and their pre-human contact food production.
“This volcanic island was formed by an eruption a million years ago, so there has been enough time for rain to wash away the potassium, phosphorus, and nitrogen needed for plant growth,” Professor Lipo said.
“Salty sea spray further reduces soil fertility.”
“The soils on Rapa Nui were not particularly productive. When people arrived on the island, they had to deal with those constraints.”
“Their first method was slash-and-burn agriculture, which involved cutting down the trees on the island.”
“This temporarily restored nutrients to the soil, but once the trees died, islanders turned to other methods, such as composting plant waste and rock mulch.”
“The fertilization benefits from composting are not enough to support a culture's food supply. Rock mulch was sufficient, but it was a very labor-intensive process.”
“The islanders chipped away some of the exposed bedrock and mixed the stone chunks into the soil, restoring nutrients and protecting the soil from further weathering.”
Rock mulch has also been traditionally used by the Maori people of New Zealand, Native Americans in the American Southwest, and in other areas such as the Netherlands.
“We do it ourselves using non-organic fertilizers. We basically use machinery to break the rocks into smaller pieces, which is more effective because it creates a larger surface area,” Professor Lipo said.
“The Rapa Nui people are literally breaking rocks by hand and burying them in the soil.”
“The gardens also grow dryland taro and yams, but the main crops are dozens of varieties of sweet potato. But not all rockeries are ancient gardens,” said Dr Robert DiNapoli from Binghamton University.
But how many gardens were there on Easter Island? When Europeans first encountered the island, they reported that 10 percent of the island was covered in gardens. Researchers have previously used satellite imagery to map the rock gardens, but this has resulted in misidentifications of things like roads.
Rapa Nui is one of the most remote human settlements on Earth, more than 2,000 km from the nearest inhabited island (Pitcairn Island) and more than 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 come up with a more precise estimate, finding that the area covered by mulch is about 180 acres, far less than previously thought.
“SWIR imagery, primarily used for geological mapping, can distinguish mineral composition and water content,” said Dr. Dylan Davis, a postdoctoral researcher at Columbia University.
“Due to unique mineralogical characteristics and moisture patches, the rock gardens stand out from their surroundings.”
Using the latest estimates of the number of gardens, the researchers calculated that around 3,000 people lived on Easter Island at the time of European contact.
The oldest European records indicate a population of between 3,000 and 4,000, which is consistent with artifacts found on the island.
“What we're actually seeing here is that ecological constraints mean that islands just can't support that many people in the first place,” Dr Davis said.
“People actually changed the landscape to increase the amount of crops they could cultivate intensively, but the numbers were still very small.”
“This is not an example of ecological catastrophe, but rather an example of how people have managed to survive for a long time in a fairly sustainable way, despite very limited natural resources.”
“The misconception about the island's population size comes from the island's large and impressive moai statues and the assumption that it takes a large number of people to build such statues,” Prof Lipo said.
“Ecologists tend to use Easter Island as a model for how population size can lead to ecological catastrophe.”
“You can't use Easter Island as an example to suit your story.”
“We need to understand the island in its own context, because what it really tells us is quite different from what people believe.”
of Investigation result Published in today's journal Scientific advances.
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Dylan S. Davis others2024. Island-wide characterization of agricultural production casts doubt on the population collapse hypothesis for Rapa Nui (Easter Island). Advances in Science 10(25):eado1459; doi:10.1126/sciadv.ado1459
Source: www.sci.news
