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Easter Island: Human Negligence or Harsh Climate ?

Easter island landscapes (Rapa Nui by its Polynesian name) © D. Hazama A row of mo’ai, these fascinating Polynesian statues © D. Hazama Deposits accumulated in water retention areas on the slopes of Te Aheru contain plants with information over climate change. © M.HorliacDeposits accumulated in water retention areas on the slopes of Te Aheru contain plants with information over climate change. © M.HorliacCollection from deposits in Lake Rano Aroi. © M.HorliacOnce abundant palm trees have disappeared from Easter Island. © M. HorliacThe paper mulberry tree, originally from Southern China, was introduced by the Polynesians and their ancestors throughout all of Oceania all the way to Easter Island. © M.HorliacThe toromiro was a sacred shrub much loved by sculptors. It remained abundant after the disappearance of the forest, but sheep annihilated it starting in 1869. © M. HorliacA current Easter Island oven. © M.HorliacResearcher Michel Orliac next to one of the colossal mo’ai located at Rano Raraku quarry. © M. HorliacAn Easter Islander friend offers a survivor of the ancient flora, the ngaoho, for the comparison series at the National Museum of Natural History in Paris. © M. HorliacCollection from deposits in Lake Rano Aroi. © M. HorliacThe island has three lakes but no permanent rivers. © D. Hazama Landscapes on Easter Island settled by Polynesian populations towards the end of the first millennium. © D. Hazama Rapa Nui’s national park is classified as a UNESCO Intangible Cultural Heritage of Humanity site. © D. Hazama The famous mo’ai, prodigious sculptures carved in stone © D. Hazama The forests on Easter Island disappeared in the middle of the 17th century, a devastation shrouded in mystery. © D. Hazama
Easter Island: Human Negligence or Harsh Climate ?
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Easter Island – Rapa Nui by its Polynesian name – is famous for its thousand giant statues. The history of the people shows how they overcame brutal trials, including the disappearance of the forest. Yet did this occur due to human negligence or a harsh climate? Michel Orliac, one of the world’s leading experts over Easter Island, is conducting a scientific investigation into what remains one of the greatest mysteries in Polynesian history.

We are enchanted by Polynesia’s mild climate and stunning landscapes; however, we must not forget that it was also home to those who crossed the Pacific in one of human history’s most immense voyages. The discovery of these islands spread across this great ocean by a people who did not have metal tools seems almost as improbable as walking on the moon. Indeed, prehistoric humans ventured very late onto the dreaded waves: it wasn’t until fifty thousand years ago after millions of years of our evolution that this adventure was a success, thanks to Homo sapiens Australia’s Aborigines and Papuans from New Guinea. However, it was only three thousand years ago that the islands of the western Pacific (New Caledonia, Fiji, Tonga …) welcomed the ancestors of Melanesians and Polynesians who arrived from Southeast Asia. A slow maturation of knowledge and techniques made it possible for them to cross thousands of kilometers by sea. This exploit was the result of a combination of talents, such as building large vessels and knowing the winds, currents and stars.

The successful settlement of these lands devoid of resources was mainly due to the transport and acclimatization of the food plants from Southeast Asia then South America (breadfruit, bananas, taro, yams, sugarcane … and sweet potatoes). Over a span of two or three centuries about a thousand years ago, Polynesians visited or populated thousands of tiny islands within the 4300 mile/7000km triangle formed by Hawaii, New Zealand and Easter Island. They even went as far as South America and brought sweet potatoes back to their islands.

This disaster occurred between the second half of the seventeenth century and the passage of Jacob Roggeveen in 1722 who described the island as completely devoid of trees. Archeology shows that during this short period, Easter Islanders used grasses instead of wood to cook their food. To explain this disappearance, some believed that the islanders roamed the country to destroy the landscape with axes in hand. It is not known why not only all the trees disappeared, but also the smallest shrubs. However, this notion is highly questionable, especially for a seafaring people who depended on the abundance of wood to build boats. It is a decidedly unbelievable theory for people who are builders working with boulders weighing several tons and for a population of sculptors who moved colossal statues over tens of kilometers. The reason for this brutal and massive plant extinction is more likely due to a climate crisis characterized by an intense drought that lasted one or more decades.

An impact from the Little Ice Age?

Indeed, the seventeenth century is characterized by a period of global climate change called the “Little Ice Age.” In Europe, these climatic disturbances resulted in a significant drop in temperatures (legend says that King Louis XIV observed wine freezing in his glass at Versailles). However, in the Pacific, the effects are lesser known. On Easter Island, the effects could have caused a substantial drop in rainfall, similar to what was reported for the same period in the Galapagos Islands and New Zealand. Severe droughts have occurred everywhere in Polynesia and stories from oral traditions show they were dreaded more than cyclones.

Estimating the amount of rainfall in the past is not easy. It is necessary to find evidence that can reconstruct the history of episodes of drought and heavy precipitation. If these changes leave traces in the landscape (such as cracks from dried soil or gully runoffs), they are quickly erased through erosion. The clues we are looking for are microscopic. Quickly buried in geological layers, they escape any deterioration. Thus, the leaf surface of some plants retains a thin white film of epicuticular wax. This permanent wax persists in the sediment while other parts of the plants undergo significant degradation. The molecular character of this wax (a proportion of its various isotopes) makes it possible to evaluate the quantity of water necessary for the plants to grow.

Easter Island: extremely remote

Around this time, a small group of Polynesians settled on remote Easter Island, thousands of kilometers from Tahiti, the Marquesas and Hawaii. On this 14 mile-long island (24 km), they found dense vegetation over 35,000 years old that has been identified through pollen (male reproductive cells of plants) and traces of palm tree trunks and their roots. Pollen reveals the presence of totora reeds and at least six types of trees, including the Sophora toromiro, which grows only on Easter Island.

Unlike microscopic pollen carried by the wind, more reliable evidence provides information about the composition of the twigs from trees and shrubs that Easter Islanders burned to prepare food. The botanical identification of several tens of thousands of charred plant fragments has allowed thirteen trees and shrubs to be added to the list of those already identified through their pollen. Extracted from an area comprised of ​​just a few square meters, these pieces of coal offer a minimal idea of Easter Island’s plant diversity, which possibly included at least several dozen trees and shrubs.

With the exception of the palm tree, which is native to Chile, the forest that was revealed came from the Society Islands where the climate is much warmer and wetter than on Easter Island, so transplants struggled to adapt. On the other hand, Easter Islanders increased their gardens according to their population growth and therefore reduced primitive vegetation. Of course, like everywhere else in Polynesia, strict taboos protected the resources, especially wood needed for boat building and transporting giant statues. Be as it may, neither prohibitions nor prayers prevented the disappearance of the forest.

Therefore, a good way to estimate the evolution of rainfall abundance in the past — on Easter Island as elsewhere — is to collect soil samples deposited in conditions favorable to the conservation of environmental variation markers (such as pollen and leaf wax), analyze them layer by layer and date them using the carbon-14 method.

On a Quest for Climate Markers

Thus, the mission that took place during April and May 2017 on Easter Island was aimed at collecting sediment samples to ensure the presence and preservation of these climate markers. One of the research sites was Lake Rano Aroi near the summit of Terevaka, the largest volcano on the island. Sediments were collected from a column about 1.5 m high (5 ft.). Preliminary studies show that this thickness covers the last five centuries, which includes most of the Little Ice Age.

In addition, other samples have been collected at the bottom of large natural depressions where water tends to stagnate. The walls of random samples in these basins present a succession of horizontal layers which proves they were formed in water by sedimentation. These sites enable the conservation of climate markers.

The results of this mission will help explain why the trees on Easter Island disappeared between the 17th and 18th centuries. For the first time, these analyses will provide tangible information about possible episodes of drought or heavy rainfall. If our results confirm a significant reduction in the amount of rainfall during the Little Ice Age, then finally the scenario that holds the islanders responsible for the destruction of the forest—a real “ecocide” according to some environmentalists— will be once and for all refuted. It will once again confirm the ingenuity of the Easter Islanders who survived, and adapted to, the most dramatic changes in their environment.

Bruno Malaizé, Assistant Professor, l’Université de Bordeaux 1 and Michel Orliac, Research Fellow for CNRS (Centre National de Recherche Scientifique)