Pace Jared Diamond, who “described Rapa Nui as ‘the clearest example of a society that destroyed itself by overexploiting its own resources’,” new archaeological results from Rapa Nui (Easter Island) suggest that it may have been rats, not humans, who deforested Rapa Nui—and Oahu in the Hawaiian Islands.
For thousands of years, most of Rapa Nui was covered with palm trees. Pollen records show that the Jubaea palm became established at least 35,000 years ago and survived a number of climatic and environmental changes. But by the time Roggeveen arrived in 1722, most of these large stands of forest had disappeared.
It is not a new observation that virtually all of the shells housing palm seeds found in caves or archaeological excavations of Rapa Nui show evidence of having been gnawed on by rats, but the impact of rats on the island’s fate may have been underestimated. Evidence from elsewhere in the Pacific shows that rats have often contributed to deforestation, and they may have played a major role in Rapa Nui’s environmental degradation as well.
Archaeologist J. Stephen Athens of the International Archaeological Research Institute conducted excavations on the Hawaiian Island of Oahu and found that deforestation of the Ewa Plain took place largely between 900 and 1100 A.D. but that the first evidence of human presence on this part of the island was not until about 1250 A.D. There were no climatic explanations for the disappearance of palm trees, but there was evidence that the Polynesian rat (Rattus exulans), introduced by the first human colonists, was present in the area by about 900 A.D. Athens showed that it was likely rats that deforested large areas of Oahu.
Paleobotanists have demonstrated the destructive effect of rats on native vegetation on a number of other islands as well, even those as ecologically diverse as New Zealand. In areas where rats are removed, vegetation often recovers quickly. And on Nihoa Island, in the northwest Hawaiian Islands, where there is no evidence that rats ever became established, the island’s native vegetation still survives despite prehistoric human settlement.
Whether rats were stowaways or a source of protein for the Polynesian voyagers, they would have found a welcoming environment on Rapa Nui—an almost unlimited supply of high-quality food and, other than people, no predators. In such an ideal setting, rats can reproduce so quickly that their population doubles about every six or seven weeks. A single mating pair could thus reach a population of almost 17 million in just over three years. On Kure Atoll in the Hawaiian Islands, at a latitude similar to Rapa Nui but with a smaller supply of food, the population density of the Polynesian rat was reported in the 1970s to have reached 45 per acre. On Rapa Nui, that would equate to a rat population of more than 1.9 million. At a density of 75 per acre, which would not be unreasonable given the past abundance of food, the rat population could have exceeded 3.1 million.
The evidence from elsewhere in the Pacific makes it hard to believe that rats would not have caused rapid and widespread environmental degradation. But there is still the question of how much of an effect rats had relative to the changes caused by humans, who cut down trees for a number of uses and practiced slash-and-burn agriculture. I believe that there is substantial evidence that it was rats, more so than humans, that led to deforestation.
Our work on Anakena, as well as previous archaeological studies, found thousands of rat bones. It seems that the Polynesian rat population grew quickly, then fell more recently before becoming extinct in the face of competition from rat species introduced by Europeans. Almost all of the palm seed shells discovered on the island show signs of having been gnawed on by rats, indicating that these once-ubiquitous rodents did affect the Jubaea palm’s ability to reproduce. Reason to blame rats more than people may also be revealed in the analysis of sediments obtained at Rano Kau, which, like the Hawaiian evidence, appears to show that the forest declined (leaving less forest pollen in the sediment) before the extensive use of fire by people.