Rapa Nui palm seed endocarp chewed by a rat. Credit: Sebastian Englert Museum in Hunt and Lipo 2025
Dr. Terry Hunt from the University of Arizona and Dr. Carl Lipo from the University of Birmingham have published a study in the Journal of Archaeological Sciences, reassessing the role of Polynesian rats (Rattus exulans) in the deforestation of Rapa Nui.
They found that the role of rats has typically been massively underestimated. In fact, a single pair of rats introduced to the island may have resulted in a rat population of up to 11.2 million individuals in just 47 years, destroying 95% of the palm seeds in the process.
Rapa Nui
Rapa Nui, also known as Easter Island, is among the most contested case studies of environmental archaeology, with the role of the Polynesian rat in its deforestation being central to this debate.
It has typically been estimated that Rapa Nui once supported between 15 and 19.7 million Rapa Nui palm trees (Paschalococos disperta) before the exploitation by humans led to the near complete deforestation of the island between 1200 and 1650 CE, leaving only a few isolated trees standing by the time of European arrival in 1722 CE.
“Review of historical observations indicates that grasses and small shrubs dominated the vegetation Europeans encountered in the 18th century, a stark contrast to the palm-dominated landscape that existed at the time of colonization,” explained Dr. Lipo.
Challenging the traditional narrative
However, while the role of rats has been acknowledged, it has typically been underestimated. However, other studies and ecological examples show that rats can be incredibly destructive to an environment once introduced.
For example, once introduced to the Hawaiian island of O’ahu, researchers observed a population collapse of the Pritchardia palm between 1100 and 1200 CE, despite humans only settling on the island around 1300 CE.
Similarly, the islands of Nihoa and Huelo, where humans had settled, farmed, and engaged in fire-related activities for centuries, had never had any rats introduced to them. Here, the palm populations have thrived.
Dr. Lipo explained why the Rapa Nui palms were especially susceptible to rat predation: “Palm nuts were highly vulnerable for several reasons. Jubaea chilensis, the palm species on Rapa Nui, produces large seeds (similar to small coconuts) with thick shells but extremely high nutritional content. These seeds are rich in oils and carbohydrates. For rats, each palm nut represents a substantial energy reward worth the effort to gnaw through the shell.
“Critically, palms produce relatively few seeds per tree. When rats consume or cache most of the seeds, regeneration fails. The reproductive strategy of producing few, large, energy-rich seeds made palms particularly vulnerable to a seed predator achieving high population densities.”
When also taking into account the rat’s reproductive capability, the problem is only compounded. An ecological model developed by Dr. Hunt and Dr. Lipo showed that a single breeding rat pair introduced to the island with approximately 15 million palm trees would have had the capacity to produce a population of around 11.2 million individuals in only 47 years, effectively destroying 95% of the palm seeds.
This, in combination with human exploitation of the trees, would have exaggerated the effect, leading to the complete deforestation of the island by 1600 CE.
“Rats reached every corner of the island within decades of human arrival, chewing through palm seeds and stopping the next generation of trees before they could even sprout. People cleared land for gardens near the coast, but the rats preceded them in number and spread over the island,” noted Dr. Hunt.
Additionally, analysis of rat remains recovered from Anakena excavations between 1986 and 2005 revealed that the rat population experienced an ecological “boom-bust” pattern whereby rat populations increased dramatically upon arrival but then crashed by 93% as their food resources became depleted.
This is in direct contrast to the argument made that rat populations decreased as humans began to exploit them more often in the absence of alternative resources.
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A story of adaptation, not collapse
Dr. Hunt emphasized the broader implications of their findings: “Our study dramatically revises ‘ecocide’ into a story of an invasive species on an isolated island. But the impacts were accelerated by the actions of people using fire to clear land. People transformed a natural environment into an agricultural environment.
“This led to their success, not their collapse. Biological invasions can be independent drivers of ecological change with boom–bust dynamics.
“For island archaeology, this means we must critically evaluate the relative role of invasives plus people together, not people alone, and expect outcomes that differ from either factor in isolation. A dramatic conclusion of our ecological modeling: rats alone could have eventually led to the extinction of the palms without the actions of people.
“This is a testament to the profound impacts invasive species can have, and the problem continues today, not just on islands, but globally.”
Written for you by our author Sandee Oster, edited by Sadie Harley, and fact-checked and reviewed by Robert Egan—this article is the result of careful human work. We rely on readers like you to keep independent science journalism alive.
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More information:
Terry L. Hunt et al, Reassessing the role of Polynesian rats (Rattus exulans) in Rapa Nui (Easter Island) deforestation: Faunal evidence and ecological modeling, Journal of Archaeological Science (2025). DOI: 10.1016/j.jas.2025.106388
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Rapa Nui’s catastrophic deforestation: Invasive rats, not just humans, may be to blame (2025, November 6)
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