The forest that vanished
Pollen analysis from lakebed sediments shows that when Polynesian settlers arrived on Rapa Nui around 1200 CE, the island was covered in a dense forest. The dominant tree was a massive, now-extinct palm, Paschalococos disperta, a relative of the Chilean Wine Palm (Jubaea chilensis). Before its extinction, this palm was part of the ecosystem, with paleoecological models estimating a population of between 15 and 19.7 million individual trees. The palm was slow-growing, taking up to 70 years to mature and produce fruit, but could live for as long as 500 years.
For centuries, the palm’s disappearance was simple: the Rapa Nui people recklessly cut down every tree to build canoes and, most famously, to create rollers for transporting their giant moai statues. This "ecocide" narrative suggests islanders engineered their own societal collapse through over-exploitation of resources. By 1650, the palm had vanished from the pollen record, leaving the island a barren grassland by the time of European contact in 1722. This version of events overlooks a small co-conspirator.
Rat-icide
The Polynesian settlers did not arrive alone. Stowed away in their canoes was the Polynesian rat, Rattus exulans. With no natural predators on the island, the rats found a paradise. Their favorite food was the abundant and nutritious nuts of the Paschalococos palm. Archaeological evidence supports this; preserved palm nuts found in caves frequently show the gnaw marks of rats.
The reproductive capacity of these rats was immense. Ecological models show that a single breeding pair of rats introduced to the island could have exploded into a population of over 11 million within just 47 years. This horde of rodents would have devoured the vast majority of palm nuts, preventing the slow-growing trees from regenerating. The Rapa Nui people certainly cut down trees for agriculture and other uses, the rats' relentless seed predation made forest recovery impossible. Deforestation was likely a combination of human activity and an invasive species plague. Other factors, such as a prolonged drought that began around 1550, may have also contributed to the ecological shift.