Rats played major role in Easter Island’s deforestation, study reveals

The common myth of Easter Island goes something like this: To roll their massive moai statuary into place, the islanders deliberately cut down all the trees, causing an ecological disaster that scythed through the human population.

Binghamton University, State University of New York archaeologist Carl Lipo’s research has played a major role in debunking this story and outlining what actually happened on Rapa Nui, as the island is known in its native Polynesian language. Among his discoveries: The moai, which represented ancestors, served to bring communities together and weren’t moved by logs but upright by ropes. The human population didn’t suffer any sort of crash until the arrival of Europeans in the 18th century.

And while the Rapa Nui people did play a role in the island’s deforestation, the greater role may belong to a virtual tidal wave of rats.

“The human impact on these environments is very complex,” said Lipo, a professor of anthropology. “Sometimes there are unintended consequences, like the rats. In this case, the modification of the environment wasn’t a human disaster.”

University of Arizona anthropologist Terry L. Hunt and Lipo explore the complex situation in “Reassessing the role of Polynesian rats (Rattus exulans) in Rapa Nui (Easter Island) deforestation: Faunal evidence and ecological modeling,” which recently appeared in the Journal of Archaeological Science.

{!--[aside]1[/aside]--}Prior to human settlement, Rapa Nui was dominated by large palm trees of a variety now extinct; however, they were related to Jubaea chilensis, the Chilean wine palm. These massive trees can live up to 500 years; they’re also slow-growing, taking around 70 years to reach maturity and begin to fruit.

Few of the palms remained at the time of European contact in 1722. By the time the Europeans began to take an interest in the island’s ecology, they were gone.

“The Europeans basically describe a treeless island, but they also describe palms and palm leaves. It’s hard to know whether they’re using the term to describe some other tree,” said Lipo, adding that coconut palms weren’t introduced until the 1950s.

The rats

When setting out for a new island, the Polynesian people brought their subsistence package with them: taro, sweet potatoes, bananas, yams, dogs, chickens and pigs. Tagging along was, inevitably, the Polynesian rat. Unlike the Norway rats brought to the island after European contact, this small, arboreal species prefers to live in the tree canopy.

For researchers, the rats are a goldmine of information.

“Because of their genetics and the ‘founder’s effect,’ they have unique haplotypes. We can trace the colonization of people and, to some degree, the number of colonizations by how variable the rats are as they move across the Pacific,” Lipo explained.

Exactly how they ended up in Polynesian’ outrigger canoes is a matter of debate: Were they stowaways or brought deliberately as a backup food source?

Ethnographic evidence suggests the latter. After the arrival of Europeans, a naturalist collecting specimens for the British Museum saw a man walking down a path, carrying rats; he told the collector that they were for lunch, Lipo recounted. Rat bones are also found in midden deposits — essentially, ancient trash heaps — throughout the Pacific islands.

When the Polynesians arrived on Rapa Nui around 1200 CE, the rats found a virtual Eden devoid of predators and full of their favorite food. Capable of having multiple litters a year, the population exploded into the millions in a few years’ time.

“Palm nuts are rat candy,” Lipo said. “The rats went bananas.”

Rapa Nui’s palm trees co-evolved with birds and never developed the boom-and-bust productivity cycle that would ensure some nuts survived exploitation by rodents. The rats devoured the palm nuts, preventing the next generation of trees from taking root.

Meanwhile, the humans cut down swathes of trees to establish their sweet potato fields. The combination of the two led to the deforestation that marks the island today.

The humans

In addition to plants and animals, the Polynesian survival kit also included practices such as slash-and-burn agriculture to boost soil fertility. Older volcanic islands such as Rapa Nui can suffer from poor soils, the nutrients leached out by the rain.

Clearing and burning sections of forested land can temporarily infuse the soil with nutrients. Once the nutrients wear out, the farmers move to another site, leaving the land to recover and the trees to regrow.

“We see this in New Guinea and other places across the Pacific. But on Rapa Nui, the trees grow so slowly, and they don’t grow back due to rat predation on the palm nuts,” Lipo said.

Ultimately, the islanders resorted to a new style of farming, using stone mulch to enrich their crops.

While the loss of the palm forest was an undeniable environmental change, it wasn’t a human disaster. The islanders didn’t need the palm trees for survival; instead, their food sources depended on cleared land. Additionally, palms aren’t hardwood trees; they’re related to grasses, and cannot provide timber for canoes, houses or firewood.

“It’s a sad loss of a palm forest, but it wasn’t a disaster for the people,” Lipo said. “It wasn’t a necessary part of their survival.”

While some of the palm trees may have survived into the European occupation, the introduction of sheep ranching in the 19th century may have sounded the final knell of extinction; any remaining seedlings would have been eaten by the sheep.

Ironically, the Polynesian rats met the same fate as the palms; on most islands, they were driven to extinction, out-competed by the Norway rat, or consumed by introduced predators such as hawks. While the specific species has changed, islanders still tell stories of rodent boom and bust cycles — years when the population explodes, followed by a massive die-off.

The story of Rapa Nui is one of unintended consequences — but also of adaptation and survival on one of the world’s most remote inhabited islands, where the closest neighbors are 1,200 miles away.

“We have to be more nuanced in our understanding of environmental change,” Lipo said. “We are part of the natural world; we reshape it often for our benefit, but that doesn’t necessarily mean that we create an unsustainable world for ourselves.”

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