When Polynesians spread across the Pacific, some flourished in what became island paradises. Others deforested the islands they colonized and, as on Easter Island, sank into warfare and cannibalism. Archaeologists have long wondered what went wrong. Now a unique, Pacific-wide analysis teases out the environmental factors that stacked the deck against some colonists.
The work, by archaeologist Barry Rolett of the University of Hawaii, Honolulu, and geographer Jared Diamond of the University of California, Los Angeles, began after Diamond asked Rolett why the Marquesas, unlike Easter Island, had kept its forests. Rolett studies Polynesians' environmental impact, with a focus on the Marquesas, 1200 kilometers east of Tahiti. But Diamond's question inspired him to cast a wider net.
To answer it, the pair examined 69 islands across the Pacific. Rolett combed through the journals of early explorers such as James Cook to estimate how well forested the islands were at the time of European contact. For each island, they also quantified a range of environmental variables that might make forests fragile or resilient. After crunching the numbers, the two discovered what mattered most: Warmer, wetter islands were more likely to have resisted deforestation, as were big islands, islands whose high, rugged terrain made it hard to grow crops, and those dusted regularly with soil-enriching volcanic ash.The model, described this week in Nature, suggests that the troubles of Easter Island's colonists weren't entirely their fault. "They were in one of the most challenging situations, on one of the most environmentally fragile islands," Rolett says. Easter Island's isolation was also a factor, they concluded, by making it less likely that domesticated plants could have survived the voyage, forcing the colonists to rely on less sustainable slash-and-burn agriculture. In contrast, the equally small and dry Marquesas had retained their forests better than the model predicted because the Polynesians there cultivated breadfruit trees, Rolett says. With forests providing the main source of food, Marquesas islanders had no need to turn to slash-and-burn agriculture to sustain a growing population."It's a nice step forward," says archaeologist Patrick Kirch of University of California, Berkeley. "They are hitting on some key factors." Archaeologists had studied many of those factors for a few islands, says ecologist Peter Vitousek of Stanford University. But none had taken such a broad, quantitative look. "It's an original and valuable approach," he says.