Woolly mammoths on Alaska's St Paul Island were the architects of their own extinction after climate change and sea level rise made their habitat unsustainable. Researchers discovered the last mammoth died out on the island 5,600 years ago, having eroded the land in search of water so much that vegetation could no longer grow.
St Paul Island is 400 miles north of the Aleutian Island. It once formed part of the Bering Land Bridge, which connected Alaska and Siberia for thousands of years before the last ice age came to an end, submerging most of it beneath the sea. Because humans did not arrive there until the 1700s, the island provides an unsullied view of megafauna extinction resulting from natural factors.
In a study published in PNAS, researchers have worked out exactly when the mammoths went extinct there and what caused their demise. The remains of five mammoths discovered on the island have previously been dated back 6,480 years, but these were not necessarily the last to live and die there.
To get a more accurate picture, researchers led by Russell Graham from Penn State University looked at sediment DNA from fungus found in lake cores to work out how long mammoths were there, feeding on the land. Their findings showed they had disappeared from St Paul Island 5,600 years – give or take 100 years.
"It's amazing that everything turned out so precisely," said Graham. "We see a reduction in the three species of fungus, all of which are associated with the dung of large animals."
Researchers also used environmental information to work out what caused their extinction. They showed the island shrank rapidly as the ice age came to an end until 6,000 years ago, when it was just 42 square miles. This shrinking meant the mammoths were bunched together in a small area that had diminished drinking water.
In order to access fresh water, the mammoths – like elephants – dug holes. Along with feeding, these holes stopped vegetation growing back in the area. Eventually, they had eroded the land so much it could not sustain them anymore.
The team says the findings should serve as a warning about the effects of climate change on island populations of megafauna. "Freshwater availability may be an underappreciated driver of island extinction. This study reinforces 21st-century concerns about the vulnerability of island populations, including humans, to future warming, freshwater availability, and sea level rise," they concluded.