Bringing together climate and ancient DNA to look at a micro-instance of extinction

Ed Yong describes the results of a cool new study of mammoth extinction on Saint Paul Island, in the Bering Strait between Alaska and Russia: “The Lonely, Thirsty, Final Days of the Doomed Alaskan Mammoths”. The island was once a tiny part of the large Beringia landmass, and when sea levels rose 12,000 years ago, the mammoth population on the island was cut off from the mainland. Mammoths survived there until just under 6000 years ago, long after they became extinct on the mainland. Unlike some areas where the delayed arrival of humans could plausibly explain the later disappearance, in the case of St. Paul there seem to have been no humans until much later.

An interdisciplinary team of scientists looked at a range of climate indicators along with traces of mammoth presence. That included the fungal spores that are tracers of mammoth dung, a method that has already shown the decline of megafauna in North America (the program First Peoples relates Jacqueline Gill’s research on this).

Beth Shapiro, from the University of California, Santa Cruz, searched the samples for traces of mammoth DNA. Meanwhile, Yue Wang and John Williams from the University of Wisconsin, Madison, looked for spores from three fungi that grow in the dung of plant-eating animals. Large extinct beasts like mammoths produced a lot of dung, so scientists can track their disappearance by looking for sudden drops in the levels of these fungal spores.
To the team’s delight, the five lines of evidence—the mammoth remains, the DNA, and the three types of spores—all gave the same answer. They showed that mammoths survived on the island until 5,600 years ago, before finally going extinct. “We were really surprised that it all lined up well,” says Graham. “The nice thing about the cores is that they told us not just when the mammoths went extinct, but all this other information about climate. And that told us what caused the extinction.”

They arrive at the conclusion that salt water wedges coming up into the groundwater as sea levels rose probably drew down the fresh water supply on the island. Mammoths need a lot of water every day, and contributed to their own population decline by degrading the surface ponds, the island’s only water source.

What I like about the study is that the multiple sources of evidence all have a time depth, and the data cover a relatively large proportion of this small island. It’s a great case study of extinction. The reasons for apparent success in this study signal the great difficulty of drawing definitive conclusions for extinction across the vast area of continents, with their diversity of microclimates and habitats.

However, I hesitate on one point. I would not so quickly assume there was never a short or intermittent presence of humans on the island, and that humans may have been involved in the mammoth extinction.