Climate Is Cited as Key to Extinctions
Thursday, May 11, 2006
New evidence from Canada and Alaska suggests that climate change, rather than human hunting, may have played the key role in a great die-off of mammoths, horses and other large North American mammals that began more than 10,000 years ago.
"It was a special time of greater warmth and moisture," said paleoecologist R. Dale Guthrie of the University of Alaska at Fairbanks. "The arid steppe receded, the short grass became more lush, and then the forest came in. The mammoth, and the horses, which did well when it was cold, didn't survive."
Guthrie, reporting in the journal Nature, dated animal remains in Alaska and the Canadian Yukon, concluding that mammoths and native American horses could not find adequate forage in the forest and went extinct. Today's horses, both domesticated and "wild," are the offspring of animals brought to the New World by Europeans beginning in the 16th century.
Guthrie's research brought new insights to the debate about the extinction of large mammal species around the time that humans crossed a land bridge from Asia to populate the Americas. Besides mammoths and horses, the extinctions also include saber-toothed cats, mastodons, giant sloths and other animals.
Some scientists advocate an "overkill" theory, in which newly arrived humans rampaged through animal populations unfamiliar with human hunting talents. Others, such as Guthrie, note that the same climatic thaw that allowed humans to cross the land bridge from Asia, also caused a radical change, to vegetation that mammoths and horses could not eat.
"I don't think we've reached consensus, but most of us think there were a combination of factors," said University of Nevada at Reno archaeologist Gary Haynes. "Most scientists believe in overkill, but if you ask archaeologists, they would say climate change," because there is very little evidence that humans were killing mammoths and horses in large numbers.
Guthrie said he concentrated on collecting radiocarbon dates for the remains of mammoths, horses, elk, bison, moose and humans, focusing on the period from 13,500 years ago to 11,500 years ago.
Guthrie said mammoths and horses, unlike elk and bison, were well equipped physically to digest large quantities of low-nutrient grass when the Alaska-Yukon region was cold, treeless and arid. When the thaw came and the rains began, however, the grass became greener and richer, attracting elk and bison.
As the rains continued, Guthrie said, evergreen forests began to replace the pasture, trees leached the nutrients from the soil and armed themselves with resins, turpentine and other "defenses" that made them unpalatable as food.
Guthrie's studies showed that the horses died off first -- about 12,500 years ago -- while the mammoths lasted a thousand years longer. Elk and bison dwindled dramatically but survived. Moose, the only bark eaters among the animals, appeared unaffected.
But "it's a complex picture," Guthrie acknowledged in a telephone interview, because humans arrived while all this was happening. "It might look like humans came in and got rid of the horses and mammoth," Guthrie said, "but why are moose prospering, and elk and bison surviving?"
Paleobiologist Anthony J. Stuart of University College London said "the idea of sudden extinction doesn't seem to apply in Alaska and the Yukon," but "personally I think there's some room for human involvement." Climate may have stressed the animals, Stuart said in a telephone interview, "but humans probably finished them off."