Climate Change Caused Extinction of Big Ice Age Mammals, Scientist Says

Hillary Mayell
for National Geographic News
November 12, 2001
A renewed assault is being made on the popular idea that the mass extinction of large mammals in North America around 10,500 years ago was the result of human hunting.

The overkill hypothesis was first put forward more than a century ago and has been widely accepted for the past 30 years. But it does not square with the known facts and has become more a faith-based credo than good science, said Donald Grayson, an archaeologist at the University of Washington.

Understanding what caused the extinction has implications for conservation biology.

Grayson, who specializes in vertebrate paleontology and archaeology, argues that a call by some environmentalists to return Ice Age mammals—elephants, camels, llamas, and other large herbivores—to the southwestern United States is based on bad science.

"Overkill proponents have argued that these animals would still be around if people hadn't killed them and that ecological niches still exist for them," said Grayson. "Those niches do not exist. Otherwise the herbivores would still be there."

Grayson points to climate shifts during the late Pleistocene and related changes in weather and vegetation patterns as the likely culprits in the demise of North America's megafauna.

Islands and Continents

The number of large mammals that became extinct in North America at the end of the late Pleistocene, about 10,500 years ago, is staggering. Among some 35 different kinds of animals that disappeared from the fossil record were mammoths, mastodons, camels, horses, giant ground sloths, and bears.

A leading proponent of the overkill theory, Paul S. Martin, believes the Ice Age megafauna disappeared not because they lost their food supply but because of human hunting.

The extinction of animals as a result of human colonization in island settings has been well documented and the causes widely agreed on.

New Zealand is offered as a classic example of human impacts on island animal populations. Before it was colonized, New Zealand was home to 11 species of moas, a large flightless bird species weighing from 45 to more than 400 pounds (20 to 200 kilograms). Within a few hundred years after human settlement, moas were extinct.

The wave of extinctions that followed human colonization of New Zealand occurred as a result of several factors. Humans arriving on the islands brought with them rats, dogs, and other non-indigenous animals that competed for food or preyed on native species. Humans hunted the native animals extensively and destroyed much habitat by burning down forests.

A similar pattern of events occurred on other islands around the world. Proponents of the overkill theory argue that the same process—human colonization followed by massive extinction—also occurred in North America.

But Grayson rejects that idea. "The fossil record simply doesn't stand up to the theory, and comparing continents to islands is simply inappropriate," he said.

Arguing Against Overkill

The overkill hypothesis, Grayson says, rests on five tenets: human colonization can lead to the extinction of island species; the Clovis people were the first humans to arrive in North America, around 11,000 years ago; the Clovis people hunted a wide range of large mammals; the extinction of many species of North American megafauna occurred 11,000 years ago; and therefore, Clovis hunting caused those extinctions.

Grayson disputes several of these tenets.

There is no proof, he said, that the late Pleistocene extinctions occurred in conjunction with the arrival of the Clovis people. "Of the 35 genera to have become extinct beginning around 20,000 years ago, only 15 can be shown to have survived beyond 12,000 years ago," Grayson said. "The Clovis peoples didn't arrive until shorty before 11,000 years ago. That leaves 20 [genera] unaccounted for."

There is also no evidence that the Clovis people hunted anything other than mammoths, he said. Although numerous sites where large numbers of mammoths were killed have been uncovered, no similar sites for any other large mammals have been found in North America.

And while there is no evidence of widespread human-caused environmental change similar to that seen on island settings, there is evidence that animal populations in Siberia and Western Europe, as well as North America, were affected during the same period by climate changes and glacial retreat.

Martin, who in 1967 wrote the seminal book proposing the overkill hypothesis, disagrees that climate change could have caused the extensive extinctions.

"The climate had been changing over a million-year period, with swings from cold to warm, and back again—some much more severe than the one that occurred at the end of the Pleistocene," he said. "It doesn't make sense that just one more [climate shift swing] would make all the difference in the world."

Martin holds that the "dreadful syncopation"—humans arrive, animals disappear—seen in the islands of Oceania, Australia, New Zealand, Madagascar, and other islands fits with what happened in North America.

He suggests that because humans were responsible for the demise of large animals in the desert southwest, these animals should be reintroduced.

Grayson thinks this is dangerous thinking.

"One of the reasons people have glommed on to the overkill hypothesis is 'green' politics," said Grayson. "It plays to the Judeo-Christian theme that human beings are all-powerful and responsible for negative impacts on the environment.

"The hypothesis made a lot of sense 30 years ago," Grayson said, "but now it can be compared to the empirical record, and it just doesn't hold up."

His work appears in the current issue of the Journal of World Prehistory and an upcoming issue of the Bulletin of the Florida Museum of Natural History.

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