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Climate Change, Then Humans, Drove Mammoths Extinct

Kimberly Johnson
for National Geographic News
April 1, 2008
 
Ancient climate change cornered the woolly mammoth into a shrinking habitat, but humans delivered the final blow by hunting the species into extinction, a new study suggests.

Climate change and hunting have long been blamed for forcing the mammoth into decline at the end of the Pleistocene era about 10,000 years ago. The last mammoth died out 4,000 years ago, experts estimate.

But this study marks the first time that the massive, shaggy-haired animal's demise has been explained using combined population and climate change modeling, researchers say.

Previously, separating the individual impact of each factor on the mammoths had been difficult.

For instance, warming temperatures that emerged during and immediately after the late Pleistocene opened up new territories for human migration—ushering them into the woolly mammoth's backyard.

(See an atlas of the human journey.)

Robust Maps

David Nogués-Bravo, of the National Museum of Natural Sciences in Madrid, Spain, led the new research, which was published recently in the journal PLoS Biology.

"Only in the last [few] years we have [had] robust paleoclimatic simulations" that chart climates for historical periods, Nogués-Bravo said.

"These paleoclimatic maps allow us to model the area covered by the climatic niche of the woolly mammoth."

That area is then considered a variable when modeling human-hunting intensity, he explained.

The researchers charted the climate and distribution time line of the mammoth from 126,000 to 6,000 years ago by modeling ocean currents, rainfall, and other factors.

It's the same type of modeling used today in predicting global warming, Nogués-Bravo said.

Last Blow

Modern humans likely entered the mammoth's core habitat areas around 40,000 years ago, he said.

Before then, however, the animals' cold, dry tundra habitat in Eurasia and North America collapsed when wetter and warmer weather set in about 42,000 years ago, he said.

At the same time, forests were expanding north, eventually outcompeting the shrubby plants that mammoths depended on for survival.

"As an herbivore feeding on coarse tundra vegetation, the woolly mammoth had huge specialized teeth and a lower jaw that swung back and forward to shred plants," he said.

(Related: "Mammoth Extinction Caused by Trees, Study Suggests" [May 10, 2006].)

These changes slashed the mammoth's habitat by almost 90 percent, from about a three-million-square-mile area (about an eight-million-square-kilometer area) about 42,000 years ago to a range of about 308,000 square miles (800,000 square kilometers) about 6,000 years ago.

Nogués-Bravo stopped short of saying that the mammoths would have survived had humans not happened along. But he did point out the species adapted to an earlier collapse of favorable environmental conditions before humans emerged.

A Good Lesson

Larry Agenbroad, site director for the Mammoth Site research facility in Hot Springs, South Dakota, was not involved in the new study.

He applauded any new attempt to use modeling as a means to root out the causes of the mammoth's extinction.

But Agenbroad cautioned that no study should be considered the only answer to the extinction riddle.

"There are too many unknowns, which include the estimate of mammoth population densities through time, the even more elusive estimates of mammoth hunting, and human populations in the northern reaches of Siberia at the time intervals proposed," Agenbroad said.

Data for the earliest range and population of mammoths are thin for the northern Eurasia region, as is the documented presence of hunters in that region, he added.

The new study's findings are relevant to understanding the current trend of human-induced climate change, according to Nogués-Bravo.

"Our study shows that species could become extinct because the combined effect or climate change and direct human impacts," he said.

"It's a good [lesson] to know why we haven't [got] woolly mammoths in our backyards."
 

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