Humans And Climate Both Contributed To Large Ice-Age Mammal Extinction

UNIVERSITY PARK, Pa. -- Climate change and humans were responsible for extinction of the woolly mammoth and other large ice-age mammals, according to an international group of scientists.

Their study is the first to use genetic, archeological and climatic data together to infer the population history of large-bodied ice-age mammals, said Beth Shapiro, Shaffer Associate Professor of Biology at Penn State and a member of the research team that recently published their results in Nature.

All six of the species studied -- woolly rhinoceros, woolly mammoth, wild horse, reindeer, bison, and musk ox -- flourished about 2 million to 12,000 years ago, a period with many warm interglacial periods and cold glacial periods.

"Although these cold-adapted animals certainly fared better during the colder, glacial periods, they still managed to find places where the climate was just right --refugia -- so that they could survive during the warmer, interglacial periods."

However, after the last ice age, about 20,000 years ago, their luck ran out. The team collected several different types of data, including DNA from the animals that allowed them to estimate when and how populations grew and shrank, temperature and precipitation patterns from glacial and interglacial periods, and archeological data to show how early humans may have influenced the survival of these species.

"For example, in locations where animal bones had been cooked or converted into spears, we know that humans lived there and were using them as a resource," Shapiro said. "Even where we don't find evidence that humans were using the animals, if humans and the animals lived in the same place and at the same time, humans could have had some influence on whether the animals survived or not."

For the now-extinct woolly rhinoceros, the scientists found the ranges of humans and woolly rhinoceros never overlapped in Europe. This suggests that climate change, not humans, was the main reason this particular species went extinct, according to Shapiro. Evidence that humans did influence the population sizes of the five other species studied is clearer.

"During the most recent warming event, when the last ice age faded into the warm interval we have today, something kept these animals from doing what they had always done," Shapiro said. "That 'something' was probably us -- humans."

When these animals were declining, the human population was beginning its boom, and spreading out across not only the large-bodied mammals' cold-climate habitats, but also their warm-climate refuges, changing the landscape with agriculture and other activities. Many of these mammals suddenly had no alternative living spaces, and so, no means to maintain their populations.

After the last ice age, with warmer climate, woolly rhinoceros, woolly mammoths and wild horses became extinct. Reindeer, bison and musk ox narrowly avoided extinction, according to Shapiro.

Shapiro said the team's findings could help predict the fate of populations threatened by climate change and habitat alteration today.

"It seems that our ancestors were able to change the landscape so dramatically that these animals were effectively cut off from what they needed to survive, even when the human population was small," she said. "There are many more humans today, and we have changed and are continuing to change the planet in even more important ways."

Last Updated April 18, 2012