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Humans and climate contributed to extinctions of large Ice Age mammals

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Author Topic: Humans and climate contributed to extinctions of large Ice Age mammals  (Read 310 times)
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« on: December 03, 2014, 01:38:52 am »

The study was led by Professor Eske Willerslev of the Centre for GeoGenetics at the University of Copenhagen and includes an international team of paleontologists, geologists, geneticists and climate modelers including Beth Shapiro, the Shaffer Associate Professor of Biology at Penn State University. The study's findings are expected to shed light on the possible fates of living species of mammals as our planet continues its current warming cycle. The paper will be posted on the journal's Advance Online Publication website on 2 November 2011 at 2:00 p.m. U.S. Eastern time.

"Our findings put a final end to the single-cause theories of these extinctions," said Willerslev. "Our data suggest care should be taken in making generalizations regarding past and present species extinctions; the relative impacts of climate change and human encroachment on species extinctions really depends on which species we're looking at."

Shapiro explained that all six of the species the team studied flourished during the Pleistocene Epoch -- the period of geological time that lasted from about 2 million to 12,000 years ago. "During this time, there were lots of climatic ups and downs -- oscillations between long, warm intervals called interglacial periods, during which the climate was similar to what we have today, followed by long, cold intervals called glacial periods, or ice ages," Shapiro said. "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. Then, after the peak of the last ice age around 20,000 years ago, their luck started to run out. The question is, what changed? Why were these mammals no longer able to find safe refugia where they could survive in a warm climate?"

To answer these questions, the team collected many different types of data to test hypotheses about how, when, and why the woolly rhinoceros, woolly mammoth, and wild horse all went extinct after the last ice age, and why the reindeer, bison, and musk ox were able to survive -- albeit in much more restricted ranges than they could inhabit during the ice ages. "One source of information we used was DNA from the animals themselves," Shapiro explained. "With genetic data, it's possible to estimate when and how much populations were able to grow and shrink as the climate changed and their habitat started to disappear." The team also collected climatic data -- temperature and precipitation patterns -- from both glacial and interglacial periods, as well as archeological data, which they used to study the extent to which early humans may have influenced the survival of these six mammal 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."

In the case of the now-extinct woolly rhinoceros, the scientists found that, in Europe, the ranges of humans and woolly rhinoceros never overlapped. "These data suggest that climate change, and not humans, was the main reason why this particular species went extinct in present-day Europe," Shapiro said. "Still, we expect humans might have played a role in other regions of the world where they did overlap with woolly rhinos, and so further studies will be necessary to test this hypothesis." Much clearer was the evidence that humans did influence, and not always negatively, the population sizes of the five other species -- the woolly mammoth, wild horse, reindeer, bison, and musk ox.

Shapiro explained that population fluctuations for all six species continued until the end of the last ice age -- around 14,000 years ago -- when many of the species simply disappeared. "The take-home message is that 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, from finding alternative refugia -- less-than-ideal, but good-enough chunks of land on which to keep their populations at a critical mass," Shapiro said. "That 'something' was probably us -- humans." During the period when these animals were declining, the human population was beginning its boom, and was spreading out across not only the large-bodied mammals' cold-climate habitats, but also across their warm-climate refuges, changing the landscape with agriculture and other activities. Many large-bodied, cold-adapted mammals, including the horse -- which is considered extinct in the wild and now survives only as a domesticated animal -- suddenly had no alternative living spaces, and, as such, no means to maintain their populations.
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