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The following post doesn’t directly have much to do with anthropology. Indirectly, it sure does, especially to those out there that study human population expansions and the Pleistocene-Holocene transition or even anthropologists interested in prehistoric paleoenvironments and the context of how people were living and what they were doing during that time.

Anyways, this post is about a PLoS Biology paper. PLoS Biology is an open access journal that has just published a paper which investigates woolly mammoth extinction. Woolly MammothThe authors of the paper, “Climate Change, Humans, and the Extinction of the Woolly Mammoth,” ultimately conclude that both climate change and human hunting were critical factors in woolly mammoth extinction. Not a really Earth shattering conclusion, I know… but there has been some discussion whether or not climate change or human hunting was more impactful.

Right before the Holocene, the global climate was warming up. And most woolly mammoths died out during this time, the end of the Pleistocene (12,000 years ago). That’s what got many people to consider that warm temperatures may have driven the extinction of this cold-adapted species. But, the species had survived previous warming periods, and in places like St. Paul Island, Alaska and Wrangel Island they lasted up until 3,700 years ago. This is what got other people to think that the extinction of the woolly mammoth was due to the effects of human population expansion.

From the author’s summary,

“In this study, we combined paleo-climate simulations, climate envelope models (which describe the climate associated with the known distribution of a species—its envelope—and estimate that envelope’s position under different climate change scenarios), and a population model that includes an explicit treatment of woolly mammoth–human interactions to measure the extent to which climate changes, increased human pressures, or a combination of both factors might have been responsible. Results show a dramatic decline in suitable climate conditions for the mammoth between the Late Pleistocene and the Holocene, with hospitable areas in the mid-Holocene being restricted mainly to Arctic Siberia, where the latest records of woolly mammoths in continental Asia have been found. The population model results also support the view that the collapse of the climatically suitable area caused a significant drop in mammoth population size, making the animals more vulnerable to increasing hunting pressure from expanding human populations. The coincidence of the collapse of climatically suitable areas and the increase in anthropogenic impacts in the Holocene are most likely to have been the “coup de grâce,” which set the place and time for the extinction of the woolly mammoth.”

I’m really not clear about how the authors established their population models. I’ll do my best to review them, though. The authors compared and contrasted the population sizes to the climatic conditions. Curiously, their results differ as they increase the n, but they were able to calculate,

“that the most suitable geographic area available to woolly mammoths increased by 7.7 million km2 from the last interglacial, 126 ky BP, to 42 ky BP (from 0.3 to 8.1 million km2). There was a 0.5 million km2 decrease in the most suitable area between 42 ky BP and 30 ky BP periods, and then a 3.7 million km2 decrease between 30 ky BP and 21 ky BP (from 7.5 to 3.8 million km2). Finally, between 21 ky BP and 6 ky BP, there was a 2.9 million km2 decrease. By the 6 ky BP period, only 0.8 million km2 of the most suitable climatic conditions remained.”

This shows that with time, the available suitable habitats for the species reduced and did thus contributed to a reduction in woolly mammoth population sizes. Now the authors didn’t directly test the zooarchaeological record to directly correlate if human hunting or the side effects of human population expansion affected mammoth populations. But they did infer that their results of the incremental decrease in population sizes over time showed a “synergy” to the northward increase in human population densities during the Holocene.

So what about the mammoth groups in Alaska and the Arctic Ocean that persisted late after all the others died off? Their results actually show that these areas were largely unchanged by both climate and human impact. In fact, that climate change and human impacts were focused on mammoths in the northernmost land masses of Arctic Siberia and some arctic islands, “leaving them with nowhere to run away from extinction.”

    Nogués-Bravo, D., Rodríguez, J., Hortal, J., Batra, P., Araújo, M.B. (2008). Climate Change, Humans, and the Extinction of the Woolly Mammoth. PLoS Biology, 6(4), e79. DOI: 10.1371/journal.pbio.0060079
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