Climate change killed off mammoths, sloths, megafauna

For years, scientists have waffled on what exactly drove North America's megafauna to extinction, debating whether the blame belonged to overhunting, climate change or both.

In a new study, published Tuesday in the journal Nature Communications, researchers used statistical analysis to show climate change was the primary driver of the disappearance of North America's largest species, including mammoths and giant sloths.

In the 1950s, many researchers -- compelled, perhaps, by a growing appreciation of environmental damage caused by human development -- hypothesized that big game hunting drove mammoths, sloths and other megafauna to extinction.

Proponents of the "overkill" hypothesis argued that as human populations expanded across the continent, they easily slaughtered megafauna species that had evolved without fear of any natural predators.

Critics of the hypothesis claimed there wasn't enough archaeological evidence for such a bold claim. Megafauna hunting, they argued, wasn't widespread.

Instead, they claimed sudden climatic shifts were to blame: a period of sudden warming around 14,700 years ago, followed by a dramatic cold spell around 12,900 years ago.

Both camps attempted to bolster their claims by overlaying timelines of fossil and archaeological records with those of climate change.

"A common approach has been to try to determine the timing of megafauna extinctions and to see how they align with human arrival in the Americas or some climatic event," co-lead author Mathew Stewart, researcher with the Max Planck Extreme Events Research Group in Jena, Germany, said in a news release.

"However, extinction is a process -- meaning that it unfolds over some span of time -- and so to understand what caused the demise of North America's megafauna, it's crucial that we understand how their populations fluctuated in the lead up to extinction. Without those long-term patterns, all we can see are rough coincidences," said Stewart, a researcher with the Max Planck Extreme Events Research Group in Jena, Germany.

For the new study, Stewart and his research partners used a novel statistical analysis technique to estimate changes in the population size of several megafauna species, including: mammoths, ground sloths, giant beavers and glyptodons, humongous armadillo-like creatures.

The analysis relied on the radiocarbon record as a proxy for biological abundance. Larger populations of megafauna and humans, the thinking goes, leave behind larger amounts of datable carbon.

The new analysis method showed megafauna populations fluctuated in response to sudden shifts in climate.

"Megafauna populations appear to have been increasing as North American began to warm around 14,700 years ago," Stewart said.

"But we then see a shift in this trend around 12,900 years ago as North America began to drastically cool, and shortly after this we begin to see the extinctions of megafauna occur," Steward said.

According to the statistical analysis, the rapid expansion of glacial conditions across North America was the primary driver of megafauna extinctions.

The authors of the study suggest the simulations do leave some room for human influence -- but a more subtle influence than offered by overkill models.

"We must consider the ecological changes associated with these climate changes at both a continental and regional scale if we want to have a proper understanding of what drove these extinctions," said study senior author Huw Groucutt.

"Humans also aren't completely off the hook, as it remains possible that they played a more nuanced role in the megafauna extinctions than simple overkill models suggest," said Groucutt, who leads the Extreme Events research group at Max Planck.

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