Science.-Changes in the climate ended the megafauna of North America


The extinction of the largest mammals in North America was not driven by overhunting of rapidly expanding human populations following their entry into the Americas.

Instead, the findings of a study based on a new statistical modeling approach suggest that large mammal populations fluctuated in response to climate change, with drastic declines in temperatures some 13,000 years ago initiating the decline and extinction of these massive creatures. .

Tens of thousands of years ago, North America was home to many large and exotic creatures, such as mammoths, giant ground-dwelling sloths, larger-than-life beavers, and huge armadillo-like creatures known as glyptodonts. But about 10,000 years ago, most of the North American animals that weighed more than 150 pounds, also known as megafauna, had disappeared.

Researchers at the Max Planck Extreme Events Research Group in Jena, Germany, wanted to find out what led to these extinctions. The issue has been hotly debated for decades, with most researchers arguing that human overhunting, climate change, or some combination of the two were responsible. Using a new statistical approach, the researchers found strong evidence that climate change was the main driver of the extinction.

Since the 1960s, it has been hypothesized that as human populations grew and spread across the continents, the arrival of specialized “big game” hunters to the Americas some 14,000 years ago quickly led to extinction. many giant mammals. Large animals did not possess the appropriate anti-predator behaviors to deal with a novel, highly social, tool-wielding predator, making them particularly easy to hunt. According to proponents of this “hype hypothesis,” humans took full advantage of the easy-to-hunt prey, devastating animal populations and carelessly driving the giant creatures to extinction.

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However, not everyone agrees with this idea. Many scientists have argued that there is very little archaeological evidence to support the idea that megafauna hunting was persistent or widespread enough to cause extinctions. Instead, major climatic and ecological changes may have been to blame.

Around the time of the extinctions (15,000 to 12,000 years ago), there were two major climate changes. The first was a period of abrupt warming that began about 14,700 years ago, and the second was a cold wave about 12,900 years ago during which the Northern Hemisphere returned to near-glacial conditions. One or both of these major temperature changes, and their ecological ramifications, have been implicated in megafauna extinctions.

“A common approach has been to try to determine the timing of megafauna extinctions and see how they align with human arrival in the Americas or some climatic event,” says Mathew Stewart, co-lead author of the study, in a statement. “However, extinction is a process, meaning it unfolds over a period of time, so to understand what caused the disappearance of the North American megafauna, it is crucial that we understand how their populations fluctuated over the period. pre-extinction. Without those long-term patterns, all we can see are rough matches. “

To test these conflicting hypotheses, the authors used a new statistical approach developed by W. Christopher Carleton, the study’s other co-lead author, and published last year in the Journal of Quaternary Science. Estimating the population size of prehistoric hunter-gatherer groups and extinct animals cannot be done by counting heads or hooves. Instead, archaeologists and paleontologists use the radiocarbon record as a surrogate for previous population sizes. The reason is that the more animals and humans there are in a landscape, the more datable carbon is left behind after they are gone, which is then reflected in the archaeological and fossil records. Unlike established approaches, the new method better explains the uncertainty in fossil dates.

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The main problem with the above approach is that it combines the uncertainty associated with radiocarbon dates with the process that scientists are trying to identify.

“As a result, you can end up seeing trends in the data that don’t really exist, which makes this method quite unsuitable for capturing changes in past population levels. Using simulation studies where we know what the actual patterns are in the data, they have been able to show that the new method does not have the same problems. As a result, our method is able to do a much better job of capturing changes over time in population levels using radiocarbon recording, “explains Carleton.

The authors applied this new approach to the question of the extinctions of the North American late Quaternary megafauna. Unlike previous studies, the new findings show that megafauna populations fluctuated in response to climate change.

“Megafauna populations appear to have increased as North America began to warm up about 14,700 years ago,” says Stewart. “But then we see a change in this trend around 12,900 years ago when North America started to cool dramatically, and shortly after that we started to see megafauna extinctions happening.”

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