A new study published Feb. 16 in Nature Communications suggests that the extinction of North America’s largest mammals was not caused by overhunting by the rapidly growing human population after they arrived in the Americas.
Instead, the results, based on a new statistical modeling approach, suggest that populations of large mammals fluctuated in response to climate change, with a drastic drop in temperature around 13,000 years ago triggering the decline and extinction of these giant mammalian creatures .
Nevertheless, humans may have been involved in more complex and indirect ways than simple models of overhunting suggest.
About 10,000 years ago, North America was home to many large and exotic creatures such as mammoths, giant ground-dwelling sloths, larger than life beavers and giant armadillo-like creatures known as Glyptodons.
But by about 10,000 years ago, most of North America’s animals weighing over 44 kg, also known as megafauna, were gone.
The topic has been hotly debated for decades, with most researchers arguing that human overhunting, climate change, or a combination of both are to blame. Using a new statistical approach, researchers have now found strong evidence that climate change was the main reason for the extinctions.
Overhunting vs. climate change
Since the 1960s, this hypothesis has been put forward , as human populations grew and spread across continents, the arrival of specialized “big game” hunters in the Americas about 14,000 years ago quickly drove many giant mammals to extinction.
The large animals did not possess the appropriate anti-predator behavior to deal with a novel, highly social, tool-wielding predator, making them particularly easy to hunt. According to proponents of this “overkill hypothesis,” humans took full advantage of easy-to-hunt prey, overran animal populations, and carelessly drove the giant creatures to extinction.
Not everyone agrees with this idea, however. Many scientists have argued that there is insufficient archaeological evidence to support the idea that megafauna hunting was persistent or widespread enough to cause extinctions. Instead, significant climatic and ecological changes may have been to blame.
Around the time of the extinction (15,000 to 12,000 years ago) there were two major climatic changes. The first was a period of abrupt warming that began about 14,700 years ago, and the second was a cold snap about 12,900 years ago, during which the northern hemisphere returned to near-glacial conditions.
One or both of these important temperature fluctuations and their ecological impacts have been linked to megafauna extinctions.
“One common approach has been to try to determine the timing of megafauna extinctions and to see how they correspond to human arrival in America or a climatic event,” says Mathew Stewart, co-lead author of the study.
“However, extinction is a process – meaning that it unfolds over a period of time – and to understand what caused the decline of North American megafauna, it is crucial that we understand how.” their peak populations have fluctuated to the point of extinction. Without these long-term patterns, we can only see rough coincidences.”
A new statistical approach: tracking overhunting using carbon footprints
To test these conflicting hypotheses the authors used a new statistical approach developed by W. Christopher Carleton, the other co-lead author of the study, and published last year in the Journal of Quaternary Science.
Estimating the population size of prehistoric hunter-gatherer groups and long-extinct animals cannot be done by counting heads or hooves. Instead, archaeologists and paleontologists use the radiocarbon record as a proxy for past population sizes.
The reason for this is that the more animals and humans appear in a landscape, the more dateable carbon remains after they are gone, which is then reflected in the archaeological and fossil record. In contrast to established approaches, the new method better accounts for uncertainties in fossil dating.
The main problem with the previous approach is that it mixes the uncertainty associated with radiocarbon dates with the process scientists are trying to identify.
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“As a result, you may end up seeing trends in the data that don’t actually exist, making this method less suitable for capturing changes in past population numbers. Using simulation studies where we know the actual patterns in the data, we were able to show that the new method does not have the same problems. As a result, our method is able to capture changes in population numbers over time much better using the radiocarbon record,” explains Carleton.
Extinctions of North American megafauna
The authors applied this new approach to the question of the late Quaternary extinction of North American megafauna. In contrast to previous studies, the new results show that megafauna populations fluctuated in response to climate change.
“Megafauna populations appear to have increased when North America began warming about 14,700 years ago,” says Stewart. “But we then see a shift in this trend about 12,900 years ago, when North America began to cool drastically, and shortly thereafter we begin to observe megafauna extinctions.”
And although these results suggest the Returning to near-glacial conditions some 12,900 years ago was the proximate cause of the extinctions, the story is probably more complicated than this.
“We need to consider the ecological changes associated with these climate changes at both the continental and continental levels and regional level if we want to understand what caused these extinctions,” explains group leader Huw Groucutt, senior author of the study. “Humans aren’t entirely off the hook either, as it’s possible that they played a more subtle role in the extinction of megafauna than simple overkill models suggest.”
Many researchers have argued that it was It is an impossible coincidence that the extinction of megafauna around the world often happened around the time of the arrival of humans. However, it is important to demonstrate scientifically that there was a link, and even if there had been, the causes could have been much more indirect (e.g. through habitat modification) than a killing craze when humans came to a region.
The authors end their article with a call to arms and urge researchers to develop larger, more reliable records and robust methods for their interpretation. Only then will we have a full understanding of the late Quaternary megafauna extinction event.
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Study:“Climate change, non-human population growth, correlates with late Quaternary megafauna declines in North America”Authors: Stewart, M., Carleton, W.C., Groucutt, H.S. Published in: Nature CommunicationsPublished date:16. February 2021DOI: 10.1038/s41467-021-21201-8Photo: by Geoff Peters via Wikimedia Commons, License CC BY 2 .0
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