Climate change has probably led to the extinction of the largest animals in North America

PICTURE: The findings of the study suggest that the decline in hemisphere temperature and related ecological changes were major drivers of the extinction of late Quaternary megafauna in North America. view more

Credit: Hans Sell

A new study published in Nature Communications suggests that the extinction of the largest mammals in North America was not caused by overhunting due to the rapid expansion of human populations after their entry into America. Instead, the findings, based on a new approach to statistical modeling, suggest that populations of large mammals fluctuated in response to climate change, with drastic temperature drops about 13,000 years ago, initiating the decline and extinction of these massive creatures. Yet people may have been involved in more complex and indirect ways than simple models of exaggeration suggest.

About 10,000 years ago, North America was home to many large and exotic creatures, such as mammoths, giant lazy people living in the ground, beavers larger than life, and huge armadillo-like creatures known as glyptodons. But about 10,000 years ago, most North American animals weighing over 44 kg, also known as megafauna, have disappeared. Researchers from the Max Planck Extreme Events research group in Jena, Germany, wanted to find out what led to these extinctions. This topic has been intensively debated for decades, and most researchers claim to be responsible for human exaggeration, climate change, or some combination of the two. With the new statistical approach, researchers have found strong evidence that climate change is a major driver of extinction.

Excessive hunting versus climate change

Since the 1960s, it has been assumed that as the human population grew and spread on the continents, the arrival of specialized hunters for “big games” in America some 14,000 years ago quickly led many giant mammals to extinction. Large animals did not have the proper behavior against predators to cope with the new, very social predator handling the tool, which made it easier for them to hunt. According to proponents of this “excessive killing hypothesis,” humans have taken full advantage of prey that is easily hunted, destroying animal populations and carelessly forcing giant creatures to extinction.

However, not everyone agrees with this idea. Many scientists claim that there is too little archaeological evidence to support the idea that megafauna hunting was persistent or widespread enough to cause extinction. Instead, significant climate and environmental changes may be to blame.

Around the time of the extinction (between 15,000 and 12,000 years ago), two major climate changes occurred. The first was a period of rapid warming that began about 14,700 years ago, and the second was a cooling period about 12,900 years ago during which the northern hemisphere returned to near-glacial conditions. One or both of these important temperature changes and their ecological consequences are involved in the extinction of megafauna.

“The usual approach was to try to determine the time of megafauna extinction and see how they coincide with human arrival in America or some climatic events,” says Mathew Stewart, co-lead author of the study. “However, extinction is a process – meaning it takes place over a period of time – so to understand what caused the collapse of North America’s megafauna, it’s crucial to understand how their populations fluctuated on the eve of extinction. Without that, these long-term patterns, all that we can see they are gross coincidences. “

‘Dates as data’

To test these conflicting hypotheses, the authors used a new statistical approach developed by W. Christopher Carleton, the second co-leader of the study, and published last year in Journal of Quaternary Sciences. Estimating the size of the population of prehistoric groups of hunters and gatherers of long-extinct animals cannot be done by counting heads or hooves. Instead, archaeologists and paleontologists use the radiocarbon record as a substitute for past population size. The rationale is that the more animals and people present in the landscape, the more data is left without data after they disappear, which is then reflected in archaeological and fossil records. Unlike established approaches, the new method better explains the uncertainty in fossil dates.

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

“As a result, you can eventually see trends in the data that don’t really exist, which makes this method quite inappropriate for recording changes in the past population level. Using simulation studies in which we know what the actual patterns are in the data, we were able to show that the new the method doesn’t have the same problems. As a result, our method can do a much better job of recording time changes in population levels using a radiocarbon record, “Carleton explains.

North American megafauna extinctions

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

“The population of megafauna appears to be increasing as North America began to warm up approximately 14,700 years ago,” Stewart states. “But then we see a shift in this trend about 12,900 years ago when North America began to cool drastically, and shortly thereafter we begin to see the extinction of megafauna.”

And while these findings suggest that a return to near-glacial conditions about 12,900 years ago was the immediate cause of the extinction, the story is likely to be more complex than this.

“We need to consider the environmental changes associated with these climate changes at both the continental and regional levels if we want to properly understand what triggered these extinctions,” explains group leader Huw Groucutt, senior author of the study. “Humans have also not been completely rejected, as it is still possible that they played a more nuanced role in the extinction of the megafauna than simple models of over-killing suggest.”

Many researchers argue that it is an impossible coincidence that megafauna extinctions around the world often occurred at the time of human arrival. However, it is important to scientifically show that there was a connection, and even if it did exist, the causes may have been much more indirect (like habitat change) than the rage of killing as people arrived in the region.

The authors conclude their article with a call to arms, urging researchers to develop larger, more reliable records and robust methods for interpreting them. Only then will we develop a comprehensive understanding of the events of the extinction of the Late Quaternary megafauna.

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Max Planck’s research group “Extreme Events” aims to explore the character and impacts of extreme events from multiple perspectives using a variety of methodologies. This cross-sectional project will include research through three Jena Max Planck institutes (Chemical Ecology, Biogeochemistry, and the Science of Human History).

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