Fires led to the extinction of large mammals – 08/23/2023 – Science

Forest fires are getting worse. Scientists say parts of the United States are experiencing fires three times as often and four times as large as they were 20 years ago. Just this summer, smoke from the wildfires in Canada has turned North American skies an eerie orange color; “Whirlwinds of fire” were seen in the Mojave Desert, and a raging wildfire sparked a catastrophe on the island of Maui, Hawaii.

Past historical records can reveal what caused an increase in fire activity in the past and what the consequences may have been. In a study published last week in the journal Science, a group of paleontologists who analyzed fossil records at La Brea, a celebrated fossil excavation site in southern California, concluded that the disappearance of saber-toothed tigers, dire wolves and other large mammals from this region nearly 13,000 years ago was linked to rising temperatures and an increase in human-started fires.

“We think humans were the primary cause of the flip,” said Robin O’Keefe, an evolutionary biologist at Marshall University. “What happened at La Brea is happening now? It’s a great question, and I think we need to try to find the answer.”

Earth has experienced five mass extinction events to date. Some scientists consider that the disappearance of large mammals at the end of the last Ice Age would have been the beginning of a sixth event. “It was the biggest extinction event since an asteroid hit Earth and wiped out the dinosaurs,” said paleoecologist Emily Lindsey of the La Brea Bitumen Pits Museum and Site and one of the authors of the new study. For her, the disappearance may well represent the beginning moment of a sixth mass extinction.

Until now, scientists have not been able to define exactly what led to the extinction of these animals. La Brea Bitumen Pits is one of the few sites in the world with a fossil record large enough for scientists to investigate the issue. The tar pits, still active over a 16-acre area, are filled with bubbling black asphalt that has risen to the surface from the bowels of the Earth. Prehistoric animals that got stuck in this slime died of exhaustion or victims of predators, and the asphalt fossilized and preserved their remains. “And that continues to happen today,” O’Keefe said. “You can go to La Brea and see a squirrel in the tar. I’ve seen it myself.”

That’s bad luck for the animals, but good luck for the scientists: La Brea has a continuous fossil record of the region that goes back 55,000 years. O’Keefe and his team analyzed fossils from eight species of large mammals—including the saber-toothed tiger, American lion, and Camelops hesternus, a prehistoric camel— that lived between 10,000 and 15,600 years ago. Using radiocarbon dating, the team determined that seven of these species went extinct 13,000 years ago.

To understand why, the researchers analyzed climate, pollen and fire records in the region, alongside the growth of the continental human population at the time. They found that human occupancy began to increase rapidly at about the same time that a period of severe drought and warming began in Southern California. There were extreme wildfires, and the vegetation, once rich in junipers and oaks, eventually gave way to grasses and chaparral shrubs.

“What we see is that there was a 400-year period of tremendously increased fires,” said paleobotanist Regan Dunn of the La Brea site and museum and one of the authors of the new paper. “At the end of that period, we’re in a new ecosystem, and all the megafauna are gone.”

O’Keefe described the conditions as the perfect storm: “You have a bunch of different factors that multiply each other and lead to a huge increase in fires,” he said. Using a model similar to those that predict trends in stock markets, the scientists determined that humans were the main drivers of these fires, both by starting them directly and by eliminating herbivores, which allowed the flammable undergrowth to spread out of control. . Changes in climate exacerbated the process even further, creating the conditions for the extinction of species.

Dunn emphasized that this pattern does not explain the notable disappearance of large mammals in other parts of the world at the end of the last Ice Age. “But to understand the global event, you really need to look at what happened on a regional scale,” she said. What happened in Southern California 13,000 years ago “has striking parallels to the environmental and biodiversity crises we are facing today.”

Climate records during the Ice Age extinction point to about 10 degrees Fahrenheit warming over 1,000 years, Dunn said, with the temperature in Southern California now rising by 5.4 degrees in just the last 1000 years. years. Increased fire activity following the arrival of humans has also been documented in other places, including Australia, where fires have recently caused massive losses of that country’s unique wildlife.

“This study is a great example of how we can use the past to predict the future,” wrote paleoecologist Anthony Barnosky of the University of California at Berkeley, who was not involved in the work, in an email. “And what we’re seeing happen today — increasing human pressures combined with and causing climate change — is like a lesson from the past multiplied a thousand.” Barnosky added that these changes are not gradual, but swift and catastrophic.

The researchers noted that it is difficult to understand the similarity between current events and those in the fossil record. “Many of the most endangered animals today are the remaining large mammals that didn’t go extinct” at the end of the last Ice Age, Lindsey said. But, he said, “because we caused it, we have the power to stop it.”

^{Translated by Clara Allain}