Ancient fires drove large mammals extinct, study suggests

From the article:

Earth has seen five mass extinction events so far; some scientists argue that the disappearance of large mammals at the end of the last ice age was the start of a sixth. “It was the biggest extinction event since an asteroid slammed into Earth and wiped out all the dinosaurs,” said Emily Lindsey, a paleoecologist at La Brea Tar Pits and Museum and author of the new study, adding that the disappearance could very well represent “the first pulse” in a sixth mass extinction.

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La Brea now boasts a continuous fossil record of the region stretching as far back as 55,000 years. Dr. O’Keefe and his team analyzed fossils for eight large mammal species — including the sabertooth cat, the American lion and Camelops hesternus, an ancient camel — that lived between 10,000 and 15,600 years ago. Using radiocarbon dating, the team determined that seven of these species went extinct around 13,000 years ago.

To figure out why, the researchers analyzed climate, pollen and fire records in the region alongside continental human population growth at the time. They found that human occupation began to rise rapidly around the same time that Southern California entered a period of severe drought and warming. Extreme fires ensued, and the vegetation, once rich in juniper and oak trees, was eventually replaced by grass and chaparral shrubs.

“What we see is that you have a 400-year-long period of massively elevated wildfire,” said Regan Dunn, a paleobotanist at La Brea Tar Pits and Museum and an author of the new paper. “And at the end of that period, you’re in a different ecosystem and all of the megafauna are gone.”

Dr. O’Keefe described the conditions as the perfect storm: “You have a bunch of different factors that are multiplying each other and giving you a huge increase in fires,” he said. Using a model similar to the ones that forecast trends in the stock market, the scientists determined that humans were the primary drivers of these fires, both through direct ignition and by the elimination of herbivores, which allowed flammable underbrush to spread uncontained. Shifts in the climate exacerbated this further, setting the stage for the extinction of species.

Dr. Dunn emphasized that this pattern could not account for the notable disappearance of large mammals elsewhere in the world at the end of the last ice age. “But in order to understand the global event, you really need to look at a regional scale,” she said.