Around 13,000 years ago, during a period of a couple of hundred years, the massive mammals that roamed Southern California rapidly disappeared from the region.
One likely reason was major wildfires that were ignited by human beings, according to new research by researchers from UCLA and the La Brea Tar Pits and Museum. That counters a theory, long held by scientists, that the animals disappeared because they were hunted to extinction.
The study was published today in Science.
The unique biological record provided by fossils recovered from the La Brea Tar Pits in Los Angeles enabled researchers to pinpoint when large mammals, or megafauna, died off, said Emily Lindsey, a UCLA adjunct professor and an associate curator of the tar pits.
Over tens of thousands of years, animals became stuck in the tar, which preserved their bones well enough for researchers to accurately carbon-date them to within a few decades.
“We used samples from over 170 animals of seven megafauna species — saber-toothed cats, dire wolves, lions, camels, bison, horses and sloths,” said Lindsey, a co-author of the paper. “Around 13,000 years ago, our record of all of those animals stops, and it’s just coyote, coyote, coyote, coyote.”
The disappearance of the megafauna took place over the course of about 300 years at the end of the Pleistocene epoch, the researchers write.
Evidence from another Southern California locale suggests that extensive wildfires scorched the region during that same time period. The authors report that samples of a sediment core from Lake Elsinore, California — about 80 miles away from the tar pits — revealed the presence of charcoal, which likely came from wildfires.
Those wildfires would have coincided with the spread of human beings and the end of the ice age, according to archaeological and geological evidence and a data analysis technique called time-series modeling. Prior to that, very little charcoal is present in the geological record, said UCLA graduate student Lisa Martinez, another co-author of the paper.
“The evidence suggests unprecedented fire activity occurring with the changing climate, along with people coming into the area,” Martinez said. “It’s during this interval that we the megafauna species disappeared.”
Humans during that period would have used fire for a variety of reasons, including to clear brush for travel, to hunt and drive prey, and to promote the growth of plants useful in basketry and medicine.
The wildfires, which also coincided with a centuries-long megadrought, transformed Southern California’s landscapes, vegetation and ecosystems. Post-glacial woodlands were replaced by the shrubby chapparal that dominates the region today.
In the paper, the authors draw a parallel between that transformation and the conditions that today are leading to climate change and devastating wildfires.
“Throughout history, fire has magnified the impact of humans, for better or worse,” said Glen MacDonald, a UCLA geography professor and co-author of the study. “Humans today are responsible for at least three factors that produce wildfires: the ignitions themselves, climate change and the introduction of invasive species, which changes the fuel structure.”
The disappearance of megafauna in Southern California during the time studied was part of a larger phenomenon: Two-thirds of the world’s large mammals went extinct at the end of the last ice age.
Lindsey said there are significant parallels between the time period the researchers analyzed and conditions today — chief among them that climate change has created conditions favorable to more severe, extensive wildfires. In Africa, for example, areas where plant-devouring elephants and wildebeest have been eliminated have quickly become dense, shrubby environments.
“That completely changes the fire regime because suddenly there’s all this fuel to burn,” Lindsey said.
The study was made possible by a partnership between the UCLA Institute of the Environment and Sustainability, the La Brea Tar Pits and other research institutions. Future studies of the age-old specimens in the tar pits could provide additional revelations about the possible effects of a warming planet.
“This site is uniquely positioned to inform a lot of questions that are of key environmental significance today — things like what the long-term impacts of climate change will be,” Lindsey said.