Around 13,000 years ago, humans and fire changed LA’s ecosystem forever

By about 11,700 years ago, most large land mammals outside of Africa had gone extinct. Scientists have long debated whether these extinctions were primarily caused either by human activities or a changing climate as the last ice age came to a close (SN: 11/13/14; SN: 2/6/14).

A new study of the remains of animals trapped long ago in the La Brea tar pits, in what’s now Los Angeles, suggests both factors worked in concert to bring about the demise of the region’s megafauna. A warming, drying climate plus humans’ hunting and burning of the landscape led to large fires that precipitated the end-Pleistocene die-offs there around 13,000 years ago and forever changed the ecosystem, researchers report in the Aug. 18 Science.

The findings “reflect the reality of nature, which is that phenomena are rarely, if ever, driven by a single factor,” says Danielle Fraser, a paleoecologist at the Canadian Museum of Nature in Ottawa who was not involved with the research.

The type of “climate-human synergy” implicated in the demise of California’s biggest beasts may warn of dramatic upheaval in modern ecosystems subjected to ongoing human-caused climate change, the researchers say. Southern California, for instance, has warmed more than 2 degrees Celsius over the last century, a more rapid change than the area faced during that earlier time period.

In the new study, F. Robin O’Keefe, a paleontologist and evolutionary biologist at Marshall University in Huntington, W.Va., and colleagues were initially studying the remains of ancient carnivores that had become stuck and died in the asphalt seeps of La Brea, investigating how the animals had physically changed over many thousands of years. Then the researchers found evidence of an extinction event recorded in the tar pit fossil record.

“We had lots and lots of megafauna, and then suddenly they were gone,” O’Keefe says.

The team started gathering data on more species. In all, the researchers dated remains from 172 individuals from eight megafauna species from 10,000 to about 15,600 years ago. Included were extinct animals like saber-toothed cats (Smilodon fatalis), dire wolves (Aenocyon dirus) and ground sloths (Paramylodon harlani), and a single species that survived to today, the coyote (Canis latrans). Sure enough, about 13,000 years ago, the seven of the eight megafaunal species all vanished from the tar pit fossil record, the team found.

To understand what was going on in the environment long ago, the researchers turned to sediment cores from nearby Lake Elsinore. The cores serve as a record of regional vegetation, climate and fire frequency changes over tens of thousands of years. O’Keefe and his colleagues also compared the extinction timing with computer modeling of human population growth on the continent built from a database of many thousands of radiocarbon dates of archaeological sites across North America. 

The sediment cores revealed that over the millennium preceding the extinction, the region warmed by 5.6 degrees Celsius and dried out. The area’s juniper and oak woodlands gave way to more drought and fire-tolerant plants. Shortly after this shift started, Southern California went through a 300-year-long period of intense fires, evidenced by a spike in charcoal in the lake records. The team’s modeling on human populations shows their numbers rapidly grew right before the burning started. That the population upswing so closely coincides with the fires suggests the two are linked.

What’s more, the changing climate and human activities not only precipitated the extinctions, the team found, but also converted the region’s woodlands into chaparral scrubland for good. 

O’Keefe describes it as a feedback loop, noting that hunting herbivores also makes the ecosystem more fire prone as plants go uneaten. “You get this vicious cycle,” he says. “You add more people and it gets hotter and drier, and you’re killing more herbivores. So there’s more fuel [to burn].”

The seven megafauna species vanished from Southern California about 1,000 years before they did elsewhere in North America. Those other populations may have met a similar end, the researchers say. “There is evidence for a continent-wide event, not just in Southern California but across the continent right about at the same time,” O’Keefe says.

Sandra Brügger, a paleoecologist at the University of Basel in Switzerland who wasn’t involved in the research, notes that similarly rapid ecological transformations have been documented in the Mediterranean and a broader swathe of the U.S. West during the transition between the Pleistocene and the following Holocene Epoch.

The new findings not only provide a glimpse into the past but are also a “cautionary tale” relevant to the present and to the survival of modern biodiversity, says O’Keefe, pointing to recent large, intense fires in Hawaii, the U.S. West and Canada (SN: 6/9/23). “So the parallels are certainly there. The one thing that’s different about today is that we know what happened before, and if we can learn something from that, maybe we can change our trajectory.”