About 11,700 years ago most large land mammal species outside of Africa were extinct. Scientists have debated for years whether the extinctions of large land mammals outside Africa were caused by humans or other factors. Human ActivitiesYou can also use a Climate ChangeThe last ice age ended (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 The end-Pleistocene mass extinctions were precipitatedResearchers in the Aug. 18, 2018 issue report that there was a time when the ecosystem changed forever around 13,000 years ago. 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 example has warmed up by over 2 degrees Celsius since the turn of the century. That’s a much faster rate than was experienced in that time frame.
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. The researchers then found evidence that an extinction was recorded in the fossil record of the tarpit.
“We had lots and lots of megafauna, and then suddenly they were gone,” O’Keefe says.
The team then began to collect data on additional species. The researchers were able to date 172 individuals of eight megafaunas from approximately 10,000 to 15,600 years ago. Extinct animals such as saber toothed cats were included.Smilodon fatalisDire wolvesAenocyon dirusThe ground sloth (Paramylodon harlaniCoyotes are the only species that has survived up to this day.Canis latrans). The team discovered that seven out of eight species of megafauna vanished from fossil records in tar pits about 13,000 ago.
Researchers used sediment cores taken from Lake Elsinore to understand the changes that occurred in the environment over a long period of time. The cores are a record for changes in regional vegetation, climate, and fire frequency over the past tens or 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 show that, over the millennium before the extinction of the dinosaurs, the region grew warmer by 5.6° Celsius and dried up. The area’s juniper and oak woodlands gave way to more drought and fire-tolerant plants. Southern California underwent a 300-year period of intense wildfires shortly after this shift began, as shown by an increase in charcoal in lake records. The team’s modeling on human populations shows their numbers rapidly grew right before the burning started. The fact that the increase in population coincides so closely with the fires indicates a link between the two.
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 varieties vanished in Southern California 1,000 years earlier than they did in other parts of North America. Researchers say that these other populations could have suffered a similar fate. “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.”
