Most major land mammals outside of Africa had been extinct by 11,700 years ago. Since the end of the last Ice Age, scientists have long time argued about whether these Pleistocene extinctions were primarily brought on by human activity or a changing environment.
The megafauna of the area may have perished as a result of both conditions, according to a recent analysis of the remains of creatures trapped decades ago in the La Brea tar pits in what is now Los Angeles. According to research published in the journal Science, the Pleistocene extinctions around 13,000 years ago were caused by a warming and drying environment and human caused fires.
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.
According to the researchers, the type of “climate-human synergy” that led to the extinction of California’s largest animals may be a sign of significant upheaval in contemporary ecosystems that are being subjected to continuous human-caused climate change. For instance, Southern California has warmed by more over 2 degrees Celsius over the past century, a more rapid rise than the region experienced at the time.
F. Robin O’Keefe, an evolutionary biologist and paleontologist at Marshall University in Huntington, West Virginia, and colleagues first examined the remains of ancient carnivores that had perished in the asphalt seeps of La Brea in order to determine how the animals had physically changed over many thousands of years. Then they discovered proof of an extinction event preserved in the fossil record from tar pits. “We had lots and lots of megafauna, and then suddenly they were gone,” O’Keefe says.
The group began compiling information on more species. The researchers dated the remains of 172 individuals from eight different megafauna species, ranging in age from 10,000 to roughly 15,600 years ago. Included were extinct creatures including ground sloths (Paramylodon harlani), dire wolves (Aenocyon dirus), and saber-toothed cats (Smilodon fatalis), as well as one living species, the coyote (Canis latrans). The scientists discovered that all seven of the eight megafaunal species disappeared from the tar pit fossil record around 13,000 years ago.
Sediment cores from the neighboring Lake Elsinore were used by the researchers to gain insight into what was happening in the ecosystem long ago. The cores provide as a record of tens of thousands of years’ worth of changes in local vegetation, temperature, and fire frequency. O’Keefe and his colleagues also contrasted the time of the extinction with computer simulations of the expansion of the human population on the continent, which were created using a database of a large number of radiocarbon dates from ancient sites spread out across North America.
The sediment cores showed that the area warmed by 5.6 degrees Celsius and dried out during the period before the extinction. More drought- and fire-resistant species replaced the juniper and oak woods that once covered the area. A 300 year period of severe fires in Southern California occurred shortly after this change, as shown by an increase in charcoal in lake records. According to the team’s modeling of human populations, their numbers sharply increased just before the burning began. It is likely that they are connected.
Furthermore, the study discovered that human activities and climate change not only accelerated Pleistocene extinctions but also permanently changed the region’s woods into chaparral scrubland. (Chaparral is a shrubby plant community found primarily in California, southern Oregon and North America.)
O’Keefe refers to it as a feedback loop and points out that because plants go uneaten when herbivores are hunted, the ecosystem becomes more prone to fire. “You get into this vicious cycle,” he claims. “As the population grows, the environment becomes drier and hotter, killing more herbivores in the process. As a result, there is more fuel to burn.”
The seven megafauna species went extinct in Southern California roughly 1,000 years before they went extinct elsewhere in North America. According to the experts, those other populations may have died in a similar way. “There is evidence for a continent-wide event, not just in Southern California but across the continent right about at the same time.”
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.
Today we know what happened before and if we can learn from that, maybe we can change the course of the world.
Cover Photo: 13,000 years ago, as human made fires scorched the region, ancient bison (Bison antiquus) escaped the flames and became trapped in asphalt seeps. The bison, along with six other megafauna, soon disappeared from the Southern California. Cullen Townsend, Natural History Museums of Los Angeles County