La Brea megafaunal extinctions driven by fires 13,000 years ago

AMERICAN ASSOCIATION FOR THE ADVANCEMENT OF SCIENCE (AAAS)—Pleistocene megafaunal extinctions from Southern California were driven by large-scale fires in an ecosystem made increasingly vulnerable by climate change and human impacts, according to a new study*. The findings, made possible by a new radiocarbon chronology of fossils from the La Brea tar pits, not only provide insights into the dynamics that contributed to Pleistocene extinctions, but inform understandings of modern ecological change. “The conditions that led to the end-Pleistocene state shift in Southern California are recurring today across the western United States and in numerous other ecosystems worldwide,” write the authors, “and understanding the interplay of climatic and anthropogenic changes in driving this past extinction event may be helpful in mitigating future biodiversity loss in the face of similar pressures.”

At the end of the last Ice Age, roughly two-thirds of Earth’s large mammals in most regions worldwide went extinct. This extinction – the largest of the Cenozoic – coincided with both late-Quaternary climate changes and the growth and expansion of human populations across continents. However, the timing, causes, and consequences of the end-Pleistocene megafaunal extinctions have been difficult to understand. Much of what is known about the event is based on fragmentary paleontological records that lack the chronological precision required to compare the timing of species disappearances to archaeological and environmental data. Using the abundant fossils preserved at the La Brea tar pits (Rancho La Brea) – a site that contains a nearly continuous record of Pleistocene megafaunal occupation of the Los Angeles Basin from more than 55,0000 years ago to the Holocene – Frank O’Keefe and colleagues investigated the potential drivers of megafaunal extirpations in Southern California. O’Keefe et al. obtained new AMS radiocarbon dates on 172 megafaunal individuals and developed a high-resolution radiocarbon chronology for the eight most common mammal species at La Brea from 15.6 to 10 thousand years before present (ka). They found that 7 of these species were extinct in the region by 12.9 ka. Then, using local sediment core records, the authors evaluated the timing of these extirpations with regional paleoclimate and paleoenvironmental records, as well as continental-scale analyses of megafaunal extinction and human demographic growth in North America. According to the findings, the disappearance of megafauna at La Brea preceded North American megafaunal extinction by at least 1000 years, preceded the Younger Dryas climate event, and coincided with vegetation change and aridification at the end of the Bølling-Allerød – a brief climatic warming event that occurred between 14.6 and 12.8 ka. Moreover, the records reveal an increase in large-scale fire activity in the region, which the authors estimate was the primary cause of La Brea’s regional megafaunal extinction. In sum, O’Keefe et al. argue that this increase in fire may have resulted from climate change-induced warming and drying in conjunction with mounting impacts of human hunting and burning in an increasingly fire-prone ecosystem.