HARBINGER FOR THE FUTURE: DROUGHT, FIRE AND EXTINCTION IN THE LATEST PLEISTOCENE, RANCHO LA BREA, CALIFORNIA, USA

Debate over causation of the Late Pleistocene megafaunal extinction has raged for 70 years. Although multiple hypotheses have been put forth, the most widely accepted and debated are 1) over-hunting by humans, 2) climate change, and 3) some combination of the two. The extraordinary Late Pleistocene faunas preserved in the asphaltic traps at Rancho La Brea, Los Angeles, CA, are among the best in the world to test these hypotheses because of an abundance of megafaunal specimens that can be dated precisely using ¹⁴C radiometric techniques. We obtained 172 new radiocarbon dates for eight megafaunal taxa including herbivores (Bison antiquus, Equus occidentalis, Camelops hesternus, Paramylodon harlani) and carnivores (Smilodon fatalis, Aenocyon dirus, Panthera atrox, Canis latrans) spanning ~16 to 10 ka (thousand calendar years before present). Seven species of extinct megafauna disappeared by 12.9 ka, prior to the onset of the Younger Dryas (12.87 ka) while coyotes (C. latrans) persisted into the Holocene. Except for Camelops, no younger dates are known for these megafauna elsewhere in North America. Beginning ~14 ka, herbivore populations began to decline with browsers, Paramylodon and Camelops, disappearing first. From 13.25-13.0 ka, a precipitous decline of all megafauna occurred, resulting in complete extirpation/extinction by 12.9 ka.

While a concurrent record of plants from La Brea is yet lacking, sedimentological, geochemical, pollen and charcoal records from nearby Lake Elsinore provide key insights about climatic shifts and the impacts of warming and drying on vegetation and fire of the region. These data, in combination with estimates of human demographic growth based on radiocarbon dates from continental archeological records, provide an unrivaled record spanning the extinction/extirpation event. Pollen data show decreased tree cover and an increase in xeric plant taxa concurrent with the herbivore decline. The precipitous decline and disappearance of all megafauna species coincided with rapid warming, a severe drought ~250 years in duration, and a dramatic spike in wildfire activity. Time series modeling implicates large-scale fires as the primary cause of the extirpations; we argue that the catalyst of this state shift appears to have been mounting human impacts in a drying, warming and increasingly fire-prone ecosystem.