Pre-Younger Dryas megafaunal extirpation at Rancho La Brea linked to fire-driven state shift
Document Type
Article
Publication Date
8-18-2023
Abstract
INTRODUCTION
At the end of the Pleistocene, most of Earth’s large mammals (megafauna) became extinct. These extinctions occurred at different times globally, resulting in a drastic reorganization of terrestrial ecosystems. Despite decades of research on extinction causality, the relative importance of late-Quaternary climate changes and spreading human impacts have been difficult to disentangle because poor chronological resolution in the fossil record has precluded alignment of these rapidly occurring, tightly linked phenomena.RATIONALE
The Rancho La Brea (RLB) locality in Southern California provides a unique opportunity to investigate decadal-scale changes in megafaunal populations and community composition across the latest Pleistocene. At this site, naturally occurring asphalt seeps entrapped and preserved the bones of hundreds, and in some cases thousands, of individuals from numerous megafaunal species across the last 50,000 years of the Pleistocene. Nearly all of these osteological specimens preserve original collagen, which permits precise radiocarbon dating analysis.RESULTS
We obtained radiocarbon dates on 172 specimens from seven extinct and one extant species: Smilodon fatalis, Aenocyon dirus, Panthera atrox, Bison antiquus, Equus occidentalis, Paramylodon harlani, Camelops hesternus, and Canis latrans, spanning 15.6 to 10.0 thousand calendar years before present (ka). We used the resulting high-resolution chronology of entrapment at RLB to analyze population dynamics across this time interval and the timing of local disappearance for different taxa. To investigate the potential roles of late-Quaternary environmental change and human activities in driving the observed patterns, we compared our analyses of population structure and megafaunal extirpation against well-resolved regional and continental paleoclimatic proxies, vegetation records, and modeled human demographic growth. We used time-series modeling to investigate the dynamics of ecosystem change and evaluate causal relationships among these different phenomena. Modeling of extinction timing using several methods established that all taxa except coyotes were extirpated from RLB by 12.9 ka, before the onset of the Younger Dryas and well before the continental extinction of North American megafauna. The disappearance of all taxa was synchronous except for camels and sloths, which disappeared a few hundred years earlier in concert with aridification and tree loss during the Bølling–Allerød. The simultaneous disappearance of Smilodon, Aenocyon, Panthera, Equus, and Bison antiquus coincided with a regional ecological state shift characterized by floral community reorganization and unprecedented fire activity. Time-series modeling strongly implicates humans as the primary cause of the state shift and resulting extinctions.CONCLUSION
Our data document a transition from a postglacial megafaunal woodland to a human-mediated chaparral ecosystem in Southern California before the onset of the Younger Dryas. This transition began with gradual opening and drying of the landscape over two millennia, and terminated in an abrupt (300-year) regime shift characterized by the complete extirpation of megafauna and unprecedented fire activity. This state shift appears to have been triggered by human-ignited fires in an ecosystem stressed by rapid warming, a megadrought, and a millennial-scale trend toward the loss of large herbivores from the landscape. This event parallels processes occurring in Mediterranean ecosystems today.Original Publication Citation
F. Robin O’Keefe et al., Pre–Younger Dryas megafaunal extirpation at Rancho La Brea linked to fire-driven state shift.Science381,eabo3594(2023).DOI:10.1126/science.abo3594
Digital Commons @ LMU & LLS Citation
O’Keefe, F. Robin; Dunn, Regan E.; Weitzel, Elic M.; Waters, Michael R.; Martinez, Lisa N.; Binder, Wendy; Southon, John R.; Cohen, Joshua E.; Meachen, Julie A.; DeSantis, Larisa R. G.; Kirby, Matthew E.; Ghezzo, Elena; Coltrain, Joan B.; Fuller, Benjamin T.; Farrell, Aisling B.; Takeuchi, Gary T.; MacDonald, Glen; Davis, Edward B.; and Lindsey, Emily L., "Pre-Younger Dryas megafaunal extirpation at Rancho La Brea linked to fire-driven state shift" (2023). Biology Faculty Works. 216.
https://digitalcommons.lmu.edu/bio_fac/216
