PALEOECOLOGY OF COLUMBIAN MAMMOTHS (MAMMUTHUS COLUMBI) IN SOUTHERN NEVADA AND CALIFORNIA: HOW TERMINAL PLEISTOCENE ECOSYSTEMS SHAPED MAMMOTHS AT THE POPULATION LEVEL

We use this study to examine the population dynamics of southwestern Columbian mammoths (Mammuthus columbi) by testing a novel hypothesis developed by Ripple and Van Valkenburgh (2010). This hypothesis proposes that North American Pleistocene carnivores kept megafaunal herbivore populations below carrying capacity, and a trophic cascade could have caused ecological collapse mediated by prey-switching. The more conventional hypotheses for megafaunal extinction include human over-kill and climate-change, however they both rely on the assumption that population sizes of megafaunal herbivores were resource-dependent.

We constructed age profiles to evaluate time-averaged assemblages of fossil Columbian mammoth molar teeth from the Las Vegas Formation and Rancho La Brea for population dynamics and mortality patterns. These fossils are part of collections housed at the Las Vegas Natural History Museum and La Brea Tar Pits and Museum, respectively. Fossil mammoth molariform teeth were selected for this study if they were complete or complete enough to determine the tooth assignment and percentage of wear. Tooth assignments were made based on length, width, height, and tooth plate count measurements, based on an extensive literature review of proboscidean dental progression and wear.

The age profiles from the Las Vegas Formation show selective mortality of the youngest age cohorts which suggests Pleistocene megafaunal predators may have applied top-down controls on Columbian mammoth populations that would have kept population sizes below the carrying capacity. We do not interpret our results from this site to represent death from extreme drought or environmental stress. The age profiles from Rancho La Brea show opposite mortality patterns, the cause of which needs further investigation.

Emerging results from this study and other authors’ datasets from mammoths and mastodons (Mammut americanum) in the Midwest suggest that during the Late Pleistocene, proboscidean population sizes were controlled from the top of the food chain down. Trophic cascades toward the Holocene may have made individual populations unstable, and prone to extinction. The ultimate cause for the Pleistocene megafaunal extinction remains unclear, however, we are able to add regional data to the larger framework of mammoth paleoecology.