A LATE PLEISTOCENE CHRONOLOGY OF BISON PRISCUS FROM THE KLONDIKE REGION OF NORTHERN CANADA: NICHE STABILITY THROUGH CHANGING CLIMATE

Terrestrial fossil records rarely have adequate sample size and temporal resolution to address how species respond to changing environmental conditions at millennial time scales. Here, we present a novel ~40,000 year time series of steppe bison (Bison priscus) paleoecology from the rich permafrost deposits of the Klondike region of Yukon, Canada. We hypothesized that niche flexibility through changing environmental conditions may explain how Bison survived the Late Quaternary Extinction event and persist within a wide range of grazing to mixed-feeding niches. After radiocarbon dating 88 B. priscus specimens, we tested whether environmental changes (approximated by the Greenland ice core record [temperature] and specimen δ¹⁵N values [moisture]) resulted in changes to population size, body size, and/or diet. We estimated changes in population size using modeled age frequency distributions of the radiocarbon dated fossils. We assessed body size using molar measurements and dietary niche using stable isotopes (carbon and nitrogen) and dental wear (mesowear and Dental Microwear Texture Analysis). Most specimens date from Marine Isotope Stage 3 interstadial (MIS3) with a second abundance pulse during the Bølling Allerød; abundance is lowest during the Last Glacial Maximum (LGM). These trends are consistent with ancient DNA-based population estimates. Body size is weakly positively correlated with relative abundance. The highest δ¹⁵N value (7.2 ‰) occurs during the LGM, likely indicating drier habitats. During the Bølling Allerød, δ¹⁵N values decrease significantly (mean: 2.2 ‰), indicating wetter conditions. We found that mean δ¹³C and dental wear values are consistent through time, indicating relative stability in diet. While mean values for diet and body size proxies remained stable, variance was significantly and universally reduced during the Bølling Allerød. Reduced variance suggests that B. priscus may have become restricted to graminoid-dominated ‘refugia’ within the expanding shrublands and boreal forest towards the end of the Pleistocene. We conclude that B. priscus did not respond to changing environmental conditions by shifting their dietary niche; rather, their populations thrived when conditions were favorable and suffered contraction during the coldest and driest glacial period.