NICHE NARROWS IN RECOVERY FROM POPULATION CRASH: A RETREAT TO THE ‘BIOLOGICAL OPTIMUM’ FOR KLONDIKE BISON PRISCUS?

Bison have endured dramatic changes throughout their history in North America. The Pleistocene ancestors of today’s bison, known as steppe bison (Bison priscus) from the Klondike region of Yukon, Canada experienced a catastrophic population crash and associated genetic bottleneck during the last glaciation, ca. 29 – 18 thousand years ago. Using a novel fossil timeseries (~50 – 11 thousand years ago [kya]) and a multi-proxy approach, we take advantage of this natural experiment to test the ecological response to severe population change. As the Yukon bison population recovered from their genetic bottleneck, proxies for diet (dental mesowear, microwear, and stable carbon isotope ratios) and body size (molar length) all show significantly reduced variance compared to the “baseline” period of high bison abundance ~50 – 30 kya. Our data also indicate that recovering populations of bison focused on the same dietary resources (i.e., dietary proxies maintained constant mean values) across glacial-interglacial changes in environment and in the face of dramatic population change. Reduced ecological variance may have been the phenotypic expression of lower genetic diversity. Alternatively or additionally, tighter clustering around proxy mean values could be indicative of instraspecies competitive release, allowing bison to occupy only their optimal niche during a return to ideal environmental conditions paired with low (but increasing) population size. We conclude that B. priscus did not respond to changing environmental conditions and population fluctuations by shifting their dietary niche; rather, their populations thrived when conditions were warmer and wetter and suffered contraction during the coldest and driest glacial climates.