TRIGGERS, THRESHOLDS, RUSSIAN ROULETTE, AND PLEISTOCENE EXTINCTIONS

The primary concerns for endangered species today are habitat loss and geographic range reduction. These are the same concerns that the Pleistocene megafauna faced about 11ka radiocarbon. However, in the Pleistocene, habitat loss and reductions in geographic range were driven by climate and environmental change; whereas, today humans are one of the primary driving forces. Continuous climate change throughout the Pleistocene selected for the evolution of new habitat mosaics and the "extinction" of old ones (eg., parklands, savannas, etc.). In addition, these environmental changes reduced the ranges of both large and small mammals but because the probability of extinction is a function of geographic range and body size, large mammals were preferentially selected for extinction. With this model, climate change at the end of the Pleistocene need not be unique but it probably served as a trigger for a threshold effect. Once populations dropped below critical levels, the megafauna was doomed, irrespective of human involvement!