Why The Big Animals Went Down In The Pleistocene: Was It Just The Climate?

Russell Graham, who has been working on climate models for Pleistocene extinction for almost 30 years, looked for triggers in a threshold effect that did not require a unique climate change. Graham, Chief Curator at the Denver Museum of Nature and Science, will present his research on Wednesday, November 7, at the Geological Society of America's annual meeting in Boston, Massachusetts.

"The end Pleistocene climate change, especially the Younger Dryas [a sudden cold period], was a trigger that tipped the balance," he explained. "Also, the climate model needed to answer the question of why big animals--mammoths, mastodons, ground sloths, etc., were the primary ones to go extinct and not the small ones. The answer to this question is the relationship between geographic range and body size. The larger an animal, the more real estate or geographic range it needs to support viable populations, especially in harsh environments like those of the Pleistocene…. Therefore, if the geographic range of animals decreased through time then their probability of extinction would increase with time."

Graham successfully tested his hypothesis by using a computer database of fossil ice age mammal sites linked with a geographic information system to map changes in the distribution of species throughout time.

"This is one of the first models that does not require a unique climate change at the end of the Pleistocene. To my knowledge it is one of the first to look at geographic range changes of a large number of mammal species as the primary driving factor of the extinction."