Dozens of megafauna (large animals over 100 pounds) -- such as giant tortoises, horses, elephants and cheetah -- went extinct in North America 13,000 years ago during the end of the Pleistocene. As is the case today in Africa and Asia, these megafauna likely played keystone ecological roles via predation, herbivory and other processes. What are the consequences of losing such important components of America's natural heritage?