Before humans entered the picture, North America had an impressive assortment of large mammals and birds. The herbivores of this megafauna included 3 species of elephants (woolly mammoths, giant mammoths, and mastodons), horses, camels, giant bison, giant ground sloths, giant armadillos, tapirs, giant beaver, giant tortoises (roughly the size of Volkswagon bugs), and a peccary as large as the wild boars of Europe. An entire guild of now extinct mega-predators existed to feed on these large herbivores, including cheetahs, saber-toothed tigers, giant wolves, and two species of lion (one larger than the modern lions of Africa). There also existed a truly fearsome short-nosed bear, about twice the size of a modern grizzly bear, which ran its prey down like modern wolves do. Jaguars lived far north of their current tropical latitudes, into the boreal forests of Canada, as did many of the New World cats now restricted to Central and South America. There also existed a guild of large meat-eating birds, the largest of which were the teratorns, scavengers with wingspans up to five meters. The endangered California condor is the last remnant of these giant scavenger birds. There was even a giant vampire bat adapted to feeding off the blood of these enormous beasts.
The fate of all these species has been the topic of much scientific debate, but the majority of the evidence supports the hypothesis of “Pleistocene Overkill” (Martin and Wright 1967, Flannery 2001). This hypothesis suggests that as humans spread across the two continents, they preyed upon the large herbivores, such as mammoths, ground sloths, and horses, and wiped them out. Such large animals are more vulnerable to extinction than smaller ones because they cannot hide as easily, and because their lower reproductive rates cannot compensate for the losses due to hunting. They also may have had a fearlessness of humans, somewhat like the dodo bird, because these animals evolved with out human presence. When the large herbivores disappeared, their natural predators, such as saber-toothed tigers and short-nosed bears, became extinct as well. The large scavenger bird species, adapted to eating the remains of large animals, then followed into extinction. The California condor may have held on because it had access to the carcasses of marine mammals, which did not suffer high extinction rates at this time. The loss of the megafauna also impacted the diversity of smaller animals. Because large abundant animals (such as mammoths) alter plant communities by their intense grazing practices, their disappearance caused a major shift in the plant communities (e.g., from prairie to forest) resulting in the extinction of many smaller species that depended on the habitats maintained by the large grazers. In fact, there existed a grassland ecosystem in Alaska called the mammoth steppe that disappeared entirely once the woolly mammoth went extinct in that region, which is attributed the change in ecosystem processes that occurred when this keystone herbivore was lost (Flannery 2001).