Great American Interchange - Late Pleistocene Extinctions

Late Pleistocene Extinctions

At the end of the Pleistocene epoch, about 12,000 years ago, three dramatic developments occurred in the Americas at roughly the same time (geologically speaking). Paleoindians invaded and occupied the New World, the last glacial period came to an end, and a large fraction of the megafauna of both North and South America went extinct. This wave of extinctions swept off the face of the Earth many of the successful participants of the Great American Interchange, as well as other species that had not migrated. All the pampatheres, glyptodontids, ground sloths, equids, proboscids, dire wolves, lions and Smilodon species of both continents disappeared. The last of the South and Central American notoungulates and litopterns died out, as well as North America's giant beavers, dholes, native cheetahs, scimitar cats, and many of its antilocaprid, bovid, cervid, tapirid and tayassuid ungulates. Some groups disappeared over most or all of their original range but survived in their adopted homes, e.g. South American tapirs, camelids and tremarctine bears (cougars and jaguars may have been temporarily reduced to South American ranges also). Others, such as capybaras, survived in their original range but died out in areas they had migrated to. Notably, this extinction pulse eliminated all Neotropic migrants to North America larger than about 15 kg (the size of a big porcupine), and all native South American mammals larger than about 65 kg (the size of a big capybara or giant anteater). In contrast, the largest surviving native North American mammal, the wood bison, can exceed 900 kg, and the largest surviving Nearctic migrant to South America, Baird's tapir, can reach 400 kg.

The near-simultaneity of the megafaunal extinctions with the glacial retreat and the peopling of the Americas has led to proposals that both climate change and human hunting played a role. Although the subject is contentious, a number of considerations suggest that human activities were pivotal. The extinctions did not occur selectively in the climatic zones that would have been most affected by the warming trend, and there is no plausible general climate-based megafauna-killing mechanism that could explain the continent-wide extinctions. The climate change took place worldwide, but had little effect on the megafauna in areas like Africa and southern Asia, where megafaunal species had coevolved with humans. Numerous very similar glacial retreats had occurred previously within the ice age of the last several Ma without ever producing comparable waves of extinction in the Americas or anywhere else. Similar megafaunal extinctions have occurred on other recently populated land masses (e.g. Australia, Japan, Madagascar, New Zealand, and many smaller islands around the world, such as Cyprus, Crete, Tilos and New Caledonia) at different times that correspond closely to the first arrival of humans at each location. These extinction pulses invariably swept rapidly over the full extent of a contiguous land mass, regardless of whether it was an island or a hemisphere-spanning set of connected continents. This was true despite the fact that all the larger land masses involved (as well as many of the smaller ones) contained multiple climatic zones that would have been affected differently by any climate changes ongoing at the time. However, on sizable islands far enough offshore from newly occupied territory to escape immediate human colonization, megafaunal species sometimes survived for many thousands of years after they or related species became extinct on the mainland; examples include giant kangaroos in Tasmania, giant Chelonoidis tortoises of the Galápagos Islands (formerly also of South America), giant Dipsochelys tortoises of the Seychelles (formerly also of Madagascar), giant meiolaniid turtles on Lord Howe Island, New Caledonia and Vanuatu, ground sloths on the Antilles, Steller's sea cows off the Commander Islands and woolly mammoths on Wrangel Island and Saint Paul Island. The glacial retreat may have played a primarily indirect role in the extinctions in the Americas by simply facilitating the movement of humans southeastward from Beringia down to North America. The reason that a number of groups went extinct in North America but lived on in South America (while there are no examples of the opposite pattern) appears to be that the dense rainforest of the Amazon basin and the high peaks of the Andes provided environments that afforded a degree of protection from human predation.