Towards the end of the Pleistocene era (roughly 13,000 to 10,000 years ago), nearly all megafauna of Eurasia, Australia, and South/North America, dwindled towards extinction, in what has been referred to as the Quaternary extinction event. With the loss of large herbivores and predator species, niches important for ecosystem functioning were left unoccupied. In the words of the biologist Tim Flannery, "ever since the extinction of the megafauna 13,000 years ago, the continent has had a seriously unbalanced fauna". This means, for example, that the managers of national parks in North America have to resort to culling to keep the population of ungulates under control.
Paul S. Martin (originator of the Pleistocene overkill hypothesis) states that present ecological communities in North America do not function appropriately in the absence of megafauna, because much of the native flora and fauna evolved under the influence of large mammals.
Research shows that species interactions play a pivotal role in conservation efforts. Communities where species evolved in response to Pleistocene megafauna (but now lack large mammals) may be in danger of collapse. Most living megafauna are threatened or endangered; extant megafauna have a significant impact on the communities they occupy, which supports the idea that communities evolved in response to large mammals. Pleistocene rewilding could "serve as additional refugia to help preserve that evolutionary potential" of megafauna. Reintroducing megafauna to North America could preserve current megafauna, while filling ecological niches that have been vacant since the Pleistocene.
Pleistocene rewilding aims at the promotion of extant fauna and the reintroduction of extinct genera in the southwestern and central United States. Native fauna are the first genera proposed for reintroduction. The Bolson tortoise was widespread during the Pleistocene era, and continued to be common during the Holocene epoch until recent times. Its reintroduction from northern Mexico would be a necessary step to recreate the soil humidity present in the Pleistocene, which would support grassland and extant shrub-land and provide the habitat required for the herbivores set for reintroduction. Other large tortoise species might later be introduced to fill the roll of various species of Hesperotestudo. However, to be successful, ecologists must first support fauna already present in the region.
The pronghorn, which is extant in most of the west after almost becoming extinct, is crucial to the revival of the ancient ecosystem. Pronghorns are native to the region, which once supported large numbers of the species and extinct relatives of the same family. It would occupy the great plains and other arid regions of the west and southwest.