Rewilding, or re-wilding, activities are conservation efforts aimed at restoring and protecting natural processes and wilderness areas. This may include providing connectivity between such areas, and protecting or reintroducing apex predators and keystone species.
Rewilding is a form of ecological restoration with an emphasis on humans stepping back and leaving an area to nature, as opposed to more active forms of natural resource management. Rewilding efforts can aim to create ecosystems requiring passive management. Successful long term rewilding projects can need little ongoing human attention, as successful reintroduction of keystone species creates a self-regulatory and self-sustaining stable ecosystem, possibly with near pre-human levels of biodiversity.
While re-wilding initiatives can be controversial, the United Nations have listed re-wilding as one of several methods needed to achieve massive scale restoration of natural ecosystems, which they say must be accomplished by 2030.
The word rewilding was coined by members of the grassroots network Earth First!, appearing in print by 1990, and was refined by conservation biologists Michael Soulé and Reed Noss in a paper published in 1998. According to Soulé and Noss, rewilding is a conservation method based on "cores, corridors, and carnivores." The concepts of cores, corridors, and carnivores were developed further in 1999. Dave Foreman subsequently wrote the first full-length book about rewilding as a conservation strategy.
Rewilding was developed as a method to preserve functional ecosystems and reduce biodiversity loss, incorporating research in island biogeography and the ecological role of large carnivores. In 1967, The Theory of Island Biogeography by Robert H. MacArthur and Edward O. Wilson established the importance of considering the size and isolation of wildlife conservation areas, stating that protected areas remained vulnerable to extinctions if small and isolated. In 1987, William D. Newmark's study of extinctions in national parks in North America added weight to the theory. The publications intensified debates on conservation approaches. With the creation of the Society for Conservation Biology in 1985, conservationists began to focus on reducing habitat loss and fragmentation.
Practice and interest in rewilding grew rapidly in the first two decades of the 21st century. Supporters of rewilding initiatives range from individuals, small land owners, local NGOs and authorities, to national governments and international NGOs such as IUCN. While small scale efforts are generally well regarded, the increased popularity of rewilding has generated controversary, especially regarding large scale projects. These have attracted criticism from academics, practicing conservationists, government officials and business people. In a June 2021 report for the launch of the UN Decade on Ecosystem Restoration, the UN listed rewilding as one of several restoration methods which they state should be used for the ecosystem restoration of over 1 Billion hectares ( a total area bigger than China.)  
Rewilding aims to restore three key ecological processes: trophic complexity, dispersal, and stochastic disturbances. Rewilding is important on land but perhaps more important is where land meets the water. Dam removal is the first of many steps in the process of rewilding in the riverine ecosystems. However, there are problems that should be addressed before, during, and after the dam removal. The problems are the sediments that have built up and wash out filling in spawning beds should be controlled and directed, then eliminating any and all clear cutting of trees near river banks as it raises the temperature of the water, and stopping industrial discharges. At 90 different dam sites it has been confirmed that after a dam is built the ecosystem does rebound. However, the trend will eventually slow, stop and in some cases decline. This is often due to anthropogenic chemical, light, and noise pollution as the large bodies of water draw human activity and recreation. Nemecek writes that, "researchers found that the number of species within any given area dropped by 50%. Lastly, food sources for native animals and fish need to be introduced so as to improve the long-term sustainability of native species and curtail and/or eliminate the introduction of invasive species.
The beaver is by far the most important element of a riverine ecosystem. Firstly, the dams they build create micro ecosystems that can be used as spawning beds for salmon and collect invertebrates for the salmon fry to feed on. The dams, again built by beavers, create wetlands for plant, insect, and bird life. Specific trees, alder, birch, cottonwood, and willow are important to beaver's diets and must be encouraged to grow in areas accessible by the animals. In terms of seeding the birds can do much of the rest. These animals have a trickle down effect as they create ecosystems that have the potential to grow exponentially.
Both grassroots groups and major international conservation organizations have incorporated rewilding into projects to protect and restore large-scale core wilderness areas, corridors (or connectivity) between them, and apex predators, carnivores, or keystone species (species which interact strongly with the environment, such as elephant and beaver). Projects include the Yellowstone to Yukon Conservation Initiative in North America (also known as Y2Y) and the European Green Belt, built along the former Iron Curtain; transboundary projects, including those in southern Africa funded by the Peace Parks Foundation; community-conservation projects, such as the wildlife conservancies of Namibia and Kenya; and projects organized around ecological restoration, including Gondwana Link, regrowing native bush in a hotspot of endemism in southwest Australia, and the Area de Conservacion Guanacaste, restoring dry tropical forest and rainforest in Costa Rica. European Wildlife, established in 2008, advocates the establishment of a European Centre of Biodiversity at the German-Austrian-Czech borders.
In North America, another major project aims to restore the prairie grasslands of the Great Plains. The American Prairie Reserve is reintroducing bison on private land in the Missouri Breaks region of north-central Montana, with the goal of creating a prairie preserve larger than Yellowstone National Park.
Dam removal has led to the restoration of many river systems in the Pacific Northwest. This has been done in an effort to restore salmon populations specifically but with other species in mind. "These dam removals provide perhaps the best example of large-scale environmental remediation in the twenty-first century. This restoration, however, has occurred on a case-by-case basis, without a comprehensive plan. The result has been to put into motion ongoing rehabilitation efforts in four distinct river basins: the Elwha and White Salmon in Washington and the Sandy and Rogue in Oregon."
An organization called Rewilding Australia has formed which intends to restore various marsupials and other Australian animals which have been extirpated from the mainland, such as Eastern quolls and Tasmanian devils.
In the 1980s, the Dutch government began introducing proxy species in the Oostvaardersplassen nature reserve, an area covering over 56 square kilometres (22 sq mi), in order to recreate a grassland ecology. Though not explicitly referred to as rewilding, nevertheless many of the goals and intentions of the project were in line with those of rewilding. The reserve is considered somewhat controversial due to the lack of predators and other native megafauna such as wolves, bears, lynx, elk, boar, and wisent. Between 800 and 1150 wild koniks live in the Oostvaardersplassen. The horses were reintroduced together with heck cattle and red deer to keep the landscape open by natural grazing. This provided habitat for geese who are key species in the wetlands of the area. The grazing of geese Made it possible for reetlands to remain and therefore conserved many protected birds species. This is a prime example how water and land ecosystems are connected and how reintroducing keystone species can conserve other protected species.
Since the 1980s, 8.5 million trees have been planted in the United Kingdom in an area of the Midlands around the villages of Moira and Donisthorpe, close to Leicester. The area is called The National Forest. Another, larger, reforestation project, aiming to plant 50 million trees is beginning in South Yorkshire, called The Northern Forest. Despite this, the UK government has been criticised for not achieving its tree planting goals. There have also been concerns of non-native tree planting disturbing the ecological integrity and processes of what would be a native habitat restoration. Knepp Castle and Roy Dennis Wildlife Foundation have overseen reintroductions of extinct bird species in the UK.
In 2011, the 'Rewilding Europe' initiative was established with the aim of rewilding 1 million hectares of land in ten areas including the western Iberian Peninsula, Velebit, the Carpathians and the Danube delta by 2020, mostly abandoned farmland among other identified candidate site. The present project considers only species that are still present in Europe, such as the Iberian lynx, Eurasian lynx, grey wolf, European jackal, brown bear, chamois, Iberian ibex, European bison, red deer, griffon vulture, cinereous vulture, Egyptian vulture, great white pelican and horned viper, along with a few primitive breeds of domestic horse/Przewalski's horse and cattle as proxies for the extinct tarpan and aurochs. Since 2012, Rewilding Europe is heavily involved in the Tauros Programme, which seeks to recreate the phenotype of the aurochs, the wild ancestors of domestic cattle by selectively breeding existing breeds of cattle. Many projects also employ domestic water buffalo as a grazing proxy for the extinct European water buffalo.Reviving Europe.
In 2010 and 2011, an unrelated initiative in the village of San Cebrián de Mudá (190 inhabitants) in Palencia, northern Spain released 18 European bison (a species extirpated from Spain since the Middle Ages) in a natural area already inhabited by roe deer, wild boar, red fox and grey wolf, as part of the creation of a 240-hectare "Quaternary Park". Three Przewalski's horses from a breeding center in Le Villaret, France were added to the park in October 2012. Onagers and "aurochs" were planned to follow.
In 2016 and 2018, the True Nature Foundation reintroduced in total 7 European bison of the Lowland-Caucasian breeding line in Anciles Wildlife Reserve in the Parque Regional de Picos de Europa in the Cantabrian mountains in northern Spain.
In 2020, nature writer Melissa Harrison reported a significant increase in attitudes supportive of rewilding among the British public, with plans recently approved for the release of European bison, Eurasian elk, and great bustard in England, along with calls to rewild as much as 20% of the land in East Anglia, and even return apex predators to the UK, such as the Eurasian lynx, brown bear, and grey wolf.
Pleistocene rewilding was proposed by the Brazilian ecologist Mauro Galetti in 2004. He suggested the introduction of elephants (and other proxies of Extinct megafauna) from circuses and zoos to private lands in the Brazilian cerrado and other parts of the americas. In 2005, stating that much of the original megafauna of North America including mammoths, ground sloths, and Smilodons became extinct after the arrival of humans, Paul S. Martin proposed restoring the ecological balance by replacing them with species which have similar ecological roles, such as the Asian elephant, or the African elephants.
A reserve now exists for formerly captive elephants on the Brazilian Cerrado
A controversial 2005 editorial in Nature, signed by a number of conservation biologists, took up the argument, urging that Elephants, lions, and cheetahs could be reintroduced in protected areas in the Great Plains. The Bolson tortoise, discovered in 1959 in Durango, Mexico, was the first species proposed for this restoration effort, and in 2006 the species was reintroduced to two ranches in New Mexico owned by media mogul Ted Turner. Other proposed species include various camelids such as the Wild Bactrian camel, and various equids such as the prezwalski's horse.
In 1988, researcher Sergey A. Zimov established Pleistocene Park in northeastern Siberia to test the possibility of restoring a full range of grazers and predators, with the aim of recreating an ecosystem similar to the one in which mammoths lived. Yakutian horses, reindeer, European bison, plains bison, Domestic yak, moose, and Bactrian camels were reintroduced, and reintroduction is also planned for saigas, wood bison, and Siberian tigers. The wood bison, a close relative of an ancient bison called the steppe bison that died out in Siberia 1000 or 2000 years ago, is also an important species for the ecology of Siberia. In 2006, 30 bison calves were flown from Edmonton, Alberta to Yakutsk and placed in the government-run reserve of Ust'-Buotama. This project remains controversial -- a letter published in Conservation Biology accused the Pleistocene camp of promoting "Frankenstein ecosystems," stating that "the biggest problem is not the possibility of failing to restore lost interactions, but rather the risk of getting new, unwanted interactions instead."
In 1982 Daniel Janzen and Paul S. Martin originated the concept of evolutionary anachronism in a Science article published in 1982, titled "Neotropical Anachronisms: The Fruits the Gomphotheres Ate". Eighteen years later, Connie C. Barlow in her book The Ghosts of Evolution: Nonsensical Fruit, Missing Partners, and Other Ecological Anachronisms (2000), explored the specifics of temperate North American plants whose fruits displayed the characteristics of megafauna dispersal syndrome. Barlow noted that a consequence for such native fruits following the loss of their megafaunal seed dispersal partners was range constriction during the Holocene, made increasingly severe since the mid-20th century by rapid human-driven climate change. Additional details of range contraction were incorporated in Barlow's 2001 article, "Anachronistic Fruits and the Ghosts Who Haunt Them".
A plant species beset with anachronistic features whose range had already become so restricted that it warranted classification as a glacial relict is Torreya taxifolia. For this species, Barlow and Paul S. Martin advocated for assisted migration poleward in an article published in Wild Earth in 2004, titled "Bring Torreya taxifolia North Now". Beginning in 2005 Barlow and Lee Barnes (co-founders of Torreya Guardians ) began obtaining seeds from mature horticultural plantings in states northward of Florida and Georgia and distributing seeds to volunteer planters whose lands contained forested habitats potentially suitable for this native of Florida. Documentation of seed distribution and ongoing results, state by state, are publicly available on the Torreya Guardians website.) An article published in Scientific American in 2009 referred to the actions of Torreya Guardians as an example of "rewilding". Connie Barlow expressly referred to such efforts as "rewilding" in the 2020 book by Zach St. George, The Journeys of Trees. Her earliest reference to the term "rewilding" was in her 1999 essay, "Rewilding for Evolution,"in Wild Earth. Because part of Barlow's personal seed plantings occurred on private land for which she did not expressly obtain planting permission, this form of rewilding action could be referred to as guerrilla rewilding, which is an adaptation of the established term guerrilla gardening.
A view expressed by some national governments and officials within multilateral agencies such as the United Nations, is that excessive rewilding, such as large rigorously enforced protected areas where no extraction activities are allowed, can be too restrictive on people's ability to earn sustainable livelihoods.
Some farmers have been critical of rewilding for "abandoning productive farmland when the world's population is growing". Farmers have also attacked plans to reintroduce the lynx in the United Kingdom because of fears that reintroduction will lead to an increase in sheep predation.
Rewilding has been criticized by animal rights scholars, such as Dale Jamieson, who argues that "most cases of rewilding or reintroducing are likely to involve conflicts between the satisfaction of human preferences and the welfare of nonhuman animals." Erica von Essen and Michael Allen, using Donaldson and Kymlicka's political animal categories framework, assert that wildness standards imposed on animals are arbitrary and inconsistent with the premise that wild animals should be granted sovereignty over the territories that they inhabit and the right to make decisions about their own lives. To resolve this, Essen and Allen contend that rewilding needs to shift towards full alignment with mainstream conservation and welcome full sovereignty, or instead take full responsibility for the care of animals who have been reintroduced. Ole Martin Moen argues that rewilding projects should be brought to an end because they unnecessarily increase wild animal suffering and are expensive, and the funds could be better spent elsewhere.
The environmental historian Dolly Jørgensen argues that rewilding, as it currently exists, "seeks to erase human history and involvement with the land and flora and fauna. Such an attempted split between nature and culture may prove unproductive and even harmful." She calls for rewilding to be more inclusive to combat this. Jonathan Prior and Kim J. Ward challenge Jørgensen's criticism and provide existing examples of rewilding programs which "have been developed and governed within the understanding that human and non-human world are inextricably entangled".
Some conservationists have expressed concern that rewilding "could replace the traditional protection of rare species on small nature reserves", which could potentially lead to an increase in habitat fragmentation and species loss. David Nogués-Bravo and Carsten Rahbek assert that the benefits of rewilding lack evidence and that such programs may inadvertently lead to "de-wilding", through the extinction of local and global species. They also contend that rewilding programs may draw funding away from "more scientifically supported conservation projects".