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date: 11 July 2020


Summary and Keywords

Rewilding aims at maintaining or even increasing biodiversity through the restoration of ecological and evolutionary processes using extant keystone species or ecological replacements of extinct keystone species that drive these processes. It is hailed by some as the most exciting and promising conservation strategy to slow down or stop what is considered to be the greatest mass extinction of species since the extinction of the dinosaurs 65 million years ago. Others have raised serious concerns about the many scientific and societal uncertainties and risks of rewilding. Moreover, despite its growing popularity, rewilding has made only limited inroads within the conservation mainstream and still has to prove itself in practice.

Rewilding differs from traditional restoration in at least two important respects. Whereas restoration has typically focused on the recovery of plants communities, rewilding has drawn attention to animals, particularly large carnivores and large herbivores. Whereas restoration aims to return an ecosystem back to some historical condition, rewilding is forward-looking rather than backward-looking: it examines the past not so much to recreate it, but to learn from the past how to activate and maintain the natural processes that are crucial for biodiversity conservation.

Rewilding makes use of a variety of techniques to re-establish these natural processes. Besides the familiar method of reintroducing animals in areas where populations have decreased dramatically or even gone extinct, rewilders also employ some more controversial methods, including back breeding to restore wild traits in domesticated species, taxon substitution to replace extinct species by closely related species with similar roles within an ecosystem, and de-extinction to bring extinct species back to life again using advanced biotechnological technologies such as cloning and gene editing.

Rewilding has clearly gained the most traction in North America and Europe, which have several key features in common. Both regions have recently experienced a spontaneous return of wildlife. Rewilders on both sides of the Atlantic are aware, however, that this wildlife resurgence is not that impressive, given that we are in the midst of the sixth mass extinction, which is characterized by the loss of large-bodied animals known as megafauna. The common goal is to bring back such megafaunal species because of their importance for maintaining and enhancing biodiversity. Last, both North American and European rewilders perceive the extinction crisis through the lens of the island theory, which shows that the number of species in an area depends on its size and degree of isolation—hence their special attention to the spatial aspects of rewilding.

But rewilding projects on both sides of the Atlantic not only have much in common, they also differ in certain aspects. North American rewilders have adopted the late Pleistocene as a reference period and have emphasized the role of predation by large carnivores, while European rewilders have opted for the mid-Holocene and put more focus on naturalistic grazing by large herbivores.

Keywords: wildlife recovery, sixth mass extinction, megafauna ecology, conservation biology, island biogeography, Pleistocene rewilding, naturalistic grazing, de-extinction


Rewilding aims at maintaining or even increasing biodiversity through the restoration of ecological and evolutionary processes using extant keystone species or ecological replacements of extinct keystone species that drive these processes. It has been welcomed as the most exciting and promising conservation strategy to slow down or stop what is considered to be the greatest mass extinction of species since the extinction of the dinosaurs 65 million years ago—a “Marshall Plan for the planet” (Fraser, 2009, p. 14).

The most outspoken contemporary proponent of rewilding is George Monbiot, a journalist and activist writer who published Feral: Searching for Enchantment on the Frontiers of Rewilding in 2013. This book has fueled the growing popularity of rewilding around the world by invoking the public’s passion for wildlife and wilderness. Despite this growing popularity, rewilding has yet made only limited inroads within the conservation mainstream and still has to prove itself in practice (Murray, 2017).

Moreover, rewilding has raised serious concerns. Opponents argue that there is only limited scientific support for the science behind rewilding. It is feared that the introduction of megafauna from other regions as proxies for extinct species could negatively affect native environments and its species populations. There are also concerns that the attention for the restoration of extinct species will go at the cost of the attention for the more traditional conservation strategies aimed at the protection of extant species (Nogués-Bravo, Simberloff, Rahbek, & Sanders, 2016).

Although rewilding research and interventions can be found worldwide, there is a significant geographic bias with a strong focus on North America and Europe, and, to a lesser degree, on oceanic islands (Lorimer, Sandom, Jepson, Doughty, Barua, & Kirby, 2015, p. 39; Svenning et al., 2016, p. 900). North America and Europe have featured so prominently because they share some important characteristics, on the basis of which a coherent guiding framework has emerged for rewilding initiatives in other continents.

The first similarity concerns the spontaneous return of wildlife since the Second World War. Two developments played an important role in this wildlife comeback: the reforestation and revegetation of both continents as a result of the widespread abandonment of marginal agricultural land, and the rise of the environmental movement in the 1970s, which successfully campaigned for the legal protection of species and their habitats (see section “The Recovery of Wildlife”).

The second similarity concerns the awareness on both sides of the Atlantic that this remarkable wildlife resurgence, however welcome it may be, represents little more than a drop in the ocean, and that planet Earth is in the midst of the so-called sixth mass extinction. Such an extinction event, involving the loss of three-quarters of plant and animal species on Earth, has occurred only five times in the past. Today’s extinction crisis, which started at the end of the last Ice Age, differs in two respects from the previous ones: it has mainly been caused by a single species (Homo sapiens), and it has been characterized by the loss of large-bodied animals known as megafauna (see section “The Sixth Mass Extinction”).

Both North American and European rewilders have increasingly come to understand the importance of large carnivores and large herbivores for preserving biodiversity and for maintaining ecosystem integrity. Their first priority is to “bring back” these megafaunal species that have extirpated locally or globally (see section “The Ecological Role of Megafauna”).

Finally, both North American and European rewilders perceive the actual extinction crisis through the lens of the theory of island biogeography. This theory demonstrates that the number of species on a given island will be lower as the size of the island is smaller and the distance to the mainland is longer. This theory became very popular among rewilders because nature areas have increasingly developed into “islands in a sea of cultivated land” due to the ongoing fragmentation, reduction, and destruction of wildlife habitats. Consequently, they attach great importance to the spatial aspects in their efforts to reverse the ongoing defaunation by rewilding (see section “The Influence of Island Biogeography”).

Although rewilding projects on both sides of the Atlantic have much in common, they differ in some important respects. North American and European rewilders employ two distinct historical baselines: in North America, the late Pleistocene has been adopted as baseline; in Europe, it is the mid-Holocene landscape that functions as the main benchmark. Furthermore, in North America, the emphasis is on the role of large carnivores in the top-down regulation of ecosystems; in Europe, the focus is on the role of large herbivores in this regulation (see section “Different Accents”).

Around the mid-2000s, some prominent North American conservation biologists launched the idea of “Pleistocene rewilding.” They denounce the adoption of Columbus’ arrival in 1492 as restoration baseline because this ignores the significant earlier impacts by humans. They argue that the late Pleistocene arrival of the first modern humans 13,000 bp (before present) constitutes a less arbitrary benchmark and propose the introduction of ecological substitutes for some of the North American megafauna that vanished after the arrival of these people (see section “Pleistocene Rewilding”).

European rewilders attach the utmost importance to naturalistic grazing with large and hardy herbivores outside of field-based farming systems. They dispute the widely accepted belief that lowland Europe was originally covered by dense, closed-canopy primal forests because the impact of large herbivores on the forest landscape was supposed to be negligible. By contrast, European rewilders are convinced that the impact of large herbivores on post-glacial woodland vegetation was strong enough to create and maintain a mosaic landscape of grasslands with shrubs and small forested patches. This landscape vanished with the rise of livestock farming, at the expense of wildlife habitat (see section “Naturalistic Grazing”).

An initiative that is worth mentioning separately is “Pleistocene Park,” a nature reserve located in northeastern Siberia. This project shows a somewhat mixed character: it starts from the same baseline as North-American rewilders—the Pleistocene-Holocene boundary—while at the same time using naturalistic grazing, the most important tool of European rewilders. Pleistocene Park aims at changing the current mossy tundra back to the grassy steppe that was dominant during the Ice Age by the reintroduction of large herbivores that have vanished mainly due to late Pleistocene hunters (see section “The Return of the Mammoth Ecosystem”).

The latest trend in rewilding that attracts attention and raises many questions is de-extinction—the attempt to bring extinct species back to life again. Rewilders make use of two methods from synthetic biology: cloning by somatic cell nuclear transfer (SCNT) and genome editing with the so-called CRISPR-Cas9 technique. Another, more traditional and less controversial de-extinction method is back breeding, a method that is mainly used in Europe to revive the aurochs (see section “De-Extinction”).

The Recovery of Wildlife

As already stated in the introduction, an important similarity between rewilders from both sides of the Atlantic concerns the spontaneous recovery of wildlife since the Second World War.

North America

In North America, the stage for wildlife comebacks was set by what Jim Sterba (2012, p. xxiii) has called “one of the greatest reforestations the planet has seen.” Forest regeneration started in the last century with the abandonment of marginal land that had been cleared for farming in eastern U.S., where most American forests were and are. Already by the mid-19th century, nearly half to more than two-thirds of the landscape was reforested. By early 20th century, Sterba argues, the forests in North America are about the same size as when Christopher Columbus arrived.

The transformation from farms back to forests was an important driver of wildlife renewal. Another important driver was the environmental movement, which called for the protection of wildlife threatened by pollution, overharvesting, habitat loss, and other forms of human abuse. This movement gained momentum in the 1960s and 1970s, as is evident, for example, from the adoption of the Wilderness Act (1964) and the Endangered Species Act (1973).

Sterba mentions yet another driver for wildlife comebacks: the dispersal of people out of cities that began after WWII and reached a milestone at the millennium’s end, “when an absolute majority of the American people lived not in cities, not on farms, but in an ever-expanding suburban and exurban sprawl in between” (Sterba, 2012, pp. xxii–xxiii). This sprawl had everything that species need to flourish such as food and shelter, a situation that has “turned backyards into battlegrounds,” as the subtitle of Sterba’s book suggests.

Here are some figures to illustrate the miraculous return of wildlife in the United States since the early 1900s. The number of beaver has increased from 100,000 to between 10 and 15 million today; the population of white-tailed deer has grown from about 500,000 to between 25 and 30 million; and black bears that had only survived in small remnant populations have rebounded to between 800,000 and more than 1 million.


In Europe, the stage for a massive recolonization by wildlife was also set by the reforestation and revegetation of the continent as a result of rural depopulation. The EU’s Common Agricultural Policy (CAP), created in 1962, has been a significant factor driving the intensification of farming and the ensuing abandonment of agricultural land of low productivity, especially in mountain areas (MacDonald et al., 2000). There has been a decline of 17% of the rural population since 1961, while some parishes of Mediterranean mountain areas have lost more than 50% of their population in this period (Navarro & Pereira, 2012, p. 901).

Again, just as in the North American case, another important driver of wildlife comeback next to rural abandonment has been the rise of the environmental movement in the 1970s, which provided public support for various pan-European agreements for the legal protection of species and nature areas, the most important ones being the Bern Convention, which was administered by the Council of Europe in 1982, and the Habitats Directive, which was adopted by the European Union in 1992 (Chapron et al., 2014, p. 1518).

As a result of these changes in agricultural and environmental policy, there has been a clear increment of large herbivore populations. This was a precondition for the recovery of large carnivores such as the brown bear, the grey wolf, and the Eurasian lynx—the so-called “Big Three.” These predatory species had vanished during the 18th and 19th centuries from almost all European regions because of direct persecution, ongoing deforestation, and the destruction of wild herbivore populations. Today, the sizes of these populations vary from some tens to many thousands, with current estimates being around 17,000 bears, 12,000 wolves, and 9,000 lynx in Europe (excluding Belarus, Ukraine, and Russia) (Chapron et al., 2014, pp. 1517–1518).

The Sixth Mass Extinction

Despite the picture of large-scale wildlife comeback at both sides of the Atlantic, one should be cautious not to overestimate the significance of this event. The recent increases in abundance and range of some species need to be evaluated in the context of dramatic historical declines: most recovering species have not yet reached the level necessary to secure sound and self-sustaining populations, while many other species are still declining (Deinet et al., 2013).

In fact, we are in the midst of the so-called sixth mass extinction. Paleontologists speak of a mass extinction when the Earth loses more than three-quarters of its species. This has happened only five times in the past 540 million years, the last time 65 million years ago when dinosaurs disappeared from this planet. There is a growing consensus among scientists that the 75% benchmark could be reached within just a few centuries (Barnosky et al., 2011).

Some experts have estimated the current extinction rate, at one thousand to ten thousand times the normal rate between mass extinctions, the so-called background extinction rate (see e.g., Wilson, 1992, p. 280). Such estimates have been criticized for using assumptions that might overestimate the severity of the extinction crisis. To avoid accusations of exaggerating the impact of humans on the biosphere and raising alarmist extinction fears, a relatively new study used extremely conservative assumptions to assess whether human activities are causing a mass extinction. This study confidently concludes “that modern extinction rates are exceptionally high, that they are increasing, and that they suggest a mass extinction under way—the sixth of its kind in Earth’s 4.5 billion years of history” (Ceballos et al., 2015, p. 3).

To fully assess the current situation, one should not only consider the extinction of species but also the declines in numbers and sizes of populations. The 11th edition of the World Wildlife Fund Living Planet Report from 2016 shows that the population sizes of vertebrate species have dropped by almost 60% in little more than 40 years, and are likely to reach 67% by the end of the decade.1 When we consider that population extinctions are a prelude to species extinctions, it becomes clear that “the window for effective action is very short, probably two or three decades at most” (Ceballos, Ehrlich, & Dirzo, 2017, p. 7).

It is a common misunderstanding that the current extinction crisis started during the Age of Discovery, from the end of the 15th century to the 18th century, with extensive European overseas exploration and colonization, whereas this event, in fact, stretches back to the megafauna extinctions that occurred near the Pleistocene-Holocene boundary ca. 13,000 bp (Foreman, 2004, p. 59).2

Four hypotheses have been proposed for the tremendous losses of megafauna species during the late Pleistocene and early Holocene, known tongue-in-cheek as overkill, overchill, overill, and overgrill. The overkill hypothesis puts the blame for megafaunal extinctions on the spread of modern humans (Homo sapiens). The overchill hypothesis states that climate changes at the end of the Pleistocene epoch triggered the megafaunal collapse; the overill hypothesis supposes that the megafauna were wiped out by some “hyper-disease”; and the overgrill hypothesis claims that a comet impact or airburst over North America did it (Wolverton, 2010).

With little or no evidence to support the latter two as credible explanations, recent research has focused on climate change and the expansion of hominins, especially Homo sapiens. The outcome of this research is that the megafauna extinctions are strongly linked to the prehistoric geographic distribution of hominins and only weakly to inter-glacial climate change. There is a significant difference in the magnitude of megafauna extinctions between sub-Saharan Africa, where hominins and megafauna have long coexisted, and Australia and the Americas, where Homo sapiens were the first hominin present. While megafauna extinctions were universally low in sub-Saharan Africa, they were exceptionally high in Australia and the Americas. Eurasia, where megafauna came into contact with hominins long before the arrival of Homo sapiens, falls between these extremes (Koch & Barnosky, 2006). It has been suggested that this differences in the magnitude of megafauna extinctions can be explained by the appearance of a new predator into continents with megafauna naïve to human hunting (Sandom, Faurby, Sandel, & Svenning, 2014a).

The Ecological Role of Megafauna

Megafauna play an important role in the top-down regulation of ecosystems. Large carnivores and herbivores activate trophic cascades that are essential to the preservation of biodiversity and the maintenance of ecosystem integrity. Their extensive losses have resulted in trophic downgrading on a global scale, with far-reaching negative effects, including changes in the dynamics of disease, invasive species, fire regimes, biogeochemical cycling, and carbon sequestration (Estes, Terborgh, Brashares, Power, & Berger, 2011; Malhi, Doughty, Galetti, Smith, Svenning, & Terborgh, 2016; Smith, Doughty, Malhi, Svenning, & Terborgh, 2016).

Large carnivores, who occupy the highest level in the food chain, create impacts on populations lower down the trophic ladder (Miller et al., 2001; Terborgh et al., 1999). These top or apex predators exert a strong control on prey animals that occupy the next lower trophic level, the herbivores and the medium-sized “mesopredators” that typically prey on smaller animals. They not only reduce prey numbers through predation but also cause prey to alter their behavior and avoid the “landscapes of fear” that emerge in the presence of apex predators.

Loss of large carnivores will lead to overpopulation of herbivores, which in turn may cause habitat degradation due to the resulting overgrazing and over-browsing of vegetation. Moreover, population declines of top predators are associated with the so-called mesopredator release, a term that was coined by Michael Soulé and colleagues in 1988 to describe a process of increases in the abundance of mid-sized carnivores in the absence of larger carnivores. This process generally results in decreased populations of still smaller prey species, such as birds, lizards, rodents, and rabbits. This may even lead to the extinction of whole prey populations, especially on islands (Brashares, Prugh, Stoner, & Epps, 2010; Prugh et al., 2009).

Like large carnivores, large herbivores can also exert a strong top down control on ecosystems, especially on their vegetation structure and composition. So-called megaherbivores such as elephants and rhinos, weighing ≥ 1,000 kg, are largely immune to adult predation; their numbers are only limited by food supply. These animals have a considerable impact on the structure of forests by breaking and knocking down trees. Large herbivores weighing between 100 and 1,000 kg are also not always subjected to predation. Especially herd-forming migratory ungulates often escape predation. These intermediate-size herbivores also play an important role in driving vegetation dynamics and shaping the landscape (Sandom, Ejrnæs, Hansen, & Svenning, 2014b; Svenning et al., 2016; Terborgh, Holt, & Estes, 2010).

The removal of large herbivores has adverse effects on landscape structure and ecosystem functioning. In wetter ecosystems, the loss of large herbivores is associated with an increased abundance of woody plants and the development of a closed-canopy vegetation. In drier ecosystems, reductions of large grazers, can lead to a high grass biomass and thus to an increase in the frequency and intensity of wildfires. Together with the loss of a prey base for large carnivores, these changes in vegetation structures and fire regimes, may trigger cascades of extinctions (Bakker et al., 2016; Estes et al., 2011; Hopcraft, Olff, & Sinclair, 2009; Malhi et al., 2016).

The Influence of Island Biogeography

The current high species extinction rate is mainly attributed to the ongoing fragmentation, reduction, and destruction of wildlife habitats. The increasing lack of space for wild animals explains the widespread popularity among ecologists and nature conservationists of The Theory of Island Biogeography by Robert MacArthur and Edward Wilson (1967),—“the book that changed things” (Quammen, 2004, p. 415).

The island theory, as it is referred to in shorthand, predicts the amount of species richness on a given island, using the size of the island and the distance to the mainland as its main parameters: islands will contain fewer species the smaller and more isolated they are. This theory proved highly relevant for managing nature areas that have developed more and more into “islands in a sea of cultivated land.” Consequently, it has enjoyed a lightening career in nature policy on both sides of the Atlantic.

North America

In North America, the island theory was at the heart of the new emerging field of conservation biology. Biologist Michael Soulé, who had fallen under the spell of The Theory of Island Biogeography “the day it was published,” was undoubtedly the central figure in the development of conservation biology from its origins in this theory. In 1978, he organized the First International Conference on Conservation Biology. The conference proceedings were published as Conservation Biology: An Evolutionary-Ecological Perspective (Soulé & Wilcox, 1980). This volume by a broad group of scientists, many of whom were strongly influenced by the island theory, is widely held as the foundation of the discipline of conservation biology (Quammen, 2004, pp. 530–531).

Researchers and practitioners from the new branch of biology were deeply worried about the disastrous loss of species that cannot be counterbalanced at all by the evolution of new species. As Soulé and Wilcox write in their introduction of the conference proceedings:

There is no escaping the conclusion that in our lifetime, this planet will see a suspension, if not an end, to many ecological and evolutionary processes which have been uninterrupted since the beginnings of paleontological time. We hope it is only a suspension—that the horrible onslaught can be stopped before the regenerative powers of ecosystems are also killed.

(Soulé & Wilcox, 1980, p. 8)

Given this major challenge, it will come as no surprise that Soulé and Wilcox defined conservation biology as a mission-oriented discipline. In his seminal article “What Is Conservation Biology?” Soulé (1985) referred to the emerging field, more dramatically, as a “crisis discipline,” whose primary objective is the provision of principles and tools to preserve biodiversity—“its relation to biology is analogous to that of surgery to physiology and war to political science.” Like a surgeon and a soldier, a conservation biologist must act and make decisions or recommendations before knowing all the facts. “Tolerating uncertainty is often necessary” (Soulé, 1985, p. 727).

Fortunately, conservation biologists can make use of the island theory as a compass that helps to navigate through uncertainty. The vital importance of this compass is manifest from the programmatic texts on rewilding that were produced during the 1990s.

The term rewilding was first used in 1990 in an article by Jennifer Foote, in Newsweek, that discussed radical environmentalist groups such as Earth First! These militants, Foote wrote, “vow not just to end pollution but to take back and ‘rewild’ one-third of the United States” (Foote, 1990). The term was adopted in 1992 by David Foreman, co-founder of Earth First! who helped establish the Wildlands Project in 1991. In the new journal Wild Earth, published by the Wildlands Project between 1991 and 2004, Foreman contends: “It is the goal of Wild Earth to offer the bold vision of the New Conservation Movement. It is time to rewild North America; it is past time to reweave the full fabric of life on our continent” (Foreman, 1992).

The scientific foundations for the new discipline were laid by another founder of the Wildlands Project (now the Wildlands Network), Michael Soulé, and other biologists from the Society for Conservation Biology, of which he was the first president. Together with Reed Noss, Michael Soulé formulated the essence of rewilding in a landmark paper, published in Wild Earth in 1998, that clearly shows the influence of the island theory. In it, they present three features that characterize contemporary rewilding: large, protected core reserves; connectivity between these reserves; and the presence of top predators. In simplified shorthand, these characteristics have usually been referred to as rewilding’s three C’s: “Cores, Corridors, and Carnivores” (Soulé & Noss, 1998, p. 22).3


In Europe, rewilding has gone Dutch, to paraphrase a chapter title of Andrew Balmford’s 2012 book Wild Hope. However, rewilding did not become an established concept until the start of Rewilding Europe in 2010. The Dutch term was nature development; it marked a switch from a defensive to an offensive strategy. Rather than clinging to the protection and conservation of existing nature reserves, the overriding purpose should be to create and develop “new nature.”

Like its American counterpart, nature development is based on the island theory that has risen meteorically in the ranks of Dutch nature policy since its introduction in the early 1980s. This becomes abundantly clear when we consider what can be called the three E’s of nature development that closely resemble North America’s three C’s: “ecological core areas,” “ecological corridors,” and “ecological networks” (Baerselman & Vera, 1989, pp. 45–49, 1995, pp. 41–43).

The strategy of nature development that was elaborated since the early 1980s is at the core of the ambitious Nature Policy Plan adopted in 1990 by the Dutch parliament. To give nature development a chance, this plan claimed a considerable amount of space in the form of the so-called National Ecological Network, a coherent network of international important nature areas.

With these innovative ideas, the Netherlands emerged as a pioneer of European nature policy. The Habitats Directive, including the plan of the EU-wide Natura 2000 ecological network of protected sites, was adopted in 1992, while the EU was under Dutch Presidency, and has been very much perceived as a “Dutch Directive” by many officials in the Netherlands and abroad (Wurzel, 2008, p. 267).

Different Accents

Although rewilding projects on both sides of the Atlantic have much in common, they also differ in some respects. Most importantly, they apply different historical baselines. This is hardly surprising, because from the onset American and European conservationists differed on what kind of nature should be used as a reference for conservation and restoration activities. The ideal image of American conservationists has always been the pristine wilderness as it presumably existed prior to the arrival of European settlers.4 European conservationists, who have always had much less seemingly pristine land to work with than their American colleagues, use the pre-industrial (and not the pre-settlement) landscape of the mid-19th century as baseline and aim to return ecosystems to their condition prior to large-scale industrialization.

However, the use of these baselines has become increasingly problematic and impractical. It has become evident that historical baselines are always somewhat arbitrary because of the role that cultural traditions play in their reconstruction. What is more, historical baselines are increasingly being dismissed as irrelevant as strong anthropogenic drivers such as climate change, nitrogen deposition, and habitat fragmentation make it difficult, if not impossible, to preserve or recreate historical ecosystems.

Although some hold on to traditional baselines and others try to refine or redefine the reference concept, the debate is currently dominated by two widely diverging reactions to the crisis of baselines (Keulartz, 2016). On the one hand, a growing number of conservationists feel that we have entered an era characterized more and more by so-called “novel ecosystems”; they declare the whole baseline notion obsolete and have shifted the focus from the past to the future (Hobbs, Higgs, & Hall, 2013). On the other hand, rewilders, far from abandoning history altogether, have pushed the baseline back to a deeper, more distant past—with the proviso that rewilders are not interested in recreating the past but in learning from the past how restore biodiversity by reactivating ecological and evolutionary processes using large carnivores and herbivores (see Introduction).

Yet again, North American and European rewilders have adopted different historical baselines. Whereas North American rewilders have moved the baseline back from the pre-settlement era to the pre-human past of the late Pleistocene landscape (of ca. 13,000 bp), European rewilders stay closer to human history and have replaced the pre-industrial baseline by the pre-agrarian baseline of the mid-Holocene landscape (of ca. 7,000 bp) (Lorimer et al., 2015, p. 47).

Another important difference between North American and European rewilders regards the question of which species should be given priority in rewilding projects. Whereas North American rewilders stress the important role of predation by large carnivores in the top-down regulation of ecosystems, the focus in Europe is on the role of large herbivores and naturalistic grazing in sustaining landscape diversity and providing a prey base for large carnivores.

Pleistocene Rewilding

The North American version of rewilding focused on creating large, well-connected core areas and the release of large carnivores. The reintroduction of the grey wolf in Yellowstone National Park in 1995 is commonly seen as the flagship example of this approach (Lorimer et al., 2015, p. 41). With the return of the wolf the elk herd declined 40% in five years. This decline allowed bison numbers to increase through a reduction in inter-specific competition with elk for forage. The wolves also prevented elk from overbrowsing willow and aspen near rivers and streams, and this gave rise to a substantial rebound of the beaver, itself a keystone species that may increase species diversity.5

Around the mid-2000s, this initial vision underwent a significant expansion. As we have seen, megafaunal extinctions were exceptionally high in Australia and the Americas, compared to sub-Saharan Africa and Eurasia. In North America alone more than 50 species of large mammals went extinct after the arrival of the first modern humans ca. 13,000 bp, including mammoths, mastodons, horses, giant ground sloths, American camels, lions, and saber-tooth cats. This catastrophic extirpation—generally referred to as the Pleistocene overkill—started an ecological chain reaction that led to further extinctions and hence to severe ecosystem simplification (see section “The Sixth Mass Extinction”).

To correct this dramatic megafaunal loss, Josh Donlan and ten other biologists, including David Foreman and Michael Soulé, launched the idea of “Pleistocene rewilding” (Donlan et al., 2005, 2006). They blame most conservationists and management agencies for suffering from a “post-Columbian bias,” typically turning to 1492 for a restoration baseline. If, however, we accept as benchmark for restoration measures the arrival of modern humans we could consider introducing surrogates for some of the North American megafauna that went extinct after the arrival of these people.

Pleistocene rewilders recognize that adverse anthropogenic impacts are unprecedented and show alarming signs of worsening, with the result that the megafauna that has already disappeared from Europe, Australia, and the Americas, and could eventually disappear from Africa and Asia, the only places where megafauna are still relatively intact. Given this risk of further extinction, the rewilders propose using megafauna from these regions, such as camels, cheetahs, elephants, and lions, as proxies for extinct American species. Consequently, Pleistocene rewilding is supposed to serve a dual purpose: to restore some of the evolutionary and ecological potential that was lost 13,000 bp, and to help prevent the extinction of the world’s remaining megafauna by creating new, better protected, populations in North America.

With the launching of the idea of Pleistocene rewilding, the narrow focus on large carnivores was broadened to include large herbivores as well. In fact, Donlan and colleagues see the introduction of proxies for extinct North-American herbivores such as Bolson tortoises, wild horses, wild asses, and Bactrian camels as the first stage of Pleistocene rewilding because they consider the introduction and establishment of these species as relatively easy and uncontroversial. In the second, more controversial stage of Pleistocene rewilding, Donlan and colleagues propose to introduce African cheetahs, Asian and African elephants, and lions. They admit that establishing large populations of these species will be a challenge; they will need to be provided with large protected and securely enclosed areas of appropriate habitat and with naturalistic selective regimes, including predator-prey relationships among herbivores and carnivores. Therefore, they envision, as a final and most ambitious stage, one or more enormous “ecological history parks” covering thousands of square miles in economically depressed parts of the Great Plains (Donlan et al., 2005, p. 914; see also Donlan et al., 2006, p. 674).

The Pleistocene rewilders mention not only ecological reasons but also ethical reasons to justify their conservation strategy. They believe that Pleistocene rewilding is ethically justified because human beings are to some significant degree implicated in the megafaunal extinctions in North America and thus bear a moral responsibility to vigorously redress these catastrophic losses as far as possible (Donlan et al., 2006, p. 666). In addition to the ethical justification for rewilding, Pleistocene rewilders also mention emotional and aesthetic arguments. They point to evidence of the fascination with charismatic megafauna and claim that the establishment of a “Serengeti of the New World” will create significant economic opportunities for the ecotourism industry. They further argue that wilderness without top carnivores such as cougars, wolves, or black bears can hardly be called “wild.” “Without these components, nature seems somehow incomplete, truncated, overly tame. Human opportunities to attain humility are reduced” (Soulé & Noss, 1998, p. 24).

As could be expected, Pleistocene rewilding was not only welcomed with enthusiasm but has also met with firm criticism. Many opponents challenge the science behind this conservation strategy (Rubenstein, Rubenstein, Sherman, & Gavin, 2006). They reject the possibility of restoring the evolutionary potential of North America’s extinct megafauna with the help of African and Asian proxies because of the differences in genetic makeup. They also question the claim that the ecological potential of North America’s ecosystems can, at the same time, be restored by using proxy species, because these ecosystems are different from the Pleistocene ecosystems, as well as from the ecosystems that are home to the extant African and Asian megafauna. Taken together, these differences make it probable that the proxies will act like invasive species that could devastate populations of indigenous species and wreak havoc in native environments. Rubenstein and colleagues, in fact, fear that the translocation of exotic species to non-native habitats could result in novel ecosystems with unique species compositions and unknown functional features.

Apart from ecological concerns, critics also point to the high economic costs of Pleistocene rewilding. The acquisition of land, the translocation and monitoring of animals, and the fencing of large areas will require considerable financial resources. It is feared that, given these high costs, in combination with the comparatively high salaries of North American managers and scientists, attention and funding will be diverted from more traditional conservation strategies in Asia and Africa as well as in North America (Caro, 2007). It is also feared that Pleistocene rewilding might negatively affect Africa’s ecotourism “if tourists and hunters, unwilling or unable to travel overseas, could view wildlife and hunt mega-fauna in their own North-American backyard” (Toledo, Agudelo, & Bentley, 2011, p. 566).

Possible resistance by rural communities is another area of concern. People will respond with fear rather than fascination when they, and their livestock, have to face catastrophic disease transmission and are exposed to dangerous predators. An email that Josh Donlan received during the height of the media attention for Pleistocene rewilding shows the deep-seated fear and hate of these animals: “Turning loose wild animals in USA anywhere is moronic. You must not have any children, or if you do, you must think it’s ok that they will be lion food. You are f*$#ing moron if you release killers in our homeland. I hope the cattle rancher guys shoot your ass or feed you to those lions if you release those killers into our ecosystem” (Donlan & Greene, 2010, p. 298).

Naturalistic Grazing

Whereas North American rewilders have adopted the late Pleistocene period as the historical baseline, European rewilders consider the mid-Holocene landscape as their main benchmark. They have, moreover, shifted the focus on the role in the top-down regulation of ecosystems from large carnivores to large herbivores. In the European model, great importance is afforded to naturalistic grazing with large and hardy grazing animals outside of the field-based farming system (Lorimer et al., 2015, p. 43).

The first large-scale experiment with naturalistic grazing was carried out in the Oostvaardersplassen, a polder situated 5 meters below sea level and just half an hour from Amsterdam. Reclaimed from the sea in 1968, this marshy area of 6,000 ha was initially earmarked for industry, but it soon evolved into a perfect habitat for plant and bird species that had become very rare in the Netherlands, or had completely disappeared from the country.

The site became the most important nature development area in the Netherlands, where Frans Vera and his colleagues initiated a management approach of rewilding with large ungulates. The rapid adoption of the ideas behind the Oostvaardersplassen project by agencies from other European countries gives an indication of the influence that the work of Vera and colleagues has had. These ideas have proved “immensely stimulating to conservation biology throughout northern Europe” (Smout, 2010, p. 112).

The dissemination of these ideas throughout Europe is largely due to Rewilding Europe, a foundation established in 2011 by Frans Vera’s colleague and kindred spirit Wouter Helmer. The Netherlands-based initiative aims to rewild nine abandoned agricultural areas of about 100,000 ha in ten countries across the continent, from western Spain to eastern Romania. In an interview with Elizabeth Kolbert from The New Yorker, Helmer explained that the goal of Rewilding Europe was to create giant versions of the Oostvaardersplassen, each at least fifteen times as large. “Frans Vera always says, ‘If the Dutch can do it, everyone can do it,’” he told Kolbert (2012, p. 59). Rewilding Europe focuses on naturalistic grazing as a key ecological process using wild large herbivores, such as bison, red deer, wild horse, wild bovines, beaver, ibex, and chamois.6

To understand the importance of naturalistic grazing regimes in the European context, one must realize the drastic changes in the structure and composition of plant and animal species that this continent has undergone from the late Pleistocene to the mid-Holocene period.

During the last Ice Age, large parts of Europe were covered with a mosaic of grasses, herbs, and stunted shrubs. This steppe-tundra environment was home to large cold-adapted herbivores such as reindeer, musk ox, mammoth, woolly rhino, the saiga antelope, the steppe wisent, elk, and horses. After the last Ice Age, when temperatures began to rise steeply, some of these large animals were forced to move elsewhere, while others went extinct. “Reindeer and musk ox migrated northeast, to the cooler tundra, while the saiga antelope and the Przewalski horse moved eastwards to the steppes. Mammoths and woolly rhinos shuffled on as well, but they eventually died out, never finding a climate as congenial as the steppe-tundra they left” (Vera & Buissink, 2007, p. 24).

The former steppe-tundra that these great mammals had occupied was colonized by large ungulates who had survived the Ice Age in more southern parts of Europe, like red deer, roe deer, elk, wild boar, wisent, aurochs (a wild ancient cattle species), and tarpan (the Eurasian wild horse). However, not all of the megafauna that had taken refuge in southern Europe were able to migrate back to the north. The largest animal species such as the European elephant, the rhino, and the hippo were hunted to extinction by humans that had colonized the Iberian and Italian peninsulas. The same fate also hit the mammoth and the woolly rhino when these hunters began to migrate north (Goderie, Helmer, Kerkdijk-Otten, & Widstrand, 2013; Ponomarev, Puzachenko, Bachura, Kosintsev, & Van der Plicht, 2013).

The nature developers have specifically deployed those large ungulates that succeeded to migrate back from southern to northern parts of Europe after the last Ice Age. Apart from red deer, roe deer, wild boar, and wisent, they also use proxies for the aurochs and the tarpan. In the Oostvaardersplassen, Vera and colleagues use Heck cattle, which originated in the 1920s and 1930s, in an attempt by the brothers Lutz and Heinz Heck to breed back domestic cattle to the aurochs, which went extinct in 1627 (Lorimer & Driessen, 2013, 2016). They also make use of the konik horse, the closest relative to its wild predecessor, the tarpan, the last of which died in the Moscow Zoo in 1887.

In 1983, 34 Heck cattle and 20 konik horses were introduced to the Oostvaardersplassen. In 2012, a helicopter count revealed about 350 Heck cattle and 1,150 konik horses alongside 3,400 red deer, which were introduced in 1992. Because of these large numbers of free-roaming ungulates the German magazine Der Spiegel has called the Oostvaardersplassen “the Serengeti behind the dikes.”

By stressing the key ecological importance of large herbivores for biodiversity conservation, Vera and his colleagues challenged the widely held belief that the European lowlands at the end of the Pleistocene would have been covered by closed-canopy forest. According to the prevailing succession theories, the original fauna of wild ungulates did not have any influence on the primeval forest structure and dynamics, but were supposed to follow the developments in the vegetation. Under natural conditions, all known wild ungulates were supposed to live at very low densities, such as 0.5–3 red deer per 100 ha or 4–5 roe deer per 100 ha (Vera, 2009, p. 30).

As an alternative to this “high-forest” hypothesis, Vera proposed his so-called “wood-pasture” hypothesis. Based on ecological, palynological, etymological, and historical arguments, he claimed that the mid-Holocene landscape of the European lowlands was an open park-like landscape, in which the indigenous fauna of large herbivores played an essential role in the cyclical turnover of vegetation types and the development of a shifting mosaic of open grassland, scrub, and woodland groves. Vera’s wood-pasture hypothesis, however, is at the heart of a heated debate between ecologists and archeologists that is far from settled (Mitchell, 2005; Svenning, 2002).

The open park-like landscape gradually vanished with the rise of the Neolithic Revolution and the transition from hunters and gatherers to farmers and herdsmen that started sometime between 8,000 and 7,000 bp. Wildlife populations were forced to retreat to ever more inhospitable places because they were seen as competitors with people’s domestic livestock for the best feeding grounds. As early farming cleared the natural vegetation and gradually replaced it with agriculture, many of the large ungulates were decimated, or, like the aurochs and the tarpan, went extinct altogether.

According to Vera, everywhere agriculture has led to simplification and depletion of natural ecosystems. Agriculture is selection: all over the globe, no less than about 40 species of the estimated total of approximately 50,000 natural occurring species of birds and mammals have been domesticated—that is 0.0008%! “During the past thousands of years, an enormous area has been cleared using the plough, the axe, and fire for the small number of selected species, at the cost of the space for the wild forms of the selected, domesticated species, as well as of the species that were not selected and not domesticated” (Vera, 2000, p. 379).

Apart from the scientific controversy over Vera’s “wood-pasture” hypothesis, there is also a social conflict that breaks out repeatedly. The main bones of contention are not “dangerous” animals, as in the case of Pleistocene rewilding, but “pitiful” animals. With their preference for the restoration of wild large herbivores and the introduction of naturalistic grazing, nature developers have adopted a resource-driven bottom-up approach in which the system is regulated by energy moving upward from lower to higher trophic levels, that is, from plants to herbivores to carnivores. In a bottom-up approach little ecological importance is accorded to carnivores because they sit atop the food chain. As a consequence of this bottom-up approach, the population size of large herbivores in the Oostvaardersplassen and similar reserves is regulated by limited food availability. Consequently, it is not allowed in these reserves to prevent starvation, either by proactive culling or by supplementary feeding. Only reactive culling to prevent unnecessary and prolonged suffering of moribund animals is allowed. Time and again this situation provokes fierce protests stretching from local people to the national parliament (Keulartz, 2009; Klaver, Keulartz, Van den Belt, & Gremmen, 2002).

The Return of the Mammoth Ecosystem

Remarkably, an initiative has been launched that appears to be something of a hybrid of the two rewilding strategies that were developed on both sides of the Atlantic: the “Pleistocene Park,” a 160-square-kilometer preserve in northeastern Siberia, established in 1989 by Sergey Zimov and colleagues as a radical experiment in wildlife and ecosystem restoration. On the one hand, this experiment takes the same baseline as the North-American rewilders—the Pleistocene-Holocene boundary—as its starting point; on the other hand, it shares its strong focus on naturalistic grazing with the European rewilders.

According to Zimov, the landscape of northeastern Siberia during the Ice Age was a highly productive steppe, where “mammoths, woolly rhinoceroses, bison, horses, reindeer, musk oxen, elk, moose, saiga, and yaks grazed on grasslands under the predatory gaze of cave lions and wolves.” At the beginning of the Holocene epoch, these grasslands, which Zimov refers to as “mammoth ecosystem,” were replaced by unproductive, moss-dominated tundra. The destruction of the mammoth steppe ecosystem happened when the great herbivore herds were decimated by overhunting and the rise of agriculture and cattle raising. Megaherbivores such as the mammoths disappeared, and “the only herbivores to survive were reindeer that grazed on lichens and moose that fed on willows” (Zimov, 2005, p. 796).

Not unlike Frans Vera, Zimov postulates that the Pleistocene grazers, through their grazing, trampling, and fertilizing of the soil, were maintaining the productive grassland ecosystem. The Pleistocene grasslands, he claims, would have persisted into the Holocene “had the great herds of Pleistocene animals remained in place to maintain the landscape” (Zimov, 2005, p. 798).

The main objective of the Pleistocene Park project is to convert the mossy tundra back to the grassy steppe that prevailed in the Ice Age by reintroducing large herbivores such as horses, moose, reindeer, bison, muskoxen, yaks, and wapiti.7 It is crucial to increase the density of these herbivore populations sufficiently to influence the vegetation and the soil. Only when large herds of herbivores are sufficiently abundant and successfully established in the park, the Amur tiger, presently an endangered species due to lack of suitable prey, could be introduced as the final component of a complete grazing ecosystem (Zimov, Chapin, & Chapin, 2008, p. 7).

Ecologist Paul Koch has expressed serious doubts as to whether the Pleistocene Park project will be feasible without megaherbivores weighing ≥ 1,000 kg, such as woolly mammoths and woolly rhinos. These extinct species were much more effective in maintaing the grassy steppe ecosystem by clearing snow, rooting up vegetation, and knocking down bushes and trees than the smaller herbivores that have escaped extinction. Thus Koch advocates the introduction of Asian elepants and white rhinos as proxies for these extinct megaherbivores (quoted in Stone, 1998, p. 34). However, these extant ecological replacements are not cold-adapted enough to survive the Siberian climate. Zimov has introduced his own, rather atypical surrogate for the woolly mammoth: an old Soviet army tank that is used to flatten the snow and to snap young trees, simulating the heavy footsteps of the huge woolly mammoths. But, since around 2012, there is a glimmer of hope on the horizon that the woolly mammoth can be brought back to life through new technologies.


In his 2005 book Twilight of the Mammoths, Paul Martin, the originator of the overkill hypothesis, suggested the term resurrection ecology for the Pleistocene rewilding program. He was of the opinion, however, that there was no realistic prospect of genetic resurrection, despite the recent advances in our ability to replicate and analyze ancient DNA. “Surely,” he wrote, “if the dead cannot be brought back to life, neither can the extinct” (Martin, 2005, p. 203).

Things have changed rather dramatically since the publication of Martin’s book. New technologies seem to have made it possible to revive extinct species, an effort that goes under the heading of de-extinction. Two methods, developed in the field synthetic biology, are currently used for de-extinction purposes: cloning and genetic engineering.

Cloning by somatic cell nuclear transfer (SCNT) may produce identical genetic copies of extinct species, but this technique is neither safe nor efficient and will only work in the case of very recent extinctions, because an organism’s DNA starts decaying the moment that organism dies. The problematic character of de-extinction through cloning becomes apparent when considering the case of the cloning of the bucardo, one of the four subspecies of the Spanish wild goat that was very well adapted to survive the cold and snowy winters in the Pyrenees. The last living bucardo, a female named Celia, was killed by a falling branch of a tree in 2000. However, before Celia’s death, Spanish researchers obtained some of her skin cells. They were able to inject nuclei from these cells into domestic goat eggs emptied of their own DNA. These eggs were then implanted into other subspecies of Spanish wild goat. Of 57 implantations, only seven animals became pregnant, and just one made it to term. The newborn bucardo died immediately after birth due to a defect in one of its lungs.

An alternative method for cloning is the creation of approximations of extinct species by editing the genome of closely related extant species using the so-called “CRISPR-Cas9” technique. Leading synthetic biologist George Church is employing this technique to revive the iconic passenger pigeon, a species whose last individual—“Martha”—expired in the Cincinnati Zoo in 1914. Genome editing with CRISPR-Cas9 makes it possible to transform the genome of the band-tailed pigeon, the closest relative of the passenger pigeon, gradually, gene by gene, into the genome of a passenger pigeon.

Church’s most spectacular project concerns the resurrection of the woolly mammoth by editing the genome of its closest living relative, the Indian elephant. He started this project in 2013, after meeting Sergey Zimov in the United States at a de-extinction workshop.8 Again Church and his team use CRISPR-Cas9 to transform Indian elephants into cold-resistant “mammoth-like” elephants, with the aim of releasing them into Zimov’s Pleistocene Park. By October 2014, they had succeeded in replicating and inserting 15 mammoth genes associated with cold resistance such as hairiness, ear size, subcutaneous fat, and, especially hemoglobin, into elephant genes. Church is convinced that he might need to change only 50 genes to do the whole job (Andersen, 2017).

It goes without saying that the de-extinction project, just like the whole idea of Pleistocene rewilding, has met with harsh criticism, ranging from concerns that de-extinction is too expensive and would divert attention and resources from saving extant endangered species and their habitats, to worries that the engineered species could become pests and wreak havoc when released into the environment. Opponents also warn that de-extinction reflects a new kind of Promethean vision: “a view of humans as all-powerful creators and the presumptive governors of planetary life” (Minteer, 2015, p. 15).

A third, less controversial de-extinction method is “back-breeding”: the selective breeding of domestic animals, in an attempt to achieve an animal breed with a phenotype that resembles an extinct wild ancestor. As we have seen, this method was employed by the brothers Lutz and Heinz Heck to recreate the aurochs, the largest land mammal in Europe after the woolly mammoth and the woolly rhino, which went extinct in 1627 (see section “Naturalistic Grazing”).

In 2008, the Taurus Foundation, a private Dutch organization, started a new back-breeding project. The foundation has developed a strategic partnership with Rewilding Europe, with the purpose of re-populating large European areas with wild bovines again. The foundation’s project arose out of dissatisfaction with the result of the attempts by the Heck brothers. Against the background of our current knowledge of the aurochs, the foundation argues, Heck cattle appear to bear little resemblance to the aurochs.9 The trials of the Heck brothers took place before the discovery of the world-famous caves of Lascaux and Chauvet with paintings of the aurochs, and of course also before the unraveling of the DNA of the aurochs and its nearest domestic relatives. The foundation uses DNA analysis to select European cattle breeds that are most closely related to the aurochs, and also as a yardstick to measure the progress of its back-breeding program.

Concluding Remarks

Rewilders are deeply concerned about the ongoing loss of species and their habitats. To slow down or stop the sixth mass extinction that is underway, it is necessary to tackle two problems that cause this catastrophic event. The first problem is a spatial problem: as a result of the downsizing and destruction of wildlife habitats, plant and animal species are constantly pushed back to isolated areas that increasingly take on the character of islands in a sea of cultivated land. Rewilders want to turn the tide by establishing large protected core reserves, connected by corridors, that together constitute ecological networks enabling the distribution, migration, and exchange of species.

The second problem concerns the global destruction of megafauna species that started with the arrival of Pleistocene hunters and only accelerated in the Holocene when people invented agriculture and began raising cattle. To address this problem, rewilders make use of a variety of conservation strategies: the reintroduction of species that only went extinct locally, such as the wolves; the introduction of non-native species as ecological replacements or proxies for globally extinct species; and the use of de-extinction methods such as cloning, genome editing, and back-breeding.

Given these ambitious objectives, it is apparent that rewilding represents a shift from a defensive to an offensive strategy: rather than remaining committed to the protection and conservation of existing nature reserves, the first and highest goal is to create new landscapes that are rich in wildlife. Consequently, rewilders follow a less pessimistic and more optimistic agenda, with ambitions that stretch further than merely managing species extinctions and habitat loss.


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(1.) Invertebrate patterns are in an equally dire situation: globally, 67% of monitored populations have declined by 45% in the last four decades (Dirzo, Young, Galetti, Ceballos, Isaac, & Collen, 2014).

(2.) The term megafauna refers to large-bodied mammals, weighing more than 100 pounds (45 kilograms).

(3.) The third C—Carnivores—also has a spatial meaning. As Mark L. Shaffer has remarked: “If you focus on things on top of the food chain, and save enough land for them, then probably you’re saving enough for the whole food chain” (quoted in Quammen, 2004, p. 514).

(4.) The cult of pristine wilderness, where indigenous people, under the influence of late 19th-century anthropologists, were considered as part of the fauna—“half man, half beast,” is still popular among many conservationists and the general public, although it has long been exposed as a cultural construction (Keulartz, 2016, p. 388).

(5.) Some doubts have been raised regarding this success story (Mech, 2012).

(6.) The United Kingdom has followed suit with Rewilding Britain, an organization established in 2015 whose philosophy is very similar with that of Rewilding Europe.

(7.) An additional environmental benefit of Zimov’s project is that it could prevent permafrost from thawing and could thereby mitigate some negative impacts of climate warming.

(8.) The workshop “De-extinction Projects, Techniques, and Ethics” was held in Washington, DC on October 23–24, 2012.

(9.) Nevertheless, the foundation acknowledges that Heck cattle “have proven to be a self-sufficient breed, surviving under pretty wild and hard conditions for at least 30 years in the nature reserve of Oostvaardersplassen in the Netherlands” (Goderie, Helmer, Kerkdijk-Otten, & Widstrand, 2013, p. 134).