Pleistocene rewilding describes the theory of restoring Pleistocene habitats back to their original state (at around 11,000 years ago), by reintroducing the megafauna which maintained them. The end Pleistocene saw huge megafaunal extinction (loss of 97 of the 150 megafaunal species between 50kyr and 10kyr, Barnosky 2004). This period of extinction has been linked to several causes, including disease, blitzkrieg (high killing and overexploitation), sitzkreig (associated human interference with fires etc.), climate, and meteorite explosion (Sandom, 2014). However, it has recently emerged that the most likely cause of these megafaunal extinctions lies with direct human action; Sandom (2014) found that the number of extinctions globally increased with human interaction, and was lowest where early humans and megafauna evolved together over a longer period of time (sub-Saharan Africa). It was also shown that there is only a weak link to climate change across the extinction period, one which can only be observed across Eurasia.
The idea of Pleistocene rewilding arose as a way to restore these “natural” habitats, and to repair the damage done by our ancestors. Because most of the species maintaining the habitats targeted for rewilding have become extinct, proxy organisms (stand-ins for extinct species)are used to fulfil the same ecological niche as the extinct organism (Zimov, 2005). For example, translocation of the African elephant into North America has been suggested to replace the mammoth which once roamed there during the Pleistocene (Donlan, 2006). But how realistic and feasible is this plot?
Why support rewilding?
In 2006, Donlan et al. published a paper which highlighted the potential benefits of rewilding. They claimed that as humans we have a moral obligation to restore the habitats that our ancestors once decimated, and that the most feasible way to do so is by introducing genetically related species to replace the functions of other organisms on the top trophic levels, which have been lost. As well as the moral incentive to restore these ecosystems, the study outlines other perceived benefits of the strategy, some being economic. Restoring past habitats would encourage funding into conservation and greater revenue from ecotourism – for example, when wolves were re-introduced into Yellowstone National Park in North America, an economic benefit of $9 million was recorded, compared to a societal cost of only $0.5-0.9 million (Donlan et al 2006). It is suggested that this money can then be re-invested into habitat conservation, creating further rewilding projects in a positive-feedback cycle.
The theory has also been supported on ecological grounds. A lot of pro-rewilding researchers justify restoring natural ecosystem processes and biotic interactions as a key to restoring lost habitats, and that restoring the top-predators, such as the cougar across America in place of the American tiger, would lead to a trickle-down trophic cascade effect. For example, wolves were reintroduced across Yellowstone to control populations of deer which had become unsustainably high in some natural parks. Some of the species suggested to make good replacements are themselves priorities for conservation – African elephants (as a proxy for the mammoth) are of high conservation importance, and it has been suggested that translocating these species to other continents for the purpose of habitat management may improve genetic resources across the globe and thus the genetic viability of the population (Donlan, 2006).
Does it work in practice?
There are examples of Pleistocene rewilding in practice, which could be justified as successes. The first, and most well-known, is the Pleistocene Park established in Siberia in 1998 (Zimov, 2005). This park was established in order to reconstruct the mammoth steppe habitat, a rare habitat comprising grasses, once maintained by mammoth, woolly rhino and their associated predators. Bison have been introduced to restore the ecosystem; specifically, to prevent permafrost degradation, a key justification for the creation of the park (Zimov, 2012) – permafrost degradation leads to the release of potent greenhouse gases such as methane, therefore contributing to climate change. By introducing species which are not so distantly related from the original megafauna which would have roamed the region 11,000 years ago, the ecosystem has started to be restored to its previous state, with permafrost thaw reduced (Zimov, 2012).
Another Pleistocene Park has recently been established in the Netherlands, where Heck cattle have been introduced to replace extinct auroch species (Gillson, 2015). This example has been less of a success in terms of rewilding a natural landscape – primarily through the fact that no top predators were released into the environment to control the population of other organisms. This means that there is high human intervention in maintaining the landscape – management through shooting between 30-60% of cattle and leaving their carcasses for foxes and large birds of prey mimics the role of these predators, but at a high human labour expense (Gillson, 2015).
This example illustrates the first negative point to Pleistocene rewilding as a technique: the landscape is still heavily human managed in cases where socio-political conflict prevents release of desired top predators (Donlan, 2006). This kind of conflict is expected with the introduction of potentially dangerous species such as lions and cougars; when wolves were re-introduced to Yellowstone, there was huge outrage from the farming communities around the area, who perceived their cattle to be at risk.
Through careful management of these so-called “parks”, habitats can be restored to a pre-Pleistocene condition, however it has been suggested by numerous researchers that conservation money be best spent investing in refaunation, the restoration of extant species to their original geographic ranges (Rubenstein, 2006). This would be beneficial as ecosystem services which they provide will be restored, changing the habitats in which they are found. Furthermore, it would reduce the risk of negative consequences associated with introductions of species which did not evolve to the set of conditions in which they would be translocated, causing unexpected trophic interactions or spread of zoonotic disease (Rubenstein, 2006).
Rubenstein (2006) also makes the point that rewilding would not restore habitats to their Pleistocene condition, but instead into a hybrid past-present “Frankenstein ecosystem”. This was suggested on the basis that adaptation and evolution in the landscape has happened since the extinction of the megafauna, and that it cannot be expected that these habitats remain unchanged since the start of the Holocene. As such, it is inappropriate to introduce a species which is unfamiliar with the new, adapted conditions (such as vegetation changes) to try and force back the old ecological interactions which essentially occurred between completely different vegetation and species.
Besides the economic and ecological arguments, there is a certain element of practicality which cannot be overlooked – how does one translocate a population of large organisms between continents, and expect them to adapt to the new environment as if it were their old? This question seems to be skimmed over by pro-rewilders, who justify the economic benefits of the scheme over the potentially damaging implications for the translocated animals, in terms of welfare and disease spread. For a process which aims to better protect species, and the habitats they create through their ecological relationships with other organisms, this seems counterintuitive.
To summarise, Pleistocene rewilding is an optimistic but somewhat desperate attempt at restoring past habitats, such as the mammoth steppe in Siberia, or savanna grasslands across the tropics. There have been past successes with rewilding; notably illustrated by the Pleistocene park in Siberia, or the breeding and subsequent release of specific species such as the Californian Condor, however the risks involved in translocating completely different species (or even, distantly related species) across continents to conserve them and their habitats. Not only is there the redistribution of important and limited conservation funding to a high-risk project, the likely spread of disease, altered trophic interactions and socio-political conflict associated with the technique make it infeasible in many locations. The process could be feasible if the ecological interactions of each proxy species were completely understood (Gillson, 2015), and if each area were assessed fully and comprehensively on a site-by-site basis. However, perhaps the conservation of existing trophic interactions with the landscape is a lower-risk and better ecologically justified method of restoring endangered habitats.
Donlan, J.C., Berger, J., Bock, C.E., Bock, J.H., Burney, D.A., Estes, J.A., Foreman, D., Martin, P.S., Roemer, G.W., Smith, F.A. & Soulé, M.E. (2006) Pleistocene rewilding: an optimistic agenda for twenty-first century conservation. The American Naturalist, 168(5), pp. 660-681
Rubenstein, D. R., Rubenstein, D.I., Sherman, P.W., & Gavin, T.A. (2006) Pleistocene park: does re-wilding North America represent sound conservation in the 21st century? Biological Conservation 132, pp. 232–238
Zimov, S.A. (2005) Pleistocene park: return of the mammoth’s ecosystem. Science, 308(5723), pp. 796-798
Zimov, S.A., Zimov, N.S., Tikhonov, A.N. & Chapin I.F.S., (2012) Mammoth steppe: a high-productivity phenomenon. Quaternary Science Reviews, 57, pp. 26-45
Gillson, L. (2015) Biodiversity Conservation and Environmental Change: using palaeoecology to manage dynamic landscapes in the Anthropocene. OUP Oxford.