“Innovating” Biodiversity: A brief dive into rewilding (part 3)
Here in part three of this four-part series, I delve into more detail on a few different common types of rewilding (including Passive, Translocation, and Pleistocene). In part one, I explored why environmental scientists are keen to use societal resources to bring Earth systems back into balance (i.e., to maintain healthy planetary boundaries). In part two, I focused on how rewilding is promising as a mechanism for re-balancing ecosystems within the biosphere to bring them back within self-sustaining limits. Finally, in part four, I discuss potential economic benefits and downsides of rewilding. As always, let me know your follow-up questions!
Scene 3: Rewilding on-the-ground (i.e., understanding types of rewilding)
As an innovation in conservation stewardship, rewilding offers a potentially low-cost method for restoring biodiversity to areas that have previously experienced negative impacts from human activities. Examples of this can include the over-hunting of animals (like sea otters in California’s estuaries), which in turn have damaged local environmental ecosystems that depended on that animal’s role to maintain a sustainable balance between prey, predators, flora, and fauna. While general media often only refer to translocation methods when mentioning rewilding, there are at least three different main types of rewilding that are being explored for their biodiversity and ecosystem usefulness that I will touch on here.
Rewilding methods: Passive, Translocation, and Pleistocene
Back in 1988, American conservation biologists Michael Soulé and Reed Noss described rewilding as a biodiversity restoration approach centered on strengthening three Cs: “cores, corridors, and carnivores.” Since then, multiple variations of rewilding have emerged. Among three of the best known are passive wilding, translocation, and perhaps the most radical, Pleistocene rewilding. Each of these variations come with their own set of potential advantages and difficulties, which I will briefly highlight:
Variation #1: Passive wilding
The first of these methods, passive wilding, is exactly what it sounds like: a low-to-no touch conservation intervention style where humans stop their activities within a certain conservation space and “let nature take its course.” As Gilson and colleagues outlined in their chapter in the 2011 book Conservation Biogeography, the idea here is to focus on the passive management of ecological spaces by permitting natural ecosystem processes to self-restore with minimal human influence on the landscape.
In practice, passive wilding is most frequently applied to spaces like fallow agricultural land. A recent (and, noteworthy) study by Broughton et al. (2021) examined how long it would take lowland farmland to passively re-wild into woodland. The researchers used a combination of time series of field surveys and remote sensing methodologies to study two abandoned farmland fields in England (one that had been passively wilding since 1961, the other since 1996). Surprisingly, their study found that abandoned farmland that was next to existing woodland was able to ‘re-wild’ into a closed-canopy woodland within approximately 50 years, featuring a height structure similar to that of older woods. For little-to-no human intervention, that’s a surprisingly quick turnover.
Supporters of passive wilding (such as activists, including the UK’s youngwilders) argue that applying this process-based approach to smaller restoration projects is cost-effective, inclusive, and participatory for local communities. Human interventions tend to be kept to a minimum and usually take place early on in a location’s rewilding process. Instead of reintroducing keystone species (such as in translocation or Pleistocene rewilding), these early interventions tend to focus on controlling invasive plant species, creating small pools in steams to encourage activity by local fauna, and occasionally modifying the landscape itself to create inviting habitat spaces for local species. Aside from these early interventions that are meant to ‘nudge’ along the restoration of the ecosystem’s ecological functions, the area is generally left to itself to ‘re-wild.’¹
Variation #2: Translocation rewilding
Unlike passive wilding, this second method of rewilding focuses on actively reintroducing or reinforcing species populations within a conservation area. In their review of “reversing defaunation” efforts (i.e., how to stop the loss or depletion of species from eocsystems) that was published in Science in 2014, Seddon and colleagues highlight two typical types of accepted translocation methods for restoring species’ populations:
- Reinforcements, which involve releasing a animal (or, organism) into an existing population to enhance that population’s viability within the area, and
- Reintroductions, where the intervention is meant to help reestablish a new population in an area after it had gone extinct there at a local level.
In both these cases, conservationists are aiming to prevent a cascading collapse of local animals, plants, and whole ecosystem processes by releasing new animals into a given ecosystem. This method has already been applied globally in a variety of locations (see Figure 1 below) and, in general, seems to be remaining popular. (This method’s popularity in media stories certainly doesn’t hurt.)
As I touched on briefly in part 2, a recent and currently successful example of translocation has been the reintroduction of sea otters to California’s central coast. Researchers including Dr. Brent Hughes discovered that sea otters’ high caloric needs had led them to feast on crabs within local estuaries, reducing the crabs’ numbers and enabling the California sea hare (a type of sea slug) to flourish in the area. California sea hares are important in estuaries as they eat the algae that would otherwise damage the seagrass’ health — thus, in turn, helping to restore balance within the local ecosystem. Healthy seagrass is important as a carbon sink, nursery habitat for young marine wildlife, and in controlling soil erosion, among their other ecosystem services.²
In contrast to passive rewilding, translocation rewilding is a more active approach that involves reintroducing recent species (or, the current descents of missed species) back into an ecosystem where they previously lived. Pleistocene rewilding, on the other hand, aims not to bring back recent species, but those from the last ice age – between 2.58 million to 11,700 years ago.
Variation #3: Pleistocene rewilding
Finally, Pleistocene rewilding is a variation of rewilding focused on reintroducing extinct Pleistocene Epoch megafauna into local ecosystems, particularly in North America. This rewilding strategy is built on the earlier work of Paul S. Martin, whose 1966 ‘Pleistocene overkill hypothesis’ argues that native flora and fauna in the present day do not function as well without the megafauna of the last ice age.³
Supporters of Pleistocene rewilding have included Russian scientist Sergey Zimov, who believes that this strategy could restore the mammoth steppe ecosystem (the largest biome during a period of roughly 31,000 to 16,000 years ago) and prevent further climate change impacts in the Arctic. Following his research, Zimov has established at least two known testing sites to examine the viability of his Pleistocene rewilding hypothesis: the Pleistocene Park nature reserve in Siberia, and the Wild Field wilderness park (located 250km south of Moscow).
A similar experiment was also conducted with the Oostvaardersplassen nature reserve in the Netherlands, where during the 1980s the state forestry service (Staatsbosbeheer) sought to reshape the local ecosystem by introducing large herbivores that would have existed in the area prior to modernity. This was done by introducing their modern-day equivalents to the area. These animals included wild horses (originally tarpans, replaced with Konik ponies), wild cattle (originally aurochs, replaced with Heck cattle), red deer, and European bison, along with elk and wild boar. After running for nearly 40 years, the Van Geel Commission released policy in 2018 advising that the area be ‘managed,’ rather than rewilded, with its approval resulting in over 90% of the animals in the area either being replaced or shot before they starved to death to prevent further alleged animal cruelty.
Oostvaardersplassen remains a controversial experiment. Critics have highlighted numerous issues with the former management of the project, including (to name but a few) a lack of large predators also being introduced into the ecosystem; fences preventing the migration of animals in the space; many of the grazing animals dying of starvation during harsh winters without intervention; and a general lack of political support for the project. While now under new management, questions have been raised as to whether the Oostvaardersplassen rewilding experiment was a good use of funds — a topic that I will cover in part four, where I discuss the potential economic benefits and downsides of rewilding.
This is the end of the third part of this brief dive into rewilding — part 4 TBC!
Additional Notes
[1] For an interesting paper specifically looking at the benefits and barriers of rewilding policy in practice within the UK that goes into significantly more detail than I do here, check out this article in the Journal of Applied Ecology (published 2018): https://doi.org/10.1111/1365-2664.13276 (As full disclosure, my former Master’s programme course director is a co-author!)
[2] Similarly, sea otters (with their voracious appetites) also function as a keystone species in Alaska’s Aleutian Islands, where researchers in the 1970s discovered that their regular consumption of sea urchins helps protect local kelp forests from being destroyed by sea urchin populations run amok.
[3] Criticisms of Martin’s Overkill hypothesis are many, and I do not have the space to go into them in depth within this post. If you are interested in learning more, I would suggest reading the extensive 2007 review in Quaternary International by Gary Haynes of some of these critiques, as he presents a number of both arguments for and counter-arguments against Martin’s hypothesis.
