It’s no secret that humans have spent the past few centuries relentlessly degrading environments and ruthlessly sending a variety of species extinct. Current conservation efforts are clearly not enough, and so some scientists are asking: what if we just let nature run its course again?
In a new review led by Andrea Perino from the German Centre for Integrative Biodiversity Research, an international team of ecologists and conservation biologists delve into the research behind the idea of “rewilding” complex ecosystems, ultimately presenting a framework with which to design, implement and evaluate such restorative projects.
The work is published in the journal Science.
The term “rewilding” was first conceived 20 years ago to describe the re-introduction of apex predators into various habitats, in order to exert top-down control to keep the ecosystem in balance.
Since then, the concept has changed and diversified, coming to include a range of approaches. One type, known as trophic rewilding, closely resembles the original concept, advocating for the re-introduction of key missing species – mostly large carnivores and herbivores – or their closest living proxy.
Other methods are more passive, such as creating no-hunting areas, low-impact forestry management, removing barriers within habitats, and restoring natural flood regimes.
Whether active or passive, rewilding refers to the rebuilding of critical components of ecosystems to repair the damage done by human activity. It’s distinct from other conservation methods because it seeks to restore the natural self-sustaining state of a landscape, rather than to protect a pre-defined set of species and conditions. Its ultimate goal is to create ecosystems that, in the long term, would have minimal human management.
According to Perino and colleagues, successful projects must target three key processes that influence the complexity and resilience of an ecosystem: trophic complexity, natural disturbances, and dispersal.
Trophic complexity refers to how species at the top of the food chain are able to shape their surrounding ecosystems.
“Large-bodied herbivores exert strong influences on the diversity and abundance of other taxa such as birds, small mammals, insects, and plants,” explain the authors – for example, through modifying the physical environment by grazing or trampling, and by controlling population sizes and behaviours of smaller animals through predation.
When human behaviour such as hunting, agriculture and forestry reduces the numbers of these kings of the food chain, the population of large herbivores starts to rise, which detrimentally affects other species and their habitat.
A classic example of this is the decimation of the wolf population in Yellowstone National Park in the US, which led to an explosion of elk that nearly grazed the ecosystem to death. When wolves were reintroduced in 1995, this caused a cascade of change, modifying the behaviours of the deer and literally causing rivers to reroute to a more balanced state.
Of course, issues arise when the key predator species has gone extinct. Should a “replacement” species be introduced that may occupy a similar role in the food chain? What kinds of unforeseeable consequences might that have? These are questions, the authors say, that rewilding projects need to grapple with.
The second key process involves random natural disturbances such as fires, flood and pest outbreaks. These introduce diversity and complexity into ecosystems, testing the competitiveness of species and making them more resilient to future events.
But human-dominated landscapes can suppress or alter these disturbances. This often leads to larger and even more devastating events in the long run, such as one massive wildfire instead of many smaller fires over several years. Humans also create regular artificial disturbances – through the use of fertilisers and irrigation, for example – that are so constant that species never have time to recover.
Rewilding aims to remove these artificial disturbances and allow natural, random changes to run their course.
The last key process, dispersal, is simple to understand: species depend on spreading out their population across habitat to prevent overcrowding, inbreeding and in-fighting. By degrading environments or creating barriers within them – such as highways cutting through forests – humans prevent animals from stretching their legs. Rewilding efforts would aim to re-connect habitats.
These three ecological processes inform and influence each other. For example, disturbances can create more diverse ecosystems, which in turn can lead species to diversify. Large animals can help plants disperse through their droppings, and can also be agents of disturbances by predation or grazing.
It’s clear that restoring one of these processes would also influence the other two, and choosing strategies that restore all three would lead to even greater improvement.
If only it were that easy.
Let’s not forget that humans inhabit these habitats, too, and so any rewilding project would affect local livelihoods and wellbeing.
But rewilding is likely to have positive benefits for society, say Perino and colleagues, and not just in terms of wildlife tourism.
“A growing body of literature concludes that exposure to green or natural spaces can reduce stress levels, increase positive emotions and cognitive function, encourage physical activity, and facilitate social cohesion,” the authors point out.
Even the mere existence of a symbolic species in a landscape can have a profound influence on society, inspiring spiritual, artistic, and technological development.
A growing body of evidence also points to the fact that rewilding may even help combat climate change.
But there may be detrimental consequences, too – imagine letting a fire or flood rage freely near populated areas, crops be damaged by large herbivores, or livestock killed by predators.
In a more abstract sense, there are also concerns about losing traditional cultural landscapes, and a general erasure of human history’s involvement with the land.
As always, nature presents as a paradox: since prehistoric times it has been both a constant threat and the very source human livelihoods; a dangerous place and a peaceful space in which to unwind. Rewilding has the tricky task of navigating this minefield to prevent conflicts and maximise positive outcomes.
All these considerations mean that a rewilding project isn’t just about identifying what is missing or degraded in an area and then replacing it. Perino and colleagues stress that the process must involve a huge variety of stakeholders, considerable planning, close monitoring and public education.
“Moreover, objective, evidence-based assessments of rewilding initiatives are needed to make rewilding projects fully accountable to funders, the public, and the research community,” they write.
The United Nations General Assembly recently declared 2021 to 2030 to be the “decade of ecosystem restoration”. With a bit of luck and a lot of hard work, wild spaces may soon bloom again.
Lauren Fuge is a science journalist at The Royal Institution of Australia.
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