Vast areas of degraded ecosystems could be restored to something approaching their former glory through re-introducing just 1-3 animal species, and a United Nations-funded study has identified 20 large mammals that, suitably placed, could restore much of the world.

Some species play an outsized role in their local environment, making their loss particularly devastating. The flipside of this is that when a species is locally, rather than entirely, extinct, reintroducing a population from elsewhere can produce spectacularly positive results.

The reintroduction of a few wolves to Yellowstone Park is perhaps the best-known example, although debate continues as to whether some effects attributed to the wolves could represent sampling error.

Dr Carly Vynne of the non-profit RESOLVE was commissioned by the United Nations Environment Program to lead a team identifying animals with the potential for the biggest impact. In the journal Ecography, the team reveals 13 herbivorous mammals and seven predators that could return much of the nature we have thrown away. The benefits of a single species in an area can ripple through the food chain to the point even soil invertebrates become more abundant and diverse.

“Our results give both hope and scope for reversing the depletion of intact fauna groupings,” Vynne said in a statement. 

To determine what is needed for restoration, the authors first compared modern environments with conditions 500 years ago, before intercontinental travel and the industrial revolution sent local species loss into overdrive. They found just 16 percent of the world has intact communities of wild mammals, and only 6 percent is similar to the pre-1500 state.

Sad as this is, the team concluded a 54 percent expansion in territory hosting intact mammal communities could be achieved with relative ease, mostly in the far north and parts of Africa and South America. The paper identifies the 30 ecoregions the authors consider the highest priority for such a program.

Even in Europe, where widespread degradation has gone on the longest, appropriate siting of beaver, bison, reindeer, wolf, and lynx could be transformative. The wolf and wild horse alone could restore large parts of Asia.

These examples, plus hippopotamuses, cheetahs, and lions in Africa, may appear relatively obvious, but the paper also identifies the potential of lesser-known species like the dama gazelle and common tsessebe. 

The dama gazelle is critically endangered, and many of the other animals on the list are at least threatened. Their reintroduction could ensure their own survival, but the benefits would extend to many lower-profile counterparts.

"Our recommendations may not be suitable everywhere on the ground just yet – local assessments will judge if, for example, hunting pressures or the lack of an adequate prey base mean other issues need addressing before initiating a reintroduction program," said co-author Joe Gosling of the UNEP. 

"However, our findings show there are huge areas of the world that could be suitable for large mammal restoration if mitigating factors are managed."

Restoration of keystone species can often be essential for the revival of Indigenous cultures and can benefit even urban populations. For example, thousands of small beaver dams provide more effective flood and drought control than modern engineering.

The benefits of re-wilding extend far beyond species preservation. Healthy ecosystems usually hold more carbon than degraded ones, so programs like these have the potential to fight climate change, although to what extent remains debated.

Reintroducing carnivores is frequently controversial since they pose a threat to livestock and are perceived as dangerous to humans, even where there is little evidence. Herbivores – with the possible exception of hippopotamuses, considered Africa’s most lethal mammal – usually face less resistance.

However, without control of hunting and other threats, any reintroductions are likely to be short-lived.