How Restoring Native Predators Can Help Address the Invasive Species Crisis: Dr Joshua P Twining and Professor Xavier Lambin

In Scotland, and more widely in Britain and Ireland, we have had particular success, long ago extirpating the last wolves and lynxes with which we once shared ecosystems and confining other predators to small remnants of their historic ranges.

However, there is now mounting evidence that those once-hated native predators play an essential role in regulating prey. Additionally, new evidence is emerging that the eradication of native predators has led, in part, to the invasive species crisis we face today.

At the same time, humans have transported species that we value outside of their native ranges. By introducing animals, plants, and microorganisms into ecosystems where they did not evolve, we have inadvertently created invasive species that drive native species to extinction by eating, competing with, and exposing them to new diseases. During the last century, invasive species were the main cause of the extinction of vertebrate species.

Mounting evidence suggests that once-hated native predators are essential to regulating invasive prey. In fact, our new research, published in Global Change Biology, shows that the eradication of native predators has partially caused the invasive species crisis we face today.

But we also find that all is not lost. Examining a series of surveys that included public sightings of gray squirrels and pine martens, a small carnivore and member of the weasel family that was hunted until legal protection took effect in the 1980s, between 2007 and 2019 in Ireland, we show how the return of a native predator can cause the rapid decline of a long-established invasive species (the gray squirrel) across entire landscapes.

We study populations of both species to uncover characteristics that determine a native predator’s ability to control an invasive species after restoration.

These include the predator’s ability to switch prey, the invading prey’s failure to recognize or respond to the threat of a newly recovered predator, and the availability of areas in which the prey can hide to escape.

Understanding why this is the case can reveal when and where restoring native predators will help control an invasive species. For example, sika deer are native to East Asia but became invasive in Scotland, Ireland, and throughout mainland Europe after they were introduced in the late 18th century.

The Eurasian lynx, a deer predator once widespread throughout Europe, was eradicated from most of its former range in the early 20th century. Like the gray squirrel with the marten, the sika deer evolved in the absence of the lynx and is likely to behave similarly when faced with a native predator, failing to recognize the threat.

Lynx tend to switch between the deer species it hunts and have a proven ability to suppress deer populations. There are also no areas available for deer that the lynx cannot access either. These factors combined suggest that restoring lynx populations will benefit ecosystems in which sika deer are invasive. The lynx is likely to have a greater effect on these invasive populations where alternative prey (such as roe deer) are scarce or absent, such as in Great Britain and Ireland.

The natural recovery of some large native predators in continental Europe is well advanced thanks to conservation efforts and legal protection. Despite intensive agriculture and urban sprawl, all the species has needed to recolonize their historical range is for people to stop killing them.

Unaided recovery of extinct predators is clearly not going to happen on insular Britain. Any active reintroduction of large native predators would require a social consensus that does not currently exist.

Opponents readily claim that there is not enough space for such widely distributed animals in the crowded British Isles. However, the view that large carnivores can only persist apart from people in large protected areas, such as those found in the wilderness of North America or in the fenced reserves of South Africa, has been completely falsified.

The alternative scenario of managed coexistence relies on people and predators learning to live together again. It unfolds now, with cougars and bobcats roaming San Francisco parks and Swiss towns; wolves breeding in crowded, heavily agricultural landscapes of Belgium and Holland; and black bears that frequent suburbs across the United States.

Carnivores are highly adaptable and can live alongside us, providing ecosystem services and occasional nuisances, in our highly altered and human-dominated worlds.

Opposition to predator recovery is not a space issue but a tolerance issue, reflecting a lack of awareness of the benefits of predator recovery, such as suppression of certain established invasives, and an overemphasis on the drawbacks are well tolerated in other countries.

Our research offers an ecological rationale for restoring native predators: to help control and limit the spread of invasive species.

However, it would be naive to pretend that this is the only important factor. Living alongside large carnivores has consequences, including occasional livestock and pet losses that cannot be avoided, but can be reduced with proactive management.

Achieving the broad social acceptance required for the restoration of any native predator population must encompass considerations of the likely social benefit of alleviating the crisis caused by harmful invasive species and plans to mitigate the inconveniences and losses suffered by a disproportionately affected minority.

Dr. Joshua P Twining is a Postdoctoral Research Scientist at Cornell University in the US; Professor Xavier Lambin is Professor of Zoology at the University of Aberdeen.