As humans have spread across the planet, we have killed off larger predators and other species we fear and compete with, confining them to history or to small remnants of their vast ranges. This process was particularly successful in Britain and Ireland, where wolves and lynxes once lived together have long since disappeared.
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 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.
Naive prey and versatile predators
From wolves hunting non-native Corsican mouflons in the Mercantour mountains of southeastern France to red-banded snakes preying on invasive bullfrogs in China, native predators tend to preferentially hunt invasive prey over their native counterparts by a factor of two or even three.
Understanding why this is the case can reveal when and where restoring native predators will help control an invasive species. For example, the sika deer is native to East Asia, but became invasive in Britain, Ireland, and throughout continental Europe after it was 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 confronted with a native predator, failing to recognize the threat and therefore not realizing it. that he must flee, for example.
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.
An ecological argument
The natural recovery of some large predators in continental Europe, including lynxes, bears and wolves, is underway, defying long-standing edicts on pristine habitat and space requirements for carnivores. Despite intensive agriculture and urban sprawl, all it took for this species to recolonize its former range was for people to stop killing them. Surrounded by the sea, extinct predators will not naturally recover in Britain. Any effort to reintroduce them would require a social consensus that does not currently exist.
Our research offers an ecological rationale for restoring native predators: to help control and limit the spread of invasive species. But it would be naive to pretend that this is the only important factor. Living alongside large carnivores has consequences, including occasional losses of livestock and pets. This cannot be ignored but can be reduced with proactive management.
If people are going to accept the restoration of any native predator, there must be equal consideration of the benefits, such as alleviating the damage caused by invasive species, and plans to mitigate the costs.