Saving the Spanish imperial eagle was never going to be easy. This enormous bird, which once dominated the skies above Spain, Portugal, and northern Morocco, saw its numbers drop to just 380 breeding pairs in 2014, thanks to habitat loss, poaching, poisoning from farmers and hunters, and electrocution from power lines. Now, a new study highlights a potential way of restoring eagle populations to their former glory: dropping them into long-abandoned habitat.
One common approach for bringing threatened species back from the brink is to reintroduce them to the places they were last known to live. For example, the sea eagle in Scotland—which was hunted to extinction on the Isle of Skye in 1916—was successfully reintroduced in 1975 to Rùm Island near its last known breeding ground. But not all such efforts bear fruit: When scientists tried to release the same bird to its former range in western Ireland in 2007, the newcomers fell victim to the same poisoning that had done them in 107 years earlier.
“The tendency is to think that the last place that an animal was present is the best place for the species, but this isn't always the case,” says Virginia Morandini, a biologist with the Spanish National Research Council’s Doñana Biological Station near Seville.
So Morandini and her colleagues teamed up with conservation biologist Miguel Ferrer of the Migres Foundation at Doñana to try a different approach. Along with the Andalusian government’s Spanish Imperial Eagle Action Plan, they introduced imperial eagles into a territory they last inhabited some 50 years ago, far from established populations. Their method had some strong theoretical underpinnings because relict populations that have been pushed into small, low-quality habitats—often the “last known address” of threatened species—are thought to have relatively low breeding rates.
From 2002 to 2015, the Doñana team monitored 87 eagles that had been released in the south of Cádiz province of Spain, some 85 kilometers from the nearest established eagles. Meanwhile, the researchers monitored a naturally occurring population of eagles in south-central Spain. When scientists analyzed the breeding success of the two groups—a proxy for how well the eagles might survive over the long run—they found that the relocated population produced nearly twice as many chicks, they reported last month in Ecology and Evolution. Morandini attributes their success to the ready availability of prey and breeding partners, as well as efforts to reduce threats from hunters and exposed power lines.
The results suggest such reintroductions can be helpful in recovering endangered populations, especially when natural range expansion isn’t a possibility, says Doug Armstrong, a conservation biologist at Massey University in Palmerston North, New Zealand. But Armstrong, who was instrumental in rehabilitation efforts in New Zealand of a honeyeater-like bird called the hihi, also warns that this method won’t work for every threatened species. Lots of factors can lead to failure: selecting an inappropriate site, unpredictable environmental factors, and stress after reintroduction.
Cornell University ecologist Amanda Rodewald says that—even with its upsides—the approach should be seen as a last resort. “With ongoing climate change and habitat destruction, we are likely to be turning to [reintroduction] methods more and more,” she says. “However, taking proactive conservation steps such as habitat protection before a species becomes critically endangered is always going to be the most cost-effective and successful approach.”