The greater noctule is the largest bat in Europe. It is listed as “vulnerable” on the threatened species list, but for many years these winged mammals found shelter in the hollows of trees at María Luisa Park in Seville, Spain.
Then a few years ago, they started to turn up dead.
The corpses were savaged, with holes torn in their wings, scientists who study the colony discovered. Many of them were nursing pups; one corpse was a pregnant female. Two bats that managed to survive were so injured that they could not climb or fly back up to their nests. What was behind the attacks?
The culprit turned out to be another park inhabitant: beautiful, invasive rose-ringed parakeets who also make their homes in tree hollows.
When scientists first began studying the bats more than 15 years ago, they didn’t pay much attention to the birds. But now, there are thousands of them, and they are pushing the bats out of their holes, killing some, and taking over the trees where noctules once lived.
In a new paper in Royal Society Open Science, the researchers, many of them based at the Estación Biológica de Doñana in Seville, provide evidence that unless something is done about the parakeets, the park’s group of noctules, one of the largest known breeding colonies in Europe, is likely to be obliterated in the near future. The discovery also offers a real-time view of what happens when an invasive organism finds a new niche and displaces a native species.
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In a cage, or its own habitat in Asia and Africa, the rose-ringed parakeet is a charming, sociable bird. Martina Carrete, a conservation biologist who is an author of the new paper, said the parakeets are believed to have first appeared in the park after about 10 of them were confiscated from a pet shop and released around 1990.
“At this time, nobody expected that they could become an aggressive invasive species,” she said.
Dailos Hernández-Brito, the graduate student who is the study’s lead author, first began looking into the relationship between the birds and the bats more closely in 2013.
“We were very surprised because in this year we found out that most of the trees previously occupied by noctules were at this moment occupied by parakeets,” said Dr. Carrete.
And then they started to find the bodies and witness the attacks.
The first attack Mr. Hernández-Brito saw was against a pregnant female bat. He and collaborators then saw more than 30 attacks in the last two years and collected 20 bodies presented in the current study, though there likely have been many more deaths.
Today, the number of trees that noctules live in has dropped by 81 percent since the researchers first began keeping track, with the parakeets taking over the bats’ old homes. Although it is difficult to get exact numbers for the bat population, it appears to have roughly halved since the work began, bringing their numbers down to about 250. The bats are also living in tight quarters, with double the old number occupying a given hole.
The scientists helped the city government come up with an eradication plan to get rid of the parakeets in 2016, but at the last moment, officials canceled the plans. The parakeets are popular, and citizens argued there must be some other way besides killing them — perhaps artificial nest boxes for the bats. However, the bats do not take to such nests well, and the situation is growing urgent.
The parakeet population will surpass 3,000 this year. Dr. Carrete hopes that this paper will help provide evidence that if the noctules are to be saved, eradication is the only option.
More reporting on bats and birds