Purging cats from South Atlantic islands often harms birds more than helps them
(Part I of a two-part series. See also What if an island has no cats?)
LONDON, U.K.––Fourteen years after declaring victory over feral cats on remote Ascension Island, on behalf of sooty terns, the Royal Society for the Protection of Birds has belatedly acknowledged eight years of warnings from University of Birmingham ornithologist Jim Reynolds that killing the cats only accelerated an 84% decline of sooty terns in 60 years.
“When the Royal Society for the Protection of Birds eradicated cat populations on Ascension island in 2006,” after a four-year campaign to kill them all, “it was considered a triumph in conservation,” recalled Daily Telegraph science editor Sarah Knapton.
Cats blamed for results of overfishing & global warming
“Ascension Island is the most important tropical seabird nesting station in the South Atlantic,” Knapton explained.
However, the resident bird population fell from an estimated 20 million to just 11,000 by 2002, assessed the Royal Society for the Protection of Birds, because of the combined effects of 150 years of predation by cats and mice––though a more plausible explanation would involve the combined effects of overfishing and global warming.
“Hundreds of feral cats were trapped and put down or poisoned,” wrote Knapton, “while pet cats were neutered and registered, in an ambitious two-year project partially funded by the Foreign & Commonwealth Office.”
“Explosion in rat populations”
Killing the cats is credited by the Royal Society for the Protection of Birds with having helped frigate birds and masked boobies, though adults of both species are somewhat larger than the upper extent of a cat’s normal prey range.
“But since then we have noticed an explosion in rat populations. The removal of an apex predator has allowed a mesopredator to thrive,” Reynolds told Knapton. “And [the rats] are moving away from the mountains toward where the seabird colonies are breeding,” Reynolds added, repeating a warning that he has amplified with increasing urgency through scientific media since 2008.
“Less nutritious squid”
“Reynolds believes that rats are attacking chicks who are already weakened by falling fish stocks, forcing them to turn to less nutritious squid,” Knapton summarized.
Having no bones, squid are particularly low in calcium, needed by birds to lay strong eggs.
Explained Reynolds, “Rats grow and grow and get bigger over time, so where they would not have bothered terns in the past, now they are taking their chicks and eggs.”
Said United Kingdom head of overseas territories Jonathan Hall, with unawares echoes of This Is The House That Jack Built and There Was An Old Lady Who Swallowed A Fly, “There isn’t solid evidence about the level of rat predation or population level increases at the moment. But we have carried out rat eradication programs in the past.”
Jack & the Old Lady
Unlike This Is The House That Jack Built and There Was An Old Lady Who Swallowed A Fly, the Ascension Island fiasco cannot be taken lightly because of the amount of animal suffering involved, on the parts of both cats and birds, and probably soon on the part of rats, as well.
Neither can the failure of killing all the cats on Ascension Island be considered just another demonstration of the mythical Law of Unintended Consequence, because it has many precedents, even in the same icy Antarctic and near-Antarctic latitudes, some of them involving the same agencies, organizations, and individuals.
Recounted Vox Felina blogger Peter Wolf on May 14, 2015, “It took 19 years to exterminate approximately 2,200 cats from barren, uninhabited Marion Island, which is roughly the size of Omaha, Nebraska, and located in the sub-Antarctic Indian Ocean. The methods employed included poisoning, hunting and trapping, dogs, and introducing feline distemper. In 1991, eradication of cats from Marion Island was complete. It remains the largest island from which cats have been successfully eradicated.”
But Birdlife South Africa found 14 years later that “mice have colonized almost every corner of the island, which was declared a Special Nature Reserve in 1995,” reported Tony Carnie of the KwaZulu-Natal Mercury.
Mice vs. albatross
The mice were wreaking havoc on sea birds, including albatross chicks.
Reported University of Cape Town ornithologist Ben Dilley in the journal Antarctic Science, “The first mouse-injured wandering albatross chick was found in 2003. In 2009, the first ‘scalpings’ were detected: sooty albatross fledglings were found with raw wounds on the nape. In 2015, mice attacked large chicks of all three albatross species that fledge in autumn: grey-headed, sooty, and light-mantled. Filming at night confirmed that mice were responsible for the wounds.”
BirdLife South Africa has for more than a year now discussed and debated ways and means of killing all the mice, while raising funds to try to do it.
“A similarly hard lesson was learned on Macquarie Island,” wrote Wolf of Vox Felina, “when, after the last cat was killed in 2000, the island was quickly overrun with rabbits.”
An Australian possession within the Antarctic Circle, Macquarie Island had hosted about 10,000 feral rabbits and 2,500 feral cats, both descended from animals left on the island by visiting whalers circa 1820. But the cats were blamed for killing as many as 60,000 sea birds per year, as well as preying on rabbits.
As the numbers of cats declined, the rabbits increased, to an estimated 100,000 by 2007.
Attack of the Rabbits from Hell
“Rabbits are destroying Macquarie Island’s fragile vegetation, causing erosion and exposure, which threatens its seabirds,” University of Tasmania geographer Jenny Scott warned in a report commissioned by Birds Australia.
A seven-year rabbit extermination campaign followed, featuring “releasing the calici virus into the rabbit population, large-scale bait drops, and teams of hunters and sniffer dogs,” reported Fiona Breen of the Australian Broadcasting Corporation.
The “campaign of aerial poisoning ended in 2011,” added Financial Times science editor Clive Cookson, “after 300 metric tons of bait had been dropped. Human observers and trained rodent-detecting dogs monitored the island for three years without finding a single remaining mammal, and the project was declared a success in 2014. Since then, seabirds have returned to areas where rat predation had previously made breeding impossible, native vegetation is rebounding and even spiders and moths are multiplying,” at least until some other external factor such as climate change again upsets the supposed ecological equilibrium which has now been documented for a whole five years.
“The predator-prey relationship is simple, right?”
Observed New York Times science columnist Henry Fountain in 2010, “The predator-prey relationship is simple, right? If a predator is around, that is bad for the prey, and if the predator is removed, that is good for the prey.
“Ecological theory, however, suggests that isn’t always the case, particularly if there is more than one predator species around and they share the same prey. In that case, elimination of the top predator may allow the mid-level predator to thrive, and the result may actually be worse for the prey.”
“Removing cats made life worse for petrels”
For example, Fountain continued, “Matt J. Rayner of the University of Auckland and colleagues found such a case on Little Barrier Island, off New Zealand. They studied the impact of two predators, feral cats and kiore, or Pacific rats, on a small burrowing seabird, Cook’s petrel. Kiore were introduced to the island hundreds of years ago, and cats were introduced in the 1870s. Both preyed on the petrels, with the cats also preying on the rats. Both were eventually eradicated, the cats in 1980, the rats in 2004.
“The researchers analyzed data on petrel chick survival from 1972 to 2007,” Fountain wrote. “As they reported in The Proceedings of the National Academy of Sciences, removing the cats actually made life worse for the petrels, since that left more kiore to prey on them. Only when the rats were eliminated did petrel breeding success increase.”
Yet another fiasco resulting from exterminating cats occurred on Robben Island, off Cape Town, South Africa. A hunter hired by the South African National Park Service in early 2007 shot the island cat population down from more than 100 to only two. Without cats to hunt feral rabbits on the island, the rabbit population soared from about 3,000 to more than 5,000 within the next year, and as many as 25,000 according to recent estimates, despite frequent attempts to cull them.