Birds on remote islands are sometimes better off with cats than without them
Part II of a two-part series. See also When the cats are away, the mice will play––and the rats & rabbits.
Gough Island, also in the South Atlantic, never had any cats.
“Today, the British-owned island, described as the home of the most important seabird colony in the world, still hosts 22 breeding species and is a World Heritage site,” summarized Guardian environment editor John Vidal on June 9, 2008. “But mice stowed away on whaling boats jumped ship and have since multiplied to 700,000 or more on an island of about 25 square miles. What is horrifying ornithologists is that the British house mouse has somehow evolved, growing to up to three times the size of ordinary domestic house mice, and has become a carnivore, eating albatross, petrel and shearwater chicks alive in their nests. They are now believed to be the largest mice in the world.”
Mice kill 600,000 sea birds per year on just one island
Eight years later, the Gough Island mice as of May 2016 had increased “to at least a million,” killing as many as 600,000 sea birds per year, reported Financial Times science editor Clive Cookson.
Gough Island is now slated for a $10 million mouse-killing blitz modeled after a rat-killing blitz on South Georgia Island, perhaps best known as the “other” British-held island also claimed by Argentina and involved in the 1982 Falkland Islands War.
“The South Georgia campaign, billed as ‘the world’s largest rat-eradication project,’ dropped 330 metric tons of bait in three phases over 100,000 hectares of sub-Antarctic wilderness,” summarized Cookson. “The poisoning was finished in March 2015.”
Odd numbers from the Isle of Canna
Other claimed successes for rodent eradication from remote islands include a poisoning campaign led by the National Trust of Scotland from 2005 to 2008 on the Isle of Canna, off the Scottish coast. The 15,000 resident sea birds, of 14 species, were said to be threatened by 10,000 rats, about 20 times as many as the typical sustainable ratio of predators to prey.
The rat-killing team, brought from New Zealand, “laid out 4,388 traps,” explained Raphael G. Satter of Associated Press. “Some 25 tons of rodenticide were shipped in to arm the traps,” or about five pounds per rat if there really were 10,000 rats.
Acknowledged Cookson, “All rodent eradication projects face the risk of poisoning native species as a side effect. Fortunately, most seabirds do not eat cereal bait, though a few, such as gulls and skuas, may scavenge the toxic bodies of dead rats or mice. In some campaigns on other islands, birds have suffered a short-term decline for this reason — the worst example was the 2008 rodent-eradication on Alaska’s Rat Island — but they have benefited in the longer run from the elimination of alien predators.”
Or have they?
Restoration vs. evolution
Conventional restoration ecology theory holds that the goal is to restore habitat to whatever condition it was in when humans first observed it, with the same suite of species deemed to be “native,” even if they also evolved elsewhere and arrived to establish niches at the expense of other species, much as did the human-introduced invaders.
Evolutionary theory, however, as developed by Charles Darwin chiefly from his studies of the remote Galapagos Islands, argues to the contrary that species are strongest when obliged to evolve in response to continual environmental challenges, including the threat of predation and frequent arrivals of competitor species.
From an evolutionary perspective, greater biodiversity in any given habitat is a positive, even if some newly arrived species extirpate others from niches they are no longer well-suited to hold. Adapting to the presence of both “new” top predators, such as cats, and “new” mid-level predators, such as rodents, would benefit a prey species more in the long run than not having to adapt to predation at all.
From a perspective combining the emphasis of restoration ecology on preserving endangered species with an evolutionary outlook, the most appropriate brake on a predator population believed to be putting another species at risk would be not to exterminate the predators, but rather to ensure that the predators are themselves challenged.
“When the cat is away, the mice will play”
“When the cat is away, the mice will play,” and so will the rats, mice, rabbits, and other species whose populations are held in check partly by cat predation, but mostly by fear of cat predation that keeps them from extending their range.
This ecological verity has been known for more than 400 years, variants of the expression “When the cat is away, the mice will play” having turned up in English, French, German, Italian, and Spanish literature as early as 1600.
Ironically this expression appeared to have emerged coincidental with the “Age of Exploration,” in which European sailors and colonists introduced both rodents and cats to the South Atlantic islands.
What’s in a name?
The Portuguese navigator Afonso de Albuquerque discovered Ascension Island on Ascension Day in 1503. Dutch sea captain Dirk Hartog discovered the island off Australia named for him in 1616. Fellow Dutch sea captain Abel Tasman, for whom Tasmania is named, also discovered Bruny Island in 1642. Christmas Island was discovered on Christmas Day 1643. Robben Island was discovered and colonized by 1654. Yet another Dutch sea captain, Barent Barentszoon Lam, logged the existence of Marion Island in 1663.
Kangaroo Island and French Island, off Australia, were named relatively late, both in 1802, by British explorer Matthew Flinders and French rival Jacques Hamelin.
On all of these islands rodents, cats, and often rabbits established parallel and largely separate ecologies of “non-native” species, which more-or-less co-existed with the native birds for hundreds of years.
Though the birds’ populations were somewhat diminished, the perceived ecological importance of rebuilding them at the expense of the introduced mammals results primarily from the loss of many of the same bird species from mainland habitats as result of actual human activity, including overfishing and economic development of former rookeries, not just the activities of animals brought by humans.
Killing cats to protect sheep
The cats of Dirk Hartog, Bruny, Christmas, Kangaroo, and French Islands are all now slated for eradication by 2030, Prue Adams of the Australian Broadcasting Corporation reported on October 7, 2016.
Among them, Kangaroo Island has the most cats: 3,000 to 5,000, according to the Australian federal Environment Department.
While the official pretext is protecting native wildlife, protecting sheep––another introduced species, of much more evident impact on the island habitat––may be a greater concern.
Feral cats are accused of infecting sheep with toxoplasmosis and sarcosporidiosis, the latter cutting into the profitability of the Kangaroo Island mutton industry by afflicting up to 70% of the sheep sold to slaughter.
Oddly, though, sarcosporidiosis is most common in cattle and goats, with reported infection rates of 80%-plus in other parts of the world, and in pigs and camels. It is not common in cats, and is not a disease of high priority concern for the livestock industry anywhere else.
Curiosity will kill cats
The Australian feral cat purges––and others planned for the mainland of Australia––are expected to rely on a poison called Curiosity, after the expression “curiosity killed the cat.”
Curiosity has been “developed in the main by the Federal Department of Environment and Energy,” explained Australian Broadcasting Corporation rural affairs reporter Babs McHugh in November 2016.
“There are other feral cat baits available, such as Eradicat,” McHugh said, but Curiosity is more easily distributed.
“A bandicoot won’t eat it”
Said Australian threatened species commissioner Gregory Andrews, “Testing has shown under the right conditions, 80% of the cats who take the bait die from it. Curiosity uses a meat-based sausage with a small hard plastic pellet inside, which contains the toxin PaPP (para-aminopropiophenon),” which “works by stopping oxygen bonding with haemoglobin, so when the cat eats the sausage, the pellet releases in their stomach, they get groggy, go to sleep, and die completely humanely. And the design of the sausage, and the pellet, means that a bandicoot or other carnivorous native animal won’t eat it. Or if they do nibble at it, they spit the pellet out.”
Finished McHugh, “Curiosity is currently with the Australian Pesticides and Veterinary Medicine Authority for assessment and registration. The Department is looking for an company experienced in the field to commercialize the product,” under a licensing agreement with the Australian federal government.