The cat, this formidable scourge for the biodiversity of the islands

Don’t be fooled by your cat’s clumsy, peaceful appearance when it spends its 12 to 16 hours of daily sleep on your couch. Several studies have concluded that our favorite feline is an apex predator: rodents, birds, reptiles, insects, anything that comes within its reach is potential prey. While domestic cats cause relatively limited damage – about 30 prey per year – due to their restricted territory and being fed by their owner, those that return to the wild sometimes cause great damage. Called “feral cats”, these cats that feed exclusively in the wild can capture more than 1,000 prey per year. On the islands, the risk they pose to an often fragile and endemic biodiversity can sometimes be irreversible..

At 18e and 19e centuries, the cat was introduced by European settlers to islands around the world. Even on an island as huge as Australia, it only took 200 years of its presence for it to appear on the black list of invasive species: feral cats kill 350 million birds and 600 million reptiles each year. A recent study, published on Scientific Reports on June 16, 2022, showed that feral cat predation poses a real threat to species like the Kangaroo Island dunnart, a marsupial endemic to this southern Australian island.

Threat to the dunnarts of Kangaroo Island

With its 4,400 km2, Kangaroo Island is the third largest island in Australia. Among the species it shelters, the dunnart (Sminthopsis aitkeni), a small insectivorous marsupial, is classified as critically endangered, due to its small population in a very restricted range. To estimate the proportion of dunnarts hunted by feral cats, zoologists set traps just after a bush fire between 2019 and 2020, which consumed a third of the island including 98% of the dunnart’s habitat. dunnart. “Fire tends to concentrate predators and their prey in “oasis” spared by the flames“explains Louis Lignereux, a researcher at the School of Animal and Veterinary Sciences at the University of Adelaide (Australia). “We do not know if the dunnarts were pushed to these refuge areas or if they were there before the fire, but they are small mammals not much larger than mice, which I imagine less able to move over great distances. .

They analyzed the contents of the digestive tract of 86 feral cats, captured and euthanized in 2020 under Australian pest control law. The results revealed the remains of 263 different prey items, including a majority of small mammals, birds, reptiles and arthropods. The remains of 8 dunnarts were found in 7 cats, or 8% of the sample taken. Given the rarity of the animal, this is a significant rate and confirms for the first time that cats consume dunnarts. Also, southern brown bandicoot remains (Isoodon obesulus), another endangered species, were found in cat food when researchers believed it was protected on the island, out of reach of the mainland red fox. At this point, Kangaroo Island has many species hunted by the cat, which is not in competition with the red fox, absent on the island.“, continues Louis Lignereux.

The researchers calculated that the number of cats living on the island had reached a higher relative abundance than on the mainland. A situation that could push the authorities to adopt measures against its proliferation. “A 2020 study estimates there are between 660 and 1,600 feral cats on Kangaroo Island, specifies Louis Lignereux. For now, the biggest control plane is taking place on the Dudley Peninsula, in the east of the island. The authorities would like to see feral cats disappear by 2030, but this is ambitious. This requires a coordinated effort and the endorsement of residents, although the local population as a whole is supportive of this program. The native fauna is really considered an asset that must be preserved.

Predation of the dunnart of Kangaroo Island (Australia) by the feral cat threatens the disappearance of this endemic marsupial. © Patrick Hodgens & all / Scientific Reports

On Molène Island, storm petrels targeted

Among the Breton islands in the Iroise Sea, Molène seems the most exposed to this phenomenon. Spread over 72 hectares, it concentrates 75% of the national population of storm petrels (Hydrobates pelagicus), a protected species that is also the smallest seabird in Europe. In 2013 and 2015, more than 300 individuals were killed from the claw of feral cats. An observation published on the journal of the League of Protection of Birds Ornithos in 2018 noticed traces of predation on storm petrels from 2007, prompting the first measures to limit the presence of cats.

In 2011, the municipality of Molène had started a regulation program in cooperation with the Iroise Marine Nature Park: the 160 annual residents had received a letter raising awareness of the fauna of their island, urging them to identify and sterilize their cat to limit their proliferation. Due to an upsurge in predation, with an average of 300 corpses identified each year, the municipality has authorized the installation of capture cages. Veterinarians have been commissioned to capture feral or ownerless cats to sterilize them or send them up for adoption on the mainland. Wildlife protection associations appeal to the responsibility of cat owners to have their animals sterilized. At the time of the study, between 80 and 100 cats, mostly domestic, had been listed on the island.

The storm petrel nests on the ground, between rocks, which makes it easy prey for cats. © Jean-Louis Le Moigne / BIOSPHOTO / BIOSPHOTO VIA AFP

Other islands in mainland France are concerned, such as Port-Cros and Porquerolles off the city of Toulon. Feral cats hunt there, for example, the Mediterranean shearwater (Puffinus yelkouan), a seabird classified as vulnerable that also nests on the ground, in rock cavities. Cat identification and sterilization operations also began in early 2022 on the Breton islands of Sein and Ouessant, which could experience similar phenomena.

Dozens of islands affected

In the majority of cases, the fauna of the islands has developed in isolation over thousands of years, without major disruption of the food chain. In the absence of predators, island species have not acquired flight reflexes or avoidance behaviors: many have not resisted the introduction of predators or the denaturation of their habitat. Some birds that have become accustomed to nesting on the ground have, for example, lost their ability to fly, particularly in New Zealand, where the arrival of the cat was catastrophic: Stephens’ xenic (Xenicus lyalli), a small, flightless passerine bird, died out within ten years of the arrival of Europeans on Stephens Island. Today, the emblematic New Zealand kiwi, which does not fly, is also threatened by the feral cat. A small locality had also suggested, in 2018, to ban residents from owning a cat to protect the fauna of a surrounding reserve.

The pattern is similar on many islands. In New Caledonia, the feral cat hunts lizards, geckos and fruit bats. In Reunion, two birds, the Barau petrel and the tuit-tuit, are threatened with extinction. In Guadeloupe and Martinique, the cat attacks the Lesser Antilles iguana. On the Seychelles archipelago, which has one of the greatest diversities of birds and lizards in the world, cats are the number one enemy and several total eradication measures have been considered on certain islands. On the sub-Antarctic islands of Kerguelen and the Crozet archipelago, the feral cat kills more than a million birds each year, including many young albatrosses: in 1976, a study confirmed that cats were responsible for the extinction of a dozen species of seabirds. On the island of Guam, in the Philippine Sea, the reintroduction of the Guam rail, an extinct bird in the wild, is made impossible by too many feral cats. A 2004 study published on Conservation Biology reported that cats had been eradicated from 48 islands around the world, since the first decisions to restore their original fauna. They took place mainly in Mexico, New Zealand, the Pacific, the Seychelles or the Caribbean.

However, the eradication of cats is a decision of last resort, often considered cruel, which must go through the approval of local authorities and residents. In 2020, three French deputies tabled an amendment to classify the cat in the category of “harmful” animals. It had been deemed inadmissible by the National Assembly. To avoid going to such extremes, the question of limiting the number of cats in these fragile ecosystems must be raised, through the accountability of animal owners.

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