8 Ways Domestic Cats Are a Serious Threat to Nature

Fat Cat via Wikimedia Commons // CC BY-SA 2.0
Fat Cat via Wikimedia Commons // CC BY-SA 2.0 / Fat Cat via Wikimedia Commons // CC BY-SA 2.0

Who doesn’t love a Grumpy Cat video or sneezing kittens? But offline, domestic cats represent a threat to the natural world—and a much more serious one than you might think. Here’s a look at some of the environmental risks cats pose that might encourage you to keep Kitty inside.


Thanks to their isolation, islands generally boast high levels of biodiversity and endemic species found nowhere else. Island species evolve based on a very specific set of circumstances; on islands without large predators, for example, some birds lose the ability to fly because they simply don’t need it. This makes the cat problem particularly acute on islands, where free-ranging cats have caused or contributed to 33 of the modern bird, mammal, and reptile extinctions recorded by the International Union for Conservation of Nature (IUCN) Red List.


Felines down under have become a major pain: A recent study in Biological Conservation finds that feral cats roam 99.8 percent of Australia. This means that at any given time, there are somewhere between 2.1 to 6.3 million cats ranging around the continent, ignoring any and all posted signs about environmentally protected areas. Since Australia is the only continent other than Antarctica where wildlife evolved without wild cat species, its wild creatures are particularly vulnerable. John Woinarski of Charles Darwin University, the deputy director of the National Environmental Science Programme’s Threatened Species Recovery Hub and the study’s co-author, tells mental_floss, “Australia has the worst record of extinctions of native mammal species over the last two centuries.” Approximately 30 species have gone extinct. “Introduced cats were a significant factor in most of these extinctions, and feral cats continue to be one of the most serious threats to many of Australia’s threatened animal species,” he says.

In fact, some of Australia’s endangered species survive only within specialized cat-proof exclosures or on cat-free islands. According to a 2012 government plan, species at risk include the endangered woylie, night parrot, bridled nail-tail and black-footed rock wallabys, and the vulnerable greater bilby. To combat the feline threat, the country’s Threatened Species Commission recently launched an ambitious plan to cull feral cats using baits, traps, and even trained cat-finding dogs.


New Zealand has a number of rare endemic species, including the iconic flightless kiwi. The island also has an estimated 2.5 million feral cats. It’s a problematic equation. Cats have already contributed to the extinction of nine native bird species and have affected 33 endangered bird species. The National Cat Management Strategy Group (NCMSG), which includes the Royal New Zealand Society for the Prevention of Cruelty to Animals, Local Government New Zealand and the Morgan Foundation, set a goal of eliminating feral cats by 2025. Government officials have been quick to point out that no one is limiting cat ownership and the former Prime Minister, John Key, has a pet cat.

 "We all agree on what we are trying to achieve,” Geoff Simmons, spokesperson for Morgan Foundation, an NCMSG partner, told Radio New Zealand, “which is making sure all cats are loved pets and are looked after well, and that we minimize the stray and feral population and ensure that the cats that we do own have the least possible impact on our environment."

The government also plans to eliminate all invasive vertebrate predators, including rats and brushtail possums, by 2050. Worldwide, more than 1,000 islands have been cleared of invasive species, including more than 100 around New Zealand. But the largest island ever successfully cleared, Australia’s Macquarie Island, is only about 49 square miles; in contrast, New Zealand is 103,483 square miles. To accomplish this daunting task, the government will turn to new methods, including drones and genetic biocontrol.


In the United States, an estimated 60 to 100 million cats range free, and the number of domestic cats has tripled during the past 40 years. Scientists from the Smithsonian Institution and U.S. Fish and Wildlife Service estimate that domesticated cats—both free-roaming pets and feral—kill as many as 4 billion birds and 22 billion small mammals in the U.S. each year. This makes them “likely the single greatest source of anthropogenic mortality for U.S. birds and mammals,” according to the Smithsonian. While controlling feral cats poses a daunting problem, simply keeping pet cats indoors at least reduces the slaughter. The Humane Society of the United States has developed tips to help indoor cats stay happy, and suggest that you always neuter even indoor animals, because…


Female cats can reproduce as young as four months old, and on average have two to three litters per year of four to six kittens each. One cat can produce as many as 100 kittens in her lifetime, and one pair of cats and their kittens can account for 420,000+ kittens in just seven years (it’s an exponential thing).


Peter Marra, director of the Smithsonian Migratory Bird Center and co-author of Cat Wars: The Devastating Consequences of a Cuddly Killer, tells mental_floss that cats are known to carry plague, rabies, and the parasite Toxoplasma gondii. All are zoonotic diseases, carried by animals and capable of jumping to people. Toxoplasma gondii only reproduces in cats, Marra said, producing virtually indestructible oocysts—cysts containing a zygote formed from an egg and sperm. “They persist for years, in frozen soil, in saltwater environments. In the U.S., about 20 percent of the human population is infected, and globally, about a third of the population.” There is no cure.

While most infected people seem to have no symptoms, recent research links the parasite with behavioral changes, including depression and bipolar disorder, Marra reports. For example, one recent study showed infection causes disruption of a major neurotransmitter in the brain. Infections in pregnant women can cause death or serious health issues in the fetus; the parasite damages the eyes of some 3000 infants in the U.S. each year. Toxoplasmosis can cause fever, fatigue, headaches, blindness, and, in people with compromised immune systems, death. Like a ticking time bomb, the parasite can hide in brain tissue, putting an infected person at risk if the immune system later becomes compromised.

Toxoplasmosis also affects wildlife, representing a serious threat to highly endangered Hawaiian monk seals and to sea otters.


Despite the evidence lined up against cats, pet owners have a hard time accepting that their felines may causing problems. In the UK, which has more than 10 million domestic cats, a recent study found that cat owners did not recognize the risk their pets pose to wildlife. “Cat owners failed to perceive the magnitude of their cats' impacts on wildlife and were not influenced by ecological information,” the study’s authors concluded.

Solving the problem of cat predation on wildlife obviously will require cooperation of pet owners. So it’s important to note: While one individual cat’s predation may not be an issue, the sheer number of cats equals a big problem. This is especially true since predation by cats is often not a normal ecosystem interaction (in other words, domestic cats are not part of a healthy natural ecosystem).


In Cat Wars, Marra tells the story of Stephens Island, off the south island of New Zealand, where cats drove an endemic, flightless wren to extinction in—wait for it—roughly one year. A lighthouse keeper named David Lyall arrived on the island in January 1894. A pregnant female cat arrived about the same time, likely the island’s first feline. An amateur ornithologist, Lyall studied the dead birds that this free-roaming cat brought to the lighthouse. Based on his prepared specimens, noted ornithologist Walter Rothschild described a new species, the Stephens Island Wren. Unfortunately, by that time the bird had disappeared. Subsequent lighthouse keepers began killing feral cats and had the island once again cat-free by 1925—but it was too late for the endemic wren.