Cat owners and cat admirers alike tend to be intrigued by their enigmatic companions. When we see an intrepid feline carefully stalking its prey or happily playing with other cats, we ponder the mysteries of its mind and the depths of its soul. What manic misadventures do cats engage in when we're not around?
Unfortunately, the real world of cats differs significantly from the innocent, whimsical versions depicted in movies like "Cats" and "The Aristocats." In fact, when domesticated cats are allowed to roam freely, they often leave a trail of ecological destruction in their wake. Many species have gone extinct due to domesticated cats being allowed to roam outdoors, and many other animals are suffering immensely because of it.
To understand why, it is first important to recognize that domesticated cats can never be "wild," whether they are owned by humans or "feral," meaning a domesticated cat that has no owner. Of course, wild cats exist — not only tigers, but also bobcats and literal wild cats, from which these favored pets evolved. But these cats are native, not invasive and don't spread wherever humans do. By definition, just like cows and chickens, a domesticated cat is biologically incapable of being a natural part of the wilderness.
"Just like a stray dog doesn’t become a wolf, a feral cat doesn’t become a wild cat," explained Daniel Joseph Herrera, a PhD student studying urban ecology at the University of Maryland–College Park who has co-authored a 2022 study on domesticated cats being an invasive species. "A free-roaming cat is not filling an ecological niche, or reverting to its ancestral form. It is merely a run-away pet that needs to be brought back inside."
Herrera said people often confuse the term "feral" with the term "wild" and use this confusion to justify the cat’s continued outdoor life. But it's quite clear that domesticated cats can be dangerous to ecosystems. The question is how dangerous. According to Grant Sizemore, the Director of Invasive Species Programs at the American Bird Conservancy, outdoor cats can negatively impact a wide variety of species from a diverse range of environments around the world, from backyards to oceans.
"Those impacts include direct predation, parasite and disease transmission, and indirect effects (e.g., competition)," Sizemore wrote to Salon. "Generally, however, the species most detrimentally impacted by outdoor cats are island species [that] are often naïve to or ill-equipped to handle the enormous threat posed by cats and often have smaller populations than continental wildlife."
Sizemore said that this is particularly true of species that are either only as big as cats or slightly smaller, such as the endangered Newell’s Shearwater, "which has nested in burrows in the secluded mountains of Kauaʻi for eons." Outdoor cats are also responsible for the extinction of the Stephen’s Island Wren (or Lyall’s Wren), with Sizemore adding this may be the most famous example of a species now extinct because of cats.
"These birds have been ravaged by outdoor cats, even more so than other introduced predators, because cats not only kill the young in the nest but also kill adults, eliminating the chance for that adult bird, which has already survived the trials of youth, to breed again in subsequent years," Sizemore explained. "Sadly, whether on islands or elsewhere, the impacts of cats adds up, and cats have now contributed to the extinction of at least 63 species of birds, mammals and reptiles worldwide." Outdoor cats have a particularly devastating impact on birds, ranking as "the top source of direct, human-caused bird mortality in the United States, killing an estimated 2.4 billion birds every year."
As Herrera pointed out, bird species are hardly alone in being targeted by outdoor cats.
"Free-roaming cats are known for their predation on bird populations, but research has found that cats actually prey on small mammals more than they do birds," Herrera explained. This can include chipmunks, moles, voles, mice and squirrels, which are easier to catch than birds since they cannot fly away. "Additionally, these species may prove a more reliable source of prey since they do not migrate annually like many species of bird do."
Although cats are traditionally known for preying on non-native rodents like brown rats, Herrera said "they prefer smaller and easier-to-handle prey such as native small mammals and birds. Previous research has found that cats do not reduce the number of rats in an area, and my own research has found cats and rats to live in relative harmony where people leave cat food outside."
Even when cats are not destroying native species through hunting, they can do so through by spreading infections.
"In those parts of the world without a long evolutionary history of cats, cats also spread some cat-dependent diseases (such as toxoplasmosis) to many native birds and mammals," explained John C.Z. Woinarski, a professor at the NESP Threatened Species Recovery Hub at Charles Darwin University, in an email to Salon. He added that "in Australia, there would be no toxo were it not for cats" and that "Australia's fauna has been particularly affected by cats, with cats implicated in the extinction of many native mammal species, and the ongoing decline of many threatened birds and mammals."
If there is any good news here, it is that humans can solve the problem they helped create. According to Arie Trouwborst, a professor of nature conservation law at Tilburg University, "the only effective way to protect vulnerable wildlife from cats is for people to keep their cats indoors or otherwise within their control — just as we expect pet owners to do with any other animal." He cited the Australian city of Canberra as an example of a government that has done this effectively.
"A sympathetic way to work towards a landscape without free-roaming cats, already employed by the Australian authorities in Canberra, is to gradually phase them out," Trouwborst wrote to Salon. "Whereas current outdoor cats may keep roaming the rest of their lives, each newly acquired cat must from now on be kept indoors from the start."
He added, "Feral cats are a different category. As cats are one of the world's worst invasive alien species, biodiversity conservation laws and policies require efforts to remove them from the landscape."
Woinarski offered suggestions for cat owners who want to protect the environment including de-sexing, registration and preventing cats from roaming, strategies which can be strengthened by government regulation. While the problem of dealing with feral cats is more complicated, he suggested actions like banning the importation of domesticated cats to islands that do not already have them, eradicating cats from the islands that do have them, regulating other threats to species susceptible to cat predation, creating predator-proof enclosures for species particularly threatened by cat predation (which is being heavily done in Australia) and "intensive baiting and other cat control programs at sites of conservation significance."
While these measures may seem extreme, Woinarski pointed out that "in Australia, feral cats were most likely the principal cause of extinction of the pig-footed bandicoot, central hare-wallaby, desert bandicoot, lesser bilby and long-tailed hopping-mouse, amongst others."
Dr. Sarah Crowley, a senior lecturer at the University of Exeter's Centre for Geography and Environmental Science, argued that there are "several techniques that could help people reduce the amount of wildlife killed by owned domestic cats. The most successful were feeding a meat-rich, complete diet and playing with cats in a way that simulates hunting behaviors (e.g. with a feather wand) for 5-10 minutes a day."
It is not merely the wildlife that suffers when outdoor cats are given free rein to roam. As Sizemore observed, "cats are the top source of rabies among domestic animals in the United States and disproportionately expose more people to this disease than wildlife." The animals which roam outdoors "are also about three times more likely to be infected with parasites, which can then be spread to people. Furthermore, cats are a definitive host for the parasite Toxoplasma gondii that causes toxoplasmosis. The parasite can only complete its life cycle in a cat’s digestive tract and is then excreted via feces into the environment, where it can subsequently infect any bird or mammal."
These parasites "can cause miscarriages, fetal deformities, blindness, organ failure and death and has been associated with neurodegenerative diseases (e.g., schizophrenia)," Sizemore said. "The risk of toxoplasmosis by cats is an often overlooked but potentially serious consequence of cats roaming the landscape."
