Outdoor cats are an invasive species and a threat to themselves, scientists say

A free-roaming cat is not a happy cat. In fact, according to a new study in the journal Frontiers in Ecology and Evolution, free-roaming domesticated cats are a downright menace to local wildlife — and this is the case largely due to the irresponsibility of their human owners. The new research contributes to an ever-growing body of literature on how humanity's love for cats is changing the natural world. 

This particular study was conducted by American and Canadian researchers, who used a camera trap survey to examine the interactions of free-roaming domesticated cats in the Washington, D.C. area over a period of three years.

The scientists also analyzed the interactions of eight native mammal species, five of which are commonly preyed upon by cats and three of which are notorious vectors of disease. They found that where there are more humans, there are more likely to be cats — and where there are more cats, there are more likely to be cat interactions with disease-carrying mammals like raccoons and red foxes. Additionally, where there are more cats, there are white-footed mice and eastern cottontails being preyed upon by ferocious felines.

"Our research was driven by a desire to inform more effective and humane population management practices for outdoor cats to protect cats and wildlife alike," Daniel Joseph Herrera, a PhD student studying urban ecology at the University of Maryland–College Park, told Salon by email. Interestingly, the study also debunked certain myths about housecats, namely, the notion that they are nocturnal. Rather, researchers found that "instead of having peaks of activity at night (nocturnal) or day (diurnal), cats — as a species – maintained relatively constant activity throughout the 24-hour day. In other words, our analysis revealed that cats have no clear activity pattern and individual waking hours vary widely and overlap."

This means that, if humans begin to populate a community and allow their cats to roam freely, the odds are high that the cats will interact with the wildlife in that area — all of it.

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"From a management perspective, this means that opposing activity patterns cannot be relied upon to stop cat-wildlife conflict," Herrera pointed out. "Taking measures to ensure cats and wildlife are not in the same space, such as keeping cats indoors, will prove more effective."

Birders certainly are inclined to agree with Herrera's point. Grant Sizemore is Director of Invasive Species Programs at the American Bird Conservancy, and was quick to denounce the misconception that cats need or deserve to roam outdoors in order to be happy and healthy.

"On the contrary, cats that roam outdoors are subjected to a variety of serious risks, including vehicle traffic, poisons, injury, and disease," Sizemore explained by email. "Cats with outdoor access are nearly three times more likely to be infected with parasites," Sizemore said, citing a 2019 study from Biology Letters. "The American Veterinary Medical Association, which acknowledges the reduced lifespan for cats roaming outdoors, advises that pet cats be kept indoors, in an outdoor enclosure, or on a leash. Cats that remain indoors can live long and happy lives."

Sizemore also debunked the notion that cats do not have a significant impact on their local environment when they prey on local creatures.

'"Not only are domestic cats not native to the United States and, thus, not a natural part of the environment, their predatory impacts can be substantial," Sizemore observed. "Kays et al. (2020) identified that the impact of pet cats is actually 4-10 times greater than native predators, due in part to the incredibly high density of cats. Across the United States, outdoor cats are estimated to kill 2.4 billion birds every year." As a result, "domestic cats have already contributed to the extinction of 63 species of birds, mammals, and reptiles, and domestic cats are the top source of direct, human-caused bird mortality in the United States."

All of Sizemore's points are based on older research — but the Frontiers in Ecology and Evolution study reinforces these points.

"Advocates of free-roaming cats often claim that cats are merely stepping into a vacated ecological role," Herrera pointed out. "An interesting result from our study is that cats are positively associated with human development while wildlife tend to be negatively associated with dense human development. This finding implies that cats are dependent on humans for their survival, whereas wildlife are not." As a result, one can conclude that cats depend on humans to survive at the densities that are common in urban areas. "If cats are dependent on humans, then any hunting they do is not 'natural' since humans have facilitated it." Even worse, cats prey on wildlife at a much larger rate than native species, meaning "that they are not fulfilling an ecological role but inflicting ecological damage."

They could also help bring about another pandemic.

"The human-cat relationship further complicates ecological interactions like the transmission of zoonotic disease," Herrera noted. "While a human would never knowingly open their doors to a rabid raccoon, owners of indoor-outdoor cats routinely allow their cats to engage in activities where they might contract rabies, then welcome them back into their homes at the end of the day." Notably, cats can harbor SARS-CoV-2, the virus that causes COVID-19. Epidemiologists know that the more inter-species crossover a virus has, the more opportunities it has to mutate. SARS-CoV-2 is believed to have originated in bats and pangolins before jumping to humans.

Stephen Vantassel, a Vertebrate Pest Specialist for the Montana Department of Agriculture, was more succinct in his analysis.

"People think 'The cat just kills unwanted rodents. I need the cat to control vermin,'" Vantassel wrote to Salon. "Response: Cats are like using a grenade for hunting when a bullet would suffice. Cats kill everything they can. They don't target specific species."

As for the notion that a cat needs to go outdoors for its mental health? "No. Indoor cats can have toys and will play if you spend the time. Also their health will be better," Vantassel said.