Roaming Charges: The Environmental Costs of Outdoor Cats

by Rosemary Jann

Domesticated cats have lived in human communities for so long that they may seem like an integral part of our natural landscape. However, cats are non-native animals that can pose a significant threat to native wildlife, in the process undermining biodiversity and disrupting the balance of our natural environment.  At least in the case of owned cats, there are things owners can do to help right this balance.

Anyone who has watched a cat stalk and pounce on a toy mouse can appreciate how the quick reflexes, sharp teeth, and retractable claws of domestic cats have superbly adapted them to be hunters of small prey. These same hunting abilities played a crucial role in their domestication. According to National Geographic, cats began to frequent human communities in the Fertile Crescent area of the Middle East at least 8,000 years ago, as the development of agriculture resulted in the storage of crops that attracted rodents, and the rodents in turn attracted local wildcats. For thousands of years, cats and farmers enjoyed a mutually beneficial relationship as tamer cats in essence selected themselves for living in proximity to humans. The European domestic cat (Felis catus) was imported into the New World by mariners and colonists, leading eventually to a population explosion of domestic cats in the United States. In 2017, Statistica.com estimated that 94 million cats lived in U.S. homes. National Geographic adds that an additional 70 million feral cats may live in our communities. All these “domestic” cats are actually non-native imports that did not evolve with our local wildlife.

And therein lies the root of an environmental dilemma. Although other factors like habitat loss, pollution, and disease also endanger animals, cats play a significant role in wildlife mortality.  A 2013 review of research by biologists Scott Loss, Tom Will, and Peter Marra estimated that free-roaming cats annually kill between 1.3 and 4 billion birds and between 6.3 and 22.3 billion mammals in the United States alone, making them the largest single source of anthropogenic mortality for those animals. In addition, cats kill numerous insects, reptiles, and amphibians. Exact numbers are difficult to pin down, especially given the challenges of conducting research on feral cats, who cause the highest wildlife mortality. But even the low range represents a significant problem.

In Cat Wars (2016), Marra and Chris Santella show that cat predation has pitted proponents of native wildlife against proponents of feline welfare for over a hundred years. Today, the sharpest controversies involve Trap Neuter and Return (TNR) programs, which vaccinate and neuter feral cats and return them to the community, often in colonies supported with food and shelter. In Cats and Conservationists: The Debate about Who Owns the Outdoors (2020), Anna Peterson and Dara Wald hold out hope for finding common ground in debates over TNR, but they mostly document a deeply-entrenched standoff in which the various sides cannot even agree on what counts as scientific evidence, much less how to act upon it.

There is more clarity in what can be done about the cats that people own. Some cat owners believe that roaming and predation are natural behaviors that should be tolerated, and in a limited sense they are correct. As explained by International Cat Care, , because cats are obligate carnivores who must rely on animal protein, they have been naturally selected for effective hunting abilities. However, because small cats evolved as largely solitary hunters who never knew where their next meal might come from or how difficult it might be to capture, it made sense for them to kill whenever they had the opportunity. This means that their descendants, our domestic cats, are also hard-wired to hunt and kill regardless of whether they are hungry or not. A study in ScienceDirect  that tracked owned cats suggests that as much as 70% of what they kill is not even consumed. So, keeping a house cat well fed is no guarantee that it won’t hunt and kill smaller creatures.

And even when they don’t kill, Marra and Santella (61-62) explain that the mere presence of cats in a landscape can have indirect, sublethal effects, for instance, by reducing breeding fecundity in birds who are frightened into spending less time on the nest and hunting for food for their chicks. Even animals that escape from cats often die from bacteria in their puncture wounds, as noted by Alonso Abugattas  in his Capital Naturalist blog. Smithsonian Magazine adds that outdoor cats can also spread diseases to humans like rabies, plague, and a parasite called Toxoplasma gondii.  

Moreover, it can be argued that the position of domesticated cats in the environment is anything but “natural.” Scientists like Loss and Marra consider house cats (and supported ferals) to be “subsidized predators.” Because people give them food, shelter, medical care, and other support, they have distinct advantages over native predators and can reduce the amount of prey available to them. Cats are generalists who can switch prey more easily than can some native predators. Alonso Abugattas points out that unlike native predators, cats have the leisure to stake out and ambush the same areas (like chipmunk trails or bird feeders) repeatedly. Game camera footage of a cat carrying a dead squirrel in Barcroft Park suggests the toll that free-roaming cats can take on public lands.

Game camera footage courtesy of Alonso Abugattas.

In a natural environment, the size of the predator population would be controlled by the amount of available prey. However, subsidized cat populations can readily exceed the size that a habitat can support without experiencing environmental degradation. As conservationist Paul Noelder puts it, letting cats outdoors “is like letting semis drive in the bike lane. It’s a killer[.]”

The debate over outdoor cats is sometimes framed as a concern for protecting biodiversity versus defending the needs and rights of cats. But there is also a third consideration: free-roaming cats can be vulnerable to many threats, as this poster suggests:

Webmd estimates that on average, indoor cats can live as much as three times longer than free-roaming cats. Owned cats can find the enrichment they need indoors if their owners stimulate the cat’s natural predatory behaviors.  Cat Friendly Homesoffers useful guidelines on choosing toys that mimic a cat’s preferred prey and recommends allowing the cat to capture the toy at the end of the game to satisfy its hunting instincts. Bird videos, window perches, and food hidden in puzzle balls can provide mental stimulation for an indoor cat. The American Bird Conservancy’s “Cats Indoors” site lists safe outdoor access products for cats. These include cat harnesses and backpacks and  enclosures like the “Catio” and cat-proof fencing like the Purrfect Fence that provide outdoor spaces where cats and wildlife can be safe.

Owned cats are not the sole driver of wildlife reduction, but they are one significant factor that can be controlled, starting with the recognition that cat predation is more of a human problem than a feline one. Revoking our cats’ roaming privileges can be a crucial step in protecting biodiversity in our natural world.