Many people talk about wild-animal suffering as if it is completely intractable, or as if it is inherently opposed to environmental conservation, or as if it’s impossible to know what effects our actions could possibly have on wild animals. But, in fact, there is one simple action any cat or dog owner can do to help wildlife and the environment at the same time: keep their animals inside. (My statistics here come from this excellent review. See also Utility Farm’s excellent blog post on a similar topic.)

Both cats and dogs are predators, and their largest effect on wildlife is through predation. Most studies suggest cats primarily hunt mammals and birds, taxa which are generally considered to be moral patients. Cats kill between 1.3 and 4 billion birds annually in the United States and 100 and 300 million in Canada. In some areas, cats are the largest source of anthropogenic mortality for birds. Cats also kill between 6.3 and 22.3 billion mammals annually in the United States; they kill between 2.8 and 4.9 million native animals annually in Australia and a million per month in Finland. While the effects of dogs are less studied, dogs also hunt wildlife and no doubt kill millions or billions of animals a year.

Some people might wonder if this is part of the natural cycle of life: after all, predators are part of nature, and eliminating them may cause harm to ecosystems. Perhaps cats and dogs are preventing worse deaths through starvation. Others might wonder whether some other predator would eat the prey if the cats and dogs did not.

However, the evidence suggests that this is not the case. Cats have contributed to 14% of modern mammal, reptile, and bird extinctions. While dogs are understudied, they likely also contribute to extinctions. Since cats and dogs contribute to extinctions, one would expect the mortality rate to decrease significantly in the absence of cat and dog predation; the wildlife cats and dogs kill would have lived much longer otherwise. In addition, extinctions can make ecosystems less stable and result in an incalculable loss of ecosystem services.

Thinking just about deaths from predation underestimates the scope of the problem. It’s widely known that cats tend to play with prey, and while playing is fun for the cat, it’s horrifying for the prey. (Imagine how you would feel if a giant repeatedly injured you, then almost let you escape, then injured you again.) 43 to 68% of cat predation attempts are unsuccessful, and cat behavioral observations show a kill rate of 13%. While few victims of cats are brought into rehabilitation centers, cat-caused injuries account for 30% of the admissions into one California rehabilitation center and 28.7% of admissions into four Italian bat rehabilitation centers. Dogs also harass their prey as often as they kill it. The effects of “playing” on prey are rarely quantified, but typically include stress and behavioral changes that are indicative of poor welfare.

Even if the cat or dog never catches its prey, the cat or dog can have tremendous negative effects. Alarm calling by birds in the presence of cats can attract other predators such as corvids. The signs of dog and cat presence, such as barking or urine, increase vigilance, refuge-seeking behavior, anxiety, and wariness across multiple taxa. Therefore, even the mere presence of dogs and cats can be expected to cause fear and distress to prey animals.

Dogs and cats can also spread diseases. 90% of dog and cat pathogens are multi-host. Dogs are reservoirs for numerous pathogens, including canine distemper virus, rabies virus, canine parvovirus, and canine adenoviruses. Free-ranging owned cats excrete an astonishing 10.7 kg of feces outside each year, which transmits Toxoplasma gondii to many other species. Cats also transmit rabies and feline parvovirus. Of course, when our cats and dogs get sick, we typically treat them with antibiotics. But that contributes to antibiotic resistance and can spread antibiotic-resistant bacteria to small wild mammals.

Unsterilized cats and dogs can also interbreed with their wild cousins, producing less fit hybrids. In small populations, genetic drift and infrequent mating opportunities with members of the same species may preserve these deleterious genes. Since hybridization primarily harms relatively small populations, it is less concerning from a wild-animal welfare perspective than other harms caused by dogs and cats. However, from an ecological perspective, hybridization can help make species from dingos to wolves extinct.

How can you prevent your cat or dog from harming wildlife? If you have a cat, keep your cat strictly indoors. If you have a dog, keep control of the dog when the dog’s on a leash, allow the dog to be off-leash only in designated areas, promptly remove feces, and always be mindful of wildlife. If you must allow your cat or dog to go outside unsupervised, keep the animal well-fed, clip the animal’s nails, and keep them inside at night. If you have an outdoor cat, use a bell, sonar device, or product such as CatBibs or Birdsbesafe to protect wildlife from your cat. Regardless of your pet’s species, always vaccinate your pet to prevent the spread of diseases and sterilize the pet to prevent the existence of more feral animals.