When your house cat trots outside for a neighborhood stroll, it doesn't end well for birds, bunnies, squirrels and other wildlife. And now, thanks to a new study, we know how much damage our feline friends can do.
In fact, the study finds, they kill even more prey than wild predators similar in size to cats, and they don't have to go far to do it. The average range for pet cats allowed outside is about 328 feet from the house they live in.
"We found that house cats have a two to 10 times larger impact on wildlife than wild predators — a striking effect," said Roland Kays, lead study author and zoologist at the North Carolina Museum of Natural Sciences.
To track the behavior of house cats who were allowed to go outside, 925 cats across six countries were fitted with unobtrusive GPS devices. Scientists and citizen scientists used the data to track how far the cats ranged, as well as any prey they captured and brought home across rural and urban areas in the United States, the UK, Australia and New Zealand.
The study was published Wednesday in the journal Animal Conservation.
"Since they are fed cat food, pets kill fewer prey per day then wild predators, but their home ranges were so small that this effect on local prey ends up getting really concentrated," Kays said. "Add to this the unnaturally high density of pet cats in some areas, and the risk to bird and small mammal population gets even worse."
The study did not include feral cats.
On average, the study found that house cats killed anywhere between 14.2 and 38.9 prey per 100 acres, per year. That averages out to about 3.5 prey each month per cat. The researchers believe this large number is due to the fact that neighborhoods can include a high density of cats — more so than wild predators living in the wild.
"We knew cats were killing lots of animals — some estimates show that cats in North America kill from 10 to 30 billion wildlife animals per year — but we didn't know the area in which that was happening, or how this compared with what we see in nature," Kays said.
Much of the damage occurred in areas that already disrupt wildlife. This includes housing developments because they disturb natural habitats.
And cats weren't deterred from their ranging based on other predators, like coyotes, in the areas they lived.
"As a result, pet cats around the world have an ecological impact greater than native predators but concentrated within about 100 meters of their homes," the authors wrote in the study.
The authors also noted that it's difficult to tell which native species might persist or recolonize urban areas if domestic cats weren't around.
The study stated that domestic cats are among the most abundant carnivores on Earth, accounting for up to 600 million pet cats around the world. And that can spell doom for native species.
On the bright side, this doesn't have an impact on species living in larger protected areas because the cats were killing prey in urban and suburban habitats.
Some of the native species found at risk in the study included Brushtail possums in South Australia, as well as endangered rodents and rabbits in North America. This is because domestic cats are opportunistic hunters, pursuing small mammals that they can easily catch and kill.
Keeping cats indoors could prevent the impact on wildlife, the researchers said.
"Because the negative impact of cats is so local, we create a situation in which the positive aspects of wildlife, be they the songs of birds or the beneficial effects of lizards on pests, are least common where we would appreciate them most," said Rob Dunn, study co-author and William Neal Reynolds Distinguished Professor of Applied Ecology at North Carolina State University.
"Humans find joy in biodiversity, but we have, by letting cats go outdoors, unwittingly engineered a world in which such joys are ever harder to experience."