Everyone can agree on one thing: It's not the cats' fault they're bad for wildlife.
Cats are carnivores. Their talent for preying on rodents is a big reason their ancestors and ours started hanging around together in the first place. But then, people carried cats around the world, into ecosystems that weren't equipped for such predators.
Wherever they are, they stalk. They pounce. They kill. They eat.
Now researchers have documented the breadth of cats' global buffet. A study published recently in the journal Nature Communications found that free-ranging domestic cats (including feral ones) eat more than 2,000 species, raising renewed concerns about the ecological fallout.
Almost half of the species were birds, followed by reptiles and mammals. An unexpected number of insects were found, including monarch butterflies, pink-spotted hawk moths and emperor dragonflies. Other surprises on the menu included camels, cows and green sea turtles. (As skilled as cats are at hunting, the camels and cows were probably scavenged. The sea turtles were probably hatchlings.)
"Cats eat a lot more than we thought," said Christopher Lepczyk, an ecologist at Auburn University and one of the study's authors. "That's meaningful."
Nearly 350 of the species, including monarch butterflies and green sea turtles, were imperiled or at risk of being imperiled.
The fallout from cats is especially acute on far-flung islands, where species have often evolved without any mammalian predators. But even in the United States, research by the federal government and the Smithsonian Institution estimated that cats kill a median of 2.4 billion (yes, billion with a b) birds per year. That's especially concerning given the alarming declines in North American bird populations, which have gone down 29% since 1970.
The thorniest controversy comes over what to do with the vast number of feral cats that are driving much of the killing. Some will never be suitable for adoption.
Animal welfare organizations advocate a practice known as trap-neuter-return, in which feral cats are released after neutering to live out their lives. But research has shown that those efforts tend to have limited or no success in reducing populations unless they are performed at continuously high intensities. Well-intentioned people often feed feral cats, driving up numbers.
Then, there are cat owners who refuse to deny their pets the pleasure of roaming outdoors.
Lepczyk has advocated "science-driven management" of free-roaming cats that would designate them as an invasive species, giving wildlife officials more authority to control them. He has also argued for strengthening laws around pet ownership and banning outdoor feeding.
Manuel Nogales, a biologist with the Spanish National Research Council who has studied feral cats for more than 30 years and was not involved with the new paper, praised the work.
"These numbers are totally new for the scientific community," he said. "This paper is quite useful."
The researchers gathered their data through an exhaustive search of published and unpublished academic papers that reported evidence of cat predation on one or more species. Sometimes the species was identified from the contents of a dissected stomach. Sometimes the information came from fecal analysis. Sometimes pet owners reported their cats' kills. Increasingly in recent research, trail cameras caught cats in the act of hunting and scavenging.
Many of the remains could not be identified, especially when it came to soft-bodied insects, which made up 6% of detected species.
Conservation groups try to educate the public about the danger of free-ranging cats while also finding common ground with cat lovers. The Cornell Lab of Ornithology's list of frequently asked questions about outdoor cats begins with the question "Is the Cornell Lab of Ornithology anti-cat?"
"Not at all," comes the response, next to a slideshow of indoor cats that belong to members of the lab.
"It's not a divide between people who love birds versus people who love cats," said Miyoko Chu, the lab's senior director of science communications. "There are so many people who love both."
Lepczyk counts himself among them. "I've had cats for 40-plus years," he said. When he was a child, his family had outdoor cats. He recalled learning about their harm in graduate school and telling his mother. After that, their cats stayed inside. Today, his family includes Mochi, a longhair Siamese, and Ahi, an orange tabby.
Indoor cats may even have an ecological benefit. Widely used rodenticides can harm or kill wildlife like hawks, owls and foxes that eat poisoned mice and rats. As long as cats stay indoors, they are a wildlife-friendly way to help keep your house free of pests.
