Exotic species such as pythons, Asian carp and cane toads often dominate the invasive species discourse. Few biological invaders, however, have wreaked as much ecological havoc as one of our most cuddly companions: cats.
Despite their small stature and memeable mugs, domestic cats (Felis catus) are perfectly adapted killing machines, armed with retractable claws, sharp fangs and night vision. And these potent predators are anything but picky. As humans have spread cats around the world over the past 9,000 years, these ferocious felines—which were likely domesticated thousands of years ago in the Near East—have terrorized native creatures on every continent except Antarctica.
A team of researchers recently added up all the species on these invaders’ menu. In a paper published on Tuesday in Nature Communications, the team compiled a database of more than 2,000 species that have fallen victim to free-ranging domestic cats. Nearly 350 of these species are of conservation concern, and several are already extinct. “We don’t really know of any other mammal that eats this many different species,” says the study’s lead author Christopher Lepczyk, an ecologist at Auburn University. “It’s almost like an indiscriminate eater; they’re eating whatever’s available.”
Because of their cherished connection with humans, cats have become some of the most widespread animals on Earth. As pure carnivores who lack the ability to process plant material, household and feral cats are always on the prowl for prey to hunt or carrion to scavenge.
Researchers have been tracking invasive cats’ tastes for more than a century. Most of this work has revolved around ecosystems in well-studied regions such as North America and Australia, however. Many of these efforts are also focused primarily on more common and well-known cat chow, such as small mammals and birds.
To help fill in the missing gaps, Lepczyk and his team analyzed more than 530 scientific papers, books and reports spanning more than 100 years—the largest database of cat diets to date. They then organized the various prey by taxonomic groups to get a view of what types of animals are targeted by killer kitties.
The researchers uncovered instances of 2,083 different species consumed by cats. Many of these animals were birds (981 species), reptiles (463) or mammals (431), with insects (119), amphibians (57) and other taxonomic groups also represented. Though common prey such as mice, rats, sparrows and rabbits were widely represented across the scientific literature, the team also found evidence of cats scavenging more surprising game such as green sea turtles, emus and even domestic cattle.
The researchers crossed-checked their new database with the International Union for Conservation of Nature’s Red List of Threatened Species to determine the conservation status of each species. They found that 347 species documented to have been consumed by cats are listed as near threatened, threatened (including some that are endangered or critically endangered) or extinct. Many of these are small birds, mammals and reptiles that are endemic to islands that lack natural catlike predators, meaning prey are naive and relatively defenseless. Eleven cat-consumed species recorded in the study, including the Hawaiian crow (Corvus hawaiiensis), New Zealand quail (Coturnix novaezelandiae) and white-footed rabbit rat (Conilurus albipes), are now classified as extinct in the wild or extinct.
“The study reaffirms that cats are the ultimate versatile generalist predator,” says Sarah Legge, a wildlife ecologist at Charles Darwin University in Australia, who was not involved in the new paper. Legge studies cats’ impact on Australian wildlife and says they are one of the most serious threats to the continent’s biodiversity. “Cats continue to cause population decline, and more extinctions are inevitable if we don’t manage cats,” she adds. “Australia’s native fauna are not equipped to withstand predation from a versatile predator with a relatively quick reproductive rate.”
Alarmingly, Lepczyk thinks the paper’s findings on cats are conservative. “We have a representation of what they’re eating, but we think that they’re eating a lot more,” he says. For example, though insects make up only a little less than 6 percent of species known to have been eaten by cats, he believes this figure is likely underestimated because of the difficulty of identifying insect remains in cats’ stomachs and scat, compared with finding feathers or mammal bones there.
The geographic bias of the literature the researchers examined is also likely obscuring the totality of species consumed. Because the majority of studies on cats’ diets were conducted in Australia or North America, animals that were native to those continents dominated the dataset. Lepczyk believes that future research to understand cats’ ecological impact in biodiverse regions of South America, Asia and Africa will uncover a multitude of at-risk critters that end up in kitty litter.
Though the new list may not be complete yet, Lepczyk hopes providing an in-depth analysis on what free-ranging cats eat will help conservationists and policy makers combat the feline invasion. “Cats are a problem that we can solve,” Lepczyk says, especially if the animals are prevented from freely roaming through native ecosystems. “At the end of the day, people need to be responsible pet owners.”