The internet supplies endless amounts of cute kitty videos. A kitten batting around a ball of yarn, a cat chasing a feather duster and fat, grumpy cats lying on their backs have generated many smiles from people throughout the world.

Those same animals, however, are decimating native wildlife populations. Free-ranging domestic cats, from feral cats to outdoor house cats, are linked to 63 wildlife extinctions spanning the globe, as they feast on birds, lizards, amphibians and small mammals.

Researchers from Oklahoma State University’s Department of Natural Resources Ecology and Management and the Smithsonian Migratory Bird Center recently came together to study how cats are impacting mainland wildlife populations.

Study authors Scott Loss, NREM assistant professor, and Pete Marra, director of SMBC, are studying the impact of cats on wildlife to support science-based conservation and cat population management. Their research findings, published in the latest issue of Frontiers in Ecology and the Environment, help fill in some gaps previously undiscovered through single studies.

“Our review shows overwhelming evidence that, beyond causing island extinctions, where there were no native predators, and massive numbers of mainland wildlife deaths, cats can exert multiple types of harmful impacts on mainland wildlife species that are reflected at the population level,” Loss said.

Based on their literature review, the researchers summarize two main direct effects and two main indirect effects of cats on wildlife populations. The first direct effect, predation, by which cats directly kill and remove individuals from the population, has some obvious evidence.

At least 14 observational studies show that cat predation is a substantial source of mortality at the population level for mainland wildlife. In addition, an experimental study in Australia compared two separate populations of small, native rodents for a two-year period. One population was fenced off, not allowing any feline predators. The other was open and fair game.

“This experimental study provides the most convincing evidence yet that cats can cause population declines for mainland wildlife,” Loss said. “In areas where cats could not access, the population persisted the whole two years of the study. For the areas where cats could access, the populations declined to extinction in both cases.”

The second main direct effect of cats on mainland wildlife populations flies a little more under the radar.

Toxoplasmosis, according to the Mayo Clinic, is a disease that results from infection with the Toxoplasma gondii parasite. It is the motivation behind recommending pregnant women should not clean out litter boxes or garden, because it can be transmitted from infected cat feces, and can be deadly to humans and also to wildlife.

Direct mortality from Toxoplasma infection has led to population declines in some cases, especially for marsupials, marine mammals, such as sea otters, and tropical primates.

“Indirect effects of Toxoplasma infection are even more surprising. For example, past research in California showed that infected sea otters were more likely to be attacked by sharks,” Loss said. “This suggests the pathogen alters otter behavior in a way that makes them more vulnerable to predation, leading to additional mortality that could affect their population.”

Finally, cats also can inflict population impacts indirectly through fear. Just the presence of a predator alters prey species’ behavior, movements or reproduction in ways that affect wildlife populations. Notably, one population modeling study showed that fear-induced reduction of bird reproductive output is capable of significantly reducing bird populations, even with low levels of direct predation.

“Based on our comprehensive review of research from around the world, there is now overwhelming evidence that cats impact populations of mainland wildlife in several ways,” Marra said. “If we hope to protect native species globally, it is essential that we start implementing more effective management of cats, including keeping owned cats on a leash or indoors and removing unowned cats from outdoor environments.”