When cats roam free, small wild animals die. And the body count in Australia exceeds 2 billion native animals per year.

Environmental researchers in Australia compiled the alarming figure by combing through hundreds of studies on the predatory habits of Australia's free-ranging pet cats as well as feral felines. The scientists documented cats' historic and ongoing toll on Australian wildlife in the book "Cats in Australia" (CSIRO Publishing, 2019).

In just one day, Australia's millions of cats kill approximately 1.3 million birds, 1.8 million reptiles and over 3.1 million mammals. [In Photos: The Peskiest Alien Mammals]

Cats were introduced to Australia in the 18th century by European colonizers, and a report in 2017 found that feral cats could be found in 99.8% of the continent, including on 80% of Australia's islands.

Current estimates of the number of feral cats in Australia range from about 2 million to more than 6 million during years with a lot of rainfall, when prey is abundant. And every feral cat kills about 740 native animals annually, co-author Sarah Legge, a principal research fellow with the School of Earth and Environmental Sciences at Australia's University of Queensland, said in a statement.

There are also about 4 million pet cats in Australia. Pet owners who allow their cats to spend time outdoors may never witness their beloved animal's killer instincts. Yet a single pet cat kills, on average, about 75 animals each year. That may not sound like much compared to the death toll racked up by feral cats. However, urban cat populations tend to be denser than in rural areas; with about 180 cats per square mile (60 per square kilometer) wildlife in urban areas pay a deadly price, Legge explained.

"As a result, cats in urban areas kill many more animals per square kilometer each year than cats in the bush," she said.

A feral cat pauses for the camera after killing an Australian rosella parrot. (Image credit: Brisbane City Council)

Australian officials are exploring multiple strategies for controlling populations of feral cats, including shooting, trapping and poisoning them with bait such as toxic sausages.

Such culls are expected to eradicate around 2 million cats by 2020, but some species of vulnerable Australian wildlife may be running out of time, said study co-author Christopher Dickman, a professor in terrestrial ecology with the School of Life and Environmental Sciences at the University of Sydney. Cats are recognized as a threat to 35 species of birds, 36 mammal species, seven reptile species and three amphibian species, according to Australia's Department of Sustainability, Environment, Water, Population and Communities (SEWPAC).

"Many of Australia’s native species cannot withstand these high levels of predation and will become increasingly at risk of extinction unless the problem of cats in Australia is solved," Dickman said in the statement.

Originally published on Live Science.