Fact check: Are feral cats killing over 20 billion native animals a year?

Updated

Environment Minister Greg Hunt has pledged new money to help community groups trap and eradicate feral cats.

"There are up to 20 million feral cats taking up to four native Australian animals a night. That is over 20 billion Australian native species being destroyed a year," Mr Hunt said on ABC's Landline on November 2.

It's well recognised that feral cats are a major threat to native wildlife, but are there really almost as many cats as people in Australia?

ABC Fact Check takes a look at the numbers.

The claim: Greg Hunt says up to 20 million feral cats are each killing four native Australian animals a night, destroying over 20 billion animals a year.

Greg Hunt says up to 20 million feral cats are each killing four native Australian animals a night, destroying over 20 billion animals a year. The verdict: The number of feral cats in Australia is impossible to calculate because of density variation, and the way the population fluctuates with prey availability and climate. The number of native Australian animals killed by feral cats each night is also difficult to calculate, due to differences in the size of prey. Mr Hunt's claim is unverifiable.



Greg Hunt's claim

Fact Check asked Mr Hunt for the basis of his claim and his office said the figures were sourced from The Action Plan for Australian Mammals published in June 2014.

However, Fact Check contacted the authors, Professor John Woinarski from Charles Darwin University and consultant biologist Andrew Burbidge, who both say the plan provides no such statistic.

"The Action Plan for Australian Mammals does not give any estimate or data on the number of feral cats in Australia or on the number of individual animals killed by those cats," Professor Woinarski said.

Dr Burbidge, who chairs the West Australian Government's Threatened Species Scientific Committee, agrees. "The action plan highlights that feral cats are the current greatest threat to Australia's mammals. But... it does not attempt to provide an estimate of the number out there," he said.

"As far as I am aware, there has been no systematic study that would allow an estimate of the number of feral cats in Australia."

The number of feral cats in Australia

Cats have been in Australia since European settlement, and may have arrived as early as the 17th century with Dutch shipwrecks. Feral cats were established in the wild by the 1850s.

Tim Doherty is completing a PhD on feral cats at Edith Cowan University. He recently wrote that "calculating an absolute number of feral cats in Australia is a very difficult exercise".

He notes that recent media reports have used 15 million, 15 to 23 million, and 20 million.

He also points out that a figure of 18 million has become entrenched in scientific and official sources. However, there does not appear to be any identified basis for the number.

The federal Environment Department's 2008 Threat Abatement Plan used the figure of "about 18 million", citing a 2004 report from the Pest Animal Control Cooperative Research Centre. The 2004 report cited a 2001 study from researchers at Cornell University. The Cornell study cited an editorial from Christchurch's The Press newspaper. The newspaper article cited a 1996 speech in federal parliament by MP Richard Evans. Mr Evans cited a 1993 speech in the NSW Parliament by MP Bob Martin.

Unlike the rest of these sources, Mr Martin did not say the number was 18 million. He cited a range of 5.6 to 18.4 million, which appeared in a pamphlet published by the Australian National Parks and Wildlife Service. The pamphlet is far from definitive. "It is difficult to estimate the numbers of cats in Australia due to lack of research," it said. "One estimate suggested that the feral population could be between 5.6 million and 18.4 million with an average of 12 million."

Numbers fluctuate with prey availability, climate

The Australian Wildlife Conservancy is a registered charity that owns land dedicated to conservation in remote areas.

Atticus Fleming, the AWC's chief executive and a member of Mr Hunt's Ministerial Council on the Environment, tells Fact Check the AWC is running "the largest feral cat project in Australia with the most up-to-date information".

Mr Fleming says Mr Hunt's figures are "in the very reasonable range" and "conservative".

Andrew Bengsen, a research scientist at NSW Department of Primary Industries, says feral cat density and abundance varies greatly across Australia.

"Cats are across Australia in sub-alpine mountains, deserts and so many other different conditions. You might be able to get a good estimate from one area but there's just so much variation you just can't extrapolate from any one area," he told Fact Check.

Christopher Johnson, a professor of wildlife conservation from University of Tasmania, echoes Mr Bengsen's view.

"The big problem is how to do the extrapolation properly, considering variation between environments and different spatial extent of those environments," he said.

Dr John Read has a PhD in ecosystem management and has been dissecting and controlling feral cats for over 25 years.

"No-one can accurately estimate the number of feral cats in Australia because the numbers fluctuate considerably with availability of prey," he said.

"[For example] cat numbers can quadruple within a few months during or after a rat, rabbit or mouse plague."

Ecology Professor Christopher Dickman from Sydney University agrees there is no reliable estimation.

"People are saying there are now more feral cats in Tasmania and in the Northern Territory. But it may not be there are more cats, they may have just become more visible. In other areas they may not want to or have learned to not reveal themselves."

Dr Tony Buckmaster, the education leader at the Invasive Animals Cooperative Research Centre, says climate has an impact.

"In drought times, the number drops considerably as there is little food available for them. When there is good rainfall, the numbers increase as there is an increase in the prey abundance."

The number of animals they kill

I've personally been in the field and had a cat cut open and had over 40 to 50 frogs in its stomach. Atticus Fleming

The Environment Department says feral cats are thought to threaten 81 vulnerable and endangered native animals - 35 bird species; 36 mammal species; 7 reptile species; and 3 amphibian species.

In a 2012 report, the AWC estimates that each feral cat kills between five and 30 animals a day. It says taking the lower figure in that range and multiplying it by a "conservative population estimate" of 15 million feral cats gives a minimum estimate of 75 million native animals killed daily by feral cats.

"We think our five [animals killed a day] is extremely conservative. I've personally been in the field and had a cat cut open [that] had over 40 to 50 frogs in its stomach," Mr Fleming said.

Professor Dickman says: "If it's difficult to put a number on the population, then it's difficult to put a number on animals they kill."

Mr Bengsen says feral cats are "compulsive killers" and so studying stomach contents will not provide the complete number of animals killed by that cat.

"You never get 100 per cent recovery from a stomach from an animal, you're probably only going to find a fraction of the animals they've killed and a lot of them would've passed through the animal or [it had] just eaten a small part of it," he said.

Dr Buckmaster says the figure is also greatly dependant on the size of the prey.

"An average feral cat requires about 300 grams of prey per day. If they eat an antechinus [small marsupial carnivore] with an average weight of 40 grams, then they need to eat about eight of them per day," he said.

"If they are eating bush rats with an average weight of 140 grams, then they only need two."

Dr Read agrees. "I have pulled over 30 freshly killed lizards from a single cat and on many occasions have found 10 or more animals," he said.

The verdict

There is no consistent figure on the number of feral cats in Australia, but estimates range from 5 to 23 million. Experts say the number is impossible to calculate because of density variation, and the way the population fluctuates with prey availability and climate.

The number of native Australian animals killed by feral cats each night is also difficult to calculate, due to differences in the size of prey. The stomachs of dissected feral cats can contain one rabbit, or up to 50 frogs.

Mr Hunt's claim that feral cats are destroying over 20 billion native Australian animals each year is unverifiable.

Sources

Editor's note (13/11/2014): This article has been amended to remove a quote from Australian Wildlife Conservancy chief executive Atticus Fleming about the population of feral cats in Astrebla Downs National Park at his request.

Editor's note (20/11/2014): The Environment Department's Threatened Species Commissioner responded to this fact check . You can read it here. It does not change our verdict.

Topics: animals, human-interest, federal-government, liberals, government-and-politics, australia

First posted