I recently published a Hot Topic on feral cats for the Ecological Society of Australia.
Hot Topics are evidence-based syntheses of topical environmental issues and they include a database of studies relating to the topic. You can find the summary text below and access the full synthesis here.
The domestic cat (Felis catus) was introduced to Australia by Europeans and now inhabits all of mainland Australia, Tasmania and a number of offshore islands. Feral cats are opportunistic, generalist carnivores and their potential impacts include: predation, competition and disease transmission.
The recent Action Plan named feral cats as the number one threat to endangered mammals and Federal Environment Minister, Greg Hunt, has called for the ‘eradicat[ion] of all significant populations of feral cats’ within a decade.
Cats prey on at least 400 native and introduced vertebrates in Australia, including 123 birds, 157 reptiles, 58 marsupials, 27 rodents and 21 frogs. Cats have played a key role in the extinction of at least 20 mammals, and are a current threat to several threatened mammal and bird species. Most evidence for the population-level impacts of feral cats has been gleaned from historical patterns of decline and extinction, or from the failure of reintroduction attempts for threatened mammal species. The small amount of experimental work that has been done confirms that cats can suppress and exterminate small mammal populations.
The impacts of competition and disease transmission are less clear. Overlap in resource use between cats and native predators suggests competition may exist, although this has not been confirmed experimentally. Cats carry the parasite Toxoplasma gondii, which can cause death for many marsupials. High prevalence of Toxoplasma antibodies has been recorded in some marsupial populations, although it is not known how infection might alter their behaviour or susceptibility to predation.
Further research into the non-lethal impacts of cats is necessary, as well as a better understanding of the relationship between cats, other predators and their prey.
Feral cats are estimated to eat tens of millions of native animals each night in Australia. But what kinds of wildlife are they eating? In research published today in the Journal of Biogeography, my colleagues and I show that cats kill hundreds of different kinds ofanimals, including at least 16 species considered globally threatened.
Recently, I was lucky enough to talk about my PhD research on feral cats on Radio National’s Science Show, which is presented by Robyn Williams on Saturdays between 12 and 1pm. I discuss a quantitative review of feral cat diet in Australia that I conducted with a group of colleagues. We found that cats consume 400 vertebrate species in Australia, including 16 globally threatened taxa. You can listen to the radio audio here and read more about the study here.
Australia is home to more than 300 species of land dwelling mammals. 85% are found nowhere else in the world. Since Europeans arrived in Australia, just over 200 years ago, 30 mammals have become extinct. This is the highest rate of mammal extinction anywhere in the world. The main culprit for the decline is predation by the introduced red fox and feral cat. Feral cats have recently been named the biggest threat to Australian native mammals. They are eating their way through our wildlife. Tim Doherty is studying the ecology of feral cats in Charles Darwin Reserve, 450km NE Perth. Dissection of cat faeces and stomach contents reveals they predate on almost anything. Tim hopes to collect information which will be useful to land managers as they attempt to control feral cats and protect native fauna from these introduced killing machines.
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?
During a speech in Parliament on October 17 1996 — 18 years ago today — then Liberal MP Richard Evans called for the “total eradication of domestic and feral cats from the Australian mainland and offshore islands by the year 2020”.
Similarly, Federal Environment Minister Greg Hunt last week announced a 10-year plan to eradicate “all of the significant populations of feral cats around Australia”.
It’s great that the national conversation is gaining momentum on this issue. Australia has lost 30 mammal species to extinction in just over 200 years, which is the worst mammal extinction record anywhere in the world. According to Professor John Woinarski at Charles Darwin University, at least 20 of these can be attributed to cats.
A recent wave of mammal declines in parts of northern Australia, such as Kakadu National Park, has sparked a renewed call for action on feral cats.
But how many cats are out there? If we look at the evidence, we really don’t know.
The media around this topic often quotes a feral cat population estimate that their followers can grab onto. I’m even guilty of it myself.
The SMH article above quotes the Australian Wildlife Conservancy (AWC) in saying there are an estimated 15 million feral cats in Australia and the Background Briefing report says there are an estimated 15 to 23 million cats.
AWC says that cats eat between five and 30 native animals a night and then they multiply this figure of five by 15 million cats to estimate that feral cats eat 75 million native animals each night.
Similarly, in a follow up to the Background Briefing report, the ABC’s Gregg Borschmann is quoted as saying “Did you know there’s 20 million of them out there, and every one of them eats five critters a night. That’s 20 billion native animals a year” (although my calculations make this closer to 40 billion).
The assumption that cats eat five native animals each night may hold true in northern Australia where European rabbits are largely absent, but it won’t always stand up in the southern half of the continent where rabbits form a staple part of cats’ diets in many areas. These are only estimates of course and they’re useful for engaging the wider community in the feral cat discussion.
A number of news reports also state that the recent Action Plan for Australian Mammals estimated that there are 15 million feral cats in Australia. I’ve read the main sections of this 1,000 page book, and I don’t believe this to be the case. It appears that these numbers are becoming conflated as they filter through various media reports.
Nevertheless, these massive numbers are great for capturing the attention of the general public and raising awareness about feral cat impacts. But from a scientific point of view, it’s important to ask how reliable these estimates are.
What do scientists say?
Professor David Pimentel and colleagues at Cornell University in 2001 quoted a figure of 18 million feral cats in Australia, with the reference being an anonymous 1996 New Zealand newspaper article.
In their 2010 report, Dr Elizabeth Denny and Professor Chris Dickman stated the following: “Pimentel et al (2001) estimated that there are approximately 18 million feral cats in Australia, although the accuracy of this estimation is unknown”.
The series of reports detailed above are all relying on a single citation of an anonymous 1996 New Zealand newspaper article.
The newspaper article in question takes its figure of 18 million from the same Parliament speech by MP Richard Evans on October 17 1996 that called for the eradication of feral cats. Evans also quoted figures of 5 and 12 million, but took the figure of 18 million from a 1993 NSW Parliament speech by then state Labor MP Bob Martin where he stated “Wildlife experts state that between 5.6 million and 18.4 million of these animals are roaming Australia”.
A number of sources incorrectly cite a 1991 report for those figures, which seem to have come from this 1993 pamphlet. In turn, the pamphlet doesn’t say where it took its numbers from. Wherever they came from, these figures seem to be the best estimate we have.
Where to from here?
Calculating an absolute number of feral cats in Australia is a very difficult exercise. Australia is a vast and diverse continent, both spatially and temporally.
Estimates of cat population densities range from 0.03 to 4.7 cats per square kilometre in relatively unmodified and pastoral landscapes, and from 0.7 to 800 cats per square kilometre in highly modified landscapes, such as rubbish dumps.
Extrapolating these figures across the entire continent would yield some fairly wide confidence intervals. Taking a conservative estimate of 1 cat per square kilometre across the entire continent would amount to nearly 8 million cats.
But calculating a more reliable estimate would involve using different density estimates for different climatic regions and would also need to consider inter-annual variation caused by rainfall.
I haven’t attempted to do this, nor do I seek to put forward a better estimate here. However, I think it’s important that we think about this more closely, especially given Greg Hunt’s decade-long feral cat eradication plan. How can we eradicate feral cats if we don’t know how many there are?
This article was updated October 17 to replace the map with a more accurate version.
Tim Doherty receives funding from Earthwatch Institute Australia and the Holsworth Wildlife Research Endowment.
I’ll be presenting some of our feral cat work at the Ecological Society of Australia’s annual conference in Alice Springs next week. My talk is on Thursday 2 October 2014, at 11:15 in the Ellery Room D.
WHAT’S FOR DINNER? A CONTINENTAL-SCALE ANALYSIS OF FERAL CAT DIET IN AUSTRALIA
Tim Doherty, Dr Dave Algar, Dr Eddie Van Etten, Dr Neil Collier, Dr Rob Davis, Professor Chris Dickman, Dr Glenn Edwards, Dr Pip Masters, Russell Palmer, Dr Sue Robinson
Reducing the harmful impact of feral cats is a priority for conservation managers across the globe and success in achieving these aims requires a detailed understanding of feral cat ecology across a broad spectrum of environmental conditions. We reviewed the diet of the feral cat in Australia, seeking to identify biogeographical patterns in diet diversity and composition. We specifically sought to examine: (1) how consumption of prey groups varies across rainfall and latitudinal gradients, (2) the relationship between consumption of rabbits and other prey groups, and (3) how trophic diversity and composition differ between different climate-habitat regions. We modelled feral cat diet against latitude, longitude and climatic variables using 49 published and unpublished data sets. Feral cats consume or predate at least 400 vertebrate species in Australia, including predation of at least 28 Red List species. Consumption of arthropods, reptiles, rabbits and rodents varied with latitude. Consumption of medium-sized mammals was highest in the southeast and consumption of birds was highest on islands. Consumption of rabbits was negatively correlated with that of rodents and dasyurids. Our findings confirm that the feral cat is an opportunistic, generalist carnivore capable of exploiting a diverse range of prey across Australia. The feral cat uses a facultative feeding strategy, feeding mainly on rabbits when they are available, but exploiting other prey like small mammals when rabbits decrease in availability or are absent altogether. We discuss these results in the context of previous dietary studies on feral cats and other medium-sized carnivores from elsewhere in the world.