The combination of feral cats and foxes is inflicting a staggering annual death toll on Australia's native wildlife.
Their ability to take down different prey, and their capacity to flourish in different habitats, means they are putting immense pressure on the survival of a number of native animals across a large portion of the country.
And where their diets and range overlap, they're delivering a one-two punch to some of our most vulnerable species.
They're some of the key lessons from new research published in the journal Diversity and Distributions today and supported by the Threatened Species Recovery Hub.
According to the findings, cats and foxes collectively kill in Australia each year:
- more than 1.4 billion mammals;
- almost 700 million reptiles;
- and around 510 million birds.
The findings highlight the urgent need for greater control of invasive species in order to curb Australia's worrying decline in native wildlife, the authors said.
"We need to deal with this problem of foxes and cats," said lead author Alyson Stobo-Wilson from Charles Darwin University.
"People think that a couple of cats and foxes aren't having an impact, but it adds up."
The researchers also mapped where the pests are found, and in what numbers, across Australia.
Cats, which range right over the expanse of the Australian mainland, are able to thrive with very little water as they can get most of their needs via the blood of their prey.
This means that, unlike foxes, they can survive in the desert.
Foxes, on the other hand, prefer cooler climates and dominate across the unmodified temperate and forested areas of southern Australia, and some peri-urban fringes.
"We've got cats going crazy in the arid zone and then foxes are impacting those forest regions in southern Australia," Dr Stobo-Wilson said.
Overall, foxes kill fewer vertebrates than cats do.
But foxes are capable of taking down larger prey like possums and macropods like wallabies.
The prey species that fall within the size range and diet of both cats and foxes — things like bettongs and quolls, some birds and reptiles — are getting hit from both sides, according to study co-author Sarah Legge from the Australian National University.
Extinction a risk if control not ramped up
The researchers estimate that foxes kill around 367 million mammals each year, with about 29 per cent of those being native.
That's about half the 815 million mammals estimated to be killed by feral cats each year. When killing by pet cats is included, that number jumps to over 1 billion mammals.
In the fox diet, possums and gliders were the most common native mammal found. Foxes also had a high incidence of macropods and potoroids in their diet.
Foxes also kill an estimated 38 million livestock animals such as lambs each year, and non-natives like rabbits.
The authors say while Australia already does a good job on pest control, their research highlights the need for an even greater effort, and they hope their findings can help to streamline control programs.
Without it, a number of species are threatened with extinction, Professor Legge said.
"The central rock rat, that's at risk from cats," she said.
"Without any cat control, it's got a pretty good chance of going extinct in the next 20 years.
"Another example is the eastern barred bandicoot — that's threatened by foxes on the mainland. It now exists only in fenced enclosures."
Others under extreme threat include things like the Gilbert's potoroo, eastern quoll, rufous-hare wallaby, and the burrowing bettong.
No 'silver bullet', but control works
While there's no "silver bullet" for controlling cat and fox numbers, ecologist John Read from the University of Adelaide, who wasn't involved in this study, says there are some things that work well.
One key approach is for councils to enforce measures to keep domestic cats contained.
"Management of domestic cats is a huge issue," Dr Read said.
"Keeping cats contained helps prevent those environmental and health impacts and stops them getting out and breeding more feral cats."
Baiting with 1080 works quite well for foxes, and there have already been some fairly large and productive fox-baiting programs in southern and western Australia.
"Baiting is largely successful with foxes and there's been some strong conservation success stories with the numbat and black-footed and yellow-footed rock wallabies," Dr Read said.
But unlike foxes, cats rarely eat carrion, making them harder to bait.
Dr Read is the founder of Thylation, which has developed,with support from NGOs and government, the Felixer: a device that is able to recognise cats and fire a dose of poison onto their fur.
"Cats are really fastidious about being clean, so if you spray poison on their fur they'll go away and lick it," he said.
Cats are also less efficient hunters where the understorey is dense and complex.
So preserving the ground layer through cooler mosaic burning, as well as by controlling damage by grazing animals and pests like goats, can also improve the survival of native animals.
But the one program that has had the greatest impact on feral cats and foxes in Australia wasn't actually targeting them, Dr Read said.
By wiping out their food source, the calicivirus inadvertently sent pest predator numbers crashing as well.
"The numbers of foxes and cats were absolutely decimated," Dr Read said.
"Several threatened mammals such as the mulgara, plains mouse and fawn hopping mouse, that were hanging on in little pockets, were able to rebound 100-fold."
The key point the researchers want to reinforce is that where efforts have been made, from fenced enclosures to culling, they have often proven very successful, according to Dr Stobo-Wilson.
"We need to do better at a landscape scale," she said.
"When we do tackle these issues we can have really positive results, such as reintroducing species [that have been lost]."
