A Question of Balance

Australia is famous for its battles with invasive species. A memorable episode of The Simpsons shows Bart releasing a bullfrog in an Australian airport; half an hour later, the country is brimming with bullfrogs (or ‘chazzwazzers’ as a local naturally names them). The impact of the poisonous cane toad in its inexorable spread across the country since 1935 is already legend. Today, these amphibians inhabit approximately half the continent and are believed to number in the billions. Yet despite having been a part of the landscape for longer than the living memory of most of Australia’s human population, cane toads are still considered an invasive species. So at what point does an animal stop being considered ‘invasive’ and become ‘native’?

Dingo ( Canis dingo ). Photo: Alex Mullarky

Dingo (Canis dingo). Photo: Alex Mullarky

It’s hard to imagine an Australia without dingoes, but 4,000 years ago this country had never seen anything like a wild dog. Dingoes are believed to have migrated to Australia alongside several groups of humans, spreading across the country over the subsequent four millennia. So why are dingoes considered native when cane toads have a similar story? It appears to be a question of their ecological impact. “For a long time, dingoes were blamed for the demise of thylacines and devils from the mainland, as these losses occurred at about the same time as dingoes arrived,” says Dr Sarah Legge, one of the leaders of the National Environmental Science Program’s Threatened Species Recovery Hub. “But more recent research instead implicates human impacts and climate change as the primary agents causing the mainland extinctions of thylacines and devils.

“Whatever the case, dingoes have since taken on a key role as an apex predator in Australian ecosystems (along with some of the largest reptiles, large raptors and of course humans). Apex predators play a critical role in regulating populations of smaller predators and prey species, as well as promoting ecosystem diversity and stability.” The dingo may once have been an outsider, but it has since found a place in Australian ecosystems, making it an invaluable part of the landscape. According to Dr Legge, where dingo numbers have declined, cat and fox numbers have increased and have had a major impact on small native mammals.

Dingo tracks in the sand. Photo: Alex Mullarky

Dingo tracks in the sand. Photo: Alex Mullarky

These predators are another issue entirely. “I consider feral cats and foxes to be introduced, because they arrived extremely recently,” says Dr Legge. One school of thought places all ‘invasive’ animals in concurrence with the arrival of European colonists in Australia: 1788. Animals that were introduced after this date, including rabbits, foxes, camels, donkeys, horses, cats, and cane toads, are considered invasive. There are even dozens of species of earthworm which have been introduced for agricultural reasons (or by accident) that now populate Australian soils.

Cats and foxes in particular have upset the delicate balance of many ecosystems in the country. “They have been destabilising, causing a large number of mammal and some bird extinctions, simplifying faunal communities, and they continue to exert negative impacts on Australia’s unique fauna,” explains Dr Legge. Estimates place the number of feral cats anywhere between 5 and 23 million, and these cats aren’t happily eating kibble in urban apartments. Research has demonstrated a devastating impact on Australia’s native fauna, which aren’t adapted to this kind of predation.

While dingoes appear to have slotted relatively easily into the landscape, cats are upsetting the balance. Australia has seen as many as 30 mammal extinctions since European colonisation: a clear indicator of how ecosystems have been changed by introduced species.

Feral horses cause enormous problems for Australia's biodiversity, but an immense cultural connection remains. Photo: Alex Mullarky

Feral horses cause enormous problems for Australia's biodiversity, but an immense cultural connection remains. Photo: Alex Mullarky

Large herbivores such as horses and cattle obviously aren’t going after the quolls and numbats, but their impact has nonetheless been measured. Australia has no native large herbivores or hoofed animals that its ecosystems would be prepared for, so grazing decimates native plant life while making hunting easier for feral predators – giving native mammals nowhere to hide. Horses and cats are inadvertently working together. However, it can’t be denied that the feral horses of Australia – numbering around 400, 000 – have worked their way into the national psyche. A search for ‘High Country Victoria’ brings up images of horses cantering across streams in mountain ranges. The Man from Snowy River and The Silver Brumby are some of the country’s most famous literary works. Unsurprisingly then, large-scale removal of these particular feral animals is always likely to encounter vocal opposition.

Invasive species could be defined as those that are introduced with the movements of humans. However, we’ve already seen in the case of dingoes that it isn’t always so simple. Perhaps, then, only recent arrivals should be considered truly invasive, although this too feels simplistic. Maybe the only real test of an animal’s ‘invasiveness’ is its impact on the existing flora and fauna. Predators like cats and foxes cause more harm than native species can recover from, leading to irreversible changes in the Australian landscape; this makes them invasive. It’s a contentious issue that’s difficult to navigate, but if a species can’t exist in harmony with its surroundings, perhaps that’s what we should call ‘invasive’. ‘Native’ doesn’t mean it’s been around forever - it simply indicates an ability to coexist. 

Cover image by Billy Geary

Alex Mullarky

Alex Mullarky is a freelance journalist and works part-time in threatened species conservation. Her other passion is ex-racehorse rehabilitation and she is currently completing her Masters.

Making a killing: where to for fox and cat control?

A famous number in Australia’s history is 10.99%, but if you asked what it meant very few people would be able to tell you. Currently, 10.99% of Australia’s mammalian species have become extinct since European colonisation - a new world record. That’s a whopping 30 of 273 mammal species gone forever. These were species that represented what it meant to be quintessentially Australian and they will never return. If that doesn’t bring a tear to the eye or raise alarm bells, then nothing will.

The loss of mammals in Australia is due to various factors. Nothing in ecology is ever simple, but the main reasons tend to be habitat loss and modification, coupled with increased predation pressure from introduced species. The feral cat (Felis catus) and the invasive red fox (Vulpes vulpes) are two such predators that have decimated native species in Australia. Both introduced in the 1800s, they tend to favour smaller sized animals, and they have a very different diet to our other two extant, top order predators: the dingo (Canis lupus dingo) and the Tasmanian devil (Sarcophilus harrisii).

A feral cat carrying its dinner, a critical weight range mammal. (Image: Billy Geary)

A feral cat carrying its dinner, a critical weight range mammal. (Image: Billy Geary)

So with the demise of native predators such as the thylacine (Thylacinus cynocephalus), Tasmanian devil, eastern quoll (Dasyurus viverrinus), western quoll (Dasyurus geoffroii), northern quoll (Dasyurus hallucatus) and tiger quoll (Dasyurus maculatus), a changing of the guard has occurred that feral cats and foxes have taken advantage of. But how do they stack up against our native species? Since their introduction, both cats and foxes have impacted the land by putting additional predation pressure on many native species, especially mammals that fall into the critical weight range (CWR) of 35g to 3.5kg.

The widespread reduction of mammals in the CWR has led to a huge spate of ecological issues. Due to the nature of trophic levels, mammals within the CWR would rarely have been subjected to predation pressure from top order predators (although both foxes and cats are classified as mesopredators, in many systems void of dingoes they are the apex predators).

Foxes are distributed widely across Australia. Image: Billy Geary

Foxes are distributed widely across Australia. Image: Billy Geary

Such a dramatic reduction in small and medium-sized mammal species has potentially led to a suite of ecological services being lost. Species such as the greater bilby (Macrotis lagotis) and burrowing bettong (Bettongia lesueur) can promote the germination of native plants, as their foraging digs collect more water and seeds compared to rabbits. They also increase soil turnover, adding much needed nutrients into the system by promoting breakdown of leaf litter which may also reduce fire fuel loads.  Some of our most important species including the eastern bettong (Bettongia gaimardi gaimardi), eastern hare-wallaby (Lagorchestes leporides) and desert bandicoot (Perameles eremiana) have disappeared due to predation, and the ecological processes that they once provided are forever gone.

Subsequently, the question at the forefront of many ecologists’ minds is where to next, and how can we best control feral cats and foxes? For years, the go-to method has been shooting and poison-baiting. Although these two methods have merit in small areas when they are sustained over time, they are not a one-size-fits-all approach in a continental context. In some cases, casual baiting and shooting may lead to increases in feral cat and fox numbers due to the landscape being opened up, allowing other invasive predators to move into the vacant territory.

In more recent times, the case for rewilding our native predators, like dingoes and Tasmanian devils, has been put forward as a method for controlling the feral cats and foxes for us. Rewilding, once native predators are established, may also be more cost-effective in some cases compared to baiting, with widespread benefits provided for the multitude of species that invasive predators eat and kill. With species within the CWR released from the risk of being killed by foxes and cats, their numbers should increase in suitable habitat.

There is of course the argument that with an increase in native predators, specifically the dingo, farmers will lose more livestock due to predation. This may only be partly true. There is an emerging body of evidence suggesting that farmers and dingoes can exist in a mutually beneficial partnership. By not killing dingoes, the species’ pack structure and bonds remain strong, and they hunt and kill prey together, focusing largely on macropods. When they are killed periodically through shooting and poison-baiting their pack structure can be broken down, sometimes leading to individual dingoes targeting livestock. By having stable dingo packs in the farming landscape, overabundant herbivores such as kangaroos (Macropus sp), as well as feral goats (Capre hircus), feral donkeys (Equus asinus) and feral horses (Equus caballus) are controlled. This allows for more plant biomass to grow and be used for cattle. Researchers have even calculated that farmers may be $0.83 better off per hectare by not controlling dingoes, due to the associated reduction in grazing by overabundant herbivores.

With the future of so many of Australia’s unique species under threat of feral cats and foxes, the time for discussion and action is now. A whole host of species are depending on us to act quickly and decisively, or we risk adding to our already dismal extinction list. Lethal control techniques are just one method in the invasive predator control toolkit. Therefore, we must continue to develop new ways to think of not only how to reduce the effects of feral cats and foxes, but also how to use our natural predators to improve our biodiversity.


Grant Linley

Grant is an ecologist interested in Australia's flora and fauna. He has experience researching, trapping, tracking, identifying and handling different Australian species. Whilst experienced in terrestrial Australian ecology, he has also conducted research in Borneo and South Africa. Grant's interests centre on preserving and reintroducing extant and extinct Australian species as well as using natural predators to control mesopredators.