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’?
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.
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.
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 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.