Cane toads are poisoning freshwater crocodiles as they continue their march across Western Australia’s Kimberley region. But in Windjana Gorge, ahead of the front, researchers are gearing up to combat a mass mortality event – lacing cane toad corpses with nausea-inducing chemicals in a bid to train native crocs to stay away from the pests.
As the Kimberley’s dry season progresses through September into October, and rivers shrink to pools, “you get this huge density of crocodiles from the whole river system congregating,” Dr Georgia Ward-Fear, Macquarie University ecologist, explains.
Food portions diminish, crocodiles grow hungrier in the heat, and cane toads begin to seek water as well. When cane toads and crocs overlap, “it becomes the perfect storm”.
“You can potentially get mass mortality events of large proportions of the crocodile population, because they are all there and hungry,” Ward says.
Cane toads were introduced to Australia almost 100 years ago as a method of crop control in Queensland. Carrying foreign toxins, they began spreading from northern to Western Australia. Native apex predators that feed on frogs, such as freshwater crocodiles, iguanas, snakes and quolls, attempt to eat the toads and are killed by their toxins.
Last year, 60 freshwater crocodiles were found poisoned and killed by cane toads at Danggu Gorge in WA’s Kimberley region.
“We are seeing this impact play out again and again as cane toads move into new areas.”
The toads are expected to hit the town of Derby this year, and by 2025 they may have covered Broome. Once the Kimberley is swamped, Ward explains that a potential drop in the apex predator population is cause for concern.
“Any ecosystem is a web that exists in a delicate balance,” she says. “If you mess with one component, it sends ripples through.”
In Aboriginal culture, freshwater crocodiles carry story, totemic and ceremonial importance. They are also key sources of food.
“It comes to that time of the season where we would be looking to hunt certain types of animals, but because they are eating cane toads … there aren’t many around any more,” Bunuba ranger trainee, Lionel Marr, says.
Kristen Andrews, Bunuba ranger assistant, describes it as mucking up the whole food chain: “If the croc dies, the catfish eat everything, and there is nothing left for the barramundi, so we will lose out on the barramundi.”
Traditional methods of invasive species management include the removal of the animals from the environment by hand, or by fencing them out.
But with its females able to lay 30,000 eggs twice a year, Ward says “none of that has really worked to slow the invasion or decrease the number of toads in the environment”.
‘We know we can’t really clean up all the cane toads’
Instead, Ward is working with native species at risk and refocusing efforts towards impact mitigation.
She leads a study in Windjana Gorge by Macquarie University, the DBCA and Bunuba rangers, in which researchers are gutting the corpses of euthanised cane toads, injecting them with a non-lethal salt compound, and presenting them as bait for freshwater crocodiles. If crocodiles eat the bait, they experience nausea and learn that cane toads are harmful ahead of the invasion front. This is called conditioned taste aversion.
“It is essentially food poisoning,” Ward says.
“Obviously we know we can’t really clean up all the cane toads,” Marr says. Instead, the trial is a way “of getting the native animals to live among them”.
Nicki Mitchell, an associate professor at UWA who is independent to this study, says adapting behaviours of native animals to avoid cane toad toxins in the future “is an excellent initiative”.
“If this can be rolled out at the right scale, top order predators will naturally be less likely to consume the first toad they see,” she says. “Ideally, what it will do is … [allow] ecosystems to function as they should.”
On the ground the DBCA technical officer, Miles Bruny, says “it is a bit of a gruesome job”.
Before bait deployment along the banks, he says there are hundreds of toads to process, which includes gutting them of their toxic parts, including the stomach and eggs in the females, then injecting them.
“You are lugging all the gear into the field. Camera, stakes, baits along the banks.”
The team need to deploy the carcasses over water bodies right before dusk to avoid other nearby animals taking the bait.
“It is a push to get several hundred toads hung along the gorge.”
The next morning is spent collecting data, either by canoe or foot, up and down the gorge.
“It is quite hot there, 38C plus,” Bruny says. “It is not easy work.”
“But when you realise what is going on with the toad front, and you see the impacts, you feel really good about doing this sort of stuff.”
“We may not be able to save every single croc, but potentially we are preserving the population and diversity of the population.”
