A landmark study published in the journal Conservation Letters this month outlines a clever strategy pioneered by Macquarie University researchers to protect vulnerable native species from the devastating impacts of invasive cane toads.
The study, led by wildlife biologist Dr Georgia Ward-Fear, a Postdoctoral Research Fellow at Macquarie University, showed high survival rates in groups of goannas in the remote Kimberley region of Western Australia trained to avoid poisonous cane toads.
"Our work in adaptive management for conservation is based on decades of cane toad research and involves a broad group of people and organisations," says Dr Ward-Fear.
"Cane toads crossed into Western Australia in 2009 and have made it most of the way across the Kimberley, with disastrous results.
"When apex predators like goannas eat an adult cane toad, they die quickly and painfully – and when they disappear, it affects the whole food web."
Together with Macquarie University evolutionary biologist Professor Rick Shine, Dr Ward-Fear ran small scale field trials in 2016 showing when lizards are first exposed to younger, smaller cane toads, they get sick from eating them, but don't die. Most will then avoid eating another cane toad and survive.
"It's a form of ecological immunisation, building resilience in native wildlife against invasive species," Dr Ward-Fear says.
Teaching Taste Aversion
Using sites at the frontline of the cane toad invasion, the study field-tested this method (called 'conditioned taste aversion') on groups of yellow-spotted monitors, a tropical goanna species heavily impacted by cane toad spread.
"Cane toads have so far been unstoppable, so we wanted to scale up our success in small areas, to a landscape approach," says Dr Ward-Fear.
Initially, the team taste-trained individual goannas in the field, tracking them with radio tags during the arrival of the highly poisonous adult cane toads heading the invasion frontline. The taste-trained goannas had far better survival rates.
As cane toads arrive, we see a very rapid and huge decline in the larger predators which regulate the food web from the top. This imbalance sends ripples through the whole ecosystem.
The 'taster toad' method was then trialled at an ecosystem scale.
The results were remarkable, says Dr Ward-Fear. At control sites which did not introduce the small 'taster toads' before the larger, highly poisonous adult cane toads arrived, goanna populations plummeted by up to 94 per cent.
However, at sites where taste aversion was trialled, goanna populations ranged from 35 per cent to 140 per cent of pre-invasion levels by the time the study finished.
Group effort the key to success
Dr Ward-Fear says this study tracks Australia's largest cane toad mitigation strategy to date, so upscaling the methodology from individual trials to the ecosystem level was an exercise in logistics and collaborative partnerships.
"The strategy involved releasing thousands of eggs, tadpoles and juvenile cane toads into wild waterbodies in the Fitzroy Valley in Western Australia's Kimberley region, immediately ahead of the invasion frontline," says Dr Ward-Fear.
This ambitious approach depended heavily on collaboration with the Cane Toad Coalition, a group of research, conservation and land management organisations coordinated by Dr Ward-Fear and Professor Shine, and supported by an Australian Research Council grant.
Working with the Bunuba Rangers and the Western Australian Department of Biodiversity, Conservation and Attractions to raise then release juvenile toads, the team used camera traps and sardine-tin baits to observe goanna populations.
Food web impact
Goannas, crocodiles and other larger predators are the only species directly affected by cane toads, but the knock-on effect in Northern Australia of losing these animals has been profound, Dr Ward-Fear says.
Cane toads were introduced to Australian sugar cane farms in 1935 to control pests and have since spread across the tropics, decimating populations of native Australian predators who eat them.
"As cane toads arrive, we see a very rapid and huge decline in the larger predators which regulate the food web from the top. This imbalance sends ripples through the whole ecosystem," she says.
Free from large predators, snakes and lizards breed up in huge numbers, exerting strong pressure on their prey species like frogs and small lizards, who then decline. This can see insect species proliferate, annihilating many plant species.
"Goannas are significant cultural totems for Traditional Owners across northern Australia, and an important bush tucker food," Dr Ward-Fear says.
"In parts of the Kimberley there can be five different language words associated with a goanna burrow, for example – nesting burrows, night-time burrows and so on, so when that species disappears from the landscape, it's more difficult to practice those aspects of your culture."
Long term effects
Dr Ward-Fear says although their intervention was targeted to relatively small, specific populations in Western Australia, its results will have long-term impacts.
"By managing the initial impact of the invasion, we see ongoing survival of goanna populations because after cane toads invade and begin breeding, plenty of baby toads will "train" the next generation of goannas, without us having to keep adding more toads to the system."
Dr Ward-Fear says the research shows that behavioural interventions can be a viable alternative to traditional wildlife management approaches that try to wipe out invasive species – a goal that is often impossible to achieve.
"While it's impossible to deploy 'teacher toads' right across the Australian tropics, we can maintain pockets with healthy predator populations, and potentially these can repopulate areas where goannas have become locally extinct," she says.
"We're optimistic that even a single deployment can have long-term effects."
Dr Georgia Ward-Fear is a wildlife biologist in the School of Natural Sciences. She formed the Cane Toad Coalition and worked with a broad group of stakeholders to trial the large-scale cane toad intervention.
