Scientists have tested a new way to safeguard freshwater crocodiles from harmful invasive cane toads that are spreading across northern Australia.
A team of scientists from Macquarie University, in collaboration with Bunuba Indigenous rangers and the Department of Biodiversity, Conservation and Attractions (DBCA) in Western Australia, has explored a novel method to shield freshwater crocodiles from the dangerous invasion of cane toads in northern Australia.
Freshwater crocodiles (Crocodylus johnstoni) hold cultural significance and are featured in the Dreamtime stories of the traditional owners in the area. The decline of these predators threatens the intricate balance of local ecosystems.
The research conducted by the team, published in Proceedings of the Royal Society: B on August 14, illustrates a technique that significantly reduces crocodile mortality rates in regions where cane toads invade by training crocodiles to associate cane toads with experiencing food poisoning.
Dr. Georgia Ward-Fear, the lead author from Macquarie University’s School of Natural Sciences, along with her co-author Professor Rick Shine, have documented the successful application of a behavioral ecology method known as conditioned taste aversion (CTA).
Bunuba ranger coordinator Paul Bin Busu, along with rangers Kristen Andrews and Karl Bin Busu, observed positive changes in crocodile behavior throughout the program.
Following this successful conservation effort, Sara McAllister from DBCA’s state cane toad program can strategize future interventions in similarly ecologically sensitive areas.
Devastating Impact
Since the introduction of cane toads from South America in the 1930s, they have decimated native wildlife across northern Australia, eliminating local predator species like goannas, snakes, and freshwater crocodiles.
These animals are important parts of traditional diets and play a crucial role in their environments, yet they lack any natural defense against the poisonous toads.
“The loss of freshwater crocodiles due to cane toads will lead to a situation where the low-tier species in our rivers consume all the bait like judembah (cherrabin, a large freshwater prawn) and lardy (boney bream, an estuarine fish), failing to leave any fish for barramundi and stingrays,” explains Ranger coordinator Paul Bin Busu.
The study primarily occurred from May to October, during the dry season in Australia’s tropical north, when river systems often diminish into disconnected pools, leading to a reduced availability of the prawns, fish, and amphibians that constitute a freshwater crocodile’s typical diet, while still fostering higher numbers of crocodiles.
“Freshwater crocodiles can suffer heavily as their river environments become scarce later in the dry season,” notes Dr. Ward-Fear. “They end up gathering in large groups with limited food supply, and when toads begin using these water sources to rehydrate, the two species come into contact, resulting in substantial crocodile mortality over a few months.”
Methodology
Between 2019 and 2022, Dr. Ward-Fear and Professor Shine collaborated with local rangers and DBCA staff to use modified cane toad carcasses in four expansive gorge systems in the Kimberley region of north-western Australia.
Bunuba and DBCA gathered hundreds of cane toads, removing their poisonous components and injecting the carcasses with a chemical that induces nausea, causing any freshwater crocodile that ingests the bait to feel temporarily ill, but not fatal.
Control baits made of chicken meat—without any nausea-inducing additives—were utilized to gauge the effectiveness of the conditioned taste aversion training.
“Being involved in the ground program was a great experience,” remarks Paul Bin Busu, whose ranger team set up numerous bait stations along the riverbanks, using canoes to hang and regularly replace fresh baits while monitoring crocodile reactions.
Achieving Results
“In the first three days, we observed that crocodiles were eating the cane toads and then moving away. Over time, they began to smell the cane toad before consuming it, and on the last day, we noticed that they primarily went for the chicken necks,” he shares.
Using nighttime ‘spotlighting’ surveys and motion-activated wildlife cameras, the team observed that areas where taste aversion training was conducted experienced a significant decrease in crocodile deaths compared to sites without any baiting.
“Our baiting method completely eliminated deaths in areas where cane toads were just starting to invade and reduced deaths by 95% in regions where toads had already been established for a few years. These positive effects continued in subsequent years,” stated Dr. Ward-Fear.
“These findings are very promising as they provide land managers with tools to act proactively against the invasion as well as to mitigate impacts in areas already affected,” adds Sara McAllister from DBCA. “Collectively, we have demonstrated that collaboration among academics, indigenous rangers, and land management agencies can be highly effective for conservation efforts.”
“Post-program, we see increasing numbers of crocodiles returning, which is heartening,” states Paul Bin Busu.
The team previously applied a similar strategy using less-toxic small cane toads to shield yellow monitor lizards from invasive threats.
Professor Rick Shine, the senior author, emphasizes that the study showcases the effective application of conditioned taste aversion as a behavioral ecology approach.
“As the globalization of invasive species threatens ecosystems worldwide, behavioral ecology offers a means of protecting vulnerable environments,” says Professor Shine.
This research received funding from the Australian Research Council (LP170100013). The Cane Toad Coalition is financially supported by the WA Department of Biodiversity Conservation and Attractions (DBCA), Australian Wildlife Conservancy, World Wide Fund for Nature Australia, Kimberley Land Council, Rangelands Natural Resource Management, Dunkeld Pastoral, and Matsos Brewery.
