Scientists in Australia have come up with an unusual plan to save freshwater crocodiles that keep dying after eating invasive and poisonous toads.
By filling dead toads with a chemical that makes the crocs feel sick, they hope to train them to avoid the cane toads when they're alive and hopping and filled with a nasty toxin. A trial of this method, using over 2,000 dead toads, was successful, with the crocodiles rapidly learning to avoid both dead and alive toads, according to a paper in the journal Proceedings of the Royal Society B: Biological Sciences.
The researchers, who are from Sidney's Macquarie University, worked with Bunuba Indigenous rangers and the Department of Biodiversity, Conservation and Attractions (DBCA) in Western Australia. The team showed that they could get the crocodiles to associate the cane toads with feeling sick in areas where the toads were newly invasive, preventing the crocs from munching on them. This is a behavioral ecology method called conditioned taste aversion, where an animal comes to associate a bad taste with eating a certain animal and avoids it in the future.
Cane toads, or Rhinella marina, are an invasive species in Australia. These toads are native to Central and South America and were introduced to Australia in the 1930s. Since then, over 200 million of the critters have invaded vast areas of Queensland, the Northern Territory and Western Australia.
These toads have large glands behind their eyes that secrete a potent toxin called bufotoxin, which can be harmful or even fatal to many native Australian predators that try to eat the toads, including goannas, freshwater crocodiles, various snake species, dingoes and quolls. The spread of cane toads has led to the decline of many native species, including the freshwater crocs.
"In tropical Australia, some populations of freshwater crocodiles (Crocodylus johnstoni) have plummeted by greater than 70 percent due to lethal ingestion of toxic invasive cane toads," the researchers wrote in the paper. "Apex predators play critical ecological roles, making their conservation a high priority."
A 2016 report by the Queensland government said 75 species of Australian lizards, crocodiles and freshwater turtles face a threat by cane toads. Sixteen in particular are listed as "threatened" in Australia.
"Freshwater crocs can be heavily impacted as their river systems dry out during the late dry season," said study co-author Georgia Ward-Fear of Macquarie University's School of Natural Sciences, in a statement. "They end up congregating in large numbers with very little food, and as toads begin to use these waterbodies for rehydration, the two come into contact and we see large numbers of crocodile deaths over a few months."
In the paper, the researchers describe how they removed the toxin gland from 2,395 dead cane toads and doctored the bodies with a chemical that would make a croc feel sick but wouldn't harm it. As a control bait, they also used pieces of chicken without the nauseating chemical.
"The first three days we noticed the crocodiles were taking the cane toads then they would go away. Then we noticed they would smell the cane toad before eating, and on the last day we noticed that it was mostly the chicken necks getting eaten," said study co-author Paul Bin Busu, a Bunuba ranger coordinator, in the statement.
The researchers discovered that in areas where the dead toads had been placed, the crocodiles had significantly lower mortality rates from eating living cane toads, compared with other areas.
"Our baiting completely prevented deaths in areas where cane toads were arriving and decreased deaths by 95 percent in areas where toads had been for a couple of years. Effects which continued in the years following," Ward-Fear said.
Populations of crocodiles began to rise in the areas where the baiting had taken place.
"After the program, we see that the populations of the crocodiles are coming back, which is a good thing to see," said Bin Busu.
This is great news for the local ecosystem, as the loss of freshwater crocs could be disastrous to the food web as a whole.
"Losing freshwater crocodiles to cane toads will mean that bottom feeders in our rivers will eat all the bait such as judembah (cherrabin, a large freshwater prawn) and lardy (boney bream, an estuarine fish), leaving no fish for the barramundi and stingray to eat," Bin Busu said.
Freshwater crocodiles are also very culturally significant to traditional owners in Australia.
"These are really exciting results because it provides land managers with tools to use ahead of the invasion but also behind the invasion front," said the DBCA's Sara McAllister in a statement. "Together we've shown that collaborations between academics, indigenous rangers and land management agencies can be really effective for conservation science."
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References
Ward-Fear, G., Bruny, M., Rangers, the B., Forward, C., Cooksey, I., & Shine, R. (2024). Taste aversion training can educate free-ranging crocodiles against toxic invaders. Proceedings of the Royal Society B: Biological Sciences, 291(2028), 20232507.
Markula, A., Csurhes, S., & Hannan-Jones, M. (2016). Invasive animal risk assessment: Cane toad Bufo marinus.
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