To save wild crocodiles in Australia, scientists gave them food poisoning

AUGUST 16, 20245t :00 AM ET
Ari Daniel



Invasive cane toads like this one have fanned out across Australia, killing numerous predators in their wake, including freshwater crocodiles.
Joshua Prieto/SOPA Images/LightRocket via Getty Images

Across northern Australia, freshwater crocodiles are dying in droves, with some populations down by 70%. That's because the animals are eating a kind of super-poisonous toad that humans brought to the continent decades ago.

Georgia Ward-Fear, a conservation scientist at Macquarie University in Sydney, has witnessed the demise of the crocodiles firsthand. "It's not pretty," she says. "They go into seizures. And death is fairly quick and probably very painful because it's essentially a massive cardiac arrest."

The loss of so many crocodiles is a problem because they sit atop the food web. "When they decline," Ward-Fear says, "we see this huge hole in the ecosystem, and this kind of ripples out and sets off cascading impacts." These impacts include a surge in midlevel predators, which can negatively influence birds' ability to nest. "And so cane toads do really cause ecological havoc here in Australia."

Now, in a study published in Proceedings of the Royal Society B, Ward-Fear and her colleagues report that they've managed to reduce crocodile mortality rates by as much as 95% — by feeding the animals dead cane toads that have been altered to induce food poisoning. The crocs came to associate the toads with a temporarily unpleasant feeling, making them less inclined to eat a deadly toad in the future.

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Taste aversion training can educate free-ranging crocodiles against toxic invaders

Georgia Ward-Fear, Miles Bruny, the Bunuba Rangers, Clare Forward, Ian Cooksey and Richard Shine

Published:14 August 2024 https://doi.org/10.1098/rspb.2023.2507

Abstract
Apex predators play critical ecological roles, making their conservation a high priority. In tropical Australia, some populations of freshwater crocodiles (Crocodylus johnstoni) have plummeted by greater than 70% due to lethal ingestion of toxic invasive cane toads (Rhinella marina). Laboratory-based research has identified conditioned taste aversion (CTA) as a way to discourage consumption of toads. To translate those ideas into landscape-scale management, we deployed 2395 baits (toad carcasses with toxin removed and containing a nausea-inducing chemical) across four gorge systems in north-western Australia and monitored bait uptake with remote cameras. Crocodile abundance was quantified with surveys. Free-ranging crocodiles rapidly learned to avoid toad baits but continued to consume control (chicken) baits. Toad invasion at our sites was followed by high rates of crocodile mortality (especially for small individuals) at a control site but not at nearby treatment sites. In areas with high connectivity to other waterbodies, repeated baiting over successive years had continuing positive impacts on crocodile survival. In summary, we succeeded in buffering the often-catastrophic impact of invasive cane toads on apex predators.

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