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.
"It was a huge success," says Ward-Fear. "And it's now being deployed by Indigenous rangers and wildlife management agencies as we speak."
A tragic toad
When about a hundred cane toads were first brought from Hawaii to Australia in 1935, people had high hopes that the amphibians would feast on the beetles damaging the local sugarcane crop.
But the toads were a big flop. "They didn't control the beetles," says Ward-Fear. "Instead, they started spreading across Australia." Today, an estimated 200 million cane toads are hopping around the continent.
She says that at first, no one really noticed the invasive toads' spread. But then a whole range of animals started dying — including lizards, snakes, a furry cat-size marsupial called a quoll and lots of freshwater crocodiles.
The cause of death was a potent brew of toxins that the cane toads produce and store in their bodies.
Most of the time, the crocs and the croakers don't see much of one another. But a couple of months a year, during the dry season, the rivers and gorges of northern Australia dry out, and small pools of water attract both species.
"It's this peak period of vulnerability," says Ward-Fear. "We see these mass-mortality events of crocodiles, tens of hundreds of animals dying within a six- to eight-week period right at the end of the dry season."
A crocodile's first encounter with a toad is always lethal, so the animal has no chance of learning. But Ward-Fear and her colleagues — including a group of Indigenous Bunuba rangers and the Department of Biodiversity, Conservation and Attractions — wondered whether there might be a way to warn the crocodiles somehow.
Croc Instruction 101
To teach the crocs, the team members needed bait. But they couldn't use a full cane toad loaded up with toxins.
"So we cut off the top half of their body," says Ward-Fear, "so that [the crocodiles are] still getting the smell and the taste, but we've taken the most toxic parts off."
Then they did something sneaky. "[We] injected a compound that elicits nausea in the crocodiles," Ward-Fear explains. "So it makes them sick. It's essentially food poisoning."
The resulting taste aversion isn't conscious — it's encoded deep in the brain. (Just think of the last time it happened to you or someone you know, and an otherwise delicious or palatable food became intolerable.)
The team then had to get these drugged half-toad carcasses in front of the crocodiles. "We had to get in canoes and paddle up these gorge systems," says Ward-Fear, "so we were definitely one with the crocodiles."
The team members hung the bait from stakes at the water's edge. They also suspended chicken carcasses as a control. "There was a few times that we had to actually shut down bait stations," she says, "because there was a croc that was dominating that area and just waiting for us to set out baits at the station, which could potentially have led to a dangerous situation."
Mostly, though, the crocs behaved themselves, and the results were evident. The crocodiles learned not to eat the toads. In Danggu Geikie Gorge National Park, where the toads had shown up two years prior, "we decreased the mortality rates by 95%," says Ward-Fear, "whereas in the nearby control area where we had not done any intervention, the mortalities just kept raging unabated."
Dave Garshelis, a bear conservationist with the International Union for Conservation of Nature, applauds the study. "It's a pretty effective way of saving these crocodiles from mortality," he says.
More than two decades ago, he used taste aversion with black bears to get them to stop eating military rations. After a year, however, the association waned. He thinks the same thing may happen with the crocs, which means "this training is gonna probably have to be done over and over and over again," he says.
The authors of the paper think otherwise. Crocodile mortality rates stayed down for the full two years of the study. In addition, it takes only a year or two for adult cane toads to start breeding once they inhabit a new location. Little toadlets have far less toxin than the adults, providing at least some of the crocodiles with a nonfatal way of learning to avoid them.
Ward-Fear says this is a solid interim solution — one that manages the coexistence of crocodiles and cane toads. It's part of a growing field called conservation behavior.
"Behavioral interventions are fast acting," she says, "and they're often a lot easier to implement and more ethical than culling the invasive species."
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