ARI SHAPIRO, HOST:
In northern Australia, some freshwater crocodile populations are down by 70% because they're eating a kind of super poisonous toad that isn't even from Australia. Well, now a team of researchers has tried out a clever way of teaching the crocs to avoid the toads, and it may well save their lives. Science reporter Ari Daniel has our story.
ARI DANIEL, BYLINE: When about a hundred cane toads were first brought from Hawaii to Australia in 1935, people had high hopes that they'd feast on a local beetle pest. But the toads were a big flop, says Georgia Ward-Fear. She's a conservation scientist at Macquarie University in Sydney.
GEORGIA WARD-FEAR: They didn't control the beetles, and instead, they started spreading across Australia.
DANIEL: Today, they're estimated to be in the hundreds of millions. At first, no one really noticed the invasive toads, but then a whole range of animals started dying, including lots of freshwater crocodiles.
WARD-FEAR: It's not pretty. You know, they go into seizures, and death is fairly quick and probably very painful because it's essentially a massive cardiac arrest.
DANIEL: The cause of death is a potent brew of toxins the cane toads produce. Most of the time, the crocs and croakers don't see much of one another, but a couple months a year during the dry season, the rivers and gorges of northern Australia dry out, and small pools of water attract both species.
WARD-FEAR: We see these mass mortality events of crocodiles, tens of hundreds of animals dying.
DANIEL: Which is a problem since the crocodiles sit atop the food web.
WARD-FEAR: When they decline, we see this huge hole in the ecosystem, and this kind of ripples out. And so cane toads do really cause ecological havoc here in Australia.
DANIEL: A crocodile's first encounter with a toad is always lethal, so they have no chance of learning. But Ward-Fear and her colleagues, including a group of Indigenous rangers, wondered if there might be a way to somehow warn the crocs. To do so, the team needed bait, but they couldn't use a full cane toad loaded up with toxin.
WARD-FEAR: So we cut off the top half of their body so that they're still getting the smell and the taste, but we've taken the most toxic parts off.
DANIEL: Then they did something sneaky.
WARD-FEAR: We've injected with a compound that elicits nausea in the crocodiles. So it makes them sick. And it's essentially food poison.
DANIEL: The resulting taste diversion isn't conscious. The idea was, if the researchers could get the crocs to associate the cane toads with a temporarily unpleasant feeling, they'd be less inclined to eat a deadly toad in the future. Then the team had to get these drugged half-toad carcasses to the crocodiles.
WARD-FEAR: We had to get in canoes and paddle up these gorge systems. So we were definitely one with the crocodiles.
DANIEL: They hung the bait from stakes at the water's edge.
WARD-FEAR: There was a few times that we had to actually shut down bait stations because there was a croc that was dominating that area and just waiting for us to set out baits at the station.
DANIEL: Mostly, though, the crocs behaved themselves, and the results couldn't be clearer. The crocodiles learned not to eat the toads, including in a national park where the toads had arrived two years earlier.
WARD-FEAR: We decreased the mortality rates by 95%, whereas in the nearby control area where we had not done any intervention, the mortalities just kept raging unabated.
DANIEL: The research is published in the journal Proceedings of the Royal Society B.
DAVE GARSHELIS: It's a pretty effective way of saving these crocodiles from mortality.
DANIEL: Dave Garshelis is a bear conservationist with the International Union for Conservation of Nature. More than two decades ago, he used taste diversion with black bears to get them to stop eating military rations. After a year, the association waned, and he thinks the same thing may happen with the crocs, which means...
GARSHELIS: This training is going to probably have to be done over and over and over again.
DANIEL: The authors of the paper think otherwise. Crocodile mortality rates stayed down for the full two years of the study. Nevertheless, Georgia Ward-Fear says this is a solid interim solution.
WARD-FEAR: Behavioral interventions are fast acting, and they're often a lot easier to implement and more ethical than culling the invasive species.
DANIEL: Even if that invasive species has no problem culling other species as it conquers a continent. For NPR News, I'm Ari Daniel.
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