Massive trial shows monitor lizards will learn to avoid toxic and invasive adult toads after tasting less dangerous juveniles
A single cane toad can take down a monitor lizard, so researchers are teaching them to avoid these invasive amphibians.Releasing 200,000 eggs and young of a toxic invasive species might seem to be a sure way to make a bad situation worse.
But by doing just that in Western Australia, conservation biologists have begun to rescue the region’s largest lizard. Yellow spotted monitor lizards usually die after eating a single adult cane toad, an introduced pest with toxic skin secretions that has wreaked havoc on Australia’s native wildlife. But if these lizards first taste the species’ young, which are only slightly toxic, the predators learn to avoid eating the lethal adults. As a result,“These folks have really been thinking outside the box,” says David Skelly, an ecologist at Yale University who was not involved with the work. “The results are pretty strong.” Other scientists say the tactic could help additional threatened vertebrates, too.) are now spreading west in northern Australia at a rate of 50 kilometers per year, killing almost all monitors they’ve encountered along the way, as well as other reptiles, and. They eat or outcompete native frogs as well. “Arriving like a tsunami,” the expanding toad “front” has now reached the Kimberley region, a known biodiversity hot spot, says Georgia Ward-Fear, a conservation biologist at Macquarie University. The lizards, known locally as goannas, are an important apex predator and a food source for the Bunuba and other Indigenous people who live in the region. “If we don’t act soon … we will feel in our own selves that we will have lost everything,” says Monique Middleton, vice chair of the Bunuba Dawangarri Aboriginal Corporation. Earlier efforts to stem the spread of the toads by building fences or encouraging invertebrate predators to eat the young have largely failed, so Ward-Fear and her co-author, Richard Shine, an evolutionary biologist at Macquarie,. Young toads carry much lower doses of toxin—enough to make the lizards sick but not kill them. For her Ph.D. thesis, Ward-Fear showed that lizards avoided cane toads after tasting the young. But would that approach help lizards in the wild? To put it to the test, Ward-Fear and Shine worked with the regional wildlife department and the Bunuba Rangers, who are co-authors on the paper, to locate seven large populations of lizards that cane toads were expected to reach soon. The team counted lizards at each site, then put out more than 200,000 cane toad eggs for three of the seven populations. “It’s an ecosystem treatment,” Ward-Fear explains. Over the next 3 years, after the onslaught of adult toads hit, the monitor populations that had been initially exposed to the young toads survived and even expanded. In the other four areas, lizards virtually disappeared. “Clearly, exposure to juvenile toads gave the lizards enough of a head start,” teaching them to avoid that first wave of adults, says Alison Greggor, a conservation biologist with the San Diego Zoo Wildlife Alliance.Some researchers worry this protection may be short-lived. Adult monitors do not stay with their eggs, says Thomas Madsen, an evolutionary ecologist at Deakin University, so they can’t teach the next generation of monitors to avoid the toads. But Ward-Fear says reproducing toads that have moved in will generate their own “teacher toadlets” that young lizards will quickly and safely learn to avoid. “We are not doing anything that’s not about to happen in nature,” she explains. John Llewelyn, an ecologist at Flinders University, agrees. The monitors’ learned aversion to toads can happen naturally, “but it’s also something we can amplify so it’s more effective.” Sean Doody, an invasive species biologist at the University of South Florida, is concerned the young toads will kill other predators such as snakes, as well as juvenile lizards, as they seem to be wiping out a smaller monitor species he studies in Australia. But given the rapid growth of newborn lizards and the timing of the life cycles of lizards and toads, by the time the young toads appear, Ward-Fear expects the lizards will be big enough to survive eating them. Longer follow-up studies are needed, but so far, treated lizard populations are coexisting with the toads. Because many animals are capable of such learning, “the approach has great potential for other species as well,” Greggor says. Jorge Tobajas Gonzalez, an applied ecologist at the University of Córdoba, has taught foxes to avoid the nests of a threatened 7-kilogram grouse called the Western capercaillie by pairing the grouse’s scent with a distasteful toxin, for example. And Ward-Fear’s team is now evaluating cane toad aversion for protecting crocodiles and aquatic monitors. As the toads continue to spread, Doody also questions whether the approach can be scaled up in time. But Ward-Fear says government, Indigenous groups, and conservation organizations are now dispersing eggs and young toads around the region. “We have nothing to lose but everything to gain.”I also wish to receive emails from AAAS/Science and Science advertisers, including information on products, services and special offers which may include but are not limited to news, careers information & upcoming events.has built a global award-winning network of reporters and editors that independently cover the most important developments in research and policy.
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