In 2014, a toxic invasive species—the Asian common toad—was spotted in Madagascar’s largest seaport. Conservation biologists quickly sounded an urgent alarm, warning that the invader could devastate the African island’s unique biodiversity, which includes lemurs and hundreds of other animals found nowhere else in the world. Now, scientists have confirmed that the toad’s toxic slime will likely kill nearly everything in Madagascar that tries to eat it, according to a study that surveyed the susceptibility of 88 species.
The findings “strengthen the idea that these [toads] are a major threat,” says Guinevere Wogan, an evolutionary biologist at the University of California, Berkeley, who wasn’t involved in the study. Knowing how many species are potentially vulnerable is “critical for thinking about how to approach this invasion and save the diversity,” she says.
The toads (Duttaphrynus melanostictus) ward off predators by secreting a deadly toxin, which can trigger cardiac arrest. But some snakes, rodents, and even hedgehogs can eat the toads and come out unscathed, thanks to genetic mutations that render the toxin harmless. But no one knew whether Madagascar’s predators, which evolved on the relatively remote island, had developed similar mutations. “Madagascar has been isolated for 80 or 90 million years, and has never had toads,” says Wolfgang Wüster, a herpetologist at Bangor University in the United Kingdom.
So Wüster and his colleagues decided to inspect the DNA of a diverse subset of Madagascar’s snakes, lizards, frogs, mammals, and birds and see whether they carried the protective mutations. They discovered that only one native species—a rodent called the white-tailed antsangy—is genetically capable of eating toads, the team reports today in Current Biology. The rest of the predators lack the complete set of mutations that confers resistance, leaving lemurs, snakes, lizards, and other native species highly vulnerable should they start snacking on toads. “Small amphibians are very, very easy prey,” Wüster says. “There’s not that many things that wouldn’t eat them.”
The toads aren’t within arm’s reach of most native species just yet, as they’ve only been spotted along a roughly 350-kilometer strip of the island’s northeastern coast. But “their range is rapidly expanding,” says James Reardon, a conservation biologist with New Zealand’s Department of Conservation in Te Anau. The toads are prolific breeders—one female can produce thousands of eggs—and there are plenty of rice paddies, waterways, and drainage systems in Madagascar that are helping them expand, he says. “It’s toad heaven.”
“This [is one of] the two most invasive toad species in the world,” with the other being the infamous cane toad, says Fred Kraus, a herpetologist at the University of Michigan in Ann Arbor. “So I don’t see anything stopping it. … At this point, you’d need millions and millions of dollars.”
To make matters worse, many of Madagascar’s native species are already struggling to hang on in small pockets of habitat, as most of the island has been deforested, Wüster says. The toad, he adds, “is the kind of thing that could really knock them over the edge.”