Invasive toads will probably overrun Madagascar

Two years ago, scientists warned of a new threat to Madagascar’s unique biodiversity: invasive Asian toads that had probably arrived via container ship sometime in the 2000s and started spreading in the wild. Similar to the cane toads that have become a menace in Australia, the Asian toads (Duttaphrynus melanostictus) are toxic, and predators that eat them usually end up dead. In Australia, that toxic nature has wreaked havoc on the native wildlife, but the toads have spread too fast and too far to eliminate entirely. Efforts now focus on limiting the toads’ impacts on native wildlife and ecosystems as much as possible.

In Madagascar, where the invasion is much more recent, there may be time to do something more drastic. The bad news, though, is that a new report from an international group of scientists evaluating options for dealing with the invasion finds that eliminating the toads is “not currently feasible.”

The problem facing Madagascar is potentially huge, the report’s authors note. Predators, ranging from mammals — like the fossa and mouse lemur — to birds, such as hawks, to large snakes, may all die if they are unlucky enough to eat the toads. Populations of smaller amphibians — frogs, chameleons and skinks — could shrink from toads hunting them. There are also worries that Asian toads could spread amphibian diseases such as Ranavirus. The effects of such losses could run rampant through the food web and ecosystem.

The toads are dangerous to humans, too. Unlike in Australia, where people have dealt with the toads for a long time and are unlikely to eat them, rural Madagascans may see the toads as an easy meal. Many of these people depend on hunting and gathering for at least part of their diet and may not know that Asian toads aren’t safe. Plus, declines in large snakes could let the rat population thrive. Too many rats is never a good thing, and the rodents could devastate stores of crops and commercial items.

The report outlines four options for dealing with the toads: Do nothing, protect priority sites only, contain the spread within the 100 square kilometers or so where the toads are currently found, or eradicate them entirely. The last option would be best, the scientists say, as it would get rid of the problem entirely and protect both wildlife and people. But then they list a host of reasons why this wouldn’t work.

There are already about 4 million Asian toads in Madagascar, scientists estimate, and they are really good at making more toads. Females lay an average of 20,000 eggs — and up to 40,000 eggs — each year. Only 1 percent of those eggs survive, but that’s still a lot of toads. To keep the toad population at 4 million, wildlife managers would have to kill 1.5 million toads every year. To get rid of them, they’d have to kill 2.5 million toads every year until they were gone.

Such measures wouldn’t be horribly expensive — the report estimates it would cost about $2 million to $10 million (the effort would need only a wealthy backer from the West) — but that’s really just a guess. No one knows exactly where the toads are or precisely how many are in Madagascar. There’s no easy way to find them, and there’s no quick method of dispatching them, at least not in the numbers necessary for eradication. And then the country would need to set up good biosecurity measures to prevent another invasion (though that would have the added benefit of preventing other potentially harmful species from entering the country).

And then there’s the fact that no one has tried to remove invasive toads on such a scale before. There have been three successful removal projects, but they were all in much smaller areas.

So it looks like eradication won’t be possible, the scientists conclude, at least without a lot more research that would let managers and the government overcome many hurdles. And by that time, the toads will probably have become so numerous that, like in Australia, any such efforts would be impossible.