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Eradicating a species might sound like something that’s always bad. We put a lot of time and money into protecting life on Earth, not destroying it.

But not all eradication plans target a species’ entire population — just the parts that end up in places we don’t want them to be. Invasive species can completely wreck an ecosystem, and we’ve tried all kinds of different ways to get rid of them over the years. But some of those efforts … kind of backfired.

Spectacularly. Take, for example, Australia’s poster child for invasive species: the cane toad. Native to southern and central America, this toad is huge, averaging between 12 and 15 centimeters, and will eat almost anything, from carrion to insects to that bowl of cat food you left outside.

They’ll also eat smaller frogs and toads, snakes, snails, and small mammals. Basically, if it fits in the toad’s mouth and it can catch it, it’ll eat it. Maybe the saddest part of the cane toad story is that they were introduced to Australia on purpose.

Farmers brought them in the 1930s to control the beetles eating their sugarcane crops. Unfortunately, the toads didn’t stay in the sugarcane fields and started spreading across the continent. In their native environment, the toads are eaten by local fish, reptiles, birds, and bugs, but in Australia there are few predators that can eat them safely.

Cane toads are poisonous, and predators not adapted to their poison often die after eating them. The toads can also spray poison from glands on their shoulders, which can cause intense pain and temporary blindness in humans, making it both difficult and unpleasant to catch them. And when it comes to breeding, they’re like warty, hopping tribbles.

Females lay clutches of up to 35,000 eggs at a time, and can lay two clutches a year. Even though less than 1% of the eggs make it to maturity, that’s still a huge number of baby toads to deal with. Add all of this together, and it’s not hard to see why estimates put the number of cane toads in Australia in the hundreds of millions.

And, despite efforts to control them, they’re spreading, moving into 40-60 kilometers’ worth of new territory every year. But there may be some hope for killing off the toads in one of Australia’s native species: meat ants. These little guys can kill and eat toads without being affected by their poison, and since the toads aren’t native to Australia, they don’t know to watch out for the ants.

If scientists can encourage meat ants to colonize places where the toads like to hang out, it’s possible they could make an impact on the toad population. Some people have also considered bringing in some of the toad’s natural predators, but that’s a risky proposition considering that importing non-native predators is what got Australia into this mess in the first place. The same control problems have happened with other invasive species, too.

During the age of colonization, ships carrying explorers often brought along stowaways like mice and rats. So, to control them, sailors often introduced foreign predators to take care of the pests. You know the phrase “two wrongs don’t make a right?” That’s super applicable here.

One popular import was the small Indian mongoose, a voracious predator that was introduced throughout the Pacific islands, the Caribbean, Japan, and the Mediterranean to control invasive rodent and native snake populations. The mongoose was a favorite because it will eat practically anything it can catch, and anyone familiar with Rikki-Tikki-Tavi knows a mongoose can catch just about anything. That ability was a problem, though.

Because besides invasive rodents, they’ll also eat plants, fruit, and eggs, making them a nightmare for native wildlife. The mongoose is credited with the extinction of nearly a dozen species across the globe, which was really not what the explorers who introduced them had in mind. And they carry rabies, in case you needed another reason to wonder why we ever thought introducing them everywhere was a good idea.

Obviously, it’s best to just avoid introducing introducing foreign species in the first place, but it is possible to eradicate them if you’re really dedicated. What happened on Macquarie Island, halfway between Australia and Antarctica, gives you some idea of just how difficult eradication can be — but maybe also some hope. When the island was discovered by an Australian explorer in 1810, it became a hub for seal and penguin hunting.

Ships full of hunters brought along mice and rats, which threatened their food stores, so sailors brought in cats to control the rodents. Fast forward 60 years, when another foreign species was introduced: the rabbit. It was an old tradition, meant to provide sailors with food in case a shipwreck stranded them on a deserted island.

Of course, rabbits multiply incredibly fast. And with lots of tasty rabbits around, the cats multiplied, too. By 1970, there were more than 100,000 rabbits decimating Macquarie Island’s plant life, and the cats had hunted two species of birds to extinction.

Conservationists were so concerned about the rabbit problem that they introduced yet another foreigner to the party: a rabbit disease called myxomatosis. It was fairly successful, reducing the rabbit population to less than 20,000 within a decade. But with fewer rabbits to hunt, the cats became even more dangerous to Macquarie’s native bird population.

By 1985, they’d done enough damage that conservationists began a campaign to shoot and kill all the cats. You can probably guess where this is going. In 2000, the last cat on Macquarie was shot, and in less than a decade the rabbit population was so out of control they’d stripped nearly half the island bare.

To prevent even more damage, the Tasmanian government approved a 17 million dollar program to hunt down and kill every rabbit, mouse, and rat on the island. The first wave of the project kicked off in 2011, spreading poisoned rodent bait over the landscape. That killed off the rats, mice, and many of the rabbits.

Then, to kill off the remaining rabbits, they brought in the dogs. Before you facepalm too hard, these dogs were specially trained rabbit hunters that left the island with their trainers. In April of 2014, after two centuries of damage, the eradication was complete.

So there might be hope for controlling some of the other invasive species causing problems all over the world. But what’s happened in cases like these has taught us a lot about just how difficult, time-consuming, and expensive those efforts can be — and how badly they can backfire. So, next time you hear a story about unwanted pets being dumped into sewers, think less mutant ninjas and more ecological nightmare.

Thanks for watching this episode of SciShow. If you’re interested in learning more about how invasive species take over, you can watch. Hank tell the story of bunny and Planet Wonderful. [♪OUTRO].