[♪INTRO] 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]
