In a world that's as connected as ours,

it's no wonder that species sometimes end up in habitats that aren't their own.

But these invasive species can cause serious trouble.

They often compete with native species for food, steal their homes,

nom on them for lunch, or otherwise wreak havoc in their new habitats.

The survival of entire ecosystems can depend on getting rid of them.

But that's not so easy.

Without predators to keep them in check, invasive species tend to stick around and multiply,

despite our best efforts to fend them off.

That means that we have to get pretty creative to have any hope of eradicating them.

And in some cases, humans have risen to the challenge.

Here are five of the strangest ways that people have fought back against invasive species.

The brown tree snake invaded the tiny island of Guam in the 1940s,

after stowing away on a U.S. military plane coming from another Pacific island.

Since then, the snake population, which is now about two million strong,

has eaten its way through the island, devastating the local ecosystem.

The snakes have hunted ten of Guam's twelve native birds to extinction.

Without birds, the spider population has gone unchecked.

Forests have also thinned because there aren't enough birds to spread seeds.

To make matters worse, since these tree snakes live in trees,

it's tough to weed them out with traditional methods like traps or snake-sniffing dogs.

So to try to eradicate these predators,

the U.S. government has been experimenting with a more creative solution:

air-dropping dead mice filled with acetaminophen,

the active ingredient in Tylenol, over the jungles of Guam.

The mice come sailing down in little parachutes, which is kind of tragically adorable.

Then the parachutes get stuck in trees

and dangle the mice in places where snakes can find and eat them.

And the method is pretty deadly.

A year-2000 study by the National Wildlife Research Center found that

100% of brown tree snakes die after ingesting acetaminophen.

Unlike in humans, this medicine prevents the snakes' blood cells from carrying oxygen.

Just 80 milligrams of the stuff, about a child's dose of Tylenol,

makes the snake go into a coma and die within 60 hours.

Dead mice seem to work really well as a vessel for the poison,

because they attract the brown tree snake but not Guam's native species,

according to a preliminary study.

And so far, research suggests that this approach is pretty effective.

One study looked at a site where two rounds of poison-mice

had been dropped at different times.

The researchers found a 40% decrease in the number of baits eaten after the first round,

suggesting that there were fewer snakes to eat them by round two.

So this method could go a long way in clearing these invasive predators out of the island.

While the brown tree snake snuck its way into its co-opted home,

many invasive species waltz into foreign environments as pets.

That's probably how the spiky, colorful lionfish

got from its native Indo-Pacific seas to the Atlantic Ocean.

Lionfish have been terrorizing the Atlantic since the late 1980s,

taking over reefs, shipwrecks, and other places.

It seems likely that these invaders started out as pets

whose owners dumped them in the ocean when they didn't want them anymore.

Which sounds nice and humane!

Except, these fun, zany-looking fish are actually fierce predators.

They eat mostly fish and are super-venomous,

so they don't really have predators in their foreign territory,

which makes them pretty much unstoppable.

Lionfish have drastically reduced the populations of fish where they live.

And it doesn't stop there.

Lionfish devour other fish that typically eat algae,

and unchecked algae growth crowds out coral reefs.

So lionfish are indirectly killing our reefs.

That's a pretty serious problem.

But it's not like we can just fish them out of the ocean.

Lionfish have been moving into deeper and deeper waters,

making them hard to catch with a hook and bait.

That depth poses another problem, too:

it gets too risky for divers to catch a fish with, say, nets

because the water pressure gets dangerously high.

So, to stave off the lionfish invasion, the the CEO of iRobot,

which made the Roomba, yeah, that little robot vacuum,

has developed human-controlled, pressure-resistant robots

that can go where humans can't.

Deep underwater, the goal is for these weird tube robots to track down lionfish,

give them a good zap to stun them, and then suck them up with a vacuum.

The good news is lionfish are a pretty easy catch.

Since they aren't used to having predators around,

they don't really run away from things that approach them,

even weird robots with little clappers attached.

Which is pretty convenient for the robot.

Along with clearing the water of invasive species,

this underwater robot collects lionfish that could ultimately be served at restaurants.

Lionfish actually makes a pretty tasty dish that some people compare to snapper.

As of 2019, the robot is still being refined, so it's not on the market yet.

But its makers are perfecting the design

and preparing the machine to suck the invasive lionfish out of the seas.

The lionfish isn't the only invasive fish that's been hard to root out.

In the Upper Missouri River Basin, brook trout from the Eastern U.S.

have been crowding out the area's native trout species since the 19th century,

when people introduced them as a way of replenishing fish populations around the country.

In the past, people have gotten rid of invasive fish

by dumping toxic chemicals in the river to poison them,

but it probably doesn't surprise you that that solution leaves something to be desired.

The toxins kill invasive fish and native fish alike, including ones we rely on for food.

So researchers in Montana have developed

a much safer way of removing the trout without killing everything else.

The technique is called backpack electrofishing, and it's pretty much what it sounds like.

Essentially, researchers wade into the water wearing backpacks with electric generators.

The generators are connected to wands with positive charge flowing through them,

and researchers stick them in the water.

Fish are naturally programmed to swim toward positive charge, so they approach.

That's when the scientists unleash an electric burst

that can temporarily stun hundreds of fish in one go.

The stunned fish float to the surface and get scooped up in nets.

From there, the researchers separate out the invasives, which can become meals,

and return the native fish to the water.

Researchers have taken out tens of thousands of trout from the basin with this method,

and the best part is, it doesn't hurt the environment they're trying to protect.

It's nice to be able to tackle invasive species in bulk,

but sometimes you just have to go one by one.

That's the case in Washington state,

where invasive mountain goats are wreaking havoc in Olympic National Park.

After hunters in Alaska introduced them in the 1920s,

these goats quickly spread into their new habitat.

As of 2018, around 700 goats were roaming the peninsula,

grazing on the rare plants and eroding the landscape as they trampled across it.

Oh, and there's another detail.

The Olympic goats are addicted to human pee.

Turns out pee, and human sweat,is a rare source of salt in the park;

and the goats just love it.

As the goats have gotten used to humans, they've also gotten aggressive.

Unfortunately, getting rid of the goats is not a walk in the park.

According to the National Park Service,

typical approaches to invasive species won't cut it for all sorts of reasons.

We can't just introduce goat predators, like wolves,

because they'd happily gobble up the park's native animals too, like elk and deer.

And we can't just give the goats salt so they lay off the pee,

since they'd keep eating up native plants and eroding the landscape.

So the U.S. government has come up with a solution:

airlift the goats to their natural habitat in the Cascade Mountains.

To do that, they first paralyze the goats with a dart,

then blindfold them to keep them from panicking.

After that, they hoist the goats up in a harness attached to a helicopter

and take them to a staging area, where a truck picks them up

and delivers them to the Cascades.

And that may sound like overkill, but there is some logic to the idea.

Because of overhunting, the number of mountain goats in the Cascades has been dwindling.

Some experts believe that bringing the goats back to their natural habitat

could help the population bounce back.

Not only would this forced migration add hundreds more goats to the ecosystem,

it would also increase the genetic diversity of the goats in the region,

which could ultimately help the population thrive.

When it comes to removing invasive species,

it's often the last few stragglers that are the hardest to weed out.

But if you don't get the stragglers, the population will likely bounce back.

That was the problem on Isabela Island in the Galapagos.

Across the archipelago, goats had reduced or completely wiped out

almost 80% of plant species, which seriously messed up the ecosystem.

In 2004, helicopter crews managed to wipe out

about 90% of the goats on Isabela Island by shooting them,

but then they had to deal with the last 10 percent.

Unfortunately, it can get pretty expensive looking around for goats in a helicopter.

So researchers turned to undercover goats.

They captured around 800 female goats, neutered them,

and laced them with hormones that make them seem like they're always in heat.

These chemically altered goats are called Judas goats.

Researchers then fit them with a collar so they could track them,

then released them into the wild.

Since they were chemically aroused all of the time, they sought out other goats,

especially males, and scientists were able to follow their trackers to the stragglers.

They did that every few weeks, and then shot all of the goats who weren't Judas goats.

After all the stragglers were gone, the Judas goats, which couldn't reproduce,

were left to carry out the rest of their lives alone.

Of course, it takes a lot of resources

to be able to pull off any of these creative ways of fighting off invasive species.

But even without a helicopter or an underwater fish vacuum,

you can still do things to help control invasive species where you live,

like cleaning off your shoes after hiking or planting native plants in your garden.

Even small steps like these can help keep our ecosystems in balance.

Thanks for watching this episode of SciShow!

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