Episode 2 Nature's Weirdest Events

No matter how well we think we know our planet,

the natural world still has the ability to surprise us,

to shock us, and maybe sometimes even to scare us,

with its extraordinary events and bizarre behaviour.

And new technology means that nature's weirdest phenomena

are being caught ever more readily on camera.

So, we are going to bring you the strangest stories

our world has to offer.

We start in the southern United States,

on the trail of a tiny but fearless army.

The South American fire ant.

Having escaped their native home,

they're now busy setting up colonies all over the globe.

Arriving in the US on a cargo ship in the 1930s,

they've become an unstoppable ecological disaster,

attacking anything in their path.

Unlike many other ants,

they have a very special sting.

Using their jaws to grab hold,

the other end of their body whips round,

to inject their victim with a venom that burns like fire.

But what's worse is the ant now signals to all of the others

that an attack is underway,

and within minutes, a whole army of them show up

all joining in the fight.

It's not long before you're left with a swollen limb, full of angry stings.

Unstoppable, you might think,

but as they hit North America's urban landscape,

a very weird Achilles heel was unveiled.

A death wish for electrocution.

You see, they seem to love nothing more

than piling into electrical circuits, in their thousands,

to meet a crackly fate.

One after the other,

until the entire electrical system blows.

So, why on earth would a seemingly indestructible creature

want to commit mass suicide?

Entomologist and fire ant expert, Dr Karen Vale,

has been investigating the ants

and their fatal attraction to electricity.

Fire ants spend most of their time underground,

so they don't communicate so much visually

as they do through using chemicals,

using things like alarm pheromones to indicate a predator is nearby,

and to recruit other ants to come and defend the colony.

What this means is that they can response to danger en masse.

It just takes one ant to signal for the whole colony to respond.

Most people don't realise that they're stepping in a mound,

so they have hundreds of ants running up their legs at a time,

then that one will bite, release alarm pheromones,

causes the others to bite and sting.

It's pretty amazing how organised 245,000 workers can be.

But this doesn't explain why they are drawn to lethal electrical fields.

We used to think they were attracted to magnetic fields,

but what we think happens is that the fire ants are out scouting,

looking for food,

and they come across an electrical current,

and they get killed.

And they release an alarm pheromone,

and when they release the alarm pheromone,

other workers are attracted to that.

They come in, they get killed by the electrical current,

causes more alarm pheromone to be released,

and before you know it, the greater majority of the colony has responded

to this alarm pheromone,

and they end up shorting out equipment.

So, rather than electricity drawing the ants in,

it's actually the ants themselves,

responding to calls of help

from their accidentally-frazzled comrades.

We estimate about US$6 billion are spent a year

in paying for medical costs, controlling the ants,

replacing livestock, equipment, and other damage that they cause.

But scientists were at a loss as to how they could tackle the problem.

Electricity was never going to have any meaningful effect

on their numbers.

Then they discovered something a lot more effective,

and much, much weirder.

The ants had an even bigger nemesis.

In an incredible twist,

the very same pheromone leading them to their deaths in electrical systems

also attracts their arch enemy.

A tiny South American fly.

A fraction of the size of the ants themselves.

Found in the ants' original homeland in South America,

the mere presence of this predator can stop them leaving their nests.

The fly is attracted to the alarm pheromones.

It uses them to home in on the ant

before injecting an egg into their body.

The larvae then hatches and grows...

..feeding on the ant's brain.

When it's ready, it secretes an enzyme

that dissolves the connection between the head and the body, until...

..the head pops off.

What a wonderfully weird way to go.

So sci-fi.

Straight out of the movies.

In the end, though, it's the ant's key strength,

their ability to act as one,

in synchronicity,

all controlled by their smells,

which is leading to their demise.

You see, the flies are quite literally sniffing them out.

Next, our trail of nature's weirdest invaders

takes us to an island paradise.

The tiny island of Guam,

in the western Pacific.

Once a haven of island biodiversity,

rich in native species of birds and reptiles.

Its wildlife, having evolved in the absence of wild predators,

have little to fear...

..until the 1950s,

when species started to systematically disappear.

The native bird species simply vanished.

And the once raucous rainforest felt completely silent.

What happened next was truly bizarre.

The silent forest filled with spiders...

..their webs continually expanding and covering every available space.

So what could have stunned an island paradise

into sinister silence?

Well, to answer this, we need a quick history lesson about Guam.

CANNONS FIRE

Occupying a strategic position in the Western Pacific,

the island was used as a US military base during the Second World War.

But one of the military machines left behind

was a visitor that would change the face of Guam for ever.

The Papua New Guinean brown tree snake.

Used to a competitive world,

the snake found itself on an island full of food.

And, having never seen a predator,

the native fauna simply didn't know how to respond.

They were easy pickings.

SICKENING CRUNCH

The snake gradually spread out across the island,

decimating native bird populations as it went.

But one species' loss was another species' gain.

With fewer birds to keep them in check,

the spider population exploded.

Guam now has 40 times more spiders than its neighbouring islands.

But even the disappearance of their food source

didn't stop their population explosion.

Unusually for snakes, the brown tree snake is not a fussy eater.

They're happy to scavenge, too.

As a result, their numbers continue to skyrocket.

In the face of the onslaught,

the US government tried anything they could think of -

trapping, searching for the snakes by sight, and with dogs.

But, confronted with certain defeat,

they've now decided to deploy a radical tactic,

and engage a very covert and specialist parachute regiment...

of mice.

Yes, you heard correctly.

Lacing dead mice with a chemical found in paracetamol,

that is poisonous to the snakes.

They've started to drop them from the air into the forest.

And the snakes' relaxed eating habits means they readily take the bait.

But as effective as this might be,

it's thought it will merely control the population

rather than eradicate the snakes completely.

An accidental invader that's really overstayed its welcome.

But, then, not all invasions have been started accidentally.

Other species have been introduced as food.

The most notable example perhaps being this,

the common or garden snail.

This was brought to the UK by the Romans.

We no longer have a taste for these animals,

but they do have a taste for things that we grow,

and given that one of them can produce 480 more

in the space of a year, they've pretty much turned themselves

into the scourge of the British gardener.

But our next weird invasion is altogether more dangerous

and ruthless than the average snail.

And a whole lot more intimidating.

You all right? Yeah, go, go, go.

In August 2012, the largest Burmese python ever recorded was captured.

Weighing in at just under 75 kilograms,

it took four men to wrestle it out of the bushes.

But this massive Burmese python is nowhere near Burma.

In fact, it's being dragged out of the Florida Everglades.

And, worryingly, it's not the only one.

Argh! MAN LAUGHS

Just like on Guam,

the local wildlife had never seen anything like it before,

and scientists have recorded a massive drop of up to 99%

in some species of local mammals.

Possums, raccoons and bobcats have all been badly hit.

And even the Everglades' top predator, the alligator,

appears to have met its match.

Clashes between alligators and pythons are common.

So what is a giant snake from south-east Asia

doing over 14,000 kilometres away from home?

Well, it's all down to our desire for exotic pets.

Burmese pythons are bought as small and inoffensive snakes

at a manageable length.

But this doesn't last long.

The snakes all too often outgrow their owners' ability to keep them.

And, faced with overgrown snakes,

many owners have been known to set their animals free.

Others simply break out of their cages.

The trouble is that Florida turns out to be their ideal habitat -

plenty of food, water and cover.

The local wildlife doesn't stand a chance.

Pythons kill their prey by constriction,

squeezing the life out of it and then swallowing it whole.

Like all snakes, they have an incredibly flexible jaw,

enabling them to swallow prey many times the width of their own body.

And, as if being able to swallow bigger prey than yourself

wasn't weird enough, after a meal of this size,

the snake won't need to eat again for months.

All together, it's a recipe for success,

and the authorities are now removing pythons from the Everglades

in their hundreds.

Got it. You all right? Yeah, go, go, go.

These weird events show us what happens

when man accidentally interferes with nature's balance.

Whether it's fearless insects on a global crusade...

or snakes that develop a taste for the local cuisine...

given the right conditions,

nature can leave us feeling both powerless and awestruck.