0:00:02 > 0:00:04No matter how well we think we know our planet,
0:00:04 > 0:00:07the natural world still has the ability to surprise us,
0:00:07 > 0:00:11to shock us, and maybe sometimes even to scare us,
0:00:11 > 0:00:15with its extraordinary events and bizarre behaviour.
0:00:15 > 0:00:19And new technology means that nature's weirdest phenomena
0:00:19 > 0:00:22are being caught ever more readily on camera.
0:00:22 > 0:00:25So, we are going to bring you the strangest stories
0:00:25 > 0:00:27our world has to offer.
0:00:49 > 0:00:51We start in the southern United States,
0:00:51 > 0:00:54on the trail of a tiny but fearless army.
0:00:57 > 0:01:00The South American fire ant.
0:01:03 > 0:01:05Having escaped their native home,
0:01:05 > 0:01:08they're now busy setting up colonies all over the globe.
0:01:09 > 0:01:13Arriving in the US on a cargo ship in the 1930s,
0:01:13 > 0:01:17they've become an unstoppable ecological disaster,
0:01:17 > 0:01:19attacking anything in their path.
0:01:22 > 0:01:23Unlike many other ants,
0:01:23 > 0:01:26they have a very special sting.
0:01:26 > 0:01:28Using their jaws to grab hold,
0:01:28 > 0:01:30the other end of their body whips round,
0:01:30 > 0:01:34to inject their victim with a venom that burns like fire.
0:01:35 > 0:01:38But what's worse is the ant now signals to all of the others
0:01:38 > 0:01:40that an attack is underway,
0:01:40 > 0:01:43and within minutes, a whole army of them show up
0:01:43 > 0:01:46all joining in the fight.
0:01:46 > 0:01:50It's not long before you're left with a swollen limb, full of angry stings.
0:01:51 > 0:01:53Unstoppable, you might think,
0:01:53 > 0:01:56but as they hit North America's urban landscape,
0:01:56 > 0:01:59a very weird Achilles heel was unveiled.
0:02:02 > 0:02:04A death wish for electrocution.
0:02:08 > 0:02:10You see, they seem to love nothing more
0:02:10 > 0:02:14than piling into electrical circuits, in their thousands,
0:02:14 > 0:02:16to meet a crackly fate.
0:02:17 > 0:02:19One after the other,
0:02:19 > 0:02:22until the entire electrical system blows.
0:02:26 > 0:02:30So, why on earth would a seemingly indestructible creature
0:02:30 > 0:02:32want to commit mass suicide?
0:02:33 > 0:02:36Entomologist and fire ant expert, Dr Karen Vale,
0:02:36 > 0:02:38has been investigating the ants
0:02:38 > 0:02:40and their fatal attraction to electricity.
0:02:41 > 0:02:44Fire ants spend most of their time underground,
0:02:44 > 0:02:46so they don't communicate so much visually
0:02:46 > 0:02:48as they do through using chemicals,
0:02:48 > 0:02:52using things like alarm pheromones to indicate a predator is nearby,
0:02:52 > 0:02:55and to recruit other ants to come and defend the colony.
0:02:56 > 0:02:59What this means is that they can response to danger en masse.
0:03:01 > 0:03:05It just takes one ant to signal for the whole colony to respond.
0:03:06 > 0:03:09Most people don't realise that they're stepping in a mound,
0:03:09 > 0:03:12so they have hundreds of ants running up their legs at a time,
0:03:12 > 0:03:15then that one will bite, release alarm pheromones,
0:03:15 > 0:03:16causes the others to bite and sting.
0:03:17 > 0:03:22It's pretty amazing how organised 245,000 workers can be.
0:03:22 > 0:03:26But this doesn't explain why they are drawn to lethal electrical fields.
0:03:27 > 0:03:30We used to think they were attracted to magnetic fields,
0:03:30 > 0:03:35but what we think happens is that the fire ants are out scouting,
0:03:35 > 0:03:36looking for food,
0:03:36 > 0:03:39and they come across an electrical current,
0:03:39 > 0:03:40and they get killed.
0:03:42 > 0:03:44And they release an alarm pheromone,
0:03:44 > 0:03:46and when they release the alarm pheromone,
0:03:46 > 0:03:48other workers are attracted to that.
0:03:48 > 0:03:51They come in, they get killed by the electrical current,
0:03:51 > 0:03:53causes more alarm pheromone to be released,
0:03:53 > 0:03:56and before you know it, the greater majority of the colony has responded
0:03:56 > 0:03:57to this alarm pheromone,
0:03:57 > 0:03:59and they end up shorting out equipment.
0:04:02 > 0:04:05So, rather than electricity drawing the ants in,
0:04:05 > 0:04:07it's actually the ants themselves,
0:04:07 > 0:04:09responding to calls of help
0:04:09 > 0:04:11from their accidentally-frazzled comrades.
0:04:13 > 0:04:16We estimate about US$6 billion are spent a year
0:04:16 > 0:04:21in paying for medical costs, controlling the ants,
0:04:21 > 0:04:25replacing livestock, equipment, and other damage that they cause.
0:04:25 > 0:04:28But scientists were at a loss as to how they could tackle the problem.
0:04:29 > 0:04:32Electricity was never going to have any meaningful effect
0:04:32 > 0:04:33on their numbers.
0:04:35 > 0:04:39Then they discovered something a lot more effective,
0:04:39 > 0:04:41and much, much weirder.
0:04:44 > 0:04:47The ants had an even bigger nemesis.
0:04:49 > 0:04:50In an incredible twist,
0:04:50 > 0:04:55the very same pheromone leading them to their deaths in electrical systems
0:04:55 > 0:04:58also attracts their arch enemy.
0:04:59 > 0:05:02A tiny South American fly.
0:05:03 > 0:05:06A fraction of the size of the ants themselves.
0:05:06 > 0:05:10Found in the ants' original homeland in South America,
0:05:10 > 0:05:14the mere presence of this predator can stop them leaving their nests.
0:05:14 > 0:05:17The fly is attracted to the alarm pheromones.
0:05:17 > 0:05:19It uses them to home in on the ant
0:05:19 > 0:05:21before injecting an egg into their body.
0:05:23 > 0:05:25The larvae then hatches and grows...
0:05:26 > 0:05:29..feeding on the ant's brain.
0:05:29 > 0:05:31When it's ready, it secretes an enzyme
0:05:31 > 0:05:35that dissolves the connection between the head and the body, until...
0:05:37 > 0:05:39..the head pops off.
0:05:46 > 0:05:48What a wonderfully weird way to go.
0:05:48 > 0:05:49So sci-fi.
0:05:49 > 0:05:51Straight out of the movies.
0:05:52 > 0:05:56In the end, though, it's the ant's key strength,
0:05:56 > 0:05:58their ability to act as one,
0:05:58 > 0:06:00in synchronicity,
0:06:00 > 0:06:02all controlled by their smells,
0:06:02 > 0:06:05which is leading to their demise.
0:06:05 > 0:06:09You see, the flies are quite literally sniffing them out.
0:06:13 > 0:06:16Next, our trail of nature's weirdest invaders
0:06:16 > 0:06:18takes us to an island paradise.
0:06:20 > 0:06:22The tiny island of Guam,
0:06:22 > 0:06:23in the western Pacific.
0:06:25 > 0:06:28Once a haven of island biodiversity,
0:06:28 > 0:06:31rich in native species of birds and reptiles.
0:06:33 > 0:06:37Its wildlife, having evolved in the absence of wild predators,
0:06:37 > 0:06:38have little to fear...
0:06:41 > 0:06:42..until the 1950s,
0:06:42 > 0:06:45when species started to systematically disappear.
0:06:46 > 0:06:50The native bird species simply vanished.
0:06:50 > 0:06:54And the once raucous rainforest felt completely silent.
0:06:59 > 0:07:02What happened next was truly bizarre.
0:07:05 > 0:07:08The silent forest filled with spiders...
0:07:09 > 0:07:14..their webs continually expanding and covering every available space.
0:07:18 > 0:07:22So what could have stunned an island paradise
0:07:22 > 0:07:24into sinister silence?
0:07:26 > 0:07:30Well, to answer this, we need a quick history lesson about Guam.
0:07:33 > 0:07:35CANNONS FIRE
0:07:35 > 0:07:39Occupying a strategic position in the Western Pacific,
0:07:39 > 0:07:43the island was used as a US military base during the Second World War.
0:07:45 > 0:07:48But one of the military machines left behind
0:07:48 > 0:07:51was a visitor that would change the face of Guam for ever.
0:07:57 > 0:08:00The Papua New Guinean brown tree snake.
0:08:04 > 0:08:06Used to a competitive world,
0:08:06 > 0:08:09the snake found itself on an island full of food.
0:08:10 > 0:08:12And, having never seen a predator,
0:08:12 > 0:08:15the native fauna simply didn't know how to respond.
0:08:15 > 0:08:17They were easy pickings.
0:08:20 > 0:08:22SICKENING CRUNCH
0:08:22 > 0:08:25The snake gradually spread out across the island,
0:08:25 > 0:08:28decimating native bird populations as it went.
0:08:30 > 0:08:34But one species' loss was another species' gain.
0:08:34 > 0:08:36With fewer birds to keep them in check,
0:08:36 > 0:08:39the spider population exploded.
0:08:40 > 0:08:45Guam now has 40 times more spiders than its neighbouring islands.
0:08:49 > 0:08:52But even the disappearance of their food source
0:08:52 > 0:08:55didn't stop their population explosion.
0:08:55 > 0:08:59Unusually for snakes, the brown tree snake is not a fussy eater.
0:08:59 > 0:09:01They're happy to scavenge, too.
0:09:04 > 0:09:07As a result, their numbers continue to skyrocket.
0:09:10 > 0:09:12In the face of the onslaught,
0:09:12 > 0:09:17the US government tried anything they could think of -
0:09:17 > 0:09:20trapping, searching for the snakes by sight, and with dogs.
0:09:23 > 0:09:25But, confronted with certain defeat,
0:09:25 > 0:09:28they've now decided to deploy a radical tactic,
0:09:28 > 0:09:32and engage a very covert and specialist parachute regiment...
0:09:32 > 0:09:34of mice.
0:09:36 > 0:09:37Yes, you heard correctly.
0:09:37 > 0:09:41Lacing dead mice with a chemical found in paracetamol,
0:09:41 > 0:09:44that is poisonous to the snakes.
0:09:44 > 0:09:47They've started to drop them from the air into the forest.
0:09:48 > 0:09:53And the snakes' relaxed eating habits means they readily take the bait.
0:09:54 > 0:09:57But as effective as this might be,
0:09:57 > 0:10:00it's thought it will merely control the population
0:10:00 > 0:10:04rather than eradicate the snakes completely.
0:10:04 > 0:10:08An accidental invader that's really overstayed its welcome.
0:10:11 > 0:10:15But, then, not all invasions have been started accidentally.
0:10:15 > 0:10:18Other species have been introduced as food.
0:10:18 > 0:10:20The most notable example perhaps being this,
0:10:20 > 0:10:23the common or garden snail.
0:10:23 > 0:10:26This was brought to the UK by the Romans.
0:10:26 > 0:10:28We no longer have a taste for these animals,
0:10:28 > 0:10:31but they do have a taste for things that we grow,
0:10:31 > 0:10:34and given that one of them can produce 480 more
0:10:34 > 0:10:37in the space of a year, they've pretty much turned themselves
0:10:37 > 0:10:40into the scourge of the British gardener.
0:10:40 > 0:10:44But our next weird invasion is altogether more dangerous
0:10:44 > 0:10:47and ruthless than the average snail.
0:10:47 > 0:10:49And a whole lot more intimidating.
0:10:49 > 0:10:51You all right? Yeah, go, go, go.
0:10:53 > 0:10:59In August 2012, the largest Burmese python ever recorded was captured.
0:10:59 > 0:11:02Weighing in at just under 75 kilograms,
0:11:02 > 0:11:05it took four men to wrestle it out of the bushes.
0:11:06 > 0:11:11But this massive Burmese python is nowhere near Burma.
0:11:11 > 0:11:15In fact, it's being dragged out of the Florida Everglades.
0:11:16 > 0:11:20And, worryingly, it's not the only one.
0:11:20 > 0:11:22Argh! MAN LAUGHS
0:11:22 > 0:11:23Just like on Guam,
0:11:23 > 0:11:27the local wildlife had never seen anything like it before,
0:11:27 > 0:11:32and scientists have recorded a massive drop of up to 99%
0:11:32 > 0:11:34in some species of local mammals.
0:11:34 > 0:11:38Possums, raccoons and bobcats have all been badly hit.
0:11:39 > 0:11:42And even the Everglades' top predator, the alligator,
0:11:42 > 0:11:45appears to have met its match.
0:11:45 > 0:11:49Clashes between alligators and pythons are common.
0:11:53 > 0:11:57So what is a giant snake from south-east Asia
0:11:57 > 0:12:01doing over 14,000 kilometres away from home?
0:12:03 > 0:12:08Well, it's all down to our desire for exotic pets.
0:12:08 > 0:12:11Burmese pythons are bought as small and inoffensive snakes
0:12:11 > 0:12:13at a manageable length.
0:12:13 > 0:12:16But this doesn't last long.
0:12:17 > 0:12:22The snakes all too often outgrow their owners' ability to keep them.
0:12:22 > 0:12:24And, faced with overgrown snakes,
0:12:24 > 0:12:28many owners have been known to set their animals free.
0:12:28 > 0:12:31Others simply break out of their cages.
0:12:33 > 0:12:38The trouble is that Florida turns out to be their ideal habitat -
0:12:38 > 0:12:41plenty of food, water and cover.
0:12:42 > 0:12:45The local wildlife doesn't stand a chance.
0:12:46 > 0:12:49Pythons kill their prey by constriction,
0:12:49 > 0:12:52squeezing the life out of it and then swallowing it whole.
0:12:54 > 0:12:58Like all snakes, they have an incredibly flexible jaw,
0:12:58 > 0:13:02enabling them to swallow prey many times the width of their own body.
0:13:06 > 0:13:10And, as if being able to swallow bigger prey than yourself
0:13:10 > 0:13:13wasn't weird enough, after a meal of this size,
0:13:13 > 0:13:16the snake won't need to eat again for months.
0:13:20 > 0:13:23All together, it's a recipe for success,
0:13:23 > 0:13:26and the authorities are now removing pythons from the Everglades
0:13:26 > 0:13:28in their hundreds.
0:13:28 > 0:13:31Got it. You all right? Yeah, go, go, go.
0:13:34 > 0:13:37These weird events show us what happens
0:13:37 > 0:13:40when man accidentally interferes with nature's balance.
0:13:40 > 0:13:44Whether it's fearless insects on a global crusade...
0:13:44 > 0:13:49or snakes that develop a taste for the local cuisine...
0:13:49 > 0:13:51given the right conditions,
0:13:51 > 0:13:55nature can leave us feeling both powerless and awestruck.