Media playback is unsupported on your device Media caption VIDEO: Rebecca Morelle joins biologist James Stanford on the trail of the brown tree snake

In the dense tropical forest, a slither of movement can just be made out in the glow of our head torches.

A snake is entwined in the undergrowth. It is about 1m long, mostly dull brown but with a vivid yellow underbelly.

We are face to face with Guam's "nemesis": the brown tree snake. And the forests here are dripping with them.

The US territory, in the western Pacific, is only 50km (30 miles) long and 10km wide, but it is packed with two million snakes.

This reptile arrived here only 60 years ago but has rapidly become one of the most successful invasive species ever.

Unhealthy appetite

Wildlife biologist James Stanford, from the US Geological Survey, says: "Our belief is that they came at the end of World War II.

"We've looked at their genetics and they are all extremely closely related, and it appears they came from the Island of Manus in Papua New Guinea."

He explains that military equipment used by the US in Papua New Guinea while the war raged in the Pacific was eventually sent back to Guam to be processed. A snake probably crept on to a ship or a plane destined for the island.

Image copyright Dianna Bisset Image caption Cheryl Calaustro says the koko was easy prey for the snakes

"And from that handful, or maybe even one already impregnated female, we now have a population that is unbelievable in scale," he says.

The snakes, which are mildly venomous, have caused many problems. They get everywhere, and people have even woken up with them in their beds.

The island's power system is regularly shorted out by snakes crawling on the lines. It is so frequent the locals now call power cuts "brown outs".

But the biggest impact has been on the wildlife - it has been decimated. The forests here are eerily quiet. Now the only place where the Guam's native birds, such as the koko, can be seen on the island are in cages in a captive breeding centre.

"The brown tree snake has had a devastating impact. Ten out of 12 native forest bird species disappeared in 30 years," says Cheryl Calaustro from Guam's Department of Agriculture.

"The birds here evolved without predators. They were quite naive. And when the snake arrived on Guam it ate eggs, juveniles, adults. Whole generations disappeared."

Toxic mouse bombs

But the snakes did not stop there.

Dr Stanford explains: "We thought it would be limited: 'OK, if it wipes out the birds, it will decline.' It wasn't the case. It just switched what it was feeding on - rodents, lizards, small mammals - across the board."

Image copyright USDA/APHIS/Wildlife Services Image caption Desperate measures in Guam's battle against the brown tree snake

Now the locals are fighting back. And they are unleashing some unusual weapons in their war against the snake.

One effort has involved air-dropping mice that have been laced with poison and fitted with parachutes out of helicopters. It provides a deadly dinner for any unsuspecting snakes below.

"Right now we are using acetaminophen (paracetamol). It commonly used as a pain reliever and fever reducer in humans, but it is 100% lethal to all brown tree snakes," explains Dan Vice of the US Department of Agriculture.

"If they eat that dead mouse containing acetaminophen, they will die."

But this is a battle on two fronts. Not only is the US government trying to clear the snakes, it is also trying to prevent the problem being passed to anyone else.

And to do this, it has enlisted the help of some small dogs.

Snakes on a plane

In a busy cargo depot close to the airport, Elmo the Jack Russell, kitted out in a smart, green uniform, is sniffing box upon box of goods waiting for export.

Image caption Elmo the Jack Russell is an expert at sniffing out snakes

He is on the hunt for any unwanted stowaways.

As he catches wind of an unusual scent, he begins to scrabble, alerting the government inspector to the presence of a snake - and is rewarded with a treat.

A small army of dogs check every single item of cargo before it leaves Guam.

"It is a monumental project. We're working 24 hours a day, seven days a week," says Mr Vice.

"Cargo doesn't stop, the airport doesn't shut down, so we have to be there to make sure the cargo going on the airplane has indeed been snake inspected."

Letting the snakes on a plane could have devastating consequences.

Mr Vice says: "Economics researchers have tried to apply the impact of snakes to Hawaii. They found it could cost $400m or more if the snake became established.

Image copyright James Stanford Image caption Getting rid of the snakes is going to be an enormous job for the US government

"The impacts are running across all kinds of parts of the economy. It includes healthcare for humans because the snakes bite people, damage to the power system, lost revenue associated with declines in tourism and ecotourism."

However, with so many snakes on the island, controlling the problem is an uphill battle.

And today, Guam serves as an example to the world of what happens when an invasive species takes hold.

The worry is that it may be too late to clear the infestation, but Mr Vice says this should not stop the islanders from trying.

"Our long-term goal is to eradicate the snake," he says.

"The problems here are so profound we don't want to let them go anywhere else, and the only way to achieve that is to get rid of them completely."