Christmas Island is a busy place at the moment.

The island has been at the centre of angry debates in Federal Parliament and generated more news stories than you could begin to count in the last few weeks.

The 1,156 asylum seekers on the island rival the local population of about 1,200, and extra demountables from the Northern Territory have had to be shipped to the island's detention centre, 2,600 kilometres north-west of Perth.

About 300 extra workers are now living on Christmas Island to help deal with the influx of asylum seekers intercepted trying to reach the Australian mainland.

But some permanent island residents - oblivious to the political rows taking place - are on the move. All 50 million of them.

The red crabs have begun their yearly march from the rainforests of the rugged island to the sea to breed.

But there are concerns the increase in asylum seekers will threaten the crabs as they make their journey.

Max Orchard, Christmas Island's chief ranger, says the red crabs started their migration at a strange time this year.

"They started on Monday morning, commenced their breeding migration," he said.

"It's not early, it's just a little bit unusual in that they've set off after only a small amount of rain.

"Usually they commence their migration well and truly after the wet season rains are well and truly established.

"But this year, for some reason they've taken off without a huge amount of rain and certainly a long time before the optimum date for spawning."

Crab carnage

Scientists estimate there are anywhere between 50 million and 120 million red crabs on the island. And every year they make their pilgrimage towards the ocean to breed.

It is while they make these journeys that the crabs suffer casualties.

Mr Orchard estimates the red crab road toll is over 500,000 every migration season.

And he expects the toll will be higher this year because of increased traffic due to the packed detention centre.

"We know that they will have an impact because we went through this same exercise when the detention centre was being built," he said.

"We had similar numbers of contractors here working on it and so we've been through that exercise."

Mr Orchard says the roads to the detention centre have been closed to allow the red crabs to pass through.

"We've got two sections of roads closed at the moment, which is causing some dismay to our new residents," he said.

"It's the actual roads which are used normally to access the new detention centre here and we've had to close those roads and detour the traffic to the detention centre, but people are starting to get used to it."

The National Park has also come up with other ways of reducing carnage on the roads during the march.

"We've devised a few methods which are fairly novel in their manner. What we've got is what we call crab crossings which are essentially an open ended cattle grid," Mr Orchard said.

"We found out quite early in the piece that the crabs wouldn't go through a normal culvert system because they think it's a big burrow and there's a big crab at the end that's going to chomp them if they go into this dark hole.

"So what we've developed is an open ended cattle grid which allows light in and we've fenced the side of the road with a low sheet of plastic which keeps the crabs off the roads and we direct them through the crossing."

The island's rangers have also built overpasses to help the red crabs safely cross the roads.

Yellow crazy ants

Other than the road toll, the biggest threat to the red crabs in recent years has come from an ant. The yellow crazy ant sprays poison in the red crabs' eyes, blinding them, before it eats them.

Scientists estimate this ant has wiped out up to a third of the crab population.

But a poison has been developed to combat the ants and Parks workers have been using helicopters to do aerial drops of the bait.

Mr Orchard says the results are proving successful.

"We've won many a battle, but we haven't won the war yet," he said.

"Dropping this bait from helicopter into these high density areas of ants, we've had a 99 per cent success rate in those areas."

The red crabs time their migration with cycles of the moon and tides and the weather.

Mr Orchard says judging from the time the crabs have set off on their march this year, they will be aiming to be beach-side and spawning around December 11.

"It's a very inexact science the red crab migration. It's all very weather dependent and they can set off at any time between October and December depending on the rains," he said.

"When they do set off they actually aim for a date in the following month that coincides with tides during the last phase of the moon."

The male crabs reach the beach first and get busy digging burrows. Once the females arrive the crab couples use the burrows to mate.

But after their coupling, the males don't hang around, instead they leave the females to drop their eggs into the ocean and make the long journey back into the island's forests on their own.

After about 20 to 30 days tiny, baby red crabs emerge from the sea and head towards the forest, keeping the life cycle of Christmas Island's largest, permanent population going.