The island's land crabs - famous for migrating in their millions every year to the ocean to spawn - are blinded by the ants' formic acid and are quickly overwhelmed, eaten and left dying in rotting piles on the forest floor.

There are probably 40 million to 42 million crabs left on Christmas Island, Slip said. But those numbers mean nothing in the face of the ants' ferocity and their appetite.

"We don't actually know what the pre-ant population (of crabs) might have been...but it could have been as high as 80 million or so, which would essentially mean you've had about a 50 percent decline," Slip told Reuters by telephone yesterday.

"At the low end you've probably had at least a 25 percent decline. That's very significant."

The government has taken up the crabs' cause. Slip and his co-workers have successfully managed to wipe out crazy ant super colonies in a testing zone through a specially developed poison.

The problem is that much of the island, which lies roughly 360 km (220 miles) south of Indonesia's Java and 2,600 km (1,600 miles) northwest of Perth, is inaccessible.

Slip said the next stage of the programme to defend the crabs against the ferocious ants will probably involve spreading poison by aircraft. But it is a long-term and costly project.

STAR ATTRACTION

The land crabs are a star attraction on Christmas Island, scrambling in swarms to the ocean cliffs in the breeding season.

Christmas Island is a unique ecosystem. The red crab is only found there, and the Jackson's crab is also possibly endemic. The large robber crab used to be common throughout the Indo-Pacific region but has been wiped out almost everywhere else.

To date, their main enemy has been man. Around 15 percent of migrating crabs are crushed under tire treads while crossing roads to get to the beach.

But the crazy ants, thought to have migrated recently from Africa and known scientifically as Anoplolepis gracilipes, could spell the end of the crabs if not controlled, environmentalists say.

The cause of crazy ant invasion remains mystery.

"It's horrific, dead crabs everywhere and these ants just marauding. It's just far too awful for words," said Christine Milne, regional councillor for the International Association for the Conservation of Nature, or World Conservation Union.

Milne, who visited Christmas Island recently, said the threat posed by the ants went beyond the existence of the crabs.

The crabs are a key part of the island's delicate and unique ecosystem as they clear the forest floors of leaf litter.

When not gorging themselves on proteins from dying crabs, the ants "farm" carbohydrates from a scale insect that has proliferated and is now beginning to strip trees of foliage.

As a result, the rain forest canopy is thinning - threatening the nesting grounds of endemic birds like the Albatross-like Abbott's Booby - and weeds and undergrowth are beginning to spread through the forests.