Claws out: booming population of red crabs on the march
More than 100 million red crabs make their annual trek from Christmas Island’s rainforest to the coast, creating a migration spectacle that’s on countless bucket lists.
Their dominance over the landscape is a spectacular phenomenon, beloved by tourists and valued by local residents.
It’s a reassuring sight for the island’s environmentalists.
Brendon Tiernan, senior field program coordinator for Christmas Island National Park’s threatened species programs, said the war against yellow ant invaders that threaten red crabs is not won.
“But we made a significant impact,” he said.
Malaysian micro-wasps were introduced by scientists in 2016 to target the ants’ preferred food source.
They helped combat the pests, which were highly territorial and sprayed formic acid on passing crabs, drying them out and eventually killing them.
Mr Tiernan said tiny-winged biocontrol agents were doing a “fantastic job” of suppressing the lace scale insect, a type of symbiotic pest that produces the honeydew substance eaten by ants.
“We didn’t win the war,” he said, as ant colonies took advantage of other food sources.
Before the introduction of the wasp, roughly two-thirds of the crab population was estimated to have disappeared from the early 2000s to the mid-2010s.
Fast forward to 2025 and red crab numbers could surpass the 180 million mark, a remarkable recovery in just 10 years, Mr Tiernan said.
Years with abundant returning baby crabs further helped numbers increase.
But it’s unclear why so many crabs returned in those years, with their eggs at the mercy of the currents and predators once drifting across the ocean.
Christmas Island red crabs spend most of their lives on land and return to water each year to breed.
Before the last quarter of the month, females emerge from their nests and lay their eggs in the ocean.
Spawning usually occurs like clockwork in accordance with the lunar cycle, but the length of migration can vary, triggered by the first heavy rains of the rainy season, usually around October or November.
Migration started a little earlier than usual this year; Rainy conditions were expected to occur in the region because the Indian Ocean dipole was negative and the waters were warmer than normal.
It is not yet clear whether and how climate change affects the crabs or their migration patterns; But Mr Tiernan said the island was expected to experience more severe dry spells as global temperatures rose, and when those periods finally arrived they would be accompanied by more intense downpours.
The annual migration creates a lot of work for park rangers, who use rakes to keep themselves off the busiest roads.
Crab safety is being taken seriously, with Chris Bray, owner of eco-hotel Swell Lodge, developing a mechanism to be placed on the front of vehicles to gently move them away from harm.
Mr Bray said he had to develop a “crab mobile” to get to and from the lodge, while other residents were generally able to stick to clean, drivable roads.
The design has changed little in the six years it has been in use.
“The most important thing was that it had to be durable, simple, easy to build and repair, and work even on the pretty rough roads here, follow lines even on the worn four-wheel drive track,” he said.
