February 29, 2024

The welcome mat has been rolled out for dozens of red crabs as they make their annual coastal cruise Christmas Island.

Each year, the first significant rains of the wet season trigger tens of millions of adult red crabs to leave their forest homes in the interior of the island and march to the coast to mate and spawn.

Christmas Island is 1,500 km from the Australian mainland and lies 350 km south of the Indonesian island of Java.

Staff at Christmas Island National Park have spent months preparing for the mass migration, erecting miles of temporary roadblocks to channel the migrating crabs to the safety of underpasses and crossings.

They also direct traffic away from the crabs as they scurry toward shore.

The park manager, Derek Ball, said the red crab is the island’s keystone species and the whole community steps up to facilitate the “fascinating phenomenon”.

“Over many years, we have also targeted the red crab’s biggest threat, the invasive yellow crazy ant,” he said in a statement on Sunday.

“By reducing their numbers, the red crab population numbers are higher than we’ve seen in decades, which is an excellent result, not only for the red crabs, but for the entire island ecosystem.”

Ants were first detected on the island in the 1920s, but it wasn’t until the first supercolonies formed in the late 1980s that they became a problem.

The ants have killed tens of millions of red crabs over the years by spraying them with powerful formic acid.

But there are now fewer of the invaders after scientists deployed a micro-wasp to prey on an insect that historically provided the ants with an abundant food source.

The red crab is found on Christmas Island and, to a much lesser extent, the nearby Cocos Islands.

Its population has more than doubled in the past five years, from 50 million to more than 100 million.​

“This iconic natural phenomenon is not only amazing to see, but a fantastic success story of how we can work together to better protect our precious native species,” federal Environment Minister Tanya Plibersek said on Sunday.

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