Rescuing the red crabs of Christmas Island

A collaborative effort is trying to keep the crazy ant invasion of Christmas Island under control, saving the red crab population in the process.

The Christmas Island red crabs are known for both their great size and swarming numbers, and every year undertake a migration in large numbers from the forest to the cliffs to spawn. Dr Peter Green has been studying the ecosystem of Christmas Island since starting his PhD there in 1989, and has always held a fond fascination with the crabs.

“Seeing these crabs migrate across in waves across the island is an amazing experience,” says Dr Green. “They’re not only a wonder of the natural world, they’re a crucial part of the ecology. They keep the understorey open by eating seeds and seedlings, they clean up leaf litter on the forest floor, and they keep the population of invasive species like the giant African land snail in check.”

In the late 1990s this all changed when the yellow crazy ant, and introduced species, started forming supercolonies, putting the Christmas Island red crab at risk. Since that time the number of red crabs has been reduced by a third, around 20 million crabs.

Now the Head of the Department of Ecology, Environment and Evolution at La Trobe University, Dr Green has been working with Christmas Island National Park staff to save the red crab population before it becomes an ecological disaster.

“When we noticed the number of crabs fall and saw large groups of them lying dead on the forest floor covered in ants it wasn’t hard to establish the connection,” says Dr Green. “These ants tear through everything, and they’ve rapidly changed the island ecosystem for the worst.”

While efforts to bait the ant populations have been effective, this takes constant monitoring and surveillance, and large amounts of bait. Looking for a more long-term, sustainable solution, Dr Green looked at the food supply that fuels the ant supercolonies and found they were feeding on the sugary substance – honeydew – produced by an introduced scale insect. Dealing with these scale insects could solve the problem of the ants, and in turn, help the red crab population.

“We looked at the scale insect in its native range in southeast Asia, and found that there was a certain kind of parasitoid wasp which laid eggs in these scale insects,” says Dr Green. “This wasp could be the ideal biological control agent for the scale insects on Christmas Island, and we’ve been given permission from the Departments of Environment and Agriculture to introduce it to Christmas Island.”

Dr Green has now signed an agreement to work closely with the collaborators at the Forest Research Institute of Malaysia (FRIM) to propagate a population of wasps in Kuala Lumpur. These will be transferred to Christmas Island for a controlled field trial, and FRIM staff will train rangers from the Christmas Island National Park to propagate the wasps to build up numbers on the Island ahead of a wider release.

Dr Green’s team has undertaken extensive research into the possible unintentional effects of introducing the wasps onto Christmas Island, and is confident that the only species they would target is the introduced scale insect.

“Once these wasps have the scale insect population under control the yellow crazy ant won’t have anything to feed on,” says Dr Green. “This will hopefully improve the outlook for the red crab, and return the Christmas Island ecosystem to something close to what it originally was.”