Nicknamed ‘Land of the Red Crab’, Christmas Island—an Australian territory in the Indian Ocean—is home to some 50 million crimson crustaceans.
Each year, the crabs march en masse to spawn along its pristine shoreline.
The spectacle is considered one of the natural world’s great migrations, but the past 20 years have seen tens of millions of red crabs fall to a six-legged adversary: the yellow crazy ant (Anoplolepis gracilipes).
Ants might not seem tough, but if you were as able-bodied as one, you could easily pace a racehorse while carrying a car.
And here on Christmas Island, they run the show.
‘It’s what we call an invasive meltdown. The ants accidentally kill everything in their paths’
‘In the worst-case scenarios, it’s absolutely overwhelming what these ants can do,’ says tropical ecologist Dr Ben Hoffmann, who studied crazy ants on neighboring Samoa.
‘These places are just swarming, and in those numbers, not much else can coexist.’
To be clear, ants aren’t ruthless murder machines. They engineer soil, and are essential to many ecosystems. The problem is that these ants aren’t where they’re supposed to be.
The ant invasion
Like the tourists who come to witness the crab migration, crazy ants aren’t native to Christmas Island.
In fact, it was human visitors who brought them over from Asia. Once on the island, their numbers exploded—and now, they’ve formed vast ‘super colonies.’
For a bit of perspective, imagine looking down and seeing a dense yellow carpet made solely of ants.
Now imagine the wriggling bug rug stretches 10 square miles. At a whopping 1,000 ants per square meter, that is what the largest Christmas Island super colonies look like.
‘It’s what we call an invasive meltdown,’ says Hoffmann. ‘The ants accidentally kill everything in their paths.’
‘Crazy ants also kill lizards, invertebrates and nesting birds—but it is the crab decline that is becoming a crisis’
Crazy ants neither bite nor sting, but they’re armed with formic acid, an irritating chemical that can be used to subdue their victims.
They’re fiercely territorial, and almost any trespasser (a scuttling crab, for example) is a perceived threat.
Death by acid assault isn’t pretty: first you go blind, then your joints cease to work, rendering you unable to reach water.
But most sinister for crabs is the spray’s effect on gills. As acid soaks into soft gill tissue, breathing slowly becomes impossible.
‘It’s a pretty slow, miserable death,’ says Hoffman.
The death of the red crab
Crazy ants also kill lizards, invertebrates and nesting birds—but it is the crab decline that is becoming a crisis.
Red crabs are a keystone species on the island, meaning they keep the ecosystem in check.
Their abandoned burrows provide habitat for other animals, and by munching on leaf litter and seedlings, they keep the undergrowth clutter-free.
‘When you kill the crabs, you go from open rainforest to one that becomes very dense with vegetation,’ explains Hoffmann.
‘That looks really nice, but it’s not the natural state of the system. So all sorts of other changes begin to happen.’
Crazy ants farm bugs like we farm cattle. More specifically, they tend sap-sucking insects known as red scales that quite literally suck the life out of trees.
Scales feast on sap and, after absorbing what they need, excrete a carbohydrate-rich goo known as honeydew.
The ants protect the scales in exchange for their share of the nutritious ’dew, and it’s this relationship that’s allowed their population to skyrocket.
As the protective shepherds grow in numbers, so do their insect livestock. More scales support more ants and, left unchecked, the creepy comrades can upend an entire ecosystem.
‘A couple of tree species are particularly susceptible to the scales,’ says Hoffmann.
‘We’re in this relentless cycle of having to push the ants back’
‘Those trees have greatly declined, and ultimately we’ve seen decline in a couple of skinks and the extinction of the Christmas Island pipistrelle (a native bat).’
While the ants can’t be held solely responsible, their colonies have played a major part in the shift.
Finding a solution
Over the years, scientists have turned to poisonous bait (namely, the insecticide fipronil) to curb ant numbers. And while the neurotoxin is highly effective, it also kills land crabs.
By using it year-round, conservationists would wipe out everything they hope to save. To boot, ant colonies have a habit of rebuilding.
‘We’re in this relentless cycle of having to push the ants back,’ says Hoffman. ‘It’s a massive job, and not a sustainable solution.’
After five years of preparation and planning, a team led by La Trobe University ecologist Dr Peter Green will soon deploy a more natural remedy: a horde of parasitic micro-wasps that feed exclusively on red scales.
Measuring in at just 3 millimeters wing to wing, the wasps (Tachardiaephagus somervillei) are baby-making machines.
Females lay their eggs inside the bodies of passing sap-suckers, and three weeks later a new generation will gobble its way out.
Repeated over time, the wasps could kill enough scales to cap crazy ant numbers for good.
The release is set for the end of the year, and the team will be carefully monitoring its progress.
While it comes too late to help this round of migrating red crabs, balance could be restored to Christmas Island within a few short years.
Main photo: Steve Shattuck/flickr; body photos: Alamy, ??, frogtrail images/flickr
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