Australasia, Destinations, Nature

The slow march

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Each year, millions of red crabs migrate to the shores of Christmas Island to mate, creating one of the world’s natural wonders.

It can almost resemble a moving red carpet, gliding slowly but firmly across rocks, hills and roads. Each ‘wet season’, usually between October and December, a large portion of Christmas Island’s 120 million red crabs – or Gecarcoidea natalis – leaves the forest for the shores, instigating a synchronised five-kilometre pilgrimage that virtually crosses everything in its way. As the island’s 1,500 residents know all too well, the crabs do not like shortcuts.

This direct approach poses interesting challenges for Christmas Island, a remote Australia-governed island in the Indian Ocean on which two-thirds of the 135-square-kilometre surface is covered by the national park – home of the crabs. For days, hordes of crabs fearlessly invade roads and residences, hindering cars while turning themselves into roadkill. The task befalling the national park’s management team is to guide them to the shore while limiting casualties and general nuisance.

Their way of achieving this is rather clever. Along the main roads, plastic fences are erected to separate crabs and traffic. Swarms of crabs crash into these once emerging form the forests, then eventually follow it sideways like a floating river. At some point, they find one of the specially constructed ‘crab bridges’, or one of the many grids dug under the road. These provide safe passage.

But despite these solutions, some roads have to close for short periods at peak migration times. For residents, updates are given on public noticeboards and local radio. “It is a bit of a hassle for everyone, but the majority of people just accept it,” says Max Orchard, project manager at Christmas Island National Park. “It is that time of year and it just happens.”

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The migration’s timing is unpredictable, and relates mainly to the red crab’s need for moisture. The majority of the population lives in the humid rainforest that tops the island’s sharply rising habitat, each residing in a burrow dug into the ground. Here they stay throughout a dry season that can last up to three months. Only when the wet season arrives are conditions moist enough for their journey.

However, the departure date also depends on the breeding cycle, which relates to moon phases. Spawning – where females drop their eggs into the ocean – must occur before sunrise on spring tides on the last quarter of the moon, meaning crabs must reach the shore at certain dates. And if October isn’t wet enough, they will wait until November or December.

When ready, males lead the exodus followed by females. Most spend five to seven days. Once there, and having filled up on salt and moisture, the males dig burrows on the terraces at a density of one or two per square metre – tight enough to provoke fighting for positions. The females then retreat to these for a fortnight where they produce up to 100,000 eggs each.

Sadly, the babies’ odds of survival are unfavourable. After being released into the sea, where they hatch, these creatures spend a month floating among fish, plankton feeders and giant whale sharks – all of which find them delicious. Survivors evolve into pawn-like animals called megalopae, before returning as five-millimetre-wide crabs. But there aren’t many of them. In fact, nine out of ten years, the majority are eaten.

While the exceptional years – which happen once or twice per decade – return enough babies to preserve the endemic population, there hasn’t been one for 12 years. Additionally, the crabs have recently been threatened by yellow crazy ants – one of the world’s most invading species. “They’ve had a big impact over a very large area of the island,” says Orchard, who spent more than 20 years as manager of the national park. “It’s been estimated that during the early 1990s to the beginning of the century, between a quarter and a third of the red crab population perished as a result of ant invasion.”

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The ant is thought to have arrived with produce from Malaysia or Singapore between 1915 and 1934. It posed no problems until two decades ago, when ‘super colonies’ started to form. These can mutually join other colonies and contain thousands of queens, enabling them to increase in numbers dramatically, very quickly. These invade the crabs’ territory and their burrows, leaving crabs unable to retain moist. Eventually, they die.

This is also bad news for the island’s ecosystem, on which the crabs have a profound impact.

“Their feeding activity on seeds and seedlings of trees affects the ability of the plants to establish and grow to maturity,” says Orchard. “What’s happened is that we’ve ended up with a uniquely structured rainforest; the crabs pick the forest floor clear, so you’ve virtually got no understorey.” With crabs driven out, that composition will change.

Efforts are taken to reduce the crazy ant population. Food poisoning has been released from helicopters with some success. A possible long-term solution is to import a Malaysian wasp specie with a similar diet to the crazy ant. While research continues, the challenge remains to preserve the crab while waiting for a year where babies return in abundance. The last 12 have been bad. Hopefully, 2012 will be a good one.

Photos: Ingo Arndt, Max Orchard [both via CITA].

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