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Every year around the end of the October, the red crabs of Christmas Island begin their march. Up to 100 million individuals leave their burrows in the forest and head to the shore in a scarlet tide of legs, claws and carapaces extensive enough to be seen from the air. They are driven by the most basic of biological imperatives: to spawn.

The islanders take the crustacean migration in good humour, closing roads and erecting diversions in an effort to curtail crab casualties and the tattering of motor vehicle tyres. After all, the annual show is a symbol of the distinctiveness of the Indian Ocean territory, and a big draw for tourists, naturalists, wildlife photographers, and documentary-makers worldwide.

Many visitors are impressed both by the size of the migration and by the islanders’ obvious concern for the crabs. Indeed, an outsider could be forgiven for believing that the spectacle has been an annual occurrence at Christmas Island since time immemorial, one surely destined to astonish countless future generations.

But the grand scale of the annual march⁠— if not its very existence⁠— is threatened by a foe far more ferocious than road traffic or the clumsy feet of camera-wielding tourists. And the origin of the migration itself is perhaps more recent, and more tainted, than many would believe. The story is one of inter-species meddling, conflict, and extinction, set in the context of a fragile island ecology. It involves protagonists of a two-legged, four-legged, six-legged and eight-legged variety.

Christmas Island red crabs (Gecarcoidea natalis) are terrestrial, spending most of their lives on land. More specifically, they spend most of their lives on this one particular chunk of land, since the species can be found nowhere else on Earth. For much of the year the crabs are a very visible presence, whether migrating or not. As one of the island’s ‘keystone species’, they play a vital housekeeping role in the inland rain forest, scavenging fallen fruit, leaves, weeds, seedlings and carrion; recycling nutrients; and aerating the soil with their burrows. But the crabs must make one concession to their aquatic evolutionary origins: the larval form of the crab has primitive gills which only function in the sea. For this reason all reproductive activity takes place by the shore.

(c) Christmas Island Tourist Association

With the increase in humidity that comes in October or November, the males begin their perilous march, joined a few days later by the females. The throngs of arthropods migrate en masse, crossing the landscape as a quivering red wave. The island’s thoroughfares quickly become littered with smashed corpses, prompting locals to detour traffic and use shovels to clear roads as needed. Upon reaching the shore the males engage in frenzied bouts of digging and fighting, building and defending temporary tunnels to woo the females. When they arrive, the females peruse the wares being offered, and deeply ingrained instincts tell them to select the males with the largest, best-formed burrows. Suitably impressed females are enticed inside, where mating usually occurs. Afterwards the exhausted males take their leave and stagger back inland.

After 12-13 broody days in the burrows, the females release their eggs in the ocean at the turn of a pre-dawn high tide. They release the eggs together over six or so consecutive nights, turning the coast of Christmas Island into a huge birthing pool. Then they return inland themselves. The larvae hatch and grow in the sea for about a month before amassing back at the water’s edge, ready to moult into tiny air-breathing adult crabs. Thus in some years the renowned migration of the fully-grown crabs to the shore is matched by an equally impressive flood of dainty young minicrabs going the opposite direction a few weeks later. In other years the larval population is decimated by water-borne predators, but because it takes about four years for the young land crabs to reach breeding maturity, the fluctuations even out in the longer term and a consistently large population is maintained.

The human presence on the island dates back little more than a hundred years. Christmas Island was named by Captain William Mynors of the British East India Company when he stopped there on 25th December 1643, although it wasn’t until 1888 that the first permanent settlement was established.

Early descriptions of Christmas Island’s wildlife make an intriguing and surprising omission. In an 1888 report made to London’s Geographical Society, Captain Aldrich of the Royal Navy describes plentiful powerful coconut-cracking robber crabs –found throughout the Pacific and Indian Ocean islands⁠— and a profusion of mysterious rats on the island, but he fails to make any mention of the now-pervasive red crabs. The extent of the rat infestation can be inferred from his description of a night on the island:

“We brought hammocks with us, and slung them under the trees; mine was between two and three feet from the ground, and three rats I know came over me as I lay in it, how many more when I was asleep I have no means of telling.”

Other early accounts attest to the swarming abundance of the enigmatic rodents, but few mention the island’s red crabs. Subsequently the rats were identified as another endemic species, the Maclear’s rat. It seems that in the late 19th century, these rats were as numerous as the red crabs are today. Like the crabs they were scavenging creatures that lived in burrows on the forest floor, but the exact role they played in the ecology of the island will forever remain a mystery⁠— for by 1903 the species was extinct, wiped out by an epidemic of trypanosome parasites introduced by ship-borne black rats.

(c) Christmas Island Tourist Association

Some scientists have hypothesized a link between the decline and extinction of the Maclear’s rat in the early twentieth century and an ensuing explosion in red crab numbers. They propose that the native rats previously kept the red crab population in check, and that with the demise of the rodents, crab numbers increased– giving rise to the dramatic annual breeding migration ironically celebrated today as a ‘Wonder of Nature.’

Along with disease-carrying black rats, Christmas Island’s settlers brought other invasive species with them⁠— with one in particular having a less beneficial effect on the red crabs. The troublesome stowaways included cats, house mice, and well over 50 invertebrate species including a type of ant named the yellow crazy ant. By 1934 these intruder ants were firmly established on the island.

For fifty years the newcomer⁠— named both for its colour and for its frisky response to being poked⁠— seemed to settle quietly into the laid-back rhythms of tropical island life, enjoying relative harmony with the native flora and fauna. The ants established scattered single-queen nests, each with a discrete territory which was vigorously defended from the other ant colonies. But by 1989, the yellow crazies began to do something quite out of character: they began to form multi-queened supercolonies with numerous queens and much larger territories. This created areas with dense populations of genetically-related ants capable of close and deadly cooperation.

Supercolonies can expand rapidly, with the high ant density having a devastating effect on the surrounding wildlife. Birds and larger animals succumb to a spray of formic acid squirted by the aggressive insects, their corpses later providing a handy source of protein. Despite their impressive looking armour, red crabs are not immune: the ants’ formic acid initially blinds, then kills the crabs. When migrating across supercolony-controlled areas, almost all crustaceans perish.

The death toll among the red crabs has been high. By 2002 it was estimated that the ants had killed 20-25 million, a quarter of the original population. Other creatures facing a similar onslaught include a bird called the Abbot’s Booby, and a shy mammal named the Christmas Island Shrew. Both of these endemic species now face ant-induced extinction. In ant-infested areas of the island where the ‘keystone’ crabs no longer live, young seedlings, weeds, and a species of stinging tree grow rampant, fundamentally altering the structure of the rain forest.

Yellow crazy ant

Realizing that the loss of the crabs could spell eco-disaster (both -nomic and -logical), Christmas’ Australian authorities instituted poison-baiting schemes. In 2003, they deployed air power: a helicopter was used to drop 12 tonnes of irresistibly tasty ‘fipronil’-laced fish meal onto supercolony-held territory. Although ant density was reduced by 98% in the treated areas, recent years have seen the crazy ants reestablish themselves. It remains to be seen whether future ant abatement efforts will be equally successful.

The reasons why the crazy ants formed supercolonies in the first place are obscure⁠— but it is known that in other situations where invasive species have spread into new areas, similar transformations in ant behaviour have occurred. In 2000, researchers identified a probable 6000 km-long supercolony of invasive Argentine ants stretching along the entire coast of southern Europe.

Genetic studies of such populations may provide some answers, but nobody can yet say how the inter-species skirmishes of Christmas Island will play out. What is certain is that the last century or so of human settlement⁠— and particularly the associated introduction of invasive species⁠— has caused considerable disruption to the island’s previously isolated ecosystem. There are both winners and losers in this ongoing process, and one very visible species may well have had it both ways. Visitors intending to view the grand spectacle of the red crab migration this year may want to remember the Maclear’s rat, and hope that the crabs of Christmas manage to avoid a similar fate. And they may want to pack some ant repellent.