Located 360 kilometres (223 miles) south of Jakarta, Australia’s Christmas Island is home to one of nature’s greatest, most colourful and most spectacular natural mass migrations. Each year this pristine island’s shoreline is covered with an ankle-deep carpet of red crabs.
On the high tide of one warm moonless night in November, the female red crabs commence a frenzy of spawning, each one jettisoning their cache of 100,000 eggs or more into the sea where they hatch in seconds and swim away.
Yet while this should be increasing the numbers of these beautiful creatures, they are actually dropping. During their annual migration across the island, many of the crabs fall victim to the alien yellow crazy ants. These ants release formic acid as they crawl over and around the crabs which, with no defence against the ferocity of the onslaught, become stressed and generally die within a few hours.
The crazy ants, which were accidentally introduced by immigrants more than 50 years ago, so named because of their rapid movements and frequent changes of direction. Yet while they may be small, the impact they have on the rainforest is both enormous and rapidly increasing, with the ants spreading at a rate of one metre (three feet) per day.
Red crabs are nature’s consummate gardeners. They till the soil, clear away leaf litter and weeds, and control the regeneration of forest plants. With over 70% of Christmas Island being rainforest, scientists are concerned about the dire effects the ants’ attacks are having on the island’s ecology. However, most of the forest is almost impenetrable to humans, and controlling the spread of crazy ants will be an extraordinary challenge.
But it’s not only the ants that are making things difficult for the crabs. Some of the other natural threats include the giant tree-climbing robber crab, a creature so strong that it can split a fresh coconut in half with its claws, and the moray eel, which waits in the coastal shallows for its next tasty dinner.
With constant perils and obstacles in their way, the red crabs lives are threatened now more than ever before and the ecology of the island is at risk.