Most famous crab of the island is Christmas Island red crab Gecarcoidea natalis – dominating animal species in forest, endemic species met only on this island. In total on island live 50 – 100 million of these crabs what represents unprecedented density of land crabs in the world. As early settlers have not reported about unusual numbers of these crabs, it is possible that their number increased after extinction of it’s natural enemy – Maclear’s rat (Rattus macleari). In 19th century these rats were everywhere on the island but went extinct in early 20th century after invasion of black rat and epidemics.
One of the impressive sights not only for Christmas Island but for whole world is yearly migration of red crabs. It seems that migration takes place in the time when there is least difference between low and high tides – then it is safer to lay eggs. Migration starts on plateau, lasts up to 18 days. Crabs gather in broad columns and repeat the same route each year. They prefer to go in early morning or late afternoon – to avoid direct sunlight. The biggest danger to them is direct sunlight and heat but the rains and moist overcast conditions make their journey long and difficult too.
To save the crabs, on the roads there have been developed special crab crossings and plastic walls, funneling them to these crossings. Sections of roads where crabs cross in high numbers are closed to vehicles for short periods. But there is another deadly enemy – invasive Yellow Crazy Ant (Anoplolepis gracilipes). It is the main reason why the number of red crabs is decreasing, although numerous other species of introduced animals are changing the ecosystem too. Since 2000 there is on-going extermination of the super-colonies of crazy ants all over the island, decreasing their number per 99%.
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