The floor of the rainforest on Christmas Island is almost bare, and the reason is crabs. This limestone plateau in the Indian Ocean has no native monkeys, deer or large mammals. The leaf litter, the seeds and the seedlings are eaten instead by tens of millions of red crabs living in burrows in the soil. Without them, thickets of seedlings close the forest over. Once a year they leave it. Days after the first heavy rain of the wet season, between October and December, they come out of the trees for the sea, carpeting roads and gardens in scarlet. The sound reaches you first: a dry rustle of claws on leaf litter that does not stop. The island closes roads and puts up crab bridges to keep them moving. The timing is lunar, and the eggs go into the water before dawn on the last quarter.
From November into March young whale sharks gather offshore to feed on the billions of larvae the spawning washes out. The island's other crab is out all year: the robber crab is the largest land invertebrate alive, up to a metre across and strong enough to twist a coconut apart. It is armoured in deep blue or rust red, lives past fifty, and will carry off anything of your own that smells of food. Timber walkways take you through the Dales, the spring-fed valleys on the west side, to the mossy fall at Hugh's Dale. On the south coast the swell drives up through vents in the limestone and bursts out as spray. Golden bosunbirds wheel over the cliffs above you at Flying Fish Cove. Yellow crazy ants, brought in by accident, built supercolonies that killed ten to fifteen million red crabs by spraying acid into their eyes and joints. The answer has been a Malaysian wasp that never touches the ants: it starves them by going after the insects they feed on.