Christmas Island red crab

Red crabs live in the damp rainforest.

Each crab digs a burrow to live in. When the weather is dry, it stays inside and closes the door with leaves.

Almost all red crabs live on Christmas Island; a small group also lives on North Keeling Island, about 900 kilometers to the west. Most of them live in the moist rainforest, but some live on the flat, rocky land near the shore and even in people's gardens.

Each crab lives in a burrow it digs, or in a deep crack in the rocks. Red crabs need damp air. In the dry season they rarely come out, and they plug the entrance of their burrow with a wad of leaves to keep the moisture in.

The Australian Museum gives its habitat on Christmas Island as rainforest, coastal shore terraces and urban gardens; Green (1997) found it wherever native rainforest is still intact. Outside the breeding season, crabs live alone in burrows or deep rock crevices.

Moisture controls their lives. Green measured surface activity against humidity: none below 77 percent relative humidity, low at 87 percent and high above 95 percent. Crabs retreat into their burrows during dry periods and emerge quickly after rain, and Parks Australia notes that in the dry season they plug burrow entrances with leaves.

Red crabs eat leaves that fall on the forest floor.

They eat fruit, flowers and little seedlings too. By eating old leaves, they help keep the forest healthy.

Red crabs mostly eat fallen leaves, fruit, flowers and seedlings. They are not only plant-eaters, though: they also eat dead crabs and birds, and the giant African snail, an animal that people brought to the island.

Red crabs are forest helpers. By munching through fallen leaves, digging and dropping their waste, they clean up the forest floor, turn over the soil and add nutrients that help plants grow.

Parks Australia describes a diet mostly of fallen leaves, fruits, flowers and seedlings, supplemented by dead crabs, birds and the introduced giant African snail. Green (1997) found that falls of fruit and leaves cause dramatic local increases in the density of active crabs.

The crabs are a key species in the forest's ecology. By eating selectively among seedlings, processing leaf litter, turning the soil and fertilizing it with their droppings, they influence which plants grow and how nutrients cycle. Parks Australia warns that when crab populations collapse, the whole island ecosystem suffers.

Red crabs come out in the daytime when the air is damp.

At migration time, so many crabs walk to the sea that roads turn red! People build little bridges so the crabs can cross safely.

Red crabs are active in the daytime, but only when the air is damp. For most of the year, each crab lives alone in its own burrow.

The migration changes everything. Millions of crabs swarm across roads, streams, rocks and beaches. On some beaches there can be up to 100 crabs packed into one square meter. To keep them safe, rangers close roads and build crab bridges over them, and visitors park their cars and walk carefully among the crabs.

Green (1997) found red crabs active by day (diurnal), with humidity the main control on surface activity. Outside the breeding season they are solitary.

During the migration, Parks Australia reports densities of up to 100 crabs per square meter on beaches and rocks as egg-carrying females wait for the spawning tide, and spawning may continue on 5 to 6 nights in a row. Males often fight over burrow sites on the crowded terraces. Road closures, crab bridges and public notices on crab movements help protect the crabs, and the migration is the island's biggest tourist attraction.

Tiny ants that came to the island by accident can harm red crabs.

Rangers are working hard to stop the ants. They also close roads so the crabs can cross safely.

Red crabs have not been checked by the IUCN Red List, but they face a big problem. Yellow crazy ants were brought to Christmas Island by accident long ago. In some places they form huge colonies, and the acid the ants spray can kill red crabs. Since the late 1990s, the ants have killed tens of millions of land crabs.

Rangers put out bait to control the ants, and scientists have released a tiny wasp that attacks a scale insect whose sweet honeydew is one of the ants' key foods. Roads are closed during the migration, and crab bridges help crabs cross safely.

The species has no IUCN Red List assessment. Its main threat is the yellow crazy ant (Anoplolepis gracilipes), accidentally introduced in the first half of the twentieth century. Parks Australia reports supercolonies of up to 1,000 ants per square meter whose formic acid can wipe out local red crab populations; since the late 1990s the ants have killed tens of millions of land crabs.

Control has relied on baiting, by hand and by helicopter, which is effective but temporary. Parks Australia and La Trobe University are now trialing biological control: a 2-millimeter Malaysian micro-wasp that attacks the yellow lac scale insect, whose honeydew feeds the ants. Lac scale numbers have fallen, but the effect on the ants is not yet known.