If you like your forests tidy, consider enlisting the services of red crabs. On tiny Christmas Island, an Australian territory south of Indonesia, more than one red crab occupies each square meter of rainforest, continually grazing on fallen leaves, fruits, and flowers to keep the ground virtually litter free. For most of the year, people almost never see these land-based crustaceans, which inhabit rocky crevices or holes in the forest floor, despite their dominant influence on the island's ecology—decomposing organic matter, recycling nutrients, and tilling the soil.
In late-October or November, when the monsoon rains begin, the crabs make their presence known in grand fashion: 120 million adults, driven by the urge to procreate, fan out from the forest to the sea—a five-mile journey, lasting roughly 7 to 18 days, that takes the creatures down cliffs, through towns, and across roads, railroad tracks, and streams. Observers describe the scene as a noisy red carpet, graced by the sound of a billion scurrying legs. Creeping at a few hundred feet per hour, red crabs scramble over everything in sight: streets, parking lots, golf courses, homes, schools, and stores. Tourists are advised to tread carefully.
Male crabs reach the shore first. After dipping in the sea, they dig burrows in coastal terraces, fighting with their peers over the most propitious sites. Mating occurs within the burrows soon after the females arrive. The females stay underground for about 12 days before releasing some 100,000 eggs each in the sea. A month or so later, millions of baby crabs roughly the size of a fingernail make the return trek to the rainforest, as yet another red carpet spreads across the island. These youngsters will reach sexual maturity in four to five years, at which point they'll join the other adults in the annual drive to perpetuate their species—one of the most bizarre, and striking, spectacles in the annals of animal migration.
Efforts to save whooping cranes, the most endangered of the world's 15 crane subspecies, offer a rare instance of animals learning about migration by watching what human do. Starting in 2001, humans flying ultralight aircraft have led whooping crane chicks on their first migratory flight south from a wildlife refuge in Wisconsin to a newly-created wintering habitat in a Florida preserve--a 1,200-mile journey.