Academic Reading — Test 97

Reading passage

On Christmas Island, a small Australian territory in the Indian Ocean, one of the most remarkable natural events on the planet takes place each year. Tens of millions of red crabs, a land-dwelling species found nowhere else in such numbers, leave the shaded floor of the island's rainforest and travel towards the coast to breed. For most of the year these crabs lead solitary lives, sheltering in burrows or beneath leaf litter, where the humid, cool conditions prevent their bodies from drying out. Yet when the wet season arrives, they abandon this seclusion almost in unison, transforming roads, gardens and beaches into a moving carpet of red. The migration is so dense that local authorities temporarily close certain roads and construct special bridges and underpasses to allow the animals to pass safely. The trigger for this mass movement is the onset of the monsoon rains, which usually fall between October and December. Rain raises the humidity of the air and dampens the ground, allowing the crabs to travel long distances overland without the constant threat of dehydration. Because their breathing depends on keeping their gills moist, the crabs cannot move far across dry terrain; a sudden downpour therefore acts as a signal that conditions are finally safe. The males generally set out first, with the females following a few days later. A journey that may cover several kilometres can take a week or more, and individuals that begin too early, before sufficient rain has fallen, often perish along the way. Although rainfall determines roughly when the migration begins, the precise timing of the most critical stage is governed by the Moon rather than the weather. The release of eggs into the sea is tightly synchronised with the lunar cycle, occurring during the final quarter of the Moon, just before dawn, when the difference between high and low tide is at its smallest. At this point in the month the high tides neither rise especially far up the shore nor retreat especially far from it. Spawning at such a moment offers a clear advantage: the female crabs can approach the water's edge and release their eggs without being swept off the rocks by powerful waves, and the newly produced larvae are less likely to be stranded high on the shore as the tide withdraws. Scientists believe the crabs sense the state of the tides, and possibly changes in light at night, and use these cues to coordinate their behaviour with extraordinary accuracy. The breeding process itself unfolds in distinct stages once the animals reach the coast. The males arrive first and dig small burrows in the soil near the shore, in which mating takes place. After mating, the males begin the long return journey inland, while the females remain behind in the burrows for around two weeks, during which their eggs develop. Each female may carry as many as one hundred thousand eggs, held in a brood pouch beneath her body. When the lunar conditions are right, the females leave their burrows together, gather at the top of the shore and release their eggs into the breaking waves. The eggs hatch the instant they meet the seawater, and the tiny larvae are carried out into the ocean. The fate of these larvae is far from certain, and survival varies dramatically from one year to the next. For their first few weeks the young crabs drift in the sea as part of the plankton, where they are eaten in vast quantities by fish and other marine animals. In many years almost none survive, and the offshore waters yield no returning young at all. Occasionally, however, conditions favour the larvae, and after about a month the survivors change into a form resembling tiny crabs and emerge from the sea to begin their own march inland. These rare successful years are thought to be sufficient to maintain the island's enormous adult population, since individual crabs can live for more than twenty years and breed many times. The red crab is more than a spectacle; it plays a central role in shaping the forest it inhabits. As the dominant consumer of fallen leaves, fruit and seedlings on the rainforest floor, it influences which plants are able to grow and helps to recycle nutrients through the ecosystem. This influence has become especially clear where the crabs have been lost. An invasive ant, accidentally introduced to the island, forms large super-colonies that kill the crabs in great numbers. Where the ants have taken over, the absence of crabs allows leaf litter and seedlings to accumulate, and the character of the forest changes markedly. Protecting the red crab, and the annual migration that sustains it, has therefore become a priority for conservationists seeking to preserve the island's distinctive natural balance.