Christmas Island Land Crabs
University of New South Wales - School of Biological Life Bioscience Library
http://www.bioscience.unsw.edu.au/galle.htm
Nature: Attack of the Crazy Ants
Not even a 100 million strong population may be enough to save the red crabs of Christmas Island from hordes of alien insects
BY TIM BLAIR
Monday, Apr. 19, 1999
Six months ago, three film crews waited on Christmas Island's Greta Beach to document one of nature's wonders: the annual migration of millions of red crabs from the island's rain forest to the sea, where they spawn before lumbering back to their inland burrows. The celebrated migratory mass lures tourists from around the globe and usually causes "great excitement," says island social worker Lorraine Johnston. "The sense of awe never goes away." But the film crews were underwhelmed. Crabs were thinner on the ground than in previous years; other beaches on the tiny Indian Ocean island saw similarly low turnouts.
A decline in the crab population may have several causes, but Dennis O'Dowd, director of the Centre for Analysis and Management of Biological Invasions at Monash University in Melbourne, has identified a particularly potent crab menace: the yellow crazy ant, Anoplolepis gracilipes, introduced accidentally to Christmas Island by traders earlier this century. By O'Dowd's conservative estimate, in the past couple of years perhaps three million of the Australian territory's 100 million red crabs have been killed by the alien ant.
"They take over crab burrows, kill the crabs, then turn the burrows into their nests," says O'Dowd, who, with colleagues Peter Green and Sam Lake, has studied the red crab since 1986. "Once the crabs have been exposed to the ants, they lose condition and die. Tip them over and you'll see that their mouth parts are blackened." The ants also have a withering effect on the rain forest. While the crabs' feeding activities-- they eat mainly seedlings and leaf litter--help maintain the rain forest's structure, the ants destroy it, establishing multi-queen "supercolonies" which deplete resources for the forest's vines and trees, as well as threatening native birds, mammals and reptiles. Even the robber crab--the largest land crab of all, 1 m wide and able to cleave a coconut in two with its giant claws--is vulnerable to the 4 mm insect.
Exactly why the crazy ant (its name derives from its frenzied movements) has turned on the red crab (Gecarcoidea natalis) is a mystery; it's believed the two species coexisted harmlessly until recently. O'Dowd now fears that Christmas Island may suffer the same fate as Bird Island in the Seychelles, where crazy ants were first detected in 1991. Eight years later, they occupy half the island, threatening local species of sooty tern and skink.
O'Dowd is now urging the Australian government to investigate a chemical ant-killer. The need, he says, is vital. Super colonies now occupy about 3% of the island's 100-sq.-km rain forest, and can make inroads on that forest at a rate of 1 m a day. All in all, says O'Dowd: "This is one bad ant."
The red crab is as admired as the crazy ant is despised, even by Christmas Islanders whose homes are invaded every year by scuttling, half-kilogram crustaceans taking the most direct route to the sea. Children scoop up newborn crabs in buckets and carry them across busy roads to make safer their first journey from beach to forest. "The migration happens in the same way every year," says Johnston. "It's very humbling. It's a reminder that the island really belongs to the crabs." At least for as long as they can hold out against the ants' hostile takeover.
The Scientist 4[22]:22, Nov. 12, 1990
NewsEcology/Environment
By Peter D. MooreDepartment of Biology
King's College London, U.K.Because of the scarcity of accurate written records, it has rarely proved possible to trace the impact of the European colonists on North American woodlands in anything but general terms. The Allegheny Plateau, however, was colonized very late, and large stands of white pine and hemlock survived to the end of the last century. Written records and surveys in this area make it possible to follow woodland destruction and the spread of disturbance-tolerant trees and shrubs, like red maple and black cherry.
G.G. Whitney, "The history and status of the hemlock-hardwood forests of the Allegheny Plateau," Journal of Ecology, 78, 443-58, June 1990. (Harvard University, Petersham, Mass.)
The impact of grazing animals on the composition of vegetation has been extensively studied in domestic animals and in some past species, such as deer and rabbit. In the rain forests of Christmas Island (Indian Ocean), however, grazing is carried out by land crabs. These are fastidious creatures, grazing more intensively on some seedlings than others, and in this way playing an important part in determining the nature of the forest vegetation that eventually develops. Excluding crabs from experimental plots led to the survival of some species of seedling normally eliminated by crab depredation.
D.J. O'Dowd, P.S. Lake, "Red crabs in rain forest, Christmas Island: differential herbivory of seedlings," Oikos, 58, 289-92, August 1990. (Monash University, Clayton, Australia)
Recent analyses of the mass of data now available from fossil pollen in lake sediments have shown that no two species behaved in quite the same manner during the climatic changes of the past 12,000 years. Although the geographical ranges of hemlock and beech, for example, are now very similar in the northeastern United States, their convergence is only a very recent one. Some 10,000 years ago their main distribution centers were distinctly separate. Evidently, we must rethink our concept of plant communities. What we now see may be no more than a coincidence of distributions as species independently respond to the various environmental factors that impinge upon them.
R.W. Graham, E.C. Grimm, "Effects of global climate change on the patterns of terrestrial biological communities," Trends in Ecology and Evolution, 5, 289-92, September 1990. (Illinois State Museum, Springfield)
The Scientist 4[22]:22, Nov. 12, 1990
