Ant fever

时间:2019-03-08 02:14:25166网络整理admin

By Tim Thwaites in Melbourne AN INVASION of alien ants is changing the face of a small island in the Indian Ocean. The ants are destroying a unique rainforest habitat dominated by land crabs, say Australian ecologists. The floor of a tropical rainforest is relatively clear of leaf litter and has few seedlings and shrubs. On Australia’s Christmas Island, 350 kilometres south of Java, this delicate equilibrium is partly maintained by red land crabs ( Gecarcoidea natalis). There are some 100 million of these crustacea on the forest floor. But the forest is now under threat from a tropical invader known as the crazy ant (Anoplolepis gracilipes) because of its frenetic movements. In the past 18 months, Christmas Island’s crazy ants have gone berserk. They have begun to attack and eat the land crabs, devouring millions. The ants are also creating havoc in the forest canopy. Because they feed on a sugary secretion produced by sap-sucking scale insects, they protect this source of food from its natural predators. So numbers of scale insects are booming, and causing more and more patches of the forest canopy to die back. With the crab population decimated and the gaps in the tree canopy letting in light, a dense undergrowth of seedlings is shooting up. “The invaded and non-invaded sites are like chalk and cheese,” says Dennis O’Dowd, an ecologist at Monash University in Victoria. The sudden carnage is a mystery. The crazy ant, originally from West Africa but distributed throughout the tropics by traders, has been living on Christmas Island since the 1930s at least, and yet it has become a menace only recently. One theory links the slaughter to an El Niño that caused a prolonged drought, and subsequent plant stress, in 1997. Although less than 5 per cent of the rainforest has so far been affected, O’Dowd’s team is concerned that endangered species could be driven to extinction if the trend isn’t quickly reversed. The Christmas Island shrew and birds including Abbot’s booby and the Christmas Island hawk owl and frigate bird are particularly at risk from habitat alteration and direct attack by the ants. Nearly two-thirds of Christmas Island is an Australian national park. “We are interested in any kind of biological control, but that takes time,” says David Murray, senior planning officer of Parks Australia North,