POISON dropped by helicopter on the ant colonies rampaging round Christmas Island in the Indian Ocean has cleared the way for the land crab population to recover. The crabs are a vital part of the island鈥檚 unique ecosystem.
鈥淚t鈥檚 an amazing feat,鈥� says Mick Clout of the University of Auckland in New Zealand, who is chair of the Invasive Species Specialist Group of the World Conservation Union. 鈥淚nvasive invertebrates can be an intractable problem because they breed so quickly.鈥�
Christmas Island is dominated by 45 million red land crabs, which keep the rainforest floor clear of leaf litter. They help maintain a unique ecosystem that supports creatures such as football-sized rubber crabs and the endangered bird Abbott鈥檚 booby.
Advertisement
But almost a third of the land crabs have been killed by yellow crazy ants, named after their frenetic movements. The ants spray formic acid on any hapless crabs that happen to wander into their colonies.
Crazy ants first turned up on the island in the 1930s and underwent a mysterious population explosion in the 1990s. Supercolonies extend over hundreds of hectares with densities as high as 70 million ants per hectare.
To give the crabs a chance, a team from the Australian government agency, Environment Australia, and Monash University in Melbourne surveyed the island and used signals from Global Positioning System satellites to pinpoint the ant colonies. Then they targeted the colonies with poisoned bait dropped from a helicopter. The poison was spread during the dry season when land crabs tend to remain in their burrows. At the same time, chicken feed was laid out for the rubber crabs as a decoy to prevent them getting to the poisoned bait before the ants. The poison was not expected to harm birds and mammals.
鈥淭he trick was to get an even coverage of bait over every square metre of ground, even in difficult to get to places. That is impossible to do on foot, but with the helicopter we got a reduction in ant activity of 99.4 per cent,鈥� says Peter Green, a team member who is now at Australian National University laboratories in Atherton, Queensland.
The team will continue monitoring the island and repeat the exercise if the ant population explodes again. The technique could be used against crazy ants in Australia鈥檚 Northern Territory next. It could also be used against fire ants and other troublesome invertebrates.