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Crazy ants upsetting island ecosystem

Yellow crazy ants could be interrupting the dispersal of some plants by birds on Christmas Island, according to a new study.

Dr Dennis O'Dowd of Monash University in Melbourne and colleagues report their findings today online ahead of print publication in the Royal Society journal Biology Letters.

The studies on Christmas Island have shown that invasive species don't just have direct effects on native species, says O'Dowd.

"They can interact with other invaders and affect the web of interactions between species," he says.

"And the outcomes are unpredictable."

Yellow crazy ants were introduced to Christmas Island - an Australian territory in the Indian Ocean - in the early part of last century, says O'Dowd.

He says the long-legged yellowish ants earned the named "crazy" because when they are disturbed they run around frenetically.

O'Dowd says crazy ants form large super-colonies and cover ground and vegetation in densities of around 1000 ants per square metre.

"These ants are three-dimensional foragers," he says.

Impact on birds and fruit

O'Dowd says ants are known to annoy birds, causing them to hop around more and ruffle their feathers, but it has not been known what flow-on ecological effect this might have.

A large number of rainforest plants have fleshy fruit and are dispersed by birds and O'Dowd and colleagues studied the impact of the ants on one such fruit.

The field work carried out by Naomi Davis, a student of O'Dowd's, studied a bright red fruit called Schefflera elliptica, a native vine in rainforest on Christmas Island commonly eaten by birds.

As well as real fruit, the researchers used plasticine fruit, which took impressions of the birds' beaks and showed how often birds pecked the fruit.

Using 20 one-hectare plots across the island, the researchers surrounded some of the fruit with a barrier that excluded ants and compared these with fruit that were not protected from ants.

"We found there was a difference in the handling of fruit by birds in the presence of ants," says O'Dowd.

Those fruit not protected from ants were pecked three to six times less by birds, than those that were protected.

"This means less fruit for birds," says O'Dowd.

"It may also mean there's less seed dispersal by the birds."

Previous research

In previous years O'Dowd and colleagues have found yellow crazy ants were using formic acid to kill off the red land crab, which is a "keystone" species, that helps recycle nutrients and keep the rainforest floor from getting too crowded.

Previous bating campaigns have suppressed ant numbers by 94%, but the ants have returned with vengeance.

Since ants have dominated, the forest floor has become rowded and this indirectly affects understorey birds because it changes their habitat and food sources.

"It's an amazing transformation of the forest in a short time," says O'Dowd.

Previous research has also found that the ants are fuelled by eating "honey dew" secreted by introduced sap-sucking scale insects.

This extraordinary energy source gives the ants a "sugar high" and helps boost their numbers.

"This can really ramp up the foraging tempo of the ants," says O'Dowd

The ants in turn clean the scale insects and help maintain their energy source.

"So you have this positive feedback system," says O'Dowd.

Yellow crazy ants are regarded by the Invasive Species Specialist Group of the International Union for the Conservation of Nature as one of the top 100 most invasive species in the world.