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Argentine ants give weeds a boost

Argentine ant

An argentine ant hauling a weed seed (Source: Alexei Rowles)


Dr Alexei Rowles, an ecologist with the Victorian Department of Primary Industries in Rutherglen, reports his findings in the journal Oecologia.

"[Argentine ants] are knocking out native ants, and in doing that they are disrupting the roles that those ants have," says Rowles, who carried out the research with supervisor Dr Dennis O'Dowd as part of his PhD at Monash University.

"We've shown they disrupt dispersal of native seeds and, worse than that, they actually can aid the dispersal of an introduced weed."

No humble ant

Argentine ants, a native of South America, are dark brown ants just 2 millimetres long and have the scientific name Linepithema humile.

"It's quite ironic that their species name, which is 'humile' actually means 'humble' in Latin. But they're far from that. They're quite a dominating species," says Rowles.

He describes Argentine ants as very aggressive and highly competitive, successfully invading around the world.

Rowles says his research is the first study of the ecological impact of Argentine ants in Australia.

Weed seeds preferred

Rowles studied coastal scrub sites on the Mornington Peninsula in southeast Victoria, which had been invaded by Argentine ants from nearby urban areas.

Comparing these sites with control areas that were free of Argentine ants, Rowles analysed the impact of the invaders on native ant species, native plant species and weeds.

Rowles found that in invaded areas the number of native ants had been reduced and that one important seed-dispersing ant had almost entirely disappeared.

But unlike previous studies done elsewhere in the world, Rowles found the same number of overall seeds were being dispersed.

He found Argentine ants were favouring seeds from a weed species called Polygala myrtifolia.

"It's one of the worst weeds on the [Mornington] Peninsula," he says.

Not only were the ants dispersing the weed seeds, but they were actually taking them back to their nests and covering them in soil increasing the chance that they would germinate, says Rowles.

As part of his research, Rowles also carried out experiments using artificial seeds to test the type of seed Argentine ants prefer.

He says his findings suggest Polygala myrtifolia seeds have the right size and nutritional pay-off - in the form of a fleshy edible appendage on the seed.

Strength in numbers

In a one on one fight with Argentine ants, Australian species would always win because they are larger, but the invaders have an advantage.

Because Argentine ants in Australia all evolved from a small population base, they are genetically similar.

While such a genetic bottleneck can be a weakness, Argentine ants have turned this to an advantage, says Rowles. Their genetic similarity means they are more likely to get along.

"It's almost like one big family and there's no aggression," he says.

Despite their tiny size, separate nests of Argentine ants join forces to form formidable "super colonies", which in some countries have been found to be thousands of kilometres across.

While Australian ants are held back by infighting, Argentine ants co-operate and spend comparably more energy on collecting food and fighting off competing species.

Rowles says he is very "fond" of ants, which form such a dominant part of the Australian landscape.

"They're so diverse and their importance in an ecological sense is underrated," he says.

And he's even fond of Argentine ants.

"I think they're amazing creatures. Amazing in that certain traits they have has enabled them to travel world and be so successful."

Tags: environmental-management, weeds, ecology, entomology