Researchers at Monash University, Melbourne, have discovered a column of ants 60 miles wide under their city. The event, worthy of the worst science fiction, has two especially worrying aspects: the insects are Argentine and they all like one another.

Australia, once characterised by its whites-only immigration policy, could yet come to regret its volte-face and admit all comers. The South American ants have become so numerous that they now threaten the country’s fragile biodiversity. Worse, they are outbreeding the natives.

Melbourne and its environs are home to four million human beings, including large numbers of Greeks and Italians who add to the colourful diversity of the elegant city on the Yarra River. But the ant population outnumbers them by many million.

University researchers believe that the ant supercolony stretches from Taylor’s Lakes, northwest of the city, to Sorrento in the southern suburbs, and from Altona in the west to Blackburn in the east.

Elissa Suhr, the Monash scientist who discovered the extent of the invasion, said yesterday that the immigrants were breeding so fast because, having settled in Australia, they had lost their aggression. “In Argentina, their natural homeland, ant colonies span tens of metres, are genetically diverse and highly aggressive to one another, so population numbers never explode and they are no threat to plants and animals,” Ms Suhr said.

“However, when they arrived in Australia, a change in their structure occurred, changing their behaviour so that they are not aggressive to one another. That has resulted in the colonies becoming one supercolony.”

The Argentine ant, listed among the world’s 100 worst biological invaders, is black and barely 3mm long. It would be a harmless household pest were it not for its vast numbers.

When they no longer fight among themselves the creatures roam freely, establishing new nests and building up their numbers.

Researchers say that the effect on Australia’s ecology is likely to be profound.

“Because there are millions and millions of them they are easily able to displace smaller colonies of native ants. They are aggressive (to other species) and they will kill them off,” Ms Suhr said.

In California, where Argentine ants have also established supercolonies, they have displaced native ants, decreased the diversity of other native insects, affected dispersal of seeds and brought down lizard numbers.

Rural Victoria is expected to escape the invasion because the ants cannot tolerate dry countryside. But urban areas anywhere in Australia are in potential danger. The tiny invaders will hitch-hike on cars and on people and spread quickly.

Scientists are studying ant colonies in Perth and Adelaide to see if they share the same genetic structure and behaviour as the Melbourne ants. If they do, a supercolony several thousand miles wide could spread across Southern Australia.

The Argentine ant was first found in Australia as an illegal immigrant in 1939, possibly the result of one stowaway queen and a loyal band of worker ants. Now it is in every state, and in other countries with Mediterranean ecosystems.

Australians, who are tuned in to the latest nuances in social behaviour, are branding the Argentine invaders as “metrosexual” ants — easygoing creatures who have lost their macho aggression in favour of a live-and-let live attitude to life. They could be creating a new Australia.

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