A cane toad in Taronga Zoo, Sydney. The toads have sacs behind their heads full of a venom powerful enough to kill crocodiles and snakes.

Credit: AFP

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SYDNEY: Cane toads have adapted so well to the Australian climate that they may spread around three-quarters of the country's coastline, according to a new study.

In related news, a cane toad the size of small dog was captured in the Northern Territory's capital, Darwin, yesterday.

Bufo marinus is one of the world's most notorious examples of an animal introduction gone wrong. The toxic species was introduced to northeastern Australia in 1934 to control sugar-cane pests, but the toads have done nothing to control their intended prey and their relentless spread across the continent has devastated populations of frogs and other native species.

In February 2006, the toad's range was estimated by biologists to have spread to more than a million square kilometres in tropical and sub-tropical Australia.

But now, the same team say the unwanted anuran's empire has now expanded to 1.2 million square kilometres in a swathe across Queensland and the Northern Territory.

"Previous predictions of toad distribution in Australia were based mostly on the range of climatic conditions where toads live in their native range, in Central and South America. We just assumed that the toads in Australia would only be able to survive within that climatic envelope," said ecologist Rick Shine of the University of Sydney.

Shine is co-author of a study modelling the toad's spread which appears today in the British journal Proceedings of the Royal Society B.

Confounding those who believed that the species would be limited to hot, humid climes, B. marinus has been found to cope with a wide temperature range and dry conditions, the researchers report.

The species now occupies regions where the minimum monthly temperature falls below 5° C and the maximum monthly temperature rises to a steamy 37°C - and has colonised areas where, by all expectations, it should have died from lack of moisture.

This shows it has the capability of living in coastal Western Australia and coastal southern Australia, near the cities of Perth and Adelaide respectively.

"We predict that cane toads now have the potential to inhabit over two million square kilometers of the continent. This estimate includes three-quarters of Australia's coast, a region where most of the continent's human population and biological diversity are concentrated," the researchers wrote. "If the cane toad's advance continues, this prolific and problematic species is likely to cause further harm to Australia's unique wildlife and economy."

The authors suggest that the toads' expansion is down to evolution, the lack of predators and the fabulous empty niche they found in Australia's habitat.

In the February 2006 study, also led by Shine, toads with longer legs were found to have an evolutionary advantage of shorter-legged counterparts, enabling them to move quicker into new territory. The success of longer-legged toads helps explain why in the 1940s the species expanded its range by only 10 km a year, while today it is advancing at more than 50 km annually.

Meanwhile, yesterday, a cane toad the size of a small dog was captured in Darwin, the largest ever recorded in the Northern Territory. Green group FrogWatch, in Noonamah, Northern Territory, said the monster male was 20.5 cm long and weighed nearly a kilogram.

"The biggest toads are usually females but this one was a rampant male," said FrogWatch's Graeme Sawyer, who organises regular excursions to trap and destroy the feral pests. "He is huge. I'd hate to meet his big sister."

More information:

Proceedings of the Royal Society B.