Wednesday, 28 March 2007

Cane toads invade new Australian terrain

As the nursery rhyme goes, there was an old lady who swallowed a fly. To catch the fly, she swallowed a spider. To catch the spider she swallowed a bird … and so on, until she died, of course.

Cane toads first reached Australian soil under similar premises. When they were introduced to the Northern Territory in 1935, they were expected to combat the cane beetle, which is a pest of sugar cane crops. While the toads did not alleviate the problem with beetles, they did quickly become accustomed to the Australian climate, and they bred fast. Farmers soon found themselves faced with larger, wartier pests than cane beetles – the cane toads themselves.

For the past 70 years, residents of Perth and Adelaide have watched cane toads spread from the Northern Territory through tropical Australia, and thought it had nothing much to do with them. But new research by an international team of scientists, including ecologist Rick Shine of the Sydney University’s School of Biological Science, claims otherwise.

March of the toads

Previous predictions about the behaviour of cane toads, which go by the scientific name Bufo marinus, indicated that the species would be limited to tropical environments like those in Central and South America, from which the toads were originally imported.

Confounding these beliefs, the toads were now found to have adapted to dry conditions and temperatures ranging from 5 to 27 degrees Celsius.

“The toads have adapted rapidly to Australian conditions, and now tolerate much higher temperatures than was the case when they first arrived on our shores in 1935,” Shine said. “As a result, toads have managed to spread much further, into climatic zones also found widely through southern Australia – so there is no reason to think they will remain only in the tropics.”

Mathematical modelling by Shine and his team shows now predicts that toads will be able to live and breed in large areas of Western Australia, South Australia and western Victoria, and in several pockets along the New South Wales coast.

Current and projected range of invasive cane toads in Australia - Rick Shane et al

The researchers estimate the toads’ range to be about 1.2 million square kilometres across the Northern Territory and Queensland. This is a dramatic increase from estimates made by the same research team a little over a year ago, in which cane toads were thought to have spread to more than a million square kilometres in tropical and sub-tropical Australia.

Additionally, the rate at which cane toads are invading is not only fast – it’s also getting faster, researchers found. Compared with a rate of only 10 kilometres a year in the 1940s, the toads are now found to be advancing at more than 50 kilometres annually. Researchers speculate that this could be a result of evolution, as toads with longer legs were found to have an evolutionary advantage over shorter-legged counterparts.

Beware warty hitch-hikers

Most of the potential range of the cane toad in southern Australia is separated from the toads’ tropical range by very large expanses of desert, which the researchers expect to be too dry for a toad to cross.

But although the toads may initially be deterred by these dry expanses, Shine said it is likely that the toads will cross such areas as hitch-hikers among rubbish or equipment in the back of a truck.

According to Shine, a nationwide invasion of cane toads is ‘inevitable’.

“[The arrival of cane toads] will probably depend upon fortuitous hitch-hiking events, rather than the toads dispersing all the way under their own steam,” he said. “The toads are superb invaders, and so far, none of the attempts to slow them down seem to have been very successful.”

Due to their toxicity and eating habits, the arrival of cane toads could spell bad news for some native fauna, pets and large predators. However, Shine expects most species to be relatively unaffected.

“Our work in the Northern Territory indicates that the toads dramatically affect only a few species,” he said, “[which are] mostly large predators like goannas, that try to eat the toads and are poisoned as a result.”

The spread of cane toads is expected to provide a good model system for understanding what invasive species will do in general, Shine said. Researchers are now expecting similar shifts in other invaders as well, as they adapt to the Australian climate.

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