NEW SCIENTIST (London, UK) 09 September 10 Cane toads aren't quite the bad guys we thought (Wendy Zukerman)
It could be one of the world's most unfairly maligned creatures. Despite its invasion of Australia, the cane toad has not triggered the overwhelming ecological disaster that some predicted.
Cane toads (Bufo marinus) were brought to Australia in the 1930s in an attempt to eradicate a beetle destroying sugar cane. They quickly spread. Last year, the toads were found in Australia's most western state for the first time. One downbeat local newspaper headline lamented: "Cane toad battle lost".
Australia's frog-eating predators, including snakes, crocodiles and the northern quoll - a type of marsupial - have been dying en masse after ingesting the poisonous invaders. The worry was that mushrooming toad populations would outcompete native frogs and birds too. With the elimination of these native species seemingly imminent, an ecological catastrophe looked on the cards.
"People saw these ugly creatures moving across tropical Australia and common sense said there was going to be a huge disaster," says Richard Shine, an invasive species researcher at the University of Sydney, who has reviewed various studies on the impact of cane toads (The Quarterly Review of Biology, DOI: 10.1086/655116). "But it just hasn't happened at the scale that we feared."
"The system seems to be absorbing the toads," agrees Ross Alford of James Cook University in Townsville, Queensland. "Toads are not an overwhelming environmental disaster."
Populations of native frogs and birds do not appear to have changed significantly since the toads were introduced, and recent experimental studies reviewed by Shine show that while the toads do compete with frogs for food and egg-laying sites, they also help frogs by removing their predators. "There isn't much overall effect," says Shine.
Shine says there is also no noticeable change in populations of insects, despite both cane toads and native toads eating them. Other researchers, including Alford, believe there isn't enough evidence to be so optimistic. But Alford concedes that since populations of insect-eating species such as frogs and birds appear stable, it is probable that insect numbers have remained stable too.
Populations of goannas (monitor lizards), freshwater crocodiles, king brown snakes and northern quolls have dropped in some regions colonised by toads. But Shine believes many of these populations are reviving because the animals learn to avoid the toads. How so? According to Shine, native creatures encountering newly arrived adult cane toads will eat them voraciously, having "not grown up with them". The experience is usually fatal, but once the toads have reproduced, their predators will survive eating the immature toads - which are less toxic than adults - and so learn not to eat the species again.
Populations of many native species are reviving because they have learned to avoid cane toads
In 2008, for example, a wave of crocodile deaths was reported in the Victoria river, Northern Territory, coinciding with the toads' arrival. The following year, toad-induced deaths among the crocodiles fell. Likewise goannas are once again abundant in areas of northern Queensland, even though 96 per cent of the lizards died when they first encountered the toads. "Nothing has become extinct," says Alford.
Evidence that northern quoll numbers are recovering is weaker. This week the Nature Conservancy, a US non-profit organisation, published a report by John Woinarski, a zoologist at Charles Darwin University, Northern Territory, blaming cane toads for the quoll's decline. "Northern quolls in some areas have been extirpated," Woinarksi says.
Still, he points out that quolls were in decline even before cane toads arrived, because of a greater incidence of summer bush fires in recent years and the introduction by humans of cattle and predators such as cats. More encouragingly, Shine has evidence that young quolls can be trained to avoid cane toads by feeding them baby toads laced with a nausea-inducing chemical.
Woinarski says there is cause for optimism overall. "Our ecology is more robust than we feared," he says.
Cane toads aren't quite the bad guys we thought

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