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Cane toads: not quite the bad guy we thought
Cane toad invasion: ugly, but not so bad 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 (see map). 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”.
“Populations of many native species are reviving because they have learned to avoid cane toads” 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 18 | NewScientist | 11 September 2010
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
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. In 2008, for example, a wave –Who’s not a pretty boy then?– of crocodile deaths was reported in the Victoria river, Northern do compete with frogs for food Territory, coinciding with the and egg-laying sites, they also toads’ arrival. The following year, help frogs by removing their toad-induced deaths among the predators. “There isn’t much crocodiles fell. Likewise goannas overall effect,” says Shine. are once again abundant in areas Shine says there is also no of northern Queensland, even noticeable change in populations though 96 per cent of the lizards of insects, despite both cane toads died when they first encountered and native toads eating them. the toads. “Nothing has become Other researchers, including extinct,” says Alford. Alford, believe there isn’t enough Evidence that northern quoll evidence to be so optimistic. But numbers are recovering is weaker. Alford concedes that since This week the Nature Conservancy, populations of insect-eating a US non-profit organisation, species such as frogs and birds published a report by John appear stable, it is probable that Woinarski, a zoologist at Charles insect numbers have remained Darwin University, Northern stable too. Territory, blaming cane toads for the quoll’s decline. “Northern quolls in some areas have been Toad territory extirpated,” Woinarksi says. Cane toads are found mainly in east and Still, he points out that quolls north Australia, but they will spread to were in decline even before cane other regions where the climate suits them toads arrived, because of a greater Confirmed sightings incidence of summer bush fires in Predicted future limit of habitat 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 AUSTRALIA baby toads laced with a nauseainducing chemical. Woinarski says there is cause for optimism overall. “Our ecology is more robust than we feared,” he says. n SOURCE: KEARNEY ET AL
david gray/reuters
THIS WEEK
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 (see map). 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”.
“Populations of many native species are reviving because they have learned to avoid cane toads” 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 18 | NewScientist | 11 September 2010
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
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. In 2008, for example, a wave –Who’s not a pretty boy then?– of crocodile deaths was reported in the Victoria river, Northern do compete with frogs for food Territory, coinciding with the and egg-laying sites, they also toads’ arrival. The following year, help frogs by removing their toad-induced deaths among the predators. “There isn’t much crocodiles fell. Likewise goannas overall effect,” says Shine. are once again abundant in areas Shine says there is also no of northern Queensland, even noticeable change in populations though 96 per cent of the lizards of insects, despite both cane toads died when they first encountered and native toads eating them. the toads. “Nothing has become Other researchers, including extinct,” says Alford. Alford, believe there isn’t enough Evidence that northern quoll evidence to be so optimistic. But numbers are recovering is weaker. Alford concedes that since This week the Nature Conservancy, populations of insect-eating a US non-profit organisation, species such as frogs and birds published a report by John appear stable, it is probable that Woinarski, a zoologist at Charles insect numbers have remained Darwin University, Northern stable too. Territory, blaming cane toads for the quoll’s decline. “Northern quolls in some areas have been Toad territory extirpated,” Woinarksi says. Cane toads are found mainly in east and Still, he points out that quolls north Australia, but they will spread to were in decline even before cane other regions where the climate suits them toads arrived, because of a greater Confirmed sightings incidence of summer bush fires in Predicted future limit of habitat 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 AUSTRALIA baby toads laced with a nauseainducing chemical. Woinarski says there is cause for optimism overall. “Our ecology is more robust than we feared,” he says. n SOURCE: KEARNEY ET AL
david gray/reuters
THIS WEEK