Cane toads may look like easy victims to be preyed upon by indigenous predators. Yet these tubby amphibians have a secret weapon: fatal poison under the skin, stored in two grands on their back. A cane toad, in fact, has enough toxins to kill even a large predator like an Australian crocodile.
But what’s good for the toads may not be good for the ecosystem. The toads, which are native to South and Central America, were introduced in 1935 into Queensland in order to control the populations of cane beetles that were infesting sugarcane plantations.
There was little evidence that the newly introduced toads had much effect on the beetles. Instead, the amphibians spread so rapidly that they themselves became a pest after their populations got out of control.
The reason is that these hardy toads can survive in a wide range of habitats where they often outcompete native species for food. The amphibians can devour various kinds of terrestrial and aquatic animals, ranging from small lizards to snakes and from marsupials to mice. They can also feed on pet food or food waste left outside homes.
Worse: the toads are highly fecund. Females can each lay up to 60,000 eggs a year. Not only that but the amphibians have adopted to local conditions to such an extent that they have spread far and wide across much of Australia.
“They probably have moved about halfway through that tropical region of Western Australia,” says Rick Shine, a professor of biology at the University of Sydney. “They are in very inaccessible country now in the Kimberley. It is very hard to get detailed information on exactly where the front is but it seems to be moving at 50 to 60km (31 to 37 miles) per annum,” he continues.
Yet this is not only way in which an increase in the number of cane toads is putting indigenous animals in grave danger across Australia. The toads’ potent poison has caused a massive mortality rate among native predators in Australia, including birds, snakes and crocodiles. A recent study shows that the amphibians have caused the population densities of freshwater crocodiles in Australia to plunge by a staggering 77%.
In an attempt to get native species to avoid preying on the poisonous toads, experts have resorted to a technique called “conditioned taste aversion therapy.” Native species such as goannas and quolls are exposed to small doses of toxins produced by the toads. This can induce sickness in the target animals but does not expose them to the risk of death.
In the process they learn to avoid feeding on the toads. With several native predators the technique has been working quite well.
Some indigenous animals have also learned on their own to feed on cane toads without ingesting their potentially lethal poison. Crows have learned what the safely edible parts of the toads are, including their thighs, tongues and intestines. These clever birds also know how to handle their victims to avoid coming in contact with their poison.
Likewise, native water rats know precisely how to cut their prey open and eat their hearts, leaving their toxin-laden bodies behind.
“It was a small area of creek, three to five meters in size, and every day we were finding new dead cane toads,” says Marissa Parrott, a biologist working in the Kimberley region of Western Australia. “Up to five every single morning,” she adds.
What’s bad for the toads could well be good for the environment in Australia.
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