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The cane toad is spreading fast across Australia, killing anything that eats it. Yet the feared wildlife catastrophe hasn鈥檛 happened

AS I drive along the highway from Darwin, through a town called Humpty Doo, it鈥檚 hard to believe I鈥檓 in cane toad territory. It鈥檚 a scorching 42 掳C and the tropical savannah alongside the road is bone dry, a patchwork of fire scars, red dirt and brown leaves. It is the end of the dry season, and there has been no rain to speak of for more than three months. This isn鈥檛 the kind of place you would expect to find water-loving amphibians from the Amazon 鈥� not even killer supertoads 鈥� but they are here.

I鈥檓 heading to a nearby wildlife reserve called Fogg Dam to visit biologist Rick Shine. After spending decades studying native water pythons, Shine switched his focus to cane toads as they neared Fogg Dam a decade ago. 鈥淚t was an obvious opportunity to work out what the impact would be,鈥� he says as he shows me around his lab 鈥� an old brick building connected to a couple of large sheds. All around are small pools, cages and even ice cream containers full of toads of various ages.

As the toads invaded Fogg Dam, Shine, who is based at the University of Sydney, got some of the first hard data on their impact. What he has found is that the Australian cane toad invasion is both far worse than anyone thought 鈥� and also not nearly as bad.

Look into my eyes: I鈥檓 evolving to be faster, feistier and possibly smarter too (Image: Thomas Marent/Ardea)

The toads are spreading further and faster than anyone expected, and they do have a devastating impact when they first arrive in a region. But most animals are adapting to their presence surprisingly quickly, and some even benefit. 鈥淚f you鈥檙e a frog, the toad is your superhero,鈥� says Shine. 鈥淵ou鈥檝e got its picture up on the wall. This guy is coming in, he looks like a frog and is killing everything that attacks frogs. If you鈥檙e a green tree frog, what more could you hope for in life?鈥�

鈥淚f you鈥檙e a frog, the cane toad is your superhero. He looks like a frog and is killing everything that attacks frogs鈥�

So while many biologists still fear that the toads will have dire consequences, Shine is more optimistic. 鈥淚 came into this thinking 鈥業鈥檓 going to be documenting a catastrophe鈥�,鈥� he says. 鈥淚鈥檝e gone to thinking it鈥檚 a good-news story about the resilience of ecosystems.鈥� Could he really be right?

The toad invasion began in 1935 when 102 young cane toads were brought to Queensland from Hawaii (see map). At the time, the toads, native to South America, were being deliberately introduced to to control pests 鈥� despite there being little evidence that the ground-living toads could control pests on 3-metre-high sugarcanes. They have thrived in most of their new homes.

In Australia, the toads got little attention for decades, even as they slowly spread and multiplied. But then a pet quoll 鈥� a small carnivorous marsupial native to Australia 鈥� belonging to Mike Archer died after trying to eat a toad. Stung by denials that the toads were toxic to native wildlife, Archer, a palaeontologist at the University of New South Wales, went around collecting reports of pet dogs and native predators dying after trying to eat the toads. His 1975 paper alerted people to the potential impacts. 鈥淭hat was the turning point where we really started to say to zoologists that this is a disaster,鈥� says Archer.

No one doubts any longer that cane toads are killers. Not only do they eat any animal they can stuff in their mouths, they also produce toxins in their skin that, when absorbed through the mouth or eyes, can stop the heart. Most animals that try to eat them die 鈥� which is very bad news given that the toad army now numbers in the hundreds of millions, and in places there are more than 2000 toads per hectare.

These factors led many biologists to predict ecological meltdown. 鈥淵ou can now forget about Kakadu,鈥� biologist Mike Tyler of Adelaide University , as the toads moved into the world heritage site Kakadu National Park near Darwin. 鈥淜akadu is lost.鈥�

The toads are now racing towards a region of north-western Australia known as the Kimberley. 鈥淢any very distinctive Australian native species now only occur there,鈥� Shine says. Cats, fungi, habitat loss and other pressures have wiped them out elsewhere. 鈥淪o the Kimberley really is the last hold-out for the things that are in desperate trouble.鈥�

No one expected the toads to move so fast. Initially, they only advanced about 10 kilometres per year. Now they are moving more than 55 kilometres per year. In 2009 they crossed into Western Australia, more than 4000 kilometres from where they began.

When Shine鈥檚 team strapped radio trackers on the toads as they moved past Fogg Dam and into Darwin between 2001 and 2005, they found the invasion front was averaging an incredible 250 metres every night. Individual toads sometimes went much faster. One of the toads they tracked .

The docile, fat toads Shine knew from the farms of Queensland simply couldn鈥檛 do that, he thought. So he examined the toads on the front line and found something remarkable. In just a few decades, that allow them to move faster.

To prove we are witnessing evolution in action, Shine bred toads from different locations in captivity and compared their progeny. Lo and behold, .

The leaders of the invasion seem to be evolving in even more radical ways. When you put the toads in a cage 鈥渢he guys from Queensland sit back, you know, [saying] 鈥楤ring me another cockroach please鈥�,鈥� Shine says. 鈥淏ut when you get the toads from the invasion front, the poor buggers are bashing their noses against the wall,鈥� he says, punching his palm repeatedly with his fist to demonstrate. 鈥淭hey want to get going.鈥�

Shine鈥檚 PhD student Jodie Gruber is trying to find out if they are getting smarter too. Gruber can鈥檛 answer that question yet, she tells me when I go to visit, but she has already discovered that the toads are smart. Only one amphibian had ever passed a test called the Morris water maze, in which it had to find an underwater platform to sit on, she says. After just a week of studying the toads, she has found that they can pass the test too.

So the vanguard toads are evolving to be faster, more aggressive and possibly smarter. They do pay a price for these abilities, though, such as arthritis and . So once an area has been invaded, the population regresses back to the lazy Queensland-type toads. But in the meantime, Australia has to deal with an accelerating wave of invaders. How can Shine be optimistic? Because what happened at Fogg Dam and elsewhere wasn鈥檛 as bad as he had feared.

鈥淭he big goannas [monitor lizards] were slaughtered in droves, and quolls are hard to find. But both species are still hanging on,鈥� Shine says. Some native species have even become more common. And the same seems to be true elsewhere, including in the 鈥渓ost鈥� Kakadu park. 鈥淪o far as we know, the Kakadu story is very similar to Fogg Dam,鈥� Shine says. The doomsayers 鈥� including himself from an earlier time 鈥� haven鈥檛 been vindicated.

For example, one worry was that cane toads would outcompete native frogs, causing mass extinctions. In , Shine found toad invasions had almost no effect on native frog abundance.

What鈥檚 more, many of the predators whose population had been destroyed by the toad arrival 鈥� like quolls and goannas 鈥� have learned surprisingly quickly not to eat them (see 鈥�Winners and losers鈥�). 鈥淚f you go to northern Queensland now, quolls and goannas are common. They kick toads out of the way, grab a frog and eat it,鈥� Shine says.

One reason could be that once toad populations are established, there are a lot of baby toads hopping around. And for a young goanna or quoll, a baby toad is easier to catch than an adult. It will make the predator sick, says Shine, but is less likely to kill it, providing a life-saving lesson in bush tucker.

In the wild, for instance, adult freshwater crocodiles have died en masse after trying to eat adult toads. In the lab, Shine鈥檚 team found that . But although they showed no obvious signs of illness afterwards, half of them wouldn鈥檛 touch a toad again.

Living with the enemy

Other animals are evolving to avoid toads. A decade ago, one of Shine鈥檚 prot茅g茅s, Ben Phillips at the University of Melbourne, showed that red-bellied black snakes and tree snakes living in toad-infested areas had evolved smaller heads over time, preventing them from eating large toads.

Now the head-size shift seems to have been overtaken by a more efficient adaptation: some snakes are no longer interested in toads. It couldn鈥檛 be a learned behaviour, Phillips says, because programmes teaching animals not to eat toads don鈥檛 work with snakes.

So while the arrival of the toads is hardly good news, it hasn鈥檛 been the disaster once feared. 鈥淭oads have changed everything,鈥� Phillips says. 鈥淪ome of that change has been positive and some of it has been negative. The system is not the same as it was before.鈥�

Shine is keen to make it clear that toads have not caused a single extinction. But Simon Clulow, a biologist at the University of Newcastle in New South Wales, thinks that could change. The reason is that the toads carry a lungworm parasite. Most Australian frogs aren鈥檛 harmed by it, but it鈥檚 deadly to the magnificent tree frog, a species found in the Kimberley. 鈥淚t鈥檚 a very social species and they huddle together,鈥� Clulow says. 鈥淥nce this worm gets into the magnificent tree frog population it will rip right through it.鈥�

But even on this point, Shine is more positive. 鈥淚 think the habitat differences are quite substantial and will reduce the risk of parasite transfer,鈥� he says. 鈥淚 don鈥檛 think this is the death knell for the magnificent tree frog.鈥�

Even though Shine doesn鈥檛 think the toads will have as catastrophic an impact as some fear, he is in no doubt that it would be far better to keep the toads out. He, Clulow and others hope some key areas in the Kimberley can be kept toad-free by using pheromone traps, pheromone repellents and fences.

And while it might be too late for the Kimberley, many think the toads鈥� advance could be stopped there. To spread into the Pilbara region 1000 kilometres to the south, the toads would have to cross a desert. Models suggest that they will have no problem passing through that desert, but .

To stop this the water holes don鈥檛 even have to go 鈥� they just need to have smooth walls about 50 centimetres high to stop adult toads entering and breeding in them. 鈥淚t鈥檚 a realistic place where they could be stopped. Almost everyone is in agreement,鈥� Phillips says.

While north-western Australia fights for a toad-free life, the rest of the north has no choice but to live with the enemy. At one point the government spent millions trying to develop a genetically engineered virus to kill the toads, but . Many people around here despise the toads. Archer, for one, has never forgiven them for killing his quoll: 鈥淚 haven鈥檛 wavered a bit. I think they鈥檙e a noxious horrible pest and they鈥檙e going to cause a transformation.鈥�

In fact, they are so hated it is common practice to hit them with golf clubs or drive over them with cars rather than killing them more humanely by placing them in a bag in a freezer for a few days. 鈥淵ou know it鈥檚 not their fault that they鈥檙e here,鈥� says Shine, who has gone from hating toads to having a grudging respect for them. In fact, he seems to really love them. 鈥淲e need to treat them with the same ethical care we would a koala.鈥� He smiles and adds: 鈥淎nd if you pushed me I might say more than a koala.鈥�

Holding a Queensland cane toad, I can even start to see what Shine and his students have come to like about them. For all the talk of their viciousness, they seem remarkably tame. Because their weapon is their toxicity, as soon as they feel threatened, they just sit still. And their eyes are magnificent: they look like an exploding star with black and gold speckles. It might just be the hallucinogenic toxins, but maybe I, too, could learn to love the toad.

This article appeared in print under the headline 鈥淎liens VS predators鈥�