DEBUNKING MYTHS ABOUT CANE TOAD IMPACT

There are numerous myths about cane toads. Many of the things that people think they “know” about cane toads are actually wrong. Many other stories may or may not be true, but we simply don’t know.

Myth # 1

Toads were a failed attempt at biological control

One myth that you often hear about cane toads is that they were complete failures in the purpose they were originally brought to Australia for – that is, to control beetles that were damaging sugar cane crops in Queensland. Indeed, people often talk about the scientists involved being so stupid they didn’t think about the fact that beetles can fly but toads can’t!

The truth is a bit more complicated.

The scientists hoped that toads would eat beetles that came to the ground to breed. And whether or not the toads had any effect on beetle numbers was never studied, so far as we know – nobody really knows whether or not the toads were of any use in doing the job they were brought here to do. The fact that the beetles are still common suggests that the toads were indeed a failure - but the evidence is so flimsy that we don't think any jury would convict them!

I took a look at information on sugar-cane yields in the years around whn toads were released in the cane-fields, and I can't see any hint of an increase in yields. So, it's likely that toads didn't do the job they were brought here for - but we will never know for sure.


Graduate student, Edna Gonzalez Bernal

Graduate student, Edna Gonzalez Bernal, examined the diet of cane toads in detail (photo by Terri Shine).

Myth # 2

Toads have caused the extinction of many native Australian species

The truth is, no Australian species is known or even suspected to have gone extinct as a result of cane toads. When they first arrive in a new area, cane toads kill lots of frog-eating predators like quolls (carnivorous marsupials), goannas (varanid lizards) and snakes like death adders, red-bellied blacksnakes and king brown snakes. But over time, the Australian species adapt to deal with the toad’s presence – so if you go to places where toads have been around for many decades, most of those predator species are much more common again. They may not be as common as they were before toads arrived – we can’t tell because we don’t have any detailed information on historical numbers – but they certainly are much more common than they were a few years after the toads arrived.

How do the Australian frog-eating predators change so they can deal with toads? Two things happen. First is that many species just don’t like the taste of toads, or feel sick if they eat one but don’t die – and so they learn to stop trying to eat things that look like toads. For example, fishes (like barramundi), frogs (like the Dahl’s Aquatic Frog) and mammals (like planigales) all learn to avoid toads very quickly.

The second thing that can happen is that even if species don’t learn quickly (perhaps because eating even a small toad is fatal), they can adapt by means of rapid evolutionary change. So, for example, red-bellied blacksnakes are very slow learners but nonetheless persist even in areas where toads have occurred for a long time. The way they achieve this comes about because the only survivors when toads arrive are the few snakes that happen to have genes that say “don’t eat a toad”; or that confer resistance to the toad’s poison. When they reproduce, they pass those characteristics onto their babies – and so, we soon get a population of blacksnakes that can coexist with toads without too many problems.


Quolls

Northern quolls declined with toad invasion, but populations are recovering in Queensland (photo by Jonno Webb).

Myth Debunked!

Myth # 3

The toad's poison is deadly to all Australian predators

It’s true that Australia doesn’t have any native toads of its own (it’s the only major continent not to), and that toads have poisons different from those of Aussie frogs. So, many predators are unable to deal with these poisons, because they have never had to deal with them over the last few million years of evolutionary history. But some native frog-eating species are perfectly capable of eating toads. For example, keelback snakes can eat all the toads they like without dying. Many birds also don’t seem to have any trouble, and indeed road-killed toads are a popular diet item for hawks in our study area at Fogg Dam. Native rodents happily munch cane toads without becoming ill. And lots of invertebrates handle the toad’s poison also – sometimes you can see meat ants grabbing and eating huge numbers of tiny toads beside billabongs. A lot of insects that live in the water - like water beetles and waterbugs - actually prefer to eat toad tadpoles than frog tadpoles.

Keelback snakes can eat Cane Toads

Keelback snakes can not only eat native frogs (like this one), but also cane toads (photo by Greg Brown).

Myth Debunked!

Myth # 4

Toads are wiping out native frogs

Common sense says that frogs will be in real trouble when toads arrive – but they aren’t. Although toads do compete with frogs for food, and do sometimes eat frogs, the number of frogs doesn’t go down when the toads arrive. There have been several studies now – some relying on just counting frogs, others using more complicated systems like electronic listening posts to record the male frog’s calls before and after toads arrive – and none of them show any decline in frogs due to toads. How can that be? Surely a frog-like animal from overseas that becomes incredibly abundant in your home pond is a problem if you’re a frog? But if you think about it a little more, it makes sense. Toads DO look like frogs – but this means that many animals that eat frogs try to eat toads. Because the toads are so poisonous, the predator dies – and so, the toads act like booby-trapped frogs and kill many of the frog’s natural predators. That means that a lot more frogs survive because they aren’t being eaten by goannas and snakes and so forth – more than enough to make up for the few that are eaten by toads.


Frog eat frog

It's a frog-eat-frog world, and cane toads don't seem to make much difference to frog numbers

Myth Debunked!

One of our main aims with this website is to provide reliable evidence about cane toads in Australia, backed up by scientific research. We haven't put lots of details about that evidence on the main pages, but it's easy for you to get to it. If you want to explore a topic in more depth, just click on Rick's "official" website. You can download literally hundreds of his scientific papers there. Rick's research website

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