History in Australia

The Unwanted Amphibian

Up until 1935, Australia did not have any toad species of it's own. We had tree frogs and burrowing ground frogs - even microhylid frogs which do not have a tadpole - but none of the world's hundreds of toad species evolved here. However, not wanting to be left out, Australia acquired some - 102 toads, in fact.

These toads were supposedly being used successfully in the Carribbean islands and in Hawaii to combat the cane beetle, a pest of sugar cane crops. After rave reviews from overseas, Hawaii shipped a box of toads to Gordonvale, just south of Cairns. These were held in captivity for awhile, their numbers were increased by breeding, some experimental trials were conducted, and then they were released into the sugar cane fields of the tropic north. It was later discovered that the toads (scientific name Bufo marinus) can't jump very high so they did not eat the cane beetles which stayed up on the upper stalks of the cane plants. At the time of year when the beetle's larvae were emerging from the ground, no toads were about. So the cane toad, as it came to be known, had no impact on the cane beetles at all and farmers had to go back to the use of chemicals to kill the beetle.

Meanwhile, the 'cat was out of the bag' or, more accurately, the toads were out of the box! But there were only a few hundred of them so nobody gave any thought to catching them up again and disposing of them. The toads were on their own and they proved to be very hardy survivors. They turned out to be a lot more than we bargained for and it didn't take long to find out how well the toads would do in their new Australian home.

  • They breed like flies, as the saying goes. Each pair of cane toads can lay 33,000 eggs per spawning (some published references estimate they produce as much as 60,000 eggs!).
  • Their 'toadpoles' develop faster than many Australian frogs so they can outcompete our frogs for food.
  • Toads and toadpoles seem to be resistant to some herbicides and eutrophic water which would normally kill frogs and tadpoles.
  • All stages of a toad's life are poisonous so they have no natural predators to keep their numbers in check (although Mike Tyler's work suggests that toad juveniles are not toxic until they reach about 3cm in size but this presents a question: why would an animal lose its toxicity at the juvenile stage when it has it during larvae and adult stages?)
  • Toads not only eat the food normally available to Australian frogs, there is growing anecdotal evidence that they eat frogs as well, especially metamorphs.

Fish who eat toadpoles die. Animals who eat toad adults die. The museums have plenty of snakes preserved in jars which were killed by toad toxin so fast, the toad is still in their mouths unswallowed. Even small amounts of water, such as a pet's water dish, can be fouled by toadpoles and adults. When the pet comes along to drink from it's dish, it becomes sick. Local vets report that a couple dogs a month are brought in ill just from "mouthing" toads.

Here's a riddle: what happens if you feed a cane toad too much?

Answer: it just keeps getting bigger!

Captive cane toads will allegedly eat everything from dog food to mice and they keep growing until they reach 25cm in length and over 4 kilos. In recent years, it has been noticed that toads in the Cairns area are much smaller than they used to be (the "big mama" at right was found in Babinda - 30 minutes drive south of Cairns). A theory is that when toads first colonise a new territory, there is an abundant food supply. The toads gorge themselves and grow to full size or close to it. As the numbers of toads increase from breeding, the food resource never reaches its pre-toad levels and therefore, the toads' size and their food supply acheive a "compromise". Certainly the largest toads found in Cairns come from the suburbs which back onto bush and, therefore, have more plant life to feed more insects.

Cane toads have proven themselves to be (arguably) one of Australia's worst environmental disasters. Since 1935, they have spread across most of Queensland, they are almost entirely across the Northern Territory (only 75 km from the WA border) including the world-reknowned wetlands of Kakadu. Their numbers are profuse in the dry southeast Queensland area (or were until 2002 when another new disease problem emerged in Queensland) and they are spreading down the NSW coast. Quite a few have hitched a ride down to Sydney in vegetable trucks and they have established themselves at the 2000 Olympics site (at Homebush Bay in Sydney's inner western suburbs). This area of Sydney is also the largest remaining NSW stronghold for the endangered Green and Golden Bell frog (Litoria aurea). (One thing this endangered frog definitely does not need is another threat.)

Toads are responsible for the reduction of many species of Australian wildlife and the Northern Territory is currently a living research laboratory where researchers are documenting the changes to predator animal numbers such as crocodiles and quolls. The Northern Territory government has already taken preventative steps to help save the quoll population by relocating numbers of them to offshore islands until another toad reduction method can be found. However, there are examples of how 'nature finds a way'. Some Queensland bird and rodent species have somehow learned how to eat cane toads without exposing themselves to the toxin. They kill the toad and turn it over onto its back. They pull away the soft belly skin and partake of the internal organs, leaving the skin and the deadly paratoid glands behind. This behaviour has only taken a mere 60 years to learn - very fast on the evolutionary scales. Those native rats which do feed on animal material (such as the White-tailed and the False Water) have learned to only eat the legs of the toad and not the body.

There is also an Australian snake species called the Keelback or Freshwater snake (Tropidonophis mairi) which is reportedly immune to the toad's toxin although some debate still rages as to whether it really is immune or if it is only non-toxic juveniles that can be taken. There are other Australian snakes that may be immune and work is in progress in the Northern Territory under the supervision of Dr. Rick Shine of Sydney University.

There is also a new piece of information which has emerged because of the toad's progress across the Northern Territory: there is a beetle species there which the frogs ignore but the toads relish, and when they eat these Lavender Beetles, they die. We hope that the government will rapidly put funding into studying this event as this may be the sort of 'silver bullet' they have been looking for to get rid of the pesty toad.

An extraordinary amount of money has been spent by the Australian government on genetically manipulating a disease to kill toads (thankfully cancelled in 2008). This was a very risky project which would have cataclismic consequences if it it went wrong and severe consequences even if it worked correctly. The Queensland government has also taken a "high-tech" approach with a toxic bait project which is still in progress. We applaud the decision of the Northern Territory government who has taken a more targeted, manual approach to the problem by putting money into a trapping program which is already clearing areas of toads now!

Probably the most commonly asked question of the Frog Decline Reversal Project is how to tell the difference between the tadpoles of cane toads and Australian frogs. To find out the answers to these and other toad related questions, visit the rest of the pages in this Cane Toad section!