 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,
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 get quite large. 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 still 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 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 also 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 and is still being spent by the Australian government on
genetically manipulating a disease to kill toads. This is a very risky
project which will have cataclismic consequences if it goes wrong and
severe consequences even if it works correctly - read why in our toad
virus GMO page. Even Queensland government has jumped onto the "high-tech"
at any cost bandwagon, chosing to throw $1 million dollars into a hi-tech
bait project instead of researching the fact that frogs and cane toads
in Queensland are dying from new diseases. We applaud the decision of
the Northern Territory government, however, who has put money into a trapping
program which is already clearing areas of toads without genetic engineering
or high-risk outcomes or ten to twenty year development times.
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!
Last updated: Oct 4th, 2005
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