Killing off the cane toad
Pouring chemicals on their backs, hitting them with golf clubs, running them over, freezing them.
Australians have come up with many ways to kill cane toads, and although they may work in knocking off individual toads, the species as a whole has continued its seemingly inexorable spread throughout Australia.
But a Queensland researcher is working on the opposite approach: exterminating the species as a whole without having to kill a single cane toad.
Professor Peter Koopman, from The University of Queensland’s Institute for Molecular Bioscience, is developing a strain of “daughterless” cane toads.
As the name suggests, these are toads that can give birth only to sons.
“The simplest way of explaining it is that we hope to transplant a ‘gene’ into toads that will cause any female tadpoles they produce to change course and become males,” Professor Koopman said.
“All of the offspring of this genetically engineered toad would thus be male, and they would all be carrying the daughterless gene.”
As these males themselves begin to breed, their offspring will be limited to sons, all of whom will again have the daughterless gene.
Each new generation of toads will contain more and more males who cannot produce female offspring, and as the number of females in the population dwindles, so too will the number of toads in the next generation.
“It’s probably the greenest and safest solution to manage the cane toad problem – it doesn’t involve any toxins or pathogens, so the toads cannot develop immunity against it, and there is no risk to native frog species that we want to protect,” Professor Koopman said.
Eliminating the cane toad without adversely affecting other species is vital, as the toads themselves have had such an impact on other species of animals in Australia, especially native animals.
Their threat to native animals is twofold: not only do they eat the same food as Australian frogs, with their size, quicker breeding cycle and superiority in numbers ensuring that they come out on top in the struggle for sustenance, but the toxins they produce poison any would-be predators.
Ironically, the cane toad was itself introduced to stop a pest animal – the cane beetle. In the early 20th century these beetles were attacking sugar cane crops and in 1935 it was decided to import 102 cane toads from Hawaii to eat the cane beetles. After the toads were brought to North Queensland, they were allowed to breed with each other, and eventually thousands were released.
Even more ironically, it turned out that the cane toad couldn’t actually eat the cane beetle. The beetles lived in the tops of the sugar cane stalks and the toads couldn’t jump high enough to reach them. Farmers turned to chemical means to control cane beetles, but the supposed solution was now becoming the problem, as the cane toads went forth and multiplied.
And multiplied some more.
There are now estimated to be 200 million cane toads in Australia, in a range that covers Queensland, the Northern Territory and New South Wales.
Unfortunately for Western Australia, the toads are closing in on its border, and the latest development in controlling them will not come in time to stop the warty amphibians from entering the country’s largest state.
“Obviously, the more funding we receive the quicker the problem will be solved, but it’s not going to happen overnight,” Professor Koopman said.
“Toads have a seven-year life cycle and it will take a number of life cycles for the daughterless gene to spread throughout the population.”
The process of spreading this gene will have two stages.
The first stage is the laboratory phase, where he and his team must genetically engineer a cane toad that carries the daughterless gene.
The second stage involves extensive consultation with biologists, ecologists, park rangers and other experts to determine when and where the genetically engineered toads should be released, and how many to release.
Although it is the first stage that most people would assume would be the more difficult, Professor Koopman does not view it that way.
“We have a clear idea from our research at what molecular point in development you can change sex, and we will apply that knowledge to cane toads to create a ‘switch’ that flicks females back to males,” he said.
This isn’t a challenge that sounds insurmountable to Professor Koopman and his team, who have spent the last 15 years studying what genes and cells are required for males and females to develop in mammalian embryos. This knowledge has been enormously valuable in understanding how defects in sexual development arise in human babies, but is now being applied to protecting the Australian environment by dealing with the cane toad problem.
This is not the first time Professor Koopman has been involved in answering one of the great questions of science. In the early 1990s he was part of the team that discovered the gene that determines gender in humans; now he is planning to use his accumulated knowledge to manipulate gender in toads.
Eventually, it is hoped that the lack of females in the cane toad population will cause the species to be bred out of existence, with the last generation of males hopping forlornly through the bush in hopes of finding a mate.
“There will be a lot of lonely hearts in the cane toad world, but the nice thing about this solution is that no cane toad actually has to die,” Professor Koopman said.
Inventing new ways of killing cane toads may not be an Australian pastime for much longer.