Cold Feet Curb the March of the Cane Toad
[ The University of Melbourne Voice Vol. 3, No. 7
8 September - 13 October 2008 ] By Nerissa Hannink
Cane toads weren’t allowed to compete in the Olympics, but scientists have raced cane toads in the laboratory and calculated that they would not be able to invade Melbourne, Adelaide or Hobart and are unlikely to do well in Perth or Sydney, even with climate change.
According to research by University of Melbourne zoologist Dr Michael Kearney and collaborators from Australia and the USA, the cane toad’s march will grind to a halt once it is physically too cold for the toads to hop. The research has been published in the current edition of Ecography Journal.
“The cane toads cannot survive in much of Southern Australia because they would be too cold to move about and forage or spawn,” says Dr Kearney.
The study is unique in that it is based on an understanding of the capabilities of the toad itself whereas many other studies – some predicting that Melbourne would be invaded by the toads – are based on correlations between climate and the places the toads are living in now, which can lead to errors.
Since their introduction to Australia in the 1930s, cane toads have been steadily advancing across Australia and have already invaded Brisbane and Darwin.
Once used as pest control in Queensland canefields, the toads are now themselves a devastating pest so an accurate prediction of their final range and rate of movement is essential.
If there were a cane toad Olympics, all eyes would be on the weather: because they are cold-blooded, the toad’s ability to move depends on its body temperature, which fluctuates with its environment.
Dr Kearney and his colleagues, including Dr. Ben Phillips from the University of Sydney and Dr. Chris Tracy from Charles Darwin University, set up a 2m sprint event for toads at a range of different temperatures to see which temperatures would slow toads down the most.
The team used toads collected in the field from four populations across the invasion front.
“We found that cane toads can barely hop once they get below about 15 degrees Celsius”, said Dr. Tracy.
“Their range would also be constrained by the limited availability of water for their tadpoles in some parts of Australia”.
After racing their toads, Kearney and his colleagues used sophisticated computer models developed by Dr Warren Porter at the University of Wisconsin, Madison USA, to predict how cold toads would get at different times of the year across Australia.
They found that it is so warm and wet around Darwin that toads there can hop at the rate of more than 50km per year. However, the cooler, drier conditions around Sydney or Perth mean that toads can barely manage 1 km per year. And they couldn’t move at all under typical weather conditions in Adelaide, Melbourne and Hobart.
They found that moderate global warming by 2050 could allow toads to move 100 km further south than their present limit. This would make conditions in Sydney slightly better for toads, and the only other city at risk of toad invasion under this scenario would be Perth.
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