'Cold Feet' May Halt Cane Toad March in Australia

By Elizabeth Mitchell, 8/30/08
Science reporter, BBC News

The relentless cane toad invasion of Australia may grind to a halt once it becomes too cold for them to jump.

Scientists staged a 2m sprint event in their own laboratory "toad Olympics".

Toads from the frontline of the invasion could only hop at 0.3 km per hour at 15C, but as fast as 2km per hour at 30C, Ecography journal reports.

Areas of southern Australia that experience cooler and drier climates - such as Melbourne - may escape the invasion, they say.

Since their introduction in 1935, cane toads - Bufo marinus - have advanced across Australia - invading large cities and World Heritage areas.

Originally introduced to north-eastern Australia as pest control agents, these warty and poisonous amphibians are explosive breeders.

They have now overstayed their welcome as they pose a threat to the country's native wildlife.

So, what will be the final destination of the toad invasion?

The results of the laboratory race - where the scientists measured the speed of the toads over 2m, at five different temperatures - may have provided the answer to this question.

Progress predictions

Previous studies predicted that Melbourne and other southern regions of Australia will eventually face the toad invasion.

However, Dr Kearney from the department of zoology at the University of Melbourne told BBC News: "The toads have made it to Darwin where they are doing well. They have had trouble establishing in Sydney despite lots of toads finding their way there. But our predictions make us feel quite safe from toads in Melbourne."

Dr Kearney and his colleagues from Australia and the US used a unique approach - linking the climate to what the toads need for survival at the different stages of their life cycle:

"We considered the ability of adult toads to move around on the surface, and the potential for the toad tadpoles to develop successfully," explained Dr Kearney.

The research team used their own computer software to determine the current climatic conditions across Australia - as well as to predict the impact of future global warming.

They calculated that the toads reached "hopping speeds" equivalent to more than 50km per year in warm and wet Darwin.

But in southern Australia, "it simply becomes too cold for (them) to move around", explained Dr Kearney.

"The cane toads cannot survive in much of southern Australia because they would be too cold to move about and forage or spawn," said Dr Kearney.

The success of the tadpoles depends on the availability and duration of ponds, and the temperature of the water.

The availability of breeding sites may limit the spread of the toads into the interior of Australia - "although permanent water sources like those in the beautiful West MacDonnell Ranges and the Pilbara are of concern," said Dr Kearney.

He added: "The toads are also likely to be limited by dehydration as they move inland."

The scientists suggest that their approach could be applied to study the effect of climate on other organisms:

"We are currently using it to study butterflies, disease-causing mosquitoes and possums," commented Dr Kearney.