Archives of The Cold Blooded News

The Newsletter of the Colorado Herpetological Society

Volume 32, Number 4;   April, 2005

Cane-Toad Invasion Killing off Northern Territory's Goannas (Yellow-Spotted Monitor)

The University of Canberra ecologist Dr. Sean Doody thinks that the recent spread of cane toads into the Northern Territory has caused a massive decline in a goanna species found along the Daly River, southwest of Darwin. He said, "Just in the last year, there's been a 77 percent decline in yellow-spotted monitors." During a recent research trip, he found populations of the big yellow- spotted reptiles (also known in some areas of Australia as sand goannas) had dropped noticeably, particularly along riverbanks where cane toads were numerous. He also found a number of dead goannas." Goannas were still dying when we left, so the figure could go even higher," he said. "They often eat frogs, so they see these big toads hopping along the riverbank, run them down, eat them and die from the poison."

Dr. Doody had been studying pig-nosed turtles in the Daly River, and was able to confirm the goanna decline from a corresponding drop in turtle-egg predation. "We count the eggs, and the goannas normally take up to 20 per cent. That's dropped to zero this year."

He also found a cane toad in the stomach of a dead freshwater crocodile.

"It's too early to speculate on how crocodiles might be affected. We're only working with preliminary results, but it certainly doesn't look good for the goannas. The toads haven't bred to the stage where there will be lots of little toads along the water's edge. That will take another year or two, and then we could see similar impacts on other species." Two other Daly River goannas, Mitchell's and Merton's water monitors, feed on frogs found along the water's edge and could be affected once the cane toads increased.

CSIRO Sustainable Ecosystems is working on a biological-control method that involves splicing a gene to block metamorphosis, preventing tadpoles turning into cane toads.

Meanwhile, the Northern Territory Government has launched a national competition to invent a cane-toad trap. The competition has two categories - a winning trap and a fail-proof method for attracting toads. The winner would receive $10,000 and an additional $5000 to market the toad trap. Dr. Doody dismissed the competition as "a complete waste of time". Northern Territory Parks staff had also voiced doubts that traps would solve the cane-toad crisis.

However, Brisbane's crows may prove to be a more effective deterrent. Dr. Doody said the wily birds appeared to have developed a method of killing and eating toads without exposing themselves to the deadly toxins. "We're not sure yet how them kill them but they seem to somehow turn the toad inside out to eat it."