The researchers noted a horrifying 75 per cent reduction in the numbers of freshwater crocodiles in one river system.
Cane toads are highly poisonous, and as well as taking over the habitats of native reptiles, they generally poison any bird or animal that eats them.
The crocodiles, a top-level predator, have been attempting to eat the cane toads, but so lethal are the toads, that the predator becomes an unwitting prey.
Dr Mike Letnic’s team from the University of Sydney’s School of Biological Science found a 75 per cent reduction in the numbers of freshwater crocodiles in the Victoria River District in the northwest of the Northern Territory following the toads’ recent arrival in the river system.
A Deadly Invader
Cane toads were introduced into the sugar cane growing state of Queensland in 1935, to combat the cane beetle, a pest of sugar cane crops.
However, as the toad is not native to Australia, it has no predators, and has since spread across most of Queensland and almost entirely across the Northern Territory, including the world-renowned wetlands of Kakadu which it reached in 2006.
Dr Letnic has been conducting crocodile research since 2003 and started the current project in 2005.
He surveys the crocodiles for two weeks each year, and was horrified to see such a huge reduction in numbers since his previous survey 12 months ago.
“During surveys of the Victoria River in the semi-arid tropical region of the Northern Territory, we documented massive mortality of freshwater crocodiles at the toad invasion front,” said Dr Letnic.
His team found that population densities of crocodiles plummeted as much as 75 per cent in some areas following toad invasion.
Crocodile Numbers Crash
“Crocodile numbers crashed. They dropped from 692 in the Victoria River District pre-toad to 380 post-toad,” he said.
“In 2007 we found 34 dead crocs in just a week of surveys. The dead crocs would only be observable for a few weeks, so if this level of mortality continued, the number would be horrific.”
So lethal is the cane toad poison that some of the dead crocs were found to have undigested toads in their stomachs.
Dr Letnic said it was unclear what the follow-on effects would be of such a mass destruction of a top-level predator.
“The impact of invasive species is particularly important as the crocodiles’ removal could generate a substantial cascade of secondary effects in the local environment.
“At the least, we would expect an increase in the abundance of species normally preyed upon by crocodiles.”
For more on Cane Toads, see Radical Ways to Control Cane Toads and Cane Toads Threatened by Predatory Meat Ants
For more on Australian Crocodiles, see Crocodiles are Marathon Swimmers
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