London, June 28 : The high mortality rate of dead freshwater crocodiles in Australia's Northern Territory, is a result of the toxic and invasive cane toad, which have become a regular feature in the diet of the crocs.
The toads secrete a milky-white toxin that is lethal to many predators from glands behind their eyes and on their backs.
According to a report in New Scientist, two surveys, in 2005 and 2007, suggested that the mass croc deaths have progressively moved inland from the mouth of Victoria River, at a pace that matches that of the cane toad invasion.
Mike Letnic of the University of Sydney and his team said that a massive 77% of some populations of freshwater crocodiles - or "freshies" - have died since 2005.
"The numbers are particularly worrying, because removing top predators like freshwater crocodiles can boost the number of their prey and trigger a cascade of ecosystem changes that are difficult to predict," said Letnic.
Cane toads were introduced to Queensland in northeast Australia in 1935 to combat the cane beetle, a sugar cane pest, and have been steadily marching westward across the continent since.
They are now considered invasive pests in their own right, as they have decimated populations of Australian monitor lizards and certain species of snakes.
To try and understand the damage the toads are inflicting, Letnic and his colleagues surveyed crocodiles in four regions of the Victoria River in the Northern Territory.
Crocodile sightings in the Victoria River Gorge region, where the invasion began, dropped from 156 to 49 between 2005 and 2007. The toads moved upriver from the gorge, reaching the Longreach Lagoon region in 2007.
There, sightings dropped by 15% compared to 2005.
"We expected this. We first heard reports of dead freshies from helicopter pilots flying over rivers in the Gulf of Carpentaria (east of Victoria River), where cane toad had invaded," said Grahame Webb, director of Wildlife Management International in Darwin.
Proving a causal link between cane toads and crocodile deaths is tricky, in part because crocs rapidly digest amphibians, so traces are rarely found.
But, according to Letnic, the "wave of death" has moved upstream with the toads, strongly suggesting the toads are the cause of the dropping crocodile numbers.
According to researchers, in the long term, the high death rate may naturally select for crocodiles that have a higher tolerance to the toad toxin.
In the meantime, however, the toxin appears more lethal to younger crocs, suggesting that the reproductive rate of the populations could take a big plunge.