Pit a cane toad against a freshwater crocodile and who wins? Although the croc eats the oversized amphibian, it seems the toad has the final laugh.
Dead freshwater crocodiles in Australia's Northern Territory were once a rare sight. But since 2005, locals have witnessed mass die-offs. Researchers now say the toxic and invasive cane toad (Bufo marinus) is to blame.
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. The toads secrete a milky-white toxin which is lethal to many predators from glands behind their eyes and on their backs.
Mike Letnic of the University of Sydney and his team say a massive 77% of some populations of freshwater crocodiles – or "freshies" – have died since 2005.
The numbers are particularly worrying, says Letnic, because removing top predators like freshwater crocodiles (Crocodylus johnstoni) can boost the number of their prey and trigger a cascade of ecosystem changes that are difficult to predict.
Pest out of control
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: 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," says Grahame Webb, director of Wildlife Management International in Darwin.
"It was a disaster waiting to happen. If it had been whales or some species with big brown eyes every one would have been up in arms," he adds.
Wave of death
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 Letnic says the "wave of death" has moved upstream with the toads, strongly suggesting the toads are the cause of the dropping crocodile numbers.
Letnic's team is continuing the surveys. They say the freshies and cane toads are often seen in close proximity to each other. This is likely to be all the more true in dryer regions like the semi-arid upper reaches of the Victoria River where the two species are forced to share water holes, says Letnic.
If true, cane toads could pose an even greater threat to native species as they move south into the dry interior of Australia and the need for water brings them into close proximity.
The researchers say in the long term, the high death rate may naturally select for crocodiles that have a higher tolerance to the toad toxin. This has been seen to happen in some blacksnake populations that have also been hit hard by the cane toads.
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.