Cane toads killing Australian crocs
SYDNEY: Turning the tables on their predators, invasive cane toads are making a significant dent in northern Australia’s freshwater crocodile population, a new survey reveals.
“While there’s always talk about cane toads having an impact on Australian wildlife, in many cases what we tend to lack is the smoking gun type of evidence,” said biologist Michael Letnic, leader of the team behind the discovery at the University of Sydney in New South Wales. “But now, in our study, we may have found that smoking gun.”
Smoking gun
While the toads (Bufo marinus) are believed to be toxic to a variety of native species, including quolls, goannas and snakes, Letnic’s research – soon to be published in the journal Biological Conservation – is among the first to link the toads to both specific deaths and a population decline, he said.
Letnic’s team began studying freshwater crocodiles (Crocodylus johnstoni) two years ago in the Victoria River district of the Northern Territory, before cane toads invaded the region.
“When we first went in we counted about 700 crocodiles [in one area] and found no dead ones at all,” Letnic said. “But then we went back… after the cane toads had started moving through. This time we found less than 400 crocodiles… and, in the space of a week, we found over 30 dead.”
“On the whole there was about a 70 per cent reduction in population numbers, and the only thing that had changed was that the cane toads had arrived… they were killing the crocodiles,” he said.
Dissecting the dead crocs, the researchers found several with toads in their stomachs, suggesting that the poison-gland equipped amphibians were one of the last meals they ever made.
Bouncing back
The apparently devastating effect of the cane toad on a top predator such as the freshwater crocodile is disturbing, commented Ross Alford, a biologist from James Cook University in Townsville, Queensland.
“In many ecological systems, the top predators are very important in keeping the system in balance,” he said. A decline in crocodile numbers could cause a knock-on population boom in the species they would usually prey upon or compete with. In turn this could lead to an unchecked exploitation of resources by the newly abundant creatures, throwing the ecosystem out of sync, Alford said.
By understanding the toad’s impact, however, “we can know where to focus control efforts to reduce the most serious effects," he said.
Letnic's team plans to continue monitoring the freshwater crocs and their amphibian nemesis.
“It’s likely we’ll actually see a rapid evolution of the crocodiles. The ones that like to eat the toads… [but] don’t have any resistance to their poison will die and not pass on their genes," said Letnic. "The ones that maybe have some resistance will survive [and allow] the population to eventually bounce back.”
http://www.cosmosmagazine.com/news/2037/cane-toads-killing-australian-crocs
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