Cane toads are killing crocodiles in Australia
Jeremy Hance, mongabay.com
July 30, 2008
Australia's number one pest, the cane toad, devastates freshwater crocodile populations
Australia's freshwater crocodile is not the brawny reptile of the Crocodile Dundee movies or the ones Steve Irwin wrestled. Those are the larger, more dangerous salt water crocodiles. Freshwater crocodiles, also known as Johnston's Crocodile, are about half the size of their saltwater cousins and do not pose a threat to humans. They prey largely on fish, amphibians, and birds, yet they are the top predators in their habitat.
The study found that in four sites the average drop in freshwater crocodile population was 45 percent after the cane toad arrived. The smallest drop recorded was at Longreach Lagoon, where 15.6 percent of crocodiles died, butt the junction of Victoria and Wickham rivers, the drop was a catastrophic 76.9 percent. Prior to the cane toad invasion, the researchers did not see any dead crocodiles during their population surveys, but post-cane toad invasion they observed 34 crocodile corpses.
Bob Goninon of FrogWatch Australia holds a large cane toad captured near Darwin in Australia's Northern Territory March 26, 2007. Courtesy of FrogWatch, an Australian environmental group that to end the species' reign by capturing and killing all cane toads in the country.
A question remains: other populations of freshwater crocodiles do not appear to be affected by cane toads. Those living downstream in moisture-rich wetlands and tropics have not shown the same population plunge, although cane toads are prevalent. The researchers present a number of theories to explain the discrepancy. First, there is lower density of crocodiles in these wetter areas and they have a wider variety of food choices, perhaps allowing them to avoid cane toads. In addition cane toads, which need to find a water source frequently to avoid desiccation, are able to find sources of water outside of rivers very easily. In the drier region studied cane toads are forced into the crocodile's river to keep from drying out. Therefore, it is likely that the wetter the environment the less often freshwater crocodiles and cane toads meet. The authors believe the crocodiles susceptibility to cane toads in drier regions could have grave implications for the crocodile as Australia experiences long-term drying due to climate change.
Cane toad is its natural habitat in the Amazon rainforest
Numerous strategies have been employed to rid the continent of the toads. The difficulty is how to destroy a population of toads—greater than the human population in Brazil—without harming other species. Mass-poisoning and disease introduction have produced great concerns about Australia's native fauna, especially other amphibians. In place of such measures, people have attempted large-scale toad round-ups, barriers to stop the toad's south-east migration, and simply killing the toad on-sight with whatever is available.
One intriguing idea is currently being developed by scientists at the University of Queensland. Researchers are trying to genetically develop a cane toad which can only produce male offspring. Their male offspring, in turn, would only be able to produce males, and so on. This would slowly create a male-dominated population, incapable of effective procreation. Once developed, these genetically altered toads would be released in the wild populations. It is hoped that such a strategy would rid Australians of the toads without threatening other species.
But until an effective plan can be implemented, Australia's fauna will have to suffer through the toad's continuing expansion.
"Invasive cane toads (Bufo marinus) cause mass mortality of freshwater crocodiles (Crocodylus johnstoni) in tropical Australia". Mike Letnic, Jonathan K. Webb, Richard Shine. Biological Conservation 141 (2008) 1773—1782.
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