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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

The cane toad has been a scourge to Australian wildlife for decades. An invasive species, the cane toad competes with local endemic frog species and due to its high toxicity kills any predator who preys on it, including snakes, raptors, lizards, and the carnivorous marsupial, northern quoll. New research has uncovered another victim of the toad. The freshwater crocodile has suffered massive population declines due to consuming the irascible toad.

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.

Before the study, evidence for cane toads affecting freshwater crocodile population size was anecdotal. After an invasion of cane toads, naturalists, rangers, and tourists often reported seeing freshwater crocodile corpses. Corpses of crocodiles are usually rare (one river recorded only 3 dead crocodiles found in 20 years), but even stranger was the corpses found after cane toad invasion did not show any of the characteristic marks—i.e. puncture wounds on the head—which are caused by the crocodile's only natural enemy, itself. Freshwater crocodiles will often kill each other in battles. The finding of unmarked corpses lead to the theory—proved by this study—that the crocodiles had taken to eating cane toads and were subsequently killed by the toad's high toxicity.

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

One hundred and two cane toads were first introduced into Australia in 1935 from Hawaii, where they were also an invasive species from South America. They were brought to Australia to eat the cane beetle, which was itself devouring sugar cane crops. However in a twist worthy of a horror film, the cane toads could not jump high enough to reach the cane beetles and instead began to eat everything else. Today there is an estimated 200 million cane toads in north-western Australia, each with a voracious appetite.

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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