The catastrophic impacts of cane toads on freshwater crocodiles and efforts to stop toads spreading in northern Australia
Professor Mike Letnic
The invasion of Australia by cane toads is an ecological disaster that is still playing out. Cane toads were introduced to north Queensland in 1935 and have been slowly expanding their range across Australia ever since. In 2005, cane toads were approaching the Top End of the Northern Territory. At this time, there was concern about their impacts on predators because toads contain toxins that Australian predators have little physiological resistance against and possess no innate aversion towards toads. However, in 2005 there were only anecdotal observations that some predators died after eating cane toads and no hard scientific data linking the invasion of cane toads to the decline of native species.
To examine the impacts of cane toads on freshwater crocodiles my collaborators and I conducted surveys of crocodiles along the Victoria River in 2005 before cane toads invaded and have been conducting surveys for up to 20 years following the invasion of toads. Following toad-invasion, freshwater crocodiles died enmasse and their numbers declined by as much as 75%. Crocodile populations have shown no sign of recovery in the 20 years since toads invaded.
Frustrated by the catastrophic impacts that toads have had on freshwater crocodiles and other predators, particularly goannas and snakes, we investigated avenues to control cane toad populations and their impacts. In the vast semi-arid plains of the northern Australia, toads can only survive the dry season if they have ready access to water. Cane toads have invaded the semi-arid plains by using water-points provided for cattle as invasion hubs where they shelter in the dry season. During the wet season, the cane toads are no longer tied to dams and can disperse across the landscape. By installing small fences around dams, we showed that we could eradicate cane toads from water-points. Using modelling, we showed that if toads were strategically excluded from water, that it would be possible to halt the invasion of toads by establishing waterless tracts of land that could function as “toad-breaks”. The idea being that if water-points were spaced a sufficient distance apart so that toads which attempted to cross the “toad-break” during the wet season could not locate water by the beginning of the dry season they would perish.
The toad invasion front is currently approaching Broome in Western Australia. At this point in time, 16 years after our research on toad invasion hubs was conducted, moves are afoot to establish a toad-break (Toad Containment Zone) to prevent toads from invading the Pilbara. Fingers crossed, the Toad Containment Zone will spare the Pilbara from the ravages of cane toads.
Professor Mike Letnic is an applied ecologist and conservation biologist who has studied Australia’s arid ecosystems from more than 25 years. His primary research goal is to tackle the grand challenge of biodiversity conservation in a world with an ever-expanding human footprint. Mike’s research aims to bridge the gap between theoretical research and management and translate research findings into improved on-ground outcomes for the conservation of fauna, flora and wild landscapes.
