Sessional Committee on Environment and Sustainable Development Written Submissions Received Volume 2 Issues associated with the progressive entry into the Northern Territory of Cane Toads October 2003
Tabled Paper 1123
Tabled Papers for 9th Assembly 2001 - 2005; Tabled papers for 9th Assembly 2001 - 2005; Tabled papers; ParliamentNT
Tabled by Delia Lawrie
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Dr Brown Written Submissions Cane Toad Inquiry Report Volume 2 146 In addition to the populations of reptiles which we study intensively, marking and measuring each individual, we have, over the last five years, conducted nightly surveys of frogs and other snake species. We go out each night at the same time and follow the same route and count what we see. Simply counting animals is not as robust a method of monitoring populations as marking them in some way (because you can't tell if you are counting the same individual on different nights). Nonetheless, because we have been carrying out these standardised surveys for so long, they will allow to detect and measure population declines of a wide range of species. Although cane toads have spread through numerous Australian communities, there is no detailed information on how they effect populations of animals that are likely to try to eat them. Most commonly, anecdotal reports describe long or short-term reductions in snake and lizard species soon after the arrival of cane toads. We can look in Queensland today and see there are still snakes and lizards. But because there is no information on reptile population sizes or species diversity before the toads arrived we cannot assess the toads impact other than to conclude that they did not kill everything. Now, we have the opportunity to see what happens to well-studied populations of predators subsequent to the arrival of cane toads. It offers a unique opportunity to study their impact in detail. As we wait for the arrival of cane toads at Fogg dam we continue our studies and surveys. In the meantime, we are also carrying out studies on captive animals to allow us to make an initial assessment of what might happen when toads arrive. First, we want to determine which species of snakes will try to eat cane toads when they see them. Second, we want to determine how badly each species is affected by toad toxin. The preliminary results of these studies are not encouraging for reptiles. We found that most snakes attempted to eat cane toads that were placed in their cage. Furthermore all the lizards, snakes and turtles we tested were all badly effected by toad toxin and individuals are likely to die if they swallow a toad or, in some cases, even if they bite one. There are two exceptions to this. Keelback snakes far more resistant to cane toad toxin than any other Australian snake. Slatey-grey snakes are less resistant than keelbacks still much more tolerant of toad toxin than any other species. Taken together, these preliminary results lead us to expect that most of the local populations of predatory reptiles are at risk from cane toads. In some cases, its not clear how the results of these lab studies will translate to the wild. For instance, common tree snakes specialise in eating frogs and they will readily eat cane toads. They are also extremely sensitive to toad toxin. However, as their name implies, they spend most of their time in trees where they are unlikely to encounter toads. Thus, their habitat preference may mitigate their other susceptibilities to toads. Even species that are not directly harmed by cane toads may be detrimentally affected by their presence. Native frogs and small lizards for instance, may not be able to compete with large numbers of cane toads for food or limited shelter sites. It is unknown how cane toads will impact other species through higher order interactions such as these. In addition to studying the effect of cane toads on reptile populations, we will also study the toads themselves when they arrive. Coastal floodplain habitat such as at Fogg dam should provide excellent conditions for toads and we want to monitor how rapidly their population builds, how often they are able to reproduce and what they are feeding on. We also plan to radio track a large number of toads to monitor what habitats they prefer
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