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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Written Submissions Dr Freeland Volume 2 Cane Toad Inquiry Report 159 conducted on behalf of Parks Australia. The declines reported to date can not in any sense be attributed to cane toads or any other variable, and do not allow for rejection of the null hypothesis that cane toads have no impact on frog communities. Hopefully future results will be more revealing. The above difficulties make interpretation survey/monitoring data extremely difficult and uncertain. For example Catling et al., did not find the gecko Gehyra nana (it probably never in recent times existed in the area), whereas Watson and Woinarski (2003) found a negative impact. Both these studies reported negative impacts on Gilbert's Dragon, yet the species can be frequently observed to be common in areas of the Gulf Country that have had cane toads for 10 years (Freeland personal observation). Catling et al., (1999) found impacts on the frogs Litoria rubella and L rothii whereas Watson and Woinarski (2003) found none. Nor did Freeland and Kerin (1988) find impacts on these species in their detailed niche analyses and experimental population reductions. Watson and Woinarski (2003) found a weak decline in the dragon Diporiphora bilineata, Catling et al., (1999) did not find it at all yet there are records of its persistence nine years post cane toad invasion (Freeland personal observation). For most of van Dam et al's (2002) species at risk, either one, the other or both studies did not find the species (often because of the species' distributions) or there were insufficient records to allow analysis. These types of results do not allow for a clear renunciation of the null hypotheses that cane toads have no impact on the species concerned. A risk averse conclusion might be to suggest that more detailed work needs (i.e. needed to disprove the null hypothesis) to be conducted on species observed to exhibit consistent patterns of decline in the cane toad's presence. Watson and Woinarski (2003) produced data demonstrating that the Northern Quoll disappeared from the sampled quadrates following invasion by the cane toad. These data are clear and unequivocal circumstantial evidence of cane toad impact. When supported by the results of work demonstrating death of Quolls cane toads in Kakadu, it is clear that the null hypothesis can be rejected. The demonstrated disappearance or presence of a species post-invasion by cane toads is unequivocal. If this measure is used then the Northern Quoll is the only terrestrial vertebrate known (in all of Queensland and the invaded portion of the Northern Territory) that may be unable to persist in the presence of cane toads. Establishing more surveys or community monitoring styles of researches in addition to those already established seems unlikely to add greatly to our knowledge of cane toad impacts. In terms of management these studies have offered the opportunity to knowledgeably undertake the translocation of Northern Quolls to islands, and to keep the pubic informed on the known real impacts. The value of these short term studies needs to be appreciated when dealing with particular species (e.g. local endemics, rare, threatened) thought possibly to be at genuine risk and not amenable to the gross survey technique. If conducted appropriately intensive monitoring of individual populations can provide evidence of probable cane toad impacts. Spot-light surveys were established to monitor freshwater crocodiles in the Elsey National Park and Nitmiluk National Park prior to the arrival of the cane toad. In both cases individual crocodiles were known to have died, presumably because of cane toad ingestion (such frequent death had not been seen before). The pre and post invasion data were analysed but no downward trend in the population could be detected. This was interpreted as the survey method producing data with an inherently wide variability precluding detection of any trend. A similar situation occurred in the Parks & Wildlife Commission's aerial monitoring of saltwater crocodiles. On detailed analysis the data produced from helicopter surveys proved
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