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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Professor Grigg Written Submissions Cane Toad Inquiry Report Volume 2 96 indicates when we think toads reached each station, based largely on observations of toads on the roads. Our stations record toads, and mostly back up the information from the roads, but a software problem meant that there were some false positives in the first season. Figure 1 shows that some of the declines in species calling occurred before we detected toads as being present. Another problem is that the sampling effort differed between seasons and stations (see table 1), because of equipment failures. A further consideration when considering sampling effort is that frogs do not call at all in dry conditions, and thus the number of days of recordings may not be an accurate representation of the true sampling effort. Linked to this are differences between wet seasons, both in terms of total rainfall and number of rain-days. The data suggest that toads may well have a detrimental effect on frogs, but because of confounding variables and gaps in the data, combined with the short period before toads arrived, we cannot be sure. But these are very provocative results. We certainly cannot say there is no effect. The weight of our evidence is that, during the five years of our study, there has been a decrease in frog calling activity at our sites (both in terms of species present and days each species calls). The Kakadu data will be very important, as they will provide an independent replicate study, and we will have at least three years pre-toad data at each site. When we look at individual species of frogs, there have been statistically significant and substantial declines in the number of records for at least 7 of the species we have been monitoring, with possible slight increases in 2. For the remainder, 5 show no clear evidence of a trend, and in 5, there is so little data we can say nothing. As can be seen in the number of calls for some of the species (figures 2-5), there is a lot of variation between stations and wet seasons, so we need to be cautious at this stage about blaming cane toads for these changes. Figure 1: Number of frog species calling at each station Stations are ordered in distance pairs from East to West (top to bottom). The shading indicates wet seasons when cane toads were known to have reached each site. The number of recording days varied from 62 to 148 days, as is shown in the table below the figure.