Assessment of the status of threatened herpetofauna following fire in sub alpine habitat at Lake Mountain and Mount Bullfight, near Marysville, north-east Victoria

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4.1 Frog surveys

The Common Froglet was the only amphibian detected at Lake Mountain during the 2009/2010 surveys. Common Froglets were present at all 13 surveyed water bodies, with tadpoles, metamorphs and adults detected (Howard et al. 2010). Higher numbers were recorded in 2010/2011, with larger choruses of calling adults as well as hundreds of metamorphs observed. As there is no pre-fire baseline data, it is impossible to state how abundant the froglets were before the fires. The increase in numbers post fire may be a result of the increased vegetation cover, more favourable conditions in the 2010/2011 season, or simply a return to pre-fire abundance after last summer’s breeding season.

Alpine Tree Frogs were not detected at Lake Mountain in both survey years, and it is unlikely that they still occur on this plateau. The species has not been recorded at Lake Mountain for nearly 20 years (Clemann 2002), and it is unlikely that they were present during the 2009 fires. Litoria ewingii-complex frogs were not detected during the first season’s frog surveys for this project at Lake Mountain. Either this species has persisted in the area in low numbers and was not previously detected, has recently moved back into the area, or has recently been re-introduced. Construction work is being undertaken on the summit of Lake Mountain, and the arrival of construction materials

is a plausible introduction vector for frogs.

During the first season of this project, a large population of Alpine Tree Frogs was found at the lower bog at Mount Bullfight NCR. The ability for Alpine Tree Frogs to persist after fire in severely and recently burnt areas has been recently observed in the Dargo – White Timber Spur fire zone (Clemann et al. 2010). Breeding populations of Alpine Tree Frogs have been observed in that area during surveys in 2009/2010 and 2010/2011 (Clemann et al. 2010, Howard et al. in prep.). It is likely that the persistence of Alpine Tree Frogs at Mount Bullfight NCR was facilitated by the tracts of unburnt vegetation in and around the bog system (Howard et al. 2010).

Surveying the summit of Mount Bullfight at night was not possible, therefore we did not obtain a measure of the relative abundance of Alpine Tree Frogs in that area. Camping or deploying timed call recorders on the summit, that record calling frogs during the evenings, would permit a measure of calling intensity in this area.

The Litoria ewingii-complex of frogs comprises, amongst others, Verreaux’s Tree Frog, the Alpine Tree Frog, the Southern Brown Tree Frog and the Plains Brown Tree Frog (Barker et al. 1995). The identification of these taxa is often not certain due to similarities both morphologically and in their advertisement call structure. For example, the Plains Tree Frog and the Southern Brown Tree Frog are often indistinguishable morphologically, and molecular analysis is needed for species’ identification (Barker et al. 1995). Similarly, the two subspecies of Litoria verreauxii are difficult to differentiate using morphological and colour pattern characteristics. Although it has been suggested that the Alpine Tree Frog may be larger and more colourful than Verreaux’s Tree Frog (Barker et al. 1995), this is not a conclusive method for identification. Similarly, the advertisement calls are difficult to distinguish in the field. Taxa within the Litoria ewingii-complex of frogs can also hybridise where their distributions overlap, creating further uncertainty (Watson and Littlejohn 1978, Watson et al. 1985). Molecular tests and/or advertising call analysis may be necessary to accurately identify all taxa at both Lake Mountain and Mount Bullfight NCR.

The relative abundance (based on number of frogs and tadpoles observed, and size of calling choruses) of Common Froglets at the Mount Bullfight NCR lower bog was considerably less than at Lake Mountain. High abundances of Common Froglets in areas where Alpine Tree Frogs have disappeared are common (Hunter et al. 2008, Clemann et al. 2009). Conversely, in areas where Alpine Tree Frogs are present, the Common Froglet is often either absent, absent from specific waterbodies containing Alpine Tree Frogs, or occurs at lower densities than areas where Alpine Tree Frogs have been extirpated (D. Hunter pers. comm., N. Clemann pers. obs.). This relationship is evident when comparing Common Froglets at Mount Bullfight NCR and Lake Mountain. Populations of Common Froglets at Lake Mountain, where Alpine Tree Frogs are absent, occur in much higher abundances than those at Mount Bullfight NCR (based on estimation of the number of calling males). Due to its persistence in highly infected areas, the Common Froglet may be a vector of, or have a higher immunity to the fungus, and is known to be a reservoir host for the fungus (Hunter et al. 2008, Clemann et al. 2009).

The results of our sampling for Amphibian Chytrid Fungus again demonstrate that the fungus is widespread amongst Common Froglets at Lake Mountain, and probably absent from Mount Bullfight NCR. Chytrid infection at Lake Mountain is very high, whilst the prevalence of chytrid infection at Mt Bullfight NCR is likely to be less than 8 % (Figure 18). In addition to our failure to detect the fungus over two years, the greater diversity of frogs, and the co-occurrence of healthy populations of Alpine Tree Frogs and Common Froglets at Mount Bullfight NCR suggests that the site is most likely free of the fungus at the time of last sampling. However, there remains a small chance that the fungus was present but not detected at Mount Bullfight NCR.

In addition to the Alpine Tree Frog, there are historic records of the Victorian Smooth Froglet and Southern Toadlet (Pseudophryne semimarmorata) at Lake Mountain. Neither of these species were recorded during the last two season’s surveys at Lake Mountain (not surprisingly in the case of the Southern Toadlets, as their breeding season extends from March to April). The Victorian Smooth Froglet has been recorded at Mount Bullfight NCR in both survey seasons (once at both the lower and upper bog). Failure to detect both Alpine Tree Frogs and Victorian Smooth Froglets at Lake Mountain, where fungal infection rates are high, compared to the detection of good numbers of both taxa at Mount Bullfight NCR, where Amphibian Chytrid Fungus is yet to be detected, may explain this localised pattern of apparent decline at Lake Mountain.

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