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RESEARCH ARTICLE
Contents Vol 37(1)

Population age structure of the spotted tree frog (Litoria spenceri): insights into population declines

Graeme Gillespie
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Melbourne Zoo, PO Box 74, Parkville, Vic. 3052, Australia. Email: ggillespie@zoo.org.au

Wildlife Research 37(1) 19-26 https://doi.org/10.1071/WR08178
Submitted: 20 December 2008  Accepted: 4 December 2009   Published: 1 March 2010

Abstract

Context. Despite increased scientific attention on amphibian conservation in recent years, knowledge of population demography of amphibians remains scarce, hampering evaluation of population declines and development of appropriate management responses.

Aims. The aims of this research were to examine variation in population demography of the spotted tree frog (Litoria spenceri), a critically endangered species in Australia, and to evaluate the role of various factors potentially responsible for population declines such as introduced trout, chytridiomycosis and habitat changes.

Methods. Skeletochronology combined with mark–recapture sampling were undertaken in two different river systems, Bogong Creek and Taponga River, to determine population age structure. Age-specific survival estimates were derived from each population and were then used to examine variance in age-specific mortality.

Key results. Relative population density per 200 m of stream was 67.7 adults and 131.3 juveniles at Bogong Creek and 10.7 adults and 33.8 juveniles at Taponga River. Ages were determined for 578 frogs across the two populations. Age-specific survival was lowest in the first year of life compared to all other age classes, and highest in sexually mature adults. Differences in age-specific survival were similar between the populations, with the exception of first-year survivorship, which averaged 1.9% at Bogong Creek and 0.4% at Taponga River. This difference was large enough to explain most of the marked difference in population density between the two streams.

Key conclusions. The difference in first-year age-specific survival is consistent with trout predation as the most parsimonious explanation for the large differences in population density between the populations, and lends further weight to the role of introduced trout in the decline of this species.

Implications. This study has contributed to informing management actions for conservation of this species, and demonstrates that population age structure data may provide valuable insights into demographic variability within and between populations and species. This may have important implications for interpretation of population declines, and conservation and management responses.


Acknowledgements

Funding was provided by the Department of Environment, Heritage, Water and the Arts, Australia, and the Department of Sustainability and Environment, Victoria. The research was carried out under research permits RP 94–063, RP 95–120 and RP 96–187 from the Department of Sustainability and Environment, Victoria, and A1350 from NSW National Parks & Wildlife Service, and Animal Experimental Ethics Committee approval nos. 115/94 and 96/002 (AEC, Department of Sustainability and Environment, Victoria). Assistance with data collection was provided by G. Hollis, D. Hunter, S. Fickling and M. Scroggie. Technical advice and assistance with histology was provided by B. Abaloz and M. Scroggie.


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