Introduction
Successful conservation of threatened species is predicated upon mitigation of primary factors responsible for population declines. Often more than one threatening process is implicated in the decline of a species, and each threatening process may operate independently or in synergy with others. Therefore, in order to identify the factors that are ultimately responsible for the decline of a species, it is essential to differentiate the relative impacts of different processes and their interactions to identify those that are ultimately driving population dynamics. This is essential in order to develop appropriate and successful recovery plans for threatened species that effectively target mitigation of the cause of the decline.
The Spotted Tree Frog (Litoria spenceri) is a critically endangered species found in mountain streams in Victoria, and its conservation is a national priority. Captive breeding and reintroduction have been identified as a priority to assist with the recovery of this species, and this is reflected in the National Recovery Plan for the species and Zoos Victoria's conservation strategy.
The Spotted Tree Frog is one of a large number of amphibian species to have declined in recent decades, both within Australia and around the world. However, in contrast to many other species, empirical data exist that strongly implicate two factors in the observed decline of the Spotted Tree Frog: predation of tadpoles by introduced trout and the emergent disease chytridiomycosis. This disease has been implicated in the decline and extinction of numerous frog species in Australia and globally.
Introduced Rainbow and Brown Trout now occur throughout the range of the Spotted Tree Frog, and prey on their tadpoles. Experimental data and field observations suggest that trout have caused significant population density reduction, range contractions and possibly local extinctions of Spotted Tree Frog populations.
Chytridiomycosis is caused by a chytrid fungus, believed to have originated in Africa, and has been found in populations of Spotted Tree Frogs and sympatric species, such as the Rocky River Frog Litoria lesueuri. Demographic data has implicated chytridiomycosis as the probable cause of extinction of at least one high-density Spotted Tree Frog population in a trout-free stream; however, other low-density populations known to carry the disease have not gone extinct.
While trout predation impacts the tadpole life stage, chytridiomycosis does not kill tadpoles. Rather it is fatal to frogs. Thus these threats operate in markedly different ways, including acting upon different life stages of the species. However, so far it has not been possible to quantify the demographic impact of either of these factors on Spotted Tree Frog populations or on any other amphibian species in the wild. Furthermore, how these factors interact, either with environmental (e.g. climatic) or demographic (e.g. density dependent) factors, is also unknown.
Population models developed from low-density populations of the Spotted Tree Frog have demonstrated that most mortality occurs in the larval and juvenile frog stages of life. This is consistent with either trout predation of tadpoles or mortality due to chytridiomycosis post-metamorphosis, or a combination of the two. Due to low densities and poor detection of larval stages in the wild, to date it has not been possible to partition estimates of mortality levels between pre- and post-metamorphic stages, and therefore differentiate the relative impact of trout predation or chytridiomycosis on survivorship. This information is necessary in order to evaluate the relative importance of these factors in the observed decline of the species.
Furthermore, based on current knowledge of the biology of chytrid fungus and patterns of decline of stream-breeding frogs, infection may be mediated by host density (tadpoles or adults). The existing demographic knowledge of Spotted Tree Frogs, combined with their sedentary nature and persistence at low-densities in the presence of chytrid fungus, means that this species is suitable for investigating the epidemiology of the disease.
Objectives
This research project has the following objectives:
This information will then be used to refine population models to determine the relative importance of these factors in population declines. Ultimately this knowledge will assist in refining conservation strategies for the Spotted Tree Frog and other amphibian species.