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In recent years global declines in amphibian populations have been noted in both tropical and temperate regions. In temperate North America and Europe patterns of decline and loss have been linked with environmental stressors. Declines occurring in fairly pristine areas in tropical parts of South America and Australia are more alarming, as no obvious environmental stress is present to explain them. One hypothesis addressing the causes of these declines has suggested epidemic outbreaks of a disease. One organism currently associated with species declines in the tropics is the fungal disease chytridiomycosis, caused by the organism Batrachocyhtridium dendrobatidis, which acts upon both adults and juveniles in a population. It is not known yet whether this disease kills initially healthy individuals in the wild, or only causes mortality in individuals that are already stressed by environmental factors. The fungal organism B. dendrobatidis has been identified from adults and tadpoles of several frog species in North Queensland, and is currently proposed to be the most common cause of premature death in Australian frogs. The purpose of the proposed research is to understand the ecology of the chytrid fungus and its relationships with host frog species, particularly focussing on species that have experienced threatening population declines.

 

Regular Monitoring:

We are monitoring the status of frog populations at a number of sites in rainforests north of Townsville. Regular mark-recapture studies are being conducted on adult frogs along streams, and we are also monitoring tadpole populations in the same rainforest streams at regular intervals. These data estimate the numbers of frogs at each site on a given day, and provide information about changes in numbers over time. Our regular monitoring at four sites in the Wet Tropics region formed the basis for early concern about declining frog populations in the rainforests of northern Queensland (Richards et al 1993). At one of these sites on the Paluma Range north of Townsville we now have over 6 years' continuous monitoring data. Two species, the Waterfall Frog Litoria nannotis, and the Lace-eyed Treefrog Nyctimystes dayi disappeared from this site in 1990, but a number of other species including the green-eyed treefrog Litoria genimaculata and the Northern Barred River Frog Mixophyes schevilli are still common there in 2001. We also take small tissue samples from each frog to allow us to diagnose whether these animals are infected with chytrids.

We have also initiated studies to determine whether microhylid frogs (Cophixalus ornatus) and pond-breeding treefrogs (Litoria xanthomera) are undergoing population declines. Microhylid frogs in Australia lay large eggs in moist positions on or near the forest floor and their embryos complete development within the egg capsule. They do not use streams for reproduction. Researchers have assumed that these species are secure but no data exhist to corroborate this assumption. Our initial results indicate that they have not undergone population declines.

Our research is also using a range of approaches and techniques to compare the ecology of declining and non-declining frogs. These include:

Chytrid diagnosis:

Tissue-samples will be collected from frogs and tadpoles in the field for chytrid diagnosis. At present, the most accurate way to diagnose chytrid infection is to histologically section the tissue and examine these sections for the fungus spores. This information allows us to determine which species are infected with the chytrid fungus, and to calculate the infection rates of different species and populations.

Fluctuating assymmetry:

We will obtain assymmetry measures of frogs in the field using calipers. There is a general association between levels of assymmetry and stress, and we are hopeing to determine whether frogs show greater assymetry before undergoing population declines.

Radio-tracking:

It is difficult to determine the precise home ranges and habitat requirements of rainforest frogs because they are rarely seen when they are not at breeding sites (the use of which is only a brief phase in an adult frogs' life cycle). We are undertaking a program of radio-tracking to determine the movements and habitat requirements of several rainforest frogs including the Common Nursery-frog Cophixalus ornatus, the Green-Eyed Treefrog Litoria genimaculata, the stony creek Frog Litoria lesueuri, the Waterfall Frog Litoria nannotis, Roth's Treefrog Litoria rothi, the Common Mistfrog Litoria rheocola, the Orange-thighed Treefrog Litoria xanthomera, the Northern Barred Riverfrog Mixophyes schevilli, the Australian Lacelid Nyctimystes dayi and the Robust Whistling Frog Austrochaperina robusta. If declining and non declining species show consistent differences in their habitat requirements and shelter sites (which may, for example, expose them to different levels of UV radiation) then this may indicate the factors that should be further investigated as possible agents of declines.

One consistent pattern that has emerged from research into declining frogs in Queensland is that frogs that have undergone population declines in the tropical rainforests of northern Queensland are closely associated with upland streams. Radio-tracking may identify which non-declining species living in upland areas are now most at risk, by determining the frequency with which they enter or have contact with streams.

Ecophysiology:

Are species that have undergone population declines more susceptible to stress under conditions of drought or heat? At the time of the first reported declines in northern Queensland extremely dry winters were occurring in the rainforests north of Townsville. We will examine the capabilities of declining and non declining species to resist water loss under conditions similar to those encountered in the rainforest, and determine how sensitive chytrid fungi are to desiccation and thermal fluctuations when they are in a frog host.

Chytrid transmission:

We will conduct a series of experiments to determine if and how chytrids can be transmitted, both between different life-history stages of the same species (i.e. from adults to eggs or tadpoles), and between different species.

Effect of disease on tadpole ecology:

Infection with chytrid fungus causes immunosuppression in frogs, which in turn has the potential to alter the outcome of ecological processes to which populations are subject. Understanding how disease affects responses to competition, and changing flow rates may allow us to predict why some populations of frogs are in decline. This study attempts to examine the way in which infection with chytrid fungus effects the ability of tadpoles to compete.

 

Section of tadpole mouthparts showing chytrids.

 

 

Literature Cited:

Richards, S.J., McDonald, K.R. and Alford, R.A. 1993 Declines in populations of Australia's endemic tropical rainforest frogs. Pacific Conservation Biology 1: 66-77.

 

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For further information contact:

• Ross Alford, School of Tropical Biology, James Cook University, Townsville Qld 4811, Australia.
  Email: Ross.Alford@jcu.edu.au

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Back to Declining Frog Research at James Cook University

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Last updated September, 2001.