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Home Research and CollectionsTerrestrial Ecology
Frogs as bioindicators of environmental quality and change
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Overview: Why frogs as bioindicators?
Introduction
Frogs are likely to be affected by changes that occur in terrestrial and freshwater habitats, and to be exposed to contaminants in air, sediment and water. This makes them potential bio-indicators of environmental quality and change.
Reasons why frogs are particularly affected by changes and contamination include:
- Most frogs spend time in freshwater as aquatic eggs and larvae
- Most frogs spend time out of water as terrestrial immatures and adults
- Frogs lay naked eggs
- Frogs have semi-permeable skin
Particular environmental concerns for frogs
- Worldwide frog declines: Frog declines have now been recorded on all continents where frogs occur. This has heightened concern for frogs, especially in relation to what it tells us about environmental changes in response to human activity.
- High frequencies of physical abnormalities: There are relatively high frequencies of physical abnormalities in frog populations. In parts of North America, for example, many frogs have abnormalities such as missing body parts, extra limbs and physical rearrangements. This suggests that there 'is something in the water' which may be harmful to both frogs and humans.
Establishing links elusive
Establishing direct links between frog declines and abnormalities and particular environmental factors has proven elusive because:
- Current knowledge and understanding of frog habitat-use patterns have not separated the effects of gross habitat changes (e.g. changes in water flow) from more subtle changes (e.g. changes in the nature of the available water)
- Little information is available on the frogs that would have been at any particular site before possible or presumed environmental changes took place
- In most countries, including Australia, there have been few long-term studies of frog populations and none that have spanned periods of documented environmental change
- Studies of the 'health' of aquatic systems have focussed on macro-invertebrates and fish, with frogs given little or no attention
Progress so far
Little progress has been made towards the development of frogs as environmental bio-indicators because:
- Quantitative models of habitat-use are rare for frogs from Australia and elsewhere, so it is not possible to predict the consequences of environmental change on frogs or to infer environmental quality or change from frog surveys alone
- There is little information (some from the USA, but not elsewhere) concerning possible ecological consequences of contaminated water on frogs, frogs' eggs or tadpoles, and inferences concerning contamination cannot be made just from observations of frogs
- Relationships between human development and frogs are unclear, although the OECD has devised a draft protocol to use amphibians as a vertebrate model/screening tool for thyroid-disrupting chemicals and their impacts
What needs to be done now?
To clarify the potential of frogs as bio-indicators, we need to evaluate potential parameters that reflect the causes, nature and extent of broader environmental change, such as:
- different species: frog species that have declined might, for example, be more sensitive than frogs that have not
- different life stages: the presence of metamorphosing frogs indicates successful breeding and may therefore be more informative than the presence of other life stages
- parameters relating to individuals and populations of these species, including:
- properties of frog communities (e.g. number/identities of species present)
- measures of abundance of frog populations
- proportions of individuals in different "health" categories (e.g., sick, diseased, physically abnormal)
For example, a bio-indicator is most useful if it reflects various broad categories of human land-use (e.g., natural bushland, urban, agricultural, industrial), and is complemented with an understanding of underlying causal factors (i.e. habitat alteration). For frogs, a particular factor is likely to be chemical pollution of their aquatic environment.
The project: goals
This project aims to:
- determine relationships between human land-use and a range of frog parameters, including abundance and frequency of physical abnormalities
- develop detailed habitat models relating differences in a range of frog parameters to variation in available frog habitat
- determine, through lab experiments, the roles of various factors that have apparent or implicated effects on frogs and the most efficient methods for evaluating these factors
- determine, through field experiments, the roles of various factors that have apparent or implicated effects on frogs
- examine frog specimens from the Australian Museum collection for physical abnormalities and evaluate possible relationships between these abnormalities and factors such as: frog species, collection location and collection date
- compare frogs' effectiveness as bio-indicators with aquatic macro-invertebrates and small fish
The project: methods
Developing habitat models
Frog habitat will be assessed in terms of:
- aquatic areas suitable for breeding
- adjacent terrestrial areas
- predators
- competitors
- potential shelter sites
- chemical and physical composition of the water
Chemical analyses will focus on substances that have previously been found to adversely affect the eggs or tadpoles of frogs.
Lab experiments
Lab experiments will attempt to:
- replicate apparent effects through the creation of conditions closely resembling wild conditions
- determine causal factors responsible for observed effects through selective removal and inclusion of various components of the environment and through evaluation of the impacts on development of specific physical and chemical conditions
Targeted chemicals will be those known to affect frog development and detected during the chemical surveys.
Through consideration of a range of frog species and developmental stages, these experiments will enable comparison of the sensitivities of different species, development of efficient methods for assessing environmental impacts on frogs, and development of procedures and protocols appropriate to Australian circumstances.
Field experiments
Field experiments will involve the use of cages set up within water bodies that are associated both with and without deleterious impacts on frogs (eg. upstream/downstream of potential contaminants).
Through appropriate manipulation, these cages will have physical, biological and chemical factors varied.
The project: study sites
Sydney region
This study focuses on the region around Sydney, because here we already have evidence for frog abnormalities, established study sites, and familiarity with frogs and their habitats. As Sydney is the largest Australian city and has a wide variety of land-use patterns in the region around it, it should provide ample opportunities to carry out this project. Also, the results should be widely applicable, as similar patterns of human development have occurred throughout Australia.
Riverina region
This study also considers the Riverina region of New South Wales where irrigation is used to grow various crops. Here the relative simplicity of the irrigation landscape should enable us to identify the effects of factors such as water management and use of pesticides.
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