Disappearing Frogs

Frogs have existed for tens of millions of years and were well established globally long before the breakup of Gondwana. Throughout this period frog populations have survived natural catastrophes, climate change, desertification and massive changes to waterways, just to name a few! Why then are we suddenly seeing populations of frogs disappearing before our eyes? In Australia alone, 27 frog species (13%) have become threatened, 14 species have declined and eight have disappeared, all in the past 30 years.

Who cares about frogs?

You should!! Frogs play a very important role in ensuring the healthy functioning of many of the inland waterways in Australia, the very same waterways that we depend on! Frogs also play very important roles in the food chain. They help keep insect populations in check and some of the larger species consume vast quantities of insects each night. Frogs and tadpoles are also an important food resource for other native animals such as predatory birds and snakes.

Why are they vanishing?

There are many factors that affect the health of frog populations. Serious declines in frog numbers are influenced by many things including:

• increased exposure to ultraviolet light due to the thinning of the ozone layer;
• global warming and climate change;
• emerging diseases;
• chemical pollution (for example the use of pesticides and herbicides near waterways);\
• deforestation;
• road kills;
• introduced exotic pests (such as cane toads);
• water channelling and agricultural expansion;
• habitat loss for development (residential housing for example);
• populations of introduced predatory animals (dogs and cats); and
• lack of sufficient clean fresh water sources (caused by drought and pollution).

These factors are all serious issues that need to be addressed. However! they don't help to solve the mystery behind the loss of whole frog populations in pristine natural areas such as the tropical rainforests of northern Queensland.

This mystery is of great concern to scientists in Australia, why are we seeing the disappearance of frogs in areas free of pollution and predators? What is even more worrying is that similar declines in frog populations have been reported in North and South America, Europe, New Zealand and South Africa.

Chytrid Fungus

About 20 years after this problem of disappearing frogs was first reported, the mystery is starting to unravel. The problem appears to be a particular type of chytrid (pronounced KIT-trid) fungus called Batrachochytrium dendrobatidis. This newly discovered disease was first reported in 1998, after adult frogs were collected from mass deaths in Panama and northern Queensland between 1993 and 1998.

Although scientists have only recently described this fungus it now appears that it may have been responsible for frog declines dating back to the 1970s and has been present in Australia since at least 1978. Frogs infected with the virus have been identified throughout the world.

How does it work? This is the first time that a chytrid fungus has been found infecting vertebrates and the actual mechanism by which chytrid kills the frogs that it infects is still unknown. What is known however, is that the fungus in frogs attacks a substance called keratin (the same protein that forms hair and nails in humans). In frogs, keratin forms a resistant layer in the skin and it is thought that the fungus attacks this layer. Frogs use their skin to breathe and drink, so it's possible that this damage that eventually kills individuals infected by the fungus. The fungus may also kill frogs by releasing toxins that are absorbed.

Can it be cured? Currently there is no cure for the chytrid fungus after frogs have been infected. What is interesting though is that whereas adult frogs have keratin in their skin, tadpoles only have in their mouths. This means that tadpoles can carry the fungus but that they do not succumb to it. (the fungus only spreads after they metamorphose into a frog).

CSIRO 's Australian Animal Health Laboratory is currently researching a range of anti-fungal drugs. It is hoped that these drugs will be used to treat infected tadpoles so that they can be returned to the wild.

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