4. Frogs

Posted: 5 March 2001

Author: Professor Tim Halliday

'Amphibians may be harbingers of a global ecological catastrophe' says Professor Tim Halliday.

In 1987, the Golden Toad (Bufo periglenes) failed to appear at its usual breeding sites in the Monteverde Cloud Forest Preserve in Costa Rica; it has not been seen since. The Golden Toad is the most celebrated example of an amphibian species apparently becoming extinct over the last 20 years, but it is only one of many. In Monteverde alone, 20 out of 50 frog species haveapparently vanished.Golden toadGolden Toad spawning, Monteverde, Costa Rica© And P.FogdenIn 1989, the first World Congress of Herpetology was held in Canterbury, UK,and it became clear that sudden declines among amphibians were not confined to Central America. Similar events were reported from South America, eastern Australia and the Pacific northwest of the USA. In many instances, amphibians were disappearing from nature reserves, national parks and other areas set up to protect biodiversity.

The declining amphibian phenomenon seemed to be telling us two things. First, it cast serious doubt on the assumption that animals can be protected by setting up reserves. Secondly, it suggested that amphibians are subject to some adverse environmental process that affects them on a global scale.

At first, this second conclusion proved to be highly controversial. Many ecologists working on amphibians, notably in southeastern USA, had notdetected any declines in the populations of frogs, toads and salamanders that they had been studying for many years. What they had found is that it is characteristic of most amphibians that the number of adults returning to breeding sites from year to year fluctuates enormously. They suggested, therefore, that the declines reported elsewhere in the world might simply be population fluctuations.

Task force

It was clear that issues of this kind could only be resolved by carrying out appropriate research, and that the situation required urgency. The response of the herpetological community was the setting up in 1991 of the Declining Amphibian Populations Task Force (DAPTF), a global organisation of scientists and volunteers. The DAPTF now has more that 3,000 members, most of whom belong to one of over 100 regional or national working groups. The task of these groups is to find out which amphibians are declining and where. Other, issue-based working groups are looking at potential causes of amphibian declines, such as chemical contaminants, climatic and atmospheric change, and disease.

The DAPTF has not yet answered the questions that it set itself in 1991, but it has gathered together a mass of information that is helping us to better understand what is happening. It is clear, for a start, that declines are occurring on a global scale, although there are some regions in the world where they are not.Tree frogTree frog, Manu National Park, Peru© André Bartschi/WWFIt is apparent that many declines are explicable in terms of habitat change resulting from human activities such as deforestation, draining of wetlands, and changes in agricultural land use. In Europe, for example, most population declines are explicable in terms of loss of suitable habitat, both aquatic and terrestrial.

Before the DAPTF was formed, amphibian declines were typically detected after the event; people noticed that frogs weren't there any more. More recently, however, biologists have been in position to witness some declines as they occur, notably in central America. A feature of these reports is that many are associated with outbreaks of disease.

Are amphibians being affected by totally new diseases, or by diseases that have been accidentally introduced from other places? Alternatively, are they succumbing to infections that they can normally resist, because their immune systems have been weakened by other factors, such as pollution or climate change? These are questions that require research and the DAPTF is fostering appropriate studies as a matter of urgency.

Ozone layer

A great deal of research has focused on the possibility that increased levels of UV-B radiation, caused by thinning of the Earth's ozone layer, may be a cause of amphibian declines. A number of studies have shown that thelevels of UV-B that are now reaching the ground, particularly at high altitude, are very harmful to the developing eggs and embryos of some amphibians, but not to all. Some of these studies also suggest that eggs exposed to elevated UV-B are also more susceptible to infection by fungus. It has yet to be established, however, that increased UV-B has actually caused any amphibian populations to decline and it cannot account for declines in habitats, such as tropical forest, where UV radiation does not reach ground level.

As eggs, larvae and adults, amphibians lack any kind of protection from the environment, such as a shell or a dry skin. This makes them peculiarly sensitive to chemical contamination and a lot of research carried out by members of the DAPTF has examined the harmful effects of pesticides, herbicides and fertilisers on amphibians. For example, detailed research in Yosemite National Park in California suggests strongly that dramatic declines among amphibians over the last 80 years are due to chemical contaminants blown by the prevailing wind from the agricultural area in California's Central Valley to the west of the Sierra Nevada.Water frogWaterfrog, France© M & C. Denis Huot/Still PicturesWhy should we care that amphibians are declining? There are many reasons, most of them common to other components of the Earth's biodiversity. Amphibians may, however, be sending out a special message. Because of their great sensitivity to changes in both aquatic and terrestrial habitats, they may, like the coal miner's canary, be giving us early warning of major and widespread changes taking place in the environment. The factors that have been shown to adversely affect amphibians, like solar radiation and chemical contamination, threaten all forms of life, including humans. Amphibians may be harbingers of a global ecological catastrophe that is only just beginning.

Dr Halliday is Professor in Biology at The Open University and International Director of the DAPTF.For further information on decline amphibians see the website of DAPTF