The world's amphibians are in crisis; populations of most of the world's 6200 species of frogs, toads, salamanders, newts and caecilians are in decline and one in three amphibian species are threatened with extinction. Several species have become extinct in the last 20 years. The Panamanian Golden Frog, featured in programme two of Life in Cold Blood, is just one of many species that no longer survives in its natural habitat.
The cause of this disastrous situation is human population growth and the destructive ecological 'footprint' that inevitably results from it. Natural habitats are destroyed and degraded as they are cleared for housing, agriculture and industry; surviving habitats become polluted; climate change alters natural environments, often making them hostile to their natural inhabitants.
For amphibians, there is an additional threat. A highly virulent fungus, called chytridiomycosis, has rapidly spread to all parts of the world. This invades the delicate skin of amphibians and is lethal to many species. The golden frog has lost most of its natural habitat to human development and its last populations have been overtaken by a chytridiomycosis 'wave' that is spreading through Central America.
Amphibian declines are by no means unique; planet Earth is entering what has been called the sixth mass extinction in its long history (the fifth mass extinction saw the demise of the dinosaurs). Plants and animals all over the world are rapidly declining in numbers and many are becoming extinct. In Britain, for example, we are witnessing a dramatic disappearance of insects, most notably among butterflies.
Amphibians are, however, declining faster than other groups of vertebrates, such as mammals, birds and reptiles. There are two main reasons why amphibians are more sensitive to environmental changes than other animals, making them important indicators of environmental threats that may affect all kinds of wildlife.
First, amphibians live part of their lives on land, part in water, and so are sensitive to anything that adversely affects either kind of habitat. In Britain, conservation effort has mostly focussed on the ponds in which frogs, toads and newts breed, but it is equally important to conserve the terrestrial habitat in which these animals spend much the greater part of their lives. It is becoming clear that amphibians wander quite far from their breeding ponds, up to at least 1000m in toads, meaning that amphibians require more extensive areas of terrestrial habitat than was previously realised.
Secondly, amphibians, in all their life stages, lack a protective outer layer. Their eggs have no shell and, as larvae and as adults, their skin is thin and moist. This makes amphibians very vulnerable to a variety of environmental threats. The lack of a shell around their eggs means that they are readily penetrated by ultraviolet radiation, which damages their chromosomes and causes them to develop abnormally. The skin of their larvae allows any water-borne pollutants to enter their bodies.
Amphibian larvae are very sensitive to agricultural chemicals, such as pesticides, herbicides and fertilizers, which, if they do not kill them, often stunt their growth. One widely-used herbicide, atrazine, disrupts the reproductive development of amphibians, causing males to develop female characteristics. Adult amphibians use their thin skin for gas exchange to breathe, making them susceptible to radiation, chemical pollutants and the chytrid fungus that grows in their skin.
The amphibian decline crisis first became apparent in the late 1980s, when biologists began to realise that many amphibians that had previously been abundant simply weren't there any more. Of particular concern were 'enigmatic' declines, meaning the disappearance of amphibians from national parks, nature reserves and other localities where there was no habitat destruction going on and where biodiversity is, supposedly, protected.
Clearly, there was something happening in the environment that was harmful to amphibians and which was not prevented by simple habitat protection. The brightly-coloured golden toad, once an inhabitant of the Monteverde Cloud Reserve in Costa Rica has not been seen since 1989. Several other frog species have disappeared from this reserve, largely as a result of changes in its climate which have made the forest drier, though chytridriomycosis has also affected them.
The impact of chytridiomycosis first became apparent in Central America, where an epidemic has been making its way from north to south, and in eastern Australia, where it moved from south to north. Since then, it has been detected in many parts of the world, including Europe, North America and South America. In all regions, it has a devastating effect on some species, but seems to have no adverse effects on others. Chytridiomycosis also occurs in Africa, where it does not seem to affect seriously many species, providing support for the hypothesis that the disease may have originated in Africa.
Over the last 20 years, a great deal of research has been carried out into the causes of amphibian declines. As well as habitat loss and disease, these include: climate change that has altered rainfall patterns; increased ultra-violet radiation resulting from reduced ozone in the Earth's atmosphere; pollution of amphibian habitats, particularly by agrichemicals such as pesticides, herbicides and fertilizers; and over-exploitation of amphibians for food and the pet trade.
Some of these factors adversely affect adult amphibians, but most are particularly harmful to their larvae (called tadpoles in frogs and toads), which are particularly susceptible to any degradation in their freshwater habitat. Tadpoles are indicators of severe deterioration in the world's freshwater habitats; it has been estimated that biodiversity in freshwater habitats has declined worldwide by 50% in the last 20 years.
The factors that adversely affect amphibians interact in complex ways. For example, in northwest USA, climate change has reduced springtime water levels in amphibian breeding ponds. This forces frogs and salamanders to lay their eggs in very shallow water, exposing them to abnormally high levels of ultraviolet radiation. This harms eggs and embryos directly, causing them to develop abnormally, and indirectly, making them more susceptible to fungal disease.
Amphibian declines are most dramatic in tropical regions of the world, where amphibian diversity is higher and human economic development is occurring fastest. Britain is, however, experiencing amphibian declines that mirror what is going on in the rest of the world. Britain's frogs, toads and newts used to thrive in a landscape largely devoted to agriculture which contained numerous small ponds where they could breed and abundant hedgerows and woodland in which they could live on land.
Since the Second World War, however, the modernisation of agriculture has meant that hedgerows have been destroyed and ponds filled in; in some rural parts of Britain, over 90% of ponds have been destroyed over the last 50 years. Some of our native amphibians thrive in garden ponds and it has been suggested that more of our common frogs now live in gardens than live in 'natural' habitats.
A major threat to amphibians in developed countries such as Britain is road traffic, which kills very large numbers of animals as they migrate to their breeding sites in spring. It has been estimated that a common toad has only a one in ten chance of crossing a busy motorway alive. This is thought to be one cause of a recent decline in the number of common toads in southeast England. In a number of places, this threat is counteracted by 'toad patrols', groups of people who gather up toads on roads at night and carry them to safety.
British amphibians may also be threatened by chytridiomycosis, which has recently been accidentally introduced into southern England on American bullfrogs. As well as carrying unwelcome disease, bullfrogs pose a threat to native British amphibians; they are voracious predators and their tadpoles compete with native species for food. Concerted efforts are being made to eradicate bullfrogs and work is being carried out to determine whether British amphibian species are infected by, and susceptible to chytridiomycosis.
Amphibians lead mostly secretive lives, living under ground or hidden in vegetation and coming out only at night. Only for a brief period in spring are people aware of their presence. This may account for the fact that, until recently, people have not been aware of their rapid decline. On a global scale, this decline not only raises the alarm about the deterioration of natural habitats, but also signals a growing problem for human health.
Amphibians are dependent for their survival on a plentiful supply of fresh water that is free of chemical contaminants and harmful microbes; so are people. Human population growth and climate change are combining to create a situation in which a large and increasing proportion of the human population is denied access to sufficient, clean water. The rapid decline of the world's amphibians is a warning that this most precious of natural resources is under serious threat.