Time is running out for the world’s remarkably diverse amphibians.
Almost half all known species are in decline and one in every three amphibian species is currently threatened with extinction, a far higher proportion than that of bird or mammal species.
Threatened by habitat destruction and degradation, the spread of virulent diseases, climate change and trade, amphibians may be on the verge of a near total disappearance from global ecosystems. These species are the “canaries in the coalmine” – they are highly sensitive to environmental changes which lead to extinction, and are a stark warning of things to come. If we lose them, other species will inevitably follow.
The Luristan newt, Neurergus kaiseri - surviving
in arid Iran, this species is highly sensitive to
changes in its marginal environment.
Below is information on current threats that are particularly impacting upon amphibian species.
Virulent diseases caused by viruses, fungi and bacteria are killing thousands, if not millions, of amphibians each year. Addressing the threat of diseases in wild populations is not easy, since pathogens can be difficult to remove from wild habitats. In fact, many of the most devastating effects of amphibian disease have occurred in unspoilt and protected areas. Diseases are likely to have had a major hand in driving some species to extinction in recent years.
Chytridiomycosis has been identified as the cause of the extinction of the Sharp-snouted day frog (Taudactylus acutirostris), an Australian endemic and might also have contributed to the decline of Costa Rica’s famous Golden Toad (Bufo periglenes). Bufo periglenes became extinct before the discovery of chytridiomycosis. In other cases diseases have been responsible for serious population declines. Research is urgently required to find ways of mitigating this threat in the wild.
Sharp-snouted day frog, Taudactylus acutirostris
In New Zealand, a recent outbreak of the chytrid (pronounced kit-rid) fungus is thought to have caused a mass die-off of Archey’s frog (the highest priority EDGE amphibian species) in the Coromandel, resulting in an emergency meeting of the Native Frog Recovery Group.
Archey's frog, Leioelma archeyi
Click here to learn more about the major amphibian diseases affecting species in the wild.
Climate change and UV radiation
Amphibians are especially vulnerable to desiccation – the vast majority of species must maintain moist skin surfaces because a significant amount of their breathing occurs through their skin. Their eggs do not have hardened protective shells and are also susceptible to drying out. Even the amphibians found in deserts are dependent upon seasonal rains and temporary water sources for breeding.
The timing of breeding by amphibians is governed by environmental factors such as temperature and water availability. It is thought that if global warming occurs, frogs will start breeding earlier in the season. This is already happening, with frogs coming out of hibernation earlier and being more susceptible to sudden changes in the weather. There are also predictions that in the second half of this century, carbon dioxide levels will double, which may increase the Earth’s mean surface temperature by 4°C. The sea level could then rise by over 2 metres, inundating most of the world’s coastal wetlands – there will be an alteration of rainfall patterns and more frequent and intense droughts.
Indirect effects, such as depression of immune function, will cause amphibians to become more susceptible to disease. There may be more complex subtle effects where the lower pond-water levels will expose the embryos to more ultra-violet (UV) light from the sun, causing them to be even more susceptible to fungal attacks. In fact, the widespread decline of amphibians may be linked to an increase in UV radiation (in particular UVB at a wavelength of 300 nm). UVB radiation is harmful to many species of amphibians, however research focusing on the effects of UV on frog populations is highly controversial and we are not yet sure if UV poses a serious threat.
Pet trade and overexploitation
Many amphibian species are highly attractive and have become popular among collectors in the pet trade. Generally, the rarer the species the higher the demand and, therefore, the rarer the frog becomes. Frogs caught in the wild are often kept in crowded cages awaiting final shipment to their destination. The ones that do get there alive usually only survive a couple of months in captivity before being unceremoniously replaced. Furthermore, some species are used in traditional medicines or harvested for their various properties, such as skin toxins which have numerous ceremonial and hunting-related uses.
Another possible cause of global decline is the collection of frogs as a food source. People in Africa, the United States, Germany, France and the Netherlands eat millions of frogs every year. For example, the United States imports more than 3 million kg of frog meat a year – approximately 26 million frogs! The main exporters of frogs are India, Indonesia and Bangladesh. When frogs are captured on such a large scale, it upsets the food chain and their usual prey multiplies. This results in the extensive use of insecticides and causes considerable damage to delicate ecosystems, including the further destruction of amphibian populations.
Amphibians are highly susceptible to pollutants, especially within any water sources they may use. Their skin is very sensitive and their eggs have no hardened shells, which makes them vulnerable to toxins in the environment. Aluminium, cadmium, copper, zinc and iron are all toxic to amphibians. Nickel, lead, and manganese have damaging effects on fish, so frog populations are probably similarly affected.
Human alternation of habitat is the single biggest threat to plant and animal species. Human pressure on global ecosystems is intensifying as global populations rise, settlements expand, and natural resource extraction accelerates. Furthermore, habitat degradation and fragmentation reduces the quality of what remains. It is now estimated that humans have altered between one third and one half of the Earth’s land surface. Factors such as forestry and agriculture have major effects on endangered species populations. Roads, fields and urban areas represent significant barriers to movement, and may also result in unnaturally high population densities for the available food supplies. Of longer-term importance is the probability of inbreeding and loss of genetic variation in small isolated populations. Combined, these factors make populations extremely vulnerable to other threats such as disease and environmental catastrophes.
The Sagalla caecilian is declining as a result of
habitat destructon within its tiny range
The damming of rivers and the reduction in the quality of water bodies can be extremely detrimental to the survival of many water-dependent species, such as river dolphins and amphibians. Mining is also a major threat, polluting water and resulting in massive mechanical destruction of many EDGE species and their habitat.
Introduced and invasive species
Humans have spread a great many non-native animals and plants around the world and they have done so at an unprecedented rate in the last century. The establishment and spread of non-native or “exotic” species are a major threat to worldwide biodiversity, and many species have been greatly impacted by introduced species through direct predation, competition and the introduction of diseases. Communities of species develop over a long period of time, and survive in delicately balanced ecosystems.
The Mallorcan midwife toad is now restricted to
inaccessible limestone gorges as a result of
predation by an invasive species of snake
The movement of species between separate ecosystems can cause great upset, especially if an exotic species rapidly reproduces, establishes itself competitively within the new system and becomes “invasive”. The cane toad (Bufo marinus), introduced to Australia in 1935 as a means of pest control, has had a negative effect on native amphibian populations, while the spread of black and brown rats around the globe has led to the extinction of many island species over the past 500 years.
The "General Threats" page also contains information on the major threats to EDGE species as a whole:
Introduced and invasive species