Dramatic declines in amphibian populations
, including population
crashes and mass localized extinctions
, have been noted since the 1980s from locations all over the world. These declines are perceived as one of the most critical threats to global biodiversity
, and several causes are believed to be involved, including disease
, habitat destruction
and modification, exploitation, pollution
use, introduced species
, climate change
, and increased ultraviolet-B
radiation (UV-B). However, many of the causes of amphibian declines are still poorly understood, and the topic is currently a subject of much ongoing research. Calculations based on extinction rates suggest that the current extinction rate of amphibians could be 211 times the background extinction rate and the estimate goes up to 25,039–45,474 times if endangered species are also included in the computation.
In the past three decades, declines in populations of amphibians (the group of organisms that includes frogs
, and caecilians
) have occurred worldwide. In 2004, an international convention of amphibian biologists announced that 32 percent of species are currently threatened, 43 percent were declining in population, and that 120 species are likely to have become extinct since the 1980s.
Declines have been particularly intense in the western United States, Central America, South America, eastern Australia and Fiji (although cases of amphibian extinctions have appeared worldwide). While human activities are causing a loss of much of the world’s biodiversity, amphibians appear to be suffering much greater effects than other species of organisms. Because amphibians generally have a two-staged life cycle consisting of both aquatic (larvae) and terrestrial (adult) phases, they are sensitive to both terrestrial and aquatic environmental effects. Because their skins are highly permeable, they may be more susceptible to toxins in the environment than other organisms such as birds or mammals. Many scientists believe that amphibians serve as "canaries in a coal mine," and that declines in amphibian populations and species indicate that other groups of animals and plants will soon be at risk.
Declines in amphibian populations were first widely recognized in the late 1980s, when a large gathering of herpetologists reported noticing declines in populations in amphibians across the globe. Among these species, the Golden toad of Monteverde, Costa Rica featured prominently. The Golden Toad, Bufo periglenes, was the subject of scientific investigation until populations suddenly crashed in 1987 and disappeared completely by 1989. Other species at Monteverde, including the Monteverde Harlequin Frog (Atelopus varius), also disappeared at the same time. Because these species were located in the pristine Monteverde Cloud Forest Reserve, and these extinctions could not be related to local human activities, they raised particular concern among biologists.
When amphibian declines were first presented as a conservation issue in the late 1980s, some scientists remained unconvinced of the reality and gravity of the conservation issue. Some biologists argued that populations of most organisms, amphibians included, naturally vary through time. They argued that the lack of long-term data on amphibian populations made it difficult to determine whether the anecdotal declines reported by biologists were worth the (often limited) time and money of conservation efforts.
However, since this initial skepticism, biologists have come to a consensus that declines in amphibian populations are a real and severe threat to biodiversity. This consensus emerged with an increase in the number of studies that monitored amphibian populations, direct observation of mass mortality in pristine sites that lacked apparent cause, and an awareness that declines in amphibian populations are truly global in nature.
Numerous potential explanations for amphibian declines have been proposed. Most or all of these causes have been associated with some population declines, so each cause is likely to affect in certain circumstances but not others. Many of the causes of amphibian declines are well-understood, and appear to affect other groups of organisms as well as amphibians. These causes include habitat modification and fragmentation, introduced predators or competitors, introduced species, pollution, pesticide use, or over-harvesting. However, many amphibian declines or extinctions have occurred in pristine habitats where the above effects are not likely to occur. The causes of these declines are complex, but many can be attributed to emerging diseases, climate change, increased ultraviolet-B radiation, or long-distance transmission of chemical contaminants by wind.
Habitat modification or destruction is one of the most dramatic issues affecting amphibian species worldwide. As amphibians generally need aquatic and terrestrial habitats to survive, threats to either habitat can affect populations. Hence, amphibians may be more vulnerable to habitat modification than organisms that only require one habitat type.
Habitat fragmentation occurs when habitats are isolated by habitat modification, such as when a small area of forest is completely surrounded by agricultural fields. Small populations that survive within such fragments are often susceptible to inbreeding
, genetic drift
, or extinction due to small fluctuations in the environment.
Non-native predators and competitors have also been found to affect the viability of frogs in their habitats. The mountain yellow-legged frog
which typically inhabits the Sierra Nevada
lakes have seen a decline in numbers due to stocking of non-native fish (trout
) for recreational fishing
. The developing tadpoles and froglets fall prey to the fish in large numbers. This interference in the frog's three year metamorphosis
is causing a decline that is manifest throughout their ecosystem.
Pollution and chemical contaminants
There is evidence of chemical pollutants
causing frog developmental deformities (extra limbs, or malformed eyes). Pollutants have varying effects on frogs. Some alter the central nervous system
; others like atrazine
cause a disruption in the production and secretion of hormones. Experimental studies have also shown that exposure to commonly used herbicides such as glyphosate
) or insecticides such as malathion
greatly increase mortality of tadpoles. Additional studies have indicated that terrestrial adult stages of amphibians are also susceptible to non-active ingredients in Roundup, particularly POEA
, which is a surfactant
. Atrazine has been shown to cause male tadpoles of African clawed frogs to become hermaphroditic with development of both male and female organs. Such feminization has been reported in many parts of the world. In a study conducted in a laboratory at Uppsala University in Sweden, more than 50% of frogs exposed to levels of estrogen-like pollutants existing in natural bodies of water in Europe and the United States became females. Tadpoles exposed even to the weakest concentration of estrogen were twice as likely to become females while almost all of the control group given the heaviest dose became female.
A study showed that high levels of nutrients used in farming and ranching activities fuel parasite infections that have caused frog deformities in ponds and lakes across North America. The study showed increased levels of nitrogen and phosphorus cause sharp hikes in the abundance of trematodes, and that the parasites subsequently form cysts in the developing limbs of tadpoles causing missing limbs, extra limbs and other severe malformations including five or six extra or even no limbs.
While most pesticide effects are likely to be local and restricted to areas near agriculture, there is evidence from the Sierra Nevada mountains of the western United States that pesticides are traveling long distances into pristine areas, including Yosemite National Park in California.
Some recent evidence points to ozone as a possible contributing factor to the worldwide decline of amphibian.
A number of diseases have been related to mass die-offs or declines in populations of amphibians, including "red-leg" disease (Aeromonas hydrophila
, and chytridiomycosis
. It is not entirely clear why these diseases have suddenly begun to affect amphibian populations, but some evidence suggests that these diseases may have been spread by humans, or may be more virulent when combined with other environmental factors.
There is considerable evidence that parasitic trematode platyhelminths
(a type of fluke
) have contributed to developmental abnormalities and population declines of amphibians in some regions. These trematodes of the genus Ribeiroia
have a complex life cycle with three host species. The first host includes a number of species of aquatic snails. The early larval stages of the trematodes then are transmitted into aquatic tadpoles, where the metacercariae (larvae) encyst in developing limb buds. These encysted life stages produce developmental abnormalities in post-metamorphic frogs, including additional or missing limbs. These abnormalities increase frog predation by aquatic birds, the final host of the trematode.
In 1998, following large-scale frog deaths in Australia and Central America, research teams in both areas came up with identical results: a previously undescribed species of pathogenic fungus, Batrachochytrium dendrobatidis. It is now clear that many recent extinctions of amphibians in Australia and the Americas are linked to this fungus. This fungus belongs to a family of saprobes known as chytrids that are not generally pathogenic.
The disease caused by Batrachochytrium dendrobatidis is called chytridiomycosis. Frogs infected by this disease generally show skin lesions and hyperkeratosis, and it is believed that death occurs because of interference with skin functions including maintenance of fluid balance, electrolyte homeostasis, respiration and role as a barrier to infections. The time from infection to death has been found to be 1–2 weeks in experimental tests, but infected animals can carry the fungus as long as 220 days.
Subsequent research has established that the fungus has been present in Australia since at least 1978, and present in North America since at least the 1970s. The first known record of chytrid infection in frogs is in the African Clawed Frog, Xenopus laevis.
Because Xenopus are sold in pet stores and used in laboratories around the world, it is possible that the chytrid fungus may have been exported from Africa.
Ozone depletion and ultraviolet radiation
Like many other organisms, increasing ultraviolet-B
(UVB) radiation due to stratospheric ozone depletion
and other factors may harm the DNA
of amphibians, particularly their eggs. The amount of damage depends upon the life stage, the species type and other environmental parameters. Salamanders and frogs that produce less photolyase
, an enzyme that counteracts DNA damage from UVB, are more susceptible to the effects of loss of the ozone layer. Exposure to ultraviolet radiation may not kill a particular species or life stage but may cause sublethal damage.
More than three dozen species of amphibians have been studied with severe effects reported in more than 40 publications in peer-reviewed journals representing authors from North America, Europe and Australia. Experimental enclosure approaches to determine UVB effects on egg and larval stages have been criticized, for example, egg masses were placed at water depths much shallower than is typical for natural oviposition sites. Additionally frog eggs are naturally found as egg masses, while experimental approaches might look at a number of individually-placed eggs, with the consequence that the UVB 'shading' provided by the outer layer of eggs in an egg mass has not been taken into account. While UVB radiation is an important stressor for amphibians, its effect on the egg stage has been overstated.
Increased noise levels
Frogs and toads are highly vocal, and their reproductive behaviour often involves the use of vocalizations. There have been suggestions that increased noise levels caused by human activities may be contributing to their declines. In a study in Thailand, increased ambient noise levels were shown to decrease calling in some species and to cause an increase in others. This has, however, not been shown to be a cause for the widespread decline.
On 16 February
, 2007, scientists worldwide met in Atlanta, Georgia
to form a group called the Amphibian Ark
to help save more than 6,000 species of amphibians from disappearing by starting captive breeding