Decline in amphibian populations

Dramatic declines in amphibian populations, including population crashes and mass localized extinction, 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 of causes are believed to be involved, including habitat destruction and modification, over-exploitation, pollution, pesticide use, introduced species, climate change, increased ultraviolet-B radiation (UV-B) and disease. However, many of the causes of amphibian declines are still poorly understood, and the topic is currently a topic of much ongoing research. Calculations based on extinction rates suggest that the current extinction rate of amphibians is 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.

Background
In the past three decades, declines in populations of amphibians (the group of organisms that includes frogs, toads, salamanders, newts, 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.

Initial skepticism
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.

Potential causes
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
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
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.

Introduced species
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.

Chemical contaminants
There is clear evidence that chemical pollutants are 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 (Tradename Roundup) or insecticides such as malathion or carbaryl 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 published in the September 24, 2007 issue of Proceedings of the National Academy of Sciences shows 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. According to Pieter Johnson, an ecologist and evolutionary biologist at the University of Colorado at Boulder who led the study, "You can get five or six extra limbs. You can get no hind limbs. You can get all kinds of really bizarre, sick and twisted stuff."

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.

Disease
A number of diseases have been related to mass die-offs or declines in populations of amphibians, including "red-leg" disease (Aeromonas hydrophila), Ranavirus (Family: Iridiviridae), Anuraperkinsus, and chytridiomycosis. It is not entirely clear why 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.

Trematodes
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.

Chytridiomycosis
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 the fungus makes it impossible for the amphibian to respire across the skin. The time from infection to death has been found to be 1–2 weeks in experimental tests.

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.

Climate change
Large stands of ecosystem such as rainforests are being destroyed at a phenomenal rate decreasing available habitat. Also pollutants indirectly affect frogs by way of ozone depletion causing sun burn damage to the delicate moist skins of frogs, and further affecting their immune systems.

Increased ultraviolet-B radiation
Like many other organisms, increasing ultraviolet-B (UVB) radiation due to stratospheric ozone depletion and other factors may harm amphibians. The amount of damage depends upon the life stage, the species and other environmental parameters. Thus, for some species, UVB may harm early stages such as egg or larval stages whereas for other species it may be harmful to adult stages (e.g. eye damage). Exposure to UVB radiation may not kill a particular species or life stage but may cause sublethal damage. UVB radiation may act synergistically with climate change, contaminants and diseases. However, these speculations are not supported by experimental evidence.

Recent reviews of published studies done to test effects of UVB on amphibians showed that most studies found significant mortality on embryos and increases in deformities when exposed to natural levels of UVB. This emphasizes the fact that UVB radiation is important to consider in amphibian population declines.

Over millions of years, amphibians have evolved inefficient protection against damage from UVB when faced with the recent unprecedented levels of UVB reaching the earth's surface due to human-induced destruction of the earth's protective ozone layer. 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.

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.

Amphibian Ark project
Such is the seriousness of the case, on February 16th 2007, scientists worldwide met in Atlanta, US to form a group called the Amphibian Ark to save more than 6,000 species of amphibians including frogs, salamanders and wormlike caecilians from disappearing due to the Chytridiomycosis fungus and other issues affecting the amphibian population.