Decline in amphibian populations

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The Golden Toad of Monteverde, Costa Rica was among the first casualties of amphibian declines. Formerly abundant, it  was last seen in 1989.

Dramatic declines in amphibian populations, including population crashes and  mass localized extinction, have  been noted since the 1980s from locations all over the world, and  amphibian declines are  thus perceived as one of the most critical threats to global biodiversity. A number of causes are  believed to be involved, including habitat destruction and  modification, over-exploitation, pollution, introduced species, climate change, increased ultraviolet-B radiation (UVB) and  disease. However, many of the causes of amphibian declines are  still poorly understood, and  amphibian declines are  currently a topic of much ongoing research.

[edit] Background

In the past three decades, global declines in populations of amphibians (the group of organisms that includes frogs, toads, salamanders, newts, and  caecilians) have  occurred worldwide. an  international convention of amphibian biologists in 2004 has announced that over a third of the world's amphibian species are  threatened, and  more than 120 species have  likely become extinct since the 1980s ^[1], Declines have  been particularly intense in the western United States, Central America, South America, and  eastern Australia (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 ^[2]. Among these species, the Golden toad of Monteverde, Costa Rica featured prominently. The Golden Toad, Bufo periglines, was the subject of scientific investigation until populations suddenly crashed in 1987 and  disappeared completely by 1989 ^[3]. 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.

[ or  problematic declines?">edit] Natural population fluctuations or  problematic declines?

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.^[4] 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. ^[5] 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.^[6]

[edit] Potential causes of declines

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, 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.

[edit] Habitat modification

Main article: Habitat destruction

Habitat modification or  destruction is  one of the most dramatic issue 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.

[edit] Habitat fragmentation

Main article: 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.

[edit] Introduced species

Main article: Introduced species

Non-native predators and  competitors are  also affecting 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 ("On the Rebound." Elizabeth Daerr, National Parks Magazine, January-February 2005, p. 49).

[edit] 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 Roundup or  insecticides such as malathion or  carbaryl greatly increase mortality of tadpoles ^[7]. Additional studies have  indicated that terrestrial adult stages of amphibians are  also susceptible to Roundup, particularly POEA, which is  itself not a pesticide but a surfactant ^[8]. Atrazine has been shown to causes 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. ^[9]

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.^[10]

[edit] 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 (Familiy: Iridiviridae), Anuraperkinsus, and  chytridiomycosis. it  is not entirely clear why diseases have  suddenly begun to affect amphibian populations, but some evidence suggests^[11] that these diseases may have  been spread by humans, or  may be more  virulent when combined with other environmental factors.

[edit] 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 ^[12]. 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 ^[13]. These abnormalities increase frog predation by aquatic birds, the final host of the trematode.

[edit] Chytridiomycosis

Main article: 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.

[edit] Climate change

Main article: 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.

[edit] Increased ultraviolet-B radiation

Like many other organisms, increasing ultraviolet-B (UVB)radiation due to stratospheric ozone depletion and  other factors harms amphibians. However, the amount of damage depends upon the life stage, the species and  other environmental parameters. Thus, for some species, UVB harms early stages such as egg or  larval stages whereas for other species it  is more  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.

A recent review of most published studies done to test effects of UVB on amphibians showed that very few studies found any significant mortality on embryos when exposed to natural levels of UVB. Moreover, the few studies that did report increased mortality after UVB exposure were carried out with experimental procedures that were unrealistic when considering the life history of the species being tested. For example, the eggs of a species of salamander which are  normally deposited on the undersides of submerged logs and  stones were exposed to full sun in shallow water. Amhibian tadpoles which can swim and  seek shelter from open sunlight (UVB) were exposed to UVB for long periods without any opportunity to seek refuge. Overall, amphibians are  well protected from damage from natural ambient levels of UVB by several factors: the enzyme photolyase in eggs, melanin pigment of eggs and  embryos, jelly coats around eggs and  developing embryos. Especially effective is  the dissolved organic content of the water which darkens water and  attenuates UVB within the first few centimeters of water in lakes and  ponds used for spawning by amphibians.

Over millions of years, amphibians have  evolved efficient protection against damage from UVB exposure.The death or  deformity of even a single amphibian after exposure to UVB under natural conditions has never been documented. Despite a small increase of UVB due to atmospheric ozone thinning the evidence for UVB as a causal factor in amphibian decline is  very weak. ^[14]

[edit] 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. ^[15]

[ and  references">edit] Notes and  references

[edit] External links

• FrogWeb: Amphibian Declines & Malformations
• Global Amphibian Assessment which reviews the current status of amphibian species worldwide
• AmphibiaWeb provides background information on amphibian declines.