Decline in amphibian populations

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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.

Contents 1 Background 2 Natural population fluctuations or problematic declines? 3 Potential causes of declines 3.1 Habitat modification 3.2 Habitat fragmentation 3.3 Introduced species 3.4 Chemical contaminants 3.5 Disease 3.5.1 Trematodes 3.5.2 Chytridiomycosis 3.6 Climate change 3.7 Increased ultraviolet-B radiation 3.8 Increased noise levels 4 Notes and references 5 External links

[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.

[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]

[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.