Decline in amphibian populations

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Image:Bufo periglenes1.jpg 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, 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 Chytridiomycosis 3.6 Climate change 4 Notes and references 5 External links

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 indicated that over a third of the world's amphibian species are threatened, and more than 120 species have likely become extinct since the 1980s Template:Ref, 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 Template:Ref. 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 Template:Ref. 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.

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 Template:Ref. 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 Template:Ref. 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 natureTemplate:Ref.

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, or long-distance transmission of chemical contaminants by wind.

Habitat modification

Template:Main Habitat modification or destruction is clearly the most dramatic issue affecting amphibian species worldwide. Because amphibians generally need both specific aquatic and terrestrial habitats to survive, threats to either necessary habitat can affect populations. Hence, amphibians may be more vulnerable to habitat modification than organisms that would only require one habitat type or the other.

Habitat fragmentation

Template:Main 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

Template:Main 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).

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 Template:Ref. 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 Template:Ref.

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.Template:Ref

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 that these diseases may have been spread by humans, or may be more virulent when combined with other environmental factors.

Researcher Joseph Kiesecker focused research on a rural area of Pennsylvania where trematodes were present in ponds. His research indicated that frogs sheltered from the infiltration of parasites developed normally, while those that were exposed developed deformities. He also monitored the pesticide levels in the same ponds and found a magnification in the rate of parasitism in the polluted ponds suggesting that the presence of the pollutants reduces the frog’s immune response to parasitism. ("Pesticides Mess with Immunity" J. Pickrell Science News. July 13, 2003, p. 20)

Chytridiomycosis

Template:Main 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

Template:Main 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.

Notes and references

1. Template:NoteBlaustein, A.R. and D.B. Wake. 1990. Declining amphibian populations: a global phenomenon? Trends in Ecology and Evolution 5:203-204.
2. Template:NotePechmann, J.H.K., D.E. Scott, R.D. Semlitsch, J.P. Caldwell, L.J. Vitt, and J.W. Gibbons. 1991. Declining amphibian populations: the problem of separating human impacts from natural fluctuations. Science 253:892-895.
3. Template:NoteStuart, S.N., J.S. Chanson, N.A. Cox, B.E. Young, A.S.L. Rodrigues, D.L. Fischman, and R.W. Waller. 2004. Status and trends of amphibian declines and extinctions worldwide. Science 306:1783-1786.
4. Template:NoteCrump, M.L., F.R. Hensley, and K.I. Clark. 1992. Apparent decline of the golden toad: Underground or extinct? Copeia 1992:413-420.
5. Template:NoteHoulahan, J.E., C.S. Findlay, B.R. Schmidt, A.H. Meyer, and S.L. Kuzmin. 2000. Quantitative evidence for global amphibian population declines. Nature 404:752-758.
6. Template:NoteRelyea, R.A. 2004. The impact of insecticides and herbicides on the biodiversity and productivity of aquatic communities. Ecological Applications 15(2):618–627.
7. Template:NoteRelyea, R.A. 2005. The lethal impact of Roundup on aquatic and terrestrial amphibians. Ecological Applications 15(4): 1118–1124.
8. Template:NoteDohm, M.R., et al. 2005. Effects of ozone exposure on nonspecific phagocytic capacity of pulmonary macrophages from an amphibian, Bufo marinus. Environmental Toxicology and Chemistry 24:205-210.

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.es:Declive en las poblaciones de anfibios