INTRODUCTION
Amphibians (class Amphibia) are anamniotic tetrapod vertebrates characterized by their ability to exploit both aquatic and terrestrial habitats. The name amphibian, derived from the Greek amphibios meaning “living a double life,” reflects this dual life strategy. They are the first vertebrates to colonize the terrestrial habitats.
Extinction is a common event throughout earth’s history. During the last 50 years researchers studying modern ecosystems have confronted species extinction at local and global scales more and more frequently. For amphibians the recent extinction rate is about 200 times the historical background rate (Collins, 2010).
The current wave of interest in amphibian population biology and in the possibility that there is a global pattern of decline and loss began in 1989 at the first world congress of Herpetology. There is now a consensus that alarming declines of amphibians have occurred. Because most amphibians are exposed to terrestrial and aquatic habitats at different stages of their lifecycles, and because they have highly permeable skins, they may be more sensitive to environmental toxins or to changes in patterns of temperature or rainfall than are other terrestrial vertebrates. (Alford and Richards 1999)
CAUSES FOR AMPHIBIAN DECLINING
For over 350 million year, thousands of amphibian species have lived on Earth. Amphibians have been disappearing at an alarming rate Since the 1980s. In many cases quite suddenly. Amphibians are among a number of groups providing evidence that we are likely in the midst of a sixth mass extinction. There are 6 major potential causes of modern amphibian declines and extinctions. There are different reasons for amphibian declining. In the modern era there are 6 leading causes of biodiversity loss in general. All of these are acting alone or together for modern amphibian declining.
1. 1. UV-B radiation
2. 2. Infectious disease
3. 3. Introduced/exotic species that compete with, prey on, and parasitize native amphibians
4. 4. Climate change
5. 5. Habitat modification
6. 6. Acidity and toxicity
7. 7. Commercial use
Some causes are historical in the sense that they have been operating for hundreds of years, although the rate of change due to each accelerated greatly after about the mid-20th century. Modern amphibian declines and extinction is a lens through which we can view the larger story of biodiversity loss and its consequences (Collins, 2010)
1. UV-B Radiation
UV-B radiation has significantly increased at ground level in high latitudes with the anthropogenic ozone depletion. UV-B has harmful effects on individual organisms. Embryos of some amphibian species survived much better when shielded from UV-B. Similar damaging effects of elevated UV-B have also been observed in the laboratory. Furthermore, satellite-based measures of UV radiation levels at 20 sites in Central and South America recorded increases between 1979 and 1998 that were greatest in areas where amphibian declines have been most severe. However, embryos and larvae of many declining species in tropical rainforests are not exposed to UV-B in the same way as open mountain pools and lakes of temperate countries. Another problem with attributing declines to increases in UV-B radiation is that moderate concentrations of dissolved organic matter ameliorate the effects of UV-B below the water surface. Eggs and larvae in the majority of breeding sites used by amphibians in North American mountain regions are well protected by the dissolved organic matter. (Beebee and Griffiths, 2005).
Significant variation among species in levels of photolyase, a photo reactivating DNA repair enzyme that repairs UV-B damage, is correlated with exposure of natural egg deposition sites to sunlight. In a survey of 10 Oregon amphibian species, ability to repair UV damage, was lowest in declining species and highest in non-declining species. Embryos of Hyla regilla, a non-declining species with high photolyase activity, had significantly higher hatching success than did two declining species (Rana cascadae and Bufo boreas) with low photolyase levels.
Several studies have demonstrated that enhanced UV-B radiation reduces survival or hatching success of amphibian embryos (Alford and Richards 1999). Synergistic interactions between UV-B and other environmental stresses such as pathogens and low pH may also significantly increase embryonic mortality. Rana pipiens embryos that are unaffected when exposed to UV-B and low pH separately have significantly reduced survival when exposed to these factors simultaneously. The declining frog Litoria aurea from eastern Australia has a lower photolyase activity. In many aquatic habitats UV-B radiation is largely absorbed in the first few centimeters of the water column, so increased UV-B may only affect species breeding in habitats with a narrow range of chemical and physical parameters. The relationship between UV-B and population declines have focused their attention on species that breed in shallow, clear water, where exposure to UV-B is expected to be greatest in most of the studies. But only fewer data are available to assess the indirect effects of increasing UV-B on amphibian populations. The potential indirect effects include changes in water chemistry and food supplies, and shifts in competitive and predator-prey relationships with other UV-B affected species. Exposure to increased UV-B may reduce survival rates of adult amphibians through damage to eyes, increased frequency of cancers or tumors and immunosuppression (Alford and Richards, 1999)
A large diversity of micro and macro parasites infects amphibians (Collins,2010). Diseases play major roles in dramatic mass mortalities in some declining amphibian populations in some species and regions. Rana viruses (Iridoviridae) cause high levels of mortality in tiger salamanders (Ambystoma tigrinum) but populations usually recover afterwards.
A trematode worm known as Ribeiroia ondatraeis causes leg deformities in frogs. Due to anthropogenic activities increased eutrophication favors the snails that provide a secondary host for this parasite. At this moment, water quality changes due to human activities have altered community structures, and thus predation patterns, to favour snails (Planorbella species) exclusively used as first intermediate hosts by Ribeiroia ondatraeis.
Saprolegniaceous fungi can cause high levels of egg mortality in amphibians. There may be synergistic effects between fungal infection and UV-B. Different amphibian species are vulnerable to different strains of Saprolegnia. A chytrid fungus Batrachochytrium dendrobatidis has been implicated in mass mortalities and population declines of amphibians in the Americas, Europe, Australia and New Zealand. It attacks the skin of post-metamorphic amphibians and causes death by impairing cutaneous respiration and osmoregulation. Disease outbreaks may follow either by a weakened immune response in the amphibians caused, perhaps, by another stressor or an increased virulence of the pathogen. Chytrid fungi might therefore be responsible for some amphibian population declines, but it remains uncertain as to whether they are primary or secondary causes. ( Beebee, and Griffiths, 2005)
Infection of Aeromonas hydrophila is associated with declines in populations of Bufo boreas boreas. In a Rhode Island pond Aeromonas hydrophila killed all larval Rana sylvatica. In California the same bacterium was well-documented to decline the local extinction of a population of Rana muscosa.. A Chytridiomycota fungus found on moribund anurans in Australia and Panama during mass mortality is fatal to healthy frogs. During mass mortality events different viruses have been isolated from dead and dying frogs (Alford and Richards 1999).
At least 8 Ranavirus strains may infect frogs or salamanders. Rana viruses cause epidemics in native frogs in Europe, South America, and Australia, in frogs and salamanders in North America and in aquaculture colonies of frogs in Asia. In theory, density-dependent pathogen transmission alone will not drive a host population to extinction, although extinction might occur if population size becomes so small that stochastic processes lead to the population’s demise. Rana viruses seem to fit this model. Populations regularly decline, even to local extinction (Collins,2010).
3. Introduced species
Alien species impact on amphibian populations in various ways. Biotic interactions among amphibians and between amphibians and other organisms play a significant role in determining their distribution and population dynamics. Larval amphibians are extremely vulnerable to vertebrate and invertebrate predators, and the diversity of aquatic amphibian assemblages is frequently reduced in habitats containing predatory fish. Larval amphibians that coexist with aquatic predators have evolved a range of antipredator mechanisms. However, widespread introductions of predatory fish have increasingly exposed native amphibians to predators which they have not previously interacted. Inappropriate responses to novel predators may increase mortality of native amphibians, leading to significant effects on populations. Colonization of normally fish-free water bodies by predatory fish can result in rapid extinction of amphibian assemblages. The allotropic distributions of native frogs and introduced fishes in many high-elevations in Sierra Nevada lakes indicate that introduced predatory fishes have caused the extinction of local frog populations there. 60% of lakes that frogs could formerly occupy now contain introduced fishes and no frogs. Introductions of fish have had a particularly severe impact on Rana muscosa, which breeds in the deep lakes inhabited by fishes. A similar pattern of allotopic distributions has been recorded for larval newts, Taricha torosa, and an introduced fish (Gambusia affinis) and crayfish (Procambarus clarki) (both predators of newt eggs or larvae) in. Introduced predators may also have more subtle effects. Some Rana muscosa populations persisting in fish-free environments have become isolated from other populations by surrounding aquatic habitats containing introduced fishes. This may eventually lead to regional extinction by preventing migration among local populations.
Declines and local extinctions have been variously ascribed to introduced fish, other amphibians such as bullfrogs R. catesbeiana and cane toads Bufo marinus, and crayfish. Competition and predation generate the most obvious effects, causing reduced growth or survivorship, alterations in behavior or habitat use. North American bullfrogs (Rana catesbeiana) that have become established outside their natural range have been implicated in declines of native frogs. Native frogs are consumed by adult bullfrogs and their population densities reach at a level enough to have a severe impact on local amphibian populations. The studies have shown that Rana aurora larvae gets increased larval periods, smaller mass, and, when exposed to both, lower survival whenever exposed to adult or larval bullfrogs. (Alford and Richards 1999)
Eggs and larvae are usually the most vulnerable stages. Introduction of trout, for sporting purposes, into mountain lakes in the Californian Sierra Nevada have resulted in major decline in mountain yellow legged frogs R. muscosa, by predation of their larvae. Removal of trout from some lakes has resulted in rapid recovery of R. muscosa populations.
Introduced species can bring alien pathogens to amphibian populations with them. North American bullfrog, R. catesbeiana, is an effective carrier of chytridiomycosis. Hybridization is another problem. Introduced Italian crested newts (Triturus carnifex) have hybridized with native northern crested newts (T. cristatus) in Switzerland and in southern England. But the impacts on populations appear to be local rather than regional. Introduced marsh frogs R. ridibunda have replaced the related water frog R. lessonae in several areas of western and central Europe, and probably this is at least partly a result of complex genetic consequences of hybridization. ( Beebee and Griffiths, 2005)
In habitats with exotic species, the size of native amphibian populations is often greatly reduced. Experiments show that when exotic, predatory fish and crayfish are present, native amphibians reduce activity, use different habitats, increase use of refuges, are smaller at metamorphosis, survive less well, show more injuries, and have fewer resources because of competition. Introduced trout and the decline of mountain yellow-legged frogs in California. Habitat change and the introduction of non-native mammals are causes of the extinction of the 3 largest New Zealand frog species. Overall, introduced species raise several key questions.
4. Climate change
Recent changes in the global climate might impact adversely on amphibian populations. Global mean temperature rose by about 0.6 0C over the past 100 years with an accelerating trend since the 1970s. It causes multiple effects of climate change on wildlife and ecosystems. There is no current evidence for that climate change has led to tolerance limits in temperature exceeded in amphibians. There are been detectable effects of climate change on breeding phenology.
The golden toad (B. periglenes) of the Costa Rican rainforest disappeared completely at the end of the 1980s. ( Beebee, T.J. and Griffiths, R.A., 2005). Many species of this rainforest biota declined over the past 20 years. It seems that warmer sea surface temperatures in the Pacific have caused thermal uplift in the atmosphere. The forest has consequently become drier, and amphibian breeding less successful. (Beebee and Griffiths, 2005)
Immediately prior to the disappearance of golden toads, Bufo periglenes, the rainforests of Monteverde, Costa Rica, had the lowest twelve-month rainfall in 20 years. Toads were forced to shift their habitat use. Unusual weather conditions were a cause of declines of Australian rainforest frogs. Violent storms like short-term climatic events such as can alter the dynamics of amphibian populations. Hurricane Hugo caused extensive damage to the forests of Puerto Rico in 1989. In the short term, populations of the terrestrial frog Eleutherodactylus richmondi decreased by 83%. The ecology of amphibians is affected in a number of ways due to the alterations in local weather conditions caused by global climate change. The onset of spawning in Rana temporaria in Finland between 1846 and 1986 shifted earlier by 2–13 days, following shifts in air and water temperature and dates of snow cover loss.
Decreases in summer precipitation and increased temperatures and winter rainfall effects on the amphibians in Canada. The extended dry seasons, increased temperatures and increasing inter-year rainfall variability may affect litter species by reducing prey populations and altering amphibian distributions on increasingly dry soil in the neotropics. Reproductive phenologies of pond-breeding species is affected by shifting rainfall patterns. Ponds will fill later and persist for shorter periods, leading to increased competition and predation as amphibians are concentrated at increasingly limited aquatic site. Frogs exposed to these stresses may also become more vulnerable to parasites and diseases. (Alford and Richards, 1999)
5. Habitat modification
Habitat modification is the best documented cause of amphibian population declines. Habitat loss certainly reduces amphibian abundance and diversity in the areas directly affected. Removal or modification of vegetation during forestry operations has a rapid and severe impact on some amphibian populations. Clear cutting of mature forests in the southern Appalachians has reduced salamander populations by almost 9%. Logging exposes terrestrial amphibians to drastically altered microclimatic regimes, soil compaction and desiccation, and reduction in habitat complexity. It exposes aquatic amphibians to stream environments with increased siltation and reduced woody debris. Although populations may recover as regenerating forests mature, recovery to pre disturbance levels can take many years and may not occur at all if mixed forests are replaced with monocultures. Draining wetlands directly affects frog populations by removing breeding sites, and by fragmenting populations which increases the regional probability of extinction. Amphibian populations can be eliminated or declined due to the modification of terrestrial and aquatic habitats for urban development. Populations of some amphibians in urban Florida declined after degradation of upland, dry season refuges and modification of wetlands used for breeding. Protection of aquatic breeding sites may be of little value if adjacent terrestrial habitats used by amphibians for feeding and shelter are destroyed. More subtle alterations to habitat structure can have severe impacts on amphibian populations. Bufo calamita populations in Britain declined over a 40-year period due to shifts in land use practices that altered vegetation characteristics. Changing vegetation structure and an associated increase in shading were detrimental to B. calamita and detrimental to B. calamita and provided conditions under which the common toad Bufo bufo became a successful competitor.
Although habitat alterations can reduce amphibian populations, in some cases even severe habitat modifications can have little effect. (Alford and Richards 1999)
Sri Lanka offers a clear example of how land use change and amphibian species losses are related (Meegaskumbura et al. 2002). The country has 0.013% of the world’s land surface, and >2% percent of the world’s frog species. Some 95% of its rain forests are gone. Patches now cover <2% of the island. However, 17 of Sri Lanka’s native frog species disappeared in the past decade. 50% of 34 confirmed amphibian extinctions in the past 5 centuries (Meegaskumbura et al., 2002).
There are numerous examples of how land use change and habitat loss cause the decline and extinction of many species, including amphibians (Collins,2010). All evidence indicates that more losses are expected. Land use change that results in habitat destruction is the leading cause of amphibian decline and extinction (Collins.,2010).
6. Acidity and toxicants
The different acidity levels of aquatic habitats have major impacts on amphibian distribution, reproduction, and egg and larval growth and mortality. Sensitivity to low pH varies among and within species and is influenced by complex chemical interactions among pH and other factors, particularly aluminum concentration. Both the embryonic and larval stages mortality occurs in via several mechanisms including incomplete absorption of the yolk plug, arrested development, and deformation of larvae.
Sublethal effects of acidification include delayed or early hatching, reduced larval body size, disturbed swimming behavior, and slower growth rates resulting from reduced response to, and capture of, prey. Indirect sublethal effects include changes to tadpole food sources through impacts on algal communities, and shifting predator-prey relationships resulting from differential mortality of predatory fish and invertebrates in acidified habitats. The population-level effects of acidity are less well understood.
It is possible that the effects of low pH, in combination with other abiotic factors, lead to decreased recruitment into adult populations. The acidic breeding sites often contain less diverse amphibian assemblages, at lower densities, than do less acidic sites. Long-term acidification of ponds in Britain has excluded Bufo calamita from many sites. In an Appalachian stream reduced pH and increased metal concentrations has caused elimination virtually all salamander larvae, causing severe long-term declines in populations of Desmognathus quadramaculatus and Eurycea wilderae. Low soil pH also influences the distribution, abundance, and diversity of terrestrial amphibians. There are few data to implicate acidification in recent, unexplained catastrophic population declines despite the well-documented effects of low pH on amphibians. Acid deposition is a factor in the decline of tiger salamanders, Ambystoma tigrinum, in the Rocky Mountains. Acid deposition is involved in population declines of frogs and salamanders at high altitudes in the Sierra Nevada Mountains and Rocky Mountains. Similarly, although there is an extensive literature on the toxic effects on larval amphibians of metals and chemicals used in insecticides and herbicides insufficient data exist to determine their long-term impacts on amphibian population dynamics. Environmental toxicants act directly to kill animals, or indirectly by impairing reproduction, reducing growth rates, disrupting normal development and reproduction (endocrine disruption), or increasing susceptibility to disease by immune suppression or inhibition of immune system development (Alford and Richards 1999).
7. Commercial use
From the ancient times humans have exploited amphibians, especially the larger species, as a food resource in many parts of the world. Although this is undoubtedly the most substantial direct impact of human predation, others include collection for the pet trade, education and medical research, use as bait by anglers, and even conversion into leather as fashion accessories. Millions of amphibians, mostly large frog species, are sacrificed for food each year. Only a tiny proportion of this consumption is supported by captive breeding or farming enterprises. The bulk is a result of collection, much of it illegal, from wild populations in Asia. Although local population declines have been documented in areas of intense harvesting, there is little information about long-term or large-scale consequences. ( Beebee, and Griffiths, 2005)
A 2001 UN/FAO report reached 4 main conclusions:
(1) Almost 95% of the world demand for frog legs and frog products is still supplied from wild stocks.
(2) In 1998, the international trade in frog legs involved more than 30 countries with a value of around US $49 million.
(3) The main focus of harvesting is 11 species worldwide
(4) Worldwide from 1987 to 1997, an average of about 4716 metric tons of frogs were collected annually (these data do not include the major exporting nations of China and Vietnam).
US trade records from 1998 to 2002 reinforce these conclusions. 5.2 million kg and 15 million individuals were imported and declared as wild caught. 96% of the trade was commercial, mainly for pets and food, most trade involved 9 frog families and 2 salamander families. In the western USA, Ambystoma tigrinum virus and the amphibian chytrid fungus Batrachochytrium dendrobatidis (Bd) are spread via the bait trade in tiger salamanders Ambystoma. 28 million Rana catesbeiana were imported into 3 US markets from 2000 to 2005. Commercially traded amphibians act as a source of pathogen pollution. (Collins.,2010)
Frog leg trade is very popular worldwide. Frog populations have been devastated by humans in several countries for the frog-leg trade. From Asia about two hundred million frogs were exported annually before 1995. India was still illegally exporting approximately seventy million frogs each year by 1990. It is resulting in serious population declines in the country.
DISCUSSION
Most cases the amphibian declining is caused not by only one cause but it is the result of interaction between different factors. The increased UV-B exposure levels may alter the species interactions or vulnerability to pathogens or changes in pH. Elimination of local populations can be caused by predation. It may have larger-scale effects by altering rates of migration between populations. Disease outbreaks may occur when other stresses reduce immune function. Pollutants, pesticides and environmental acidity may interact to produce unforeseen effects. All the local effects are interacting with global climate change. For the proving of the existence of these complex effects in natural populations will require well-planned programs of observation and experimentation. In planning such studies and to determine how stresses affect population behavior requires an understanding of the nature of the populations being studied and the limitations of study techniques importantly.
Population declines attracts the attention of paleontologists, conservation biologists. In the last decades of the 20th century a diversity of researchers has been studying contemporary ecological and evolutionary processes. Amphibians illustrate that it is possible for an infectious disease to emerge and place many species within a class of organisms at risk of population declining and extinction.
Modern amphibian decline and extinction bring losses that paleontologists relegate to cataclysmic events into the realm of current ecological processes. Modeling and predicting ecosystem processes in a montane, neotropical ecosystem today means having to assume that within the foreseeable future a significant fraction of the grazers (tadpoles) and predators (metamorphosed amphibians) could be gone from the aquatic and terrestrial components of the ecosystem.
Very little scientific knowledge is known about the diseases of wild amphibians. Most of the disease-causing agents are present in healthy animals, making them act as vectors at also and disease occurs when immune systems are compromised.
Even when increased levels of UV-B causes higher embryonic mortality in declining species, the ecological significance of this at the population level is difficult to assess. Much more is need to be understood about the basic natural history of amphibian species that might be at risk of population declining. As an example, more information is needed on variation in oviposition site characteristics (depth, vegetation) within local populations.
However, the recent climate patterns are not unprecedented and there is no evidence that similar conditions within the past 50 years led to amphibian declines. It is therefore uncertain as to whether recent climate change is a significant cause of amphibian declines. Recommendations should be made to address this issue, by suggesting improved methodologies to investigate climate change as an agent of amphibian declines.
There may be no documented cases which shows the effect of acidification of natural habitat on the population fluctuations of amphibian population. Studies of acid tolerance may have been biased toward species that are likely to have evolved tolerance to low pH.
Despite accumulating evidence that commercial trade causes declines and moves pathogens that may infect native species, not much examples are yet where commerce alone has decreased amphibian population sizes to extinction. But the potential is there.
It is important to determine the factors controlling amphibian population sizes and species richness as land use and land cover change. It is needed to identify how to ameliorate the impact of land use change or even block land use changes that threaten amphibian habitats.
Although they have been the subject of many experimental and monitoring studies, the autecology of amphibians in nature is poorly understood. The majority of studies of ecology and population biology of amphibians have been conducted on aggregations at reproductive sites. Relatively a very little is known of their movements or activities away from breeding sites. Less data is available on the rates of exchange between populations. Long-term combined and integrated studies are highly needed understanding ecological theory and increasing knowledge of the amphibians. Simple long-term programs that monitor the fluctuations of single populations and associated environmental factors, and then application on standard population models, are not useful for understanding the dynamics of amphibian populations as they have not worked well for whenever applied to other terrestrial vertebrate populations. It is clear that the understanding of problem of amphibian declines will highly require much more information on the ecology of the metapopulations in which many species are living.
Commercial use, introduced/exotic species, and land use change are among the historical causes of amphibian declines. These causes have been acting for centuries, and all apply to many species beyond amphibians. Contaminants, climate change, and emerging infectious diseases are important late 20th century causes of decline and even extinction. They are the primary hypothesized causes of the so called ‘enigmatic declines’ the decline of amphibians in protected areas. It is important to fulfill the gap between scientific knowledge and amphibian declining. Further studies are highly recommended for searching the reasons for the amphibian population fluctuations and relationship between declining and anthropogenic as well as natural causes for tHe declining. Since amphibians play critical roles in both aquatic and terrestrial eco systems their survival is very important for the future health of the ecosystem. Immediate necessary actions are required to address the amphibian declining issue before they reach to extinction level. Awareness, regulations, policies can be applied as a part of biodiversity conservation.
REFERENCES
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· 2. Collins, J.P., 2010. Amphibian decline and extinction: what we know and what we need to learn. Diseases of aquatic organisms, 92(2-3), pp.93-99.
· 3. Beebee, T.J. and Griffiths, R.A., 2005. The amphibian decline crisis: a watershed for conservation biology? Biological conservation, 125(3), pp.271-285.
· 4.Hays, J.B., Blaustein, A.R., Kiesecker, J.M., Hoffman, P.D., Pandelova, L., Coyle, D. and Richardson, T., 1996. Developmental responses of amphibians to solar and artificial UVB sources: a comparative study. Photochemistry and Photobiology, 64(3), pp.449-456.
· 5. Long, L.E., Saylor, L.S. and Soule, M.E., 1995. A pH/UV-B synergism in amphibians. Conservation Biology, 9(5), pp.1301-1303.
· 6. Meegaskumbura M, Bossuyt F, Pethiyagoda R, Manamendra-Arachchi K, Bahir M, Milinkovitch MC, Schneider CJ (2002) Sri Lanka: an amphibian hot spot. Science 298:379
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