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.
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