Amphibians play a pivotal
role in ecosystem as secondary consumers in many food chains. Tadpoles have
significant impact in nutritional cycling. They are herbivorous to omnivorous
and are the prey items for both invertebrates and vertebrates. Adult amphibians
are the best biological pest controllers. Invertebrates and vertebrates also
predate them. Because of their importance in ecosystem, decline or extinction of
their population has significant impact on other organisms along with them.
the ecological perspective, amphibians are regarded as good ecological
indicators. Due to high degree of sensitivity, either during tadpole stage or as
adults, they respond to very slight change in the environment. Such responses
have been used to indicate habitat fragmentation, ecosystem stress, impact of
pesticides, and various anthropogenic activities.
In human cultures,
amphibians featured through ages in the form of poetry, songs or stories.
Amphibians have been a good food source and few years ago India dominated in
frogleg export (it is completely banned now) along with Southeast Asian
countries. This has resulted in increased insect pest population. They are
exploited as model organism in ecological, embryological, physiological and
genetic research. Amines, alkaloids and polypeptides are found in skins of
amphibians and have pharmacological importance. Epibatidine, a skin
extract from Epipedobates tricolor, a south American frog blocks the pain
200 times more effectively than morphine. Poison
dart frogs of Dendrobatidae family have highly toxic skin compounds that
are smeared to arrowheads to kill larger animals.
From the past two decades,
amphibians have gained much importance for the declines in their population
worldwide. There are clear evidence for such declines from North, Central and
South America, Europe, Africa and Australia. Global amphibian decline can be
attributed to those with obvious causes like habitat destruction, alteration,
fragmentation, climate change, radiation, chemical contamination, pollution and
diseases of viral, fungal, bacterial infection as evident from various parts of
the world and those mysterious declines with no obvious cause. Looking from
another perspective, the threat to the amphibians can be attributed to changes
in physical environment and biotic environment. Changes in the physical
environment include Ultra-Violet (UV) radiation, climate change, acid rain,
pesticides, herbicides and fertilizers, where as the biotic environment
includes, fragmentation, demographic effects (barriers for the movements of one
population to other), genetic effects (fragmentation enhances isolation,
isolation emphasises inbreeding, inbreeding results in stress, stress reduces
viable population), diseases and synergistic interactions. The influence of
these factors operates at two levels, either singly and/or synergistically.
factors influencing Global Amphibian Decline
Changes in Physical
UV-B damages DNA and/or kills cells, causing egg
mortality, lesions, and increased susceptibility to disease and low pH.
The changes in the climate (due to global warming)
can lead to decreased depth of pond water and increased intensity of UV-B
radiation. This ultimately reduces the immune system of amphibians. Hence
they will be more prone to diseases.
Acid precipitation and soil
Toxins create barriers to dispersal and cause high
egg and larval mortality.
Toxins cause direct mortality of eggs and adults,
mimic endocrine hormones, reduce the prey
Changes in Biological Environment
Habitat modification, fragmentation
Deforestation and agriculture; wetlands are
drained and filled. Roads, introduced species, and low pH dissect
habitats, creating barriers to dispersal.
Introduced predators, prey on/or compete with
Disease often causes death in amphibians; what
made amphibians susceptible to disease is often unknown.
Uphill migration of birds and lizards with rise in
cloud cover elevation
We are very much puzzled over mysterious decline of
amphibians. Because, these declines have occurred in protected areas and
moreover it occurred very rapidly and selectively. This ultimately points out
that, some of these parameters are also going to threaten human welfare.
have revealed that, some amphibians exhibit abnormalities or malformations. For
example, existence of more limbs, misshapen or missing of limbs, etc. This is
due to impact of ecosystem disruption on developing individuals and plausible
agents include UV-B radiation, chemicals, pesticides and parasites
In the Visible spectra (VIBGYOR), the region of radiation from 280 to 400 nm, is called Ultraviolet radiation. It falls under three categories separated by wavelength. UV-A has wavelengths from 320-400nm (nanometers), UV-B from 280-320nm, and UV-C below 280nm. In general, the shorter the wavelength the more energetic the radiation making it damaging to plants and animals. UV-C can do great damage but fortunately poses no risk to life on earth. It is completely filtered out by oxygen and ozone in the stratosphere. Ozone also plays a vital role in filtering UV-B, the ultraviolet radiation that is the greatest threat to life on earth. But even a fully functioning ozone layer does not absorb all the UV-B rays. UV-A surpasses the stratosphere virtually unfiltered. But compared to the shorter wavelengths, UV-A causes little harm and even plays an important role synthesizing vitamin D in humans. In amphibians, the eggs and tadpoles are susceptible to UV-B radiation damages. Due to UV-B radiation, DNA of the egg or tadpoles gets damaged. To some extent photolyase enzymes reduce this damage, but severe exposure leads to irreversible changes in DNA and results in death of egg or tadpole or even malformations. This ultimately leads to population decline.
Due to the process of global warming (rise in earth's mean temperature), drastic changes in the climate occur. This lead to decreased depth of pond water, increased intensity of UV-B radiation. Which ultimately reduce the immune system of the amphibians. Due to this amphibians are more prone to diseases and death.
Frogs became hermaphrodites
Atrazine, a widely used pesticide is making leopard frogs (Rana pipiens) hermaphrodite (bearing both male and female gonads in the same organism). Atrazine gets into aquatic water body, by means of agriculture runoff. It has the ability to disrupt sex hormones. Even the exposure of 0.1 ppb (parts per billion), has led to the mixed gonads in Rana pipiens. This may be one of the causal agent for amphibian decline, world over.
T., Haston, K., Tsui, M., Hoang, A., Haeffele, C. & Vonk, A.
Feminization of male frogs in the wild. Nature, 419,
T. et al. Atrazine-Induced Hermaphroditism at 0.1 PPB in American
Leopard Frogs (Rana Pipiens): Laboratory and Field Evidence. Environmental
Health Perspectives, published online doi:10.1289/ehp.5932 (2002).
Hayes, T. et al. Hermaphroditic, demasculinized frogs after
exposure to the herbicide atrazine at low ecologically relevant doses. Proceedings
of the National Academy of Sciences, 99,