Vanishing Species - The Indian Amphibians

An article by Mohan Pai

The Indian Amphibians

‘Frog’ Thai woodcraft - pic by Mridula Pai

The First Land Animals that have survived five mass extinction cycles.

The term Amphibia means ‘having two lives’; an apt description for a class of animals who live an aquatic fish-like existence when young, and become terrestrial when adult. Thus the term ‘Amphibia' is applied to a class of vertebrates that fall between the fishes and the reptiles. Amphibians differ greatly in form, and range from the legless worm-like caecilians to the lizard-like salamanders and newts. Some of the major differences that separate amphibians from the other vertebrates are: a body covered with generally moist skin without scales, fur or feathers; soft toes with no claws; a two chambered heart in the larval stage and a three chambered heart in adults; external fertilization of eggs; and the process of METAMORPHOSIS.

The first records of the Amphibians are from the early Devonian (over 300 million years ago) and early Silurian times. These remains are associated with freshwater fish-like animals. The rise of the Amphibians from their fish-like ancestors required radical changes. For instance, the evolution of limbs from fins, a breathing apparatus that could function effectively in air, and a skin that could remain moist.

The early forms were all four-leggedand tailed but were mainly aquatic. The habitat of Eogyrinus, one of the earliest known amphibians, seems to have been temporary pools in arid areas, where the drying of the pools would have caused it to make overland journeys to other pools. From these modest beginnings, amphibians have evolved to occupy all known habitats except some of the more climatic extreme, such as the polar area.
At present the Class Amphibia is represented by three Orders: the Apoda, which encompasses the worm-like CAECILIANS, the Caudata or ‘tailed amphibians’ that are represented by NEWTS & SALAMANDERS, and the Salientia which includes FROGS & TOADS.

Mating tree frogs, Mahadayi Valley. Pic by Mohan Pai

METAMORPHOSIS

In most amphibians eggs are fertilized externally. The female may lay eggs individually or in strings. The eggs absorb water rapidly after they are laid, and after a certain stage cannot be penetrated by the sperm. This requires sperm to be in contact with the eggs immediately after laying. The familiar clasp or embrace of mating frogs enables the male to release the sperm almost directly the eggs are released.
On hatching, a larva emerges from the egg. In the frogs and toads, this is the tadpole. Most amphibian larvae usually start off with external gills but tadpoles generally have internal gills. The next major changes in the tadpole are usually seen when the adult stage is reached. Tadpole demonstrates the most striking changes with the appearance of limbs and disappearance of the tail. The major changes in the tailed and legless at metamorphosis is usually the disappearance of the external gills. Before metamorphosis, the lungs develop and are functional by the time the gills begin to disappear.

VOCALIZATION
Of the three living orders of amphibia, only two have been known to produce any vocalization - the frogs and toads, and tailed amphibians. All the amphibian calls one hears on the Indian subcontinent come from frogs and toads.Although calls to attract mates (mating calls) are the commonest uses of voice in amphibians, there are five other conditions which may give rise to sounds. These sounds are called release calls, warning sounds, rain calls, screams, and territoriality calls.

Amphibians are subject to enormous predation. From the egg to adult, they forma part of the diet of innumerable enemies. Very few individuals from a single brood live to reach maturity. The eggs are eaten by insects and even by some salamanders. The tadpole stage falls prey to dragonfly nymphs, water-beetle, giant water-bugs. Fishes, birds and crustaceans. When emerging as froglet there is heavy predation by birds and by larger frogs. The adult frogs are preyed upon mostly by snakes, monitor lizards, birds, and carnivorous mammals.
The length of life in the amphibia has been recorded to vary from as short as two years in the spadefoot toads to as long as fifty years from some salamanders; the larger animals live longer than the smaller ones.

Endemism

Out of the 219 amphibian species in India, 134 species (i.e. 61%) are endemic to the country. The Western Peninsula harbours highest number of endemic species (92) followed by the Northeast (29). Besides these, Andaman & Nicobar Islands have five endemic species while the North has three, Deccan Plateau three and the Gangetic Plains two.

Status

The status of most of the species of Indian amphibians is unknown as no population studies have been properly conducted. In 1997, the amphibian experts of our country, under the guidelines of the IUCN met in Bhubaneshwar to assess the status of the Indian species. They concluded that out of the 207 amphibian species then known from India, nine were critically endangered, 42 endangered, 39 vulnerable, 74 in the lower risk category while for 43, the data was deficient for determining their status. However, only four species, the Garo Hills tree toad (Pedostibes kempi), the Malabar black narrow mouthed frog (Melanobartrachus indicus) and the Himalayan salamander (Tylototrition verrucosus) are protected under Schedule II of the Indian Wildlife (Protection) Act, 1972 and the frogs of the genus Rana under II Schedule IV of the Act.

Distribution

The amphibian species are not evenly distributed throughout India.The highest concentrations of species are found in the Western Peninsula followed by the Northeast. Interestingly, all the three living orders of Amphibia, viz., Gymnophiona (limbless amphibians), Caudata (tailed amphibians) and Anura (tailless amphibians) are distributed in Northeast India. The Western Peninsula has Gymnophiona and Anura while the remaining geographical regions of India harbour only Anura. The order Gymnophiona include worm-like fossorial limbless amphibians living a subterranean mode of life. These are very rare and secretive, as a result of which very little is known about their habits and life history. Twenty species occur in India. The only representative of the order Caudata in India is the Himalayan salamander.

The Himalayan Salamander

Tylototrition verrucosus. It is semi-aquatic and found in the hilly lakes of Sikkim, Northern West Bengal, Khasi hills of Meghalaya, Lohit district of Arunachal Pradesh and Manipur between altitudes of 1330-2220 metres. But its population is dwindling.

Threats

Since amphibians must breed in water, their permeable skin and eggs are particularly sensitive to pollution and other changes in water quality. Therefore discharge of pesticides, fertilizers, detergents, domestic sewage and toxic industrial effluents into the water bodies take a heavy toll on the numbers of the amphibians. Besides, habitat destruction in the form of draining and filling up of wetlands, clearing of land for agriculture and felling of natural forests (canopy opening) has devastated the amphibian population of our country. In the last few decades, a large number of frogs were captured and their limbs exported as part of the frog-leg industry to gain foreign exchange. However, this activity resulted in a tremendous increase in agricultural insects/ pests. Realizing this, the Government of India banned the export of frog-legs since April 1986. Since then, the trade has declined but some illegal export still takes place through the neighbouring countries.

Small wood frog

Amphibians are considered to be the best indicators of environmental health. A decline in amphibian populations indicates ecosystem deterioration that might affect a wider spectrum of the earth's biological diversity. During the last 12 years there has been a great concern, worldwide, about the rapid decline in amphibian populations. Many reasons have been attributed to the loss of amphibians including habitat loss, UV-B radiation, global warming, toxic chemicals, pathogens that destroy eggs and larval stages, direct harvest and other. Of these, loss of habitat seems to be the most significant factor, at least in tropical countries.species known in India.

From IUCN Red List of Threatened Species:

The state of amphibians in IndiaIndia has the third largest amphibian population in Asia. The amphibian fauna of India comprises of 272 species of which 167 (66.3%) are endemic to the country. In spite of its broad variety of species, India holds second place on the list of countries having the most number of threatened amphibian species in Asia, with 67 (25%) of its species facing possible extinction. Out of the 38 species of amphibians in Asia that are confirmed to be extinct, by the Global Amphibian Assessment (GAA), 1 is from India. In addition, 13 species are listed as Critically Endangered, 31 as Endangered and 23 as Vulnerable. A further 95 species are listed under the data deficient category indicating the number of threatened species may be much higher once information becomes available.ThreatsLoss and fragmentation of habitats is the immediate threat to amphibians in India. A vast majority of Indian amphibians occupy regions that are increasingly being used for agricultural purposes. In addition to this, a vast majority of amphibian species dwell in regions that are undergoing urban development, logging and industrialization that have resulted in a drop in stable amphibian habitats. Pollution plays a major role in creating an unstable environment for amphibians in India. Excessive use of pesticides such as DDT, Dieldrin and Malathion have been shown to affect the immune systems of certain amphibian species while use of herbicides such as Atrazine has an affect on their reproductive ability by inducing sex reversal. The building of dams and water management systems disturbs stable environments by altering the natural river flow in areas populated by amphibians. Some species face a dramatic drop in number due to the introduction of alien species such as mosquito fish Gambusia affinis that destroy amphibian eggs.

Among the key findings in 2008 are: Nearly one-third (32 %) of the world's amphibian species are known to be threatened or extinct, 43 % are known to not be threatened, and 25 % have insufficient data to determine their threat status. As many as 159 amphibian species may already be extinct. At least 38 amphibian species are known to be Extinct; one is Extinct in the Wild; while at least another 120 species have not been found in recent years and are possibly extinct. At least 42 % of all species are declining in population, indicating that the number of threatened species can be expected to rise in the future. In contrast, less than one percent of species show population increases. The largest numbers of threatened species occur in Latin American countries such as Colombia (214), Mexico (211), and Ecuador (171). However, the highest levels of threat are in the Caribbean, where more than 80 % of amphibians are threatened or extinct in the Dominican Republic, Cuba, and Jamaica, and a staggering 92 % in Haiti. Although habitat loss clearly poses the greatest threat to amphibians, a newly recognized fungal disease is seriously affecting an increasing number of species. Perhaps most disturbing, many species are declining for unknown reasons, complicating efforts to design and implement effective conservation strategies.

References: Encyclopedia of Indian Natural History (BNHS), IUCN Red List, Envis Newsletter (Oct-Dec 2001)

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