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VANISHING FROGS AND TOADS (Part-2)

VANISHING FROGS AND TOADS (Part-2)
By Nagaland Post | Publish Date: 1/29/2019 5:20:16 AM IST

 Dr. J. Meren Ao

All frogs and toads are included into a group called Anura and classed under Amphibia (double life). Amphibians are able to live on land as well as in water and are thus named after their way of life. From evolutionary point of view they are intermediate between the fishes in water and reptiles on land. The group Anura which comprises of frogs and their subgroup the toads is by far the largest group, with 5200 living species currently recognized.

The number of recognized species of amphibians has grown enormously in recent year. This unprecedented growth largely reflects an increase in collecting work in previously remote locations and the application of contemporary technique such as DNA sequencing to support more traditional taxonomic methods. Unfortunately our increase in the knowledge of amphibian species diversity and biology is coincident with massive global decline in amphibian populations and their extinction.

While scientists discuss the urgency of preventing further decline, it is clear that many of us are uncertain about why we should care about the loss of these animals instead of caring for our own survival. In nature, we share and live with other living and non-living components within a balanced system called ecosystem. Disturbance or removal of a component from this well balanced system due to our own unwise short sighted activities may affect our own survival in the long run. Today due to population growth, scientific advancement and other factors men have changed ecosystems more rapidly and extensively in the last 50 years than in any comparable period of time in human history.

The Global Amphibian Assessment (GAA) has recorded that some 43% of amphibian species are declining in abundance, 32% are threatened with extinction and 122 species likely have become extinct and Northeastern region, including Nagaland is no exception. Decades ago, some of the animals which were abundant and familiar to us are no longer seen today and their population have either become reduced greatly or on the verge of disappearance. What are the causes of the decline? It may not be a single factor but a combination of many factors.

Habitat loss

Habitat loss is one of the major threat for amphibian population decline. Most of the amphibians occur in forested habitats, low land, montane forests and also in standing and running water habitat. The greatest destruction of habitats has occurred during the last century. The tremendous increase of human population of the world from a billion to more than six billion within one and a half century has lead to the gradual consumption of natural resources. Today most of the habitats that formerly occupied wide areas are now often fragmented or divided up to small patches due to expanding croplands, commercial logging, massive agricultural and forestry projects, urbanization and other development. These fragments are often isolated from one another. Unlike many other animals, some amphibians have very low ability to disperse and tend to be largely confined to small areas of moist habitat. When such habitats are fragmented due to human activities, species are reduced and lost in the long run due to insufficient area, changes in micro and macro climate which include changes in atmospheric and soil moisture, temperature, water flux, light, vegetation structure, soil and litter composition and isolation which does not allow re-colonization and inbreeding. Moreover most of the fragments continue to be under severe pressure from human activities. There has been drastic loss of forest  habitats, largely as a result of shifting cultivation. The remaining undisturbed forests are confined to steep hill slopes and hill tops, but these too are also affected. During the last few decades, the fallow period between Jhum cultivation was 10-15 years cycle. But today the fallow period has been shortened to less than 5 years. This prevents the resident animal and plant communities to re-establish. 

Climate change

The climate change is currently affecting life on Earth and has become overwhelming in the last few years. According to the scientists, in the last 500 million years of the history of Earth’s life, five great events of extinctions, starting from Ordovician period (448 m years ago) to Cretaceous period (66 m years ago) have already decimated a variety of living organisms including giant dinosaurs. The first four events of extinction are believed to have been caused by climate change and the fifth by a giant meteorite crash. The sixth extinction, which we are facing today, according to the scientists is due to human activities.

The atmospheric cover with normal concentration of carbondioxide and other four greenhouse gases such as water vapour, methane, nitrous oxide and ozone allows heat in the form of sunlight to pass through to reach the Earth’s surface but prevents a substantial amount of the heat from being re-radiated into outer space. This natural phenomenon is called green house effect which has been operating naturally for the past millennia and  is responsible for maintaining temperature of the Earth at a normal habitable level of average 150°C. But today this balance is disturbed due to rise in burning fossil fuel, land use changes and other human activities which have lead to the increase in green house gases, specially carbondioxide. The excessive increase of these gases would absorb and retain more and more heat, resulting in enhanced green house effect leading to global warming and climate change. The main characteristics of climate change, therefore are: increase in average global temperature (global warming), changes in cloud cover and precipitation and melting of ice caps and glaciers, particularly over the land mass. Over the last century, atmospheric concentration of carbon dioxide increased and average global temperature rose by 0.740°C. According to scientists, this is the largest and fastest warming trend in the history of the Earth and that the Earth would warm by 30° C towards the later part of this century. The rate of current trend in temperature shifts, therefore is vastly different from past climate change events. Could amphibian disappearance be attributed to global warming pattern? The answer may be in the interaction between climate change and diseases. Climate change is already impacting human health by facilitating the spread of disease. Similarly, evidence of a link between amphibian decline and climate change is growing. Changes in temperature or precipitation influence host-pathogen interactions. One consequences is an increase in the outbreak of lethal disease such as Chytridiomycosis caused by a fungus, which infects the skin of amphibians. The environmental stress due to various ecological factors including the climate change could suppress the immune system of frogs leaving to a reduction in disease resistance. Amphibians are cold blooded animal that is they cannot maintain a constant internal body temperature like humans; hence, especially in the higher altitude they may expose themselves to sun light in order to increase the body temperature. These characteristics including the habitat destructions could increase their exposure to ultra violet radiation, often associated with thinning of ozone layer.

Pollution 

Another significant threat to amphibian population decline is pollution and it is due to the fact that a higher proportion of amphibians are aquatic. Among the over-riding causes of increasing pollution is the increasing use of agrochemicals such as fertilizers, herbicides, fungicides and pesticides. Physiologically, the skin and egg capsules of amphibians are often highly permeable to gases and liquids and may readily absorb pollutants such as toxic chemicals and other substances from the surrounding environment. Amphibian larvae feed on a wide variety of plant and animal matter, in all parts of the water column from the bottom decomposing matter to the surface film. 

The contaminants attach with the food particles when ingested accumulate in fat deposit and during further development, when the fat deposits are drawn upon, the contaminants may significantly affect normal development. The adult amphibians largely feed on invertebrates, which are also vulnerable to accumulation of chemical pollutants such as agrochemicals. High concentration of agrochemicals is almost lethal to amphibians but even very low and sublethal concentration can also have unexpected effects on them. Analogous to the side effect of human drugs, even very low concentration of pesticides are now known to alter animal behavior in ways that can reduce feeding, impair locomotion and weaken predation avoidance and affect the immune system making amphibian more susceptible to parasites and pathogens. To this, a wide variety of stressors that occur in nature such as elevated temperature, reduced pH, increased ultraviolet radiation, competition and the threat of predation affects in amphibian population decline. Jungle clearing for Jhum cultivation, commercial logging, urbanization, development works, stone quarries, road construction and land slides, all contributes to silting in the water bodies which increases sedimentation or turbidity of streams and other water bodies that affect the adult as well as the development of larvae.

Over–exploitation

Increasing human population rapidly increase the use of food resources. Methods of harvesting have been dramatically modified to have maximum gains. In traditional societies, there existed some controls to prevent over exploitation in several ways. In contrast to this, in much of the world to day resources are exploited as rapidly as possible. To many people, it may be surprising to hear that amphibians, specially the frogs are valued food sources throughout much of the world. Frogs are delicacy in Southeast Asia including the tribal belt of Northeastern region specially Nagaland. Amphibians have long been recognized for their value in traditional medicine by tribal and local people, often to meet primary health needs. Besides there are some areas, notably Southeast Asia where commercial markets exist for the use of amphibians in traditional medicinal practice. More than 30 species of amphibian have been recorded in traditional Chinese medicine alone. Commercial collection of wild amphibians specially the frogs and toads have been unsustainable and in many cases appears to have resulted in significant reduction in their population. The international trade in frog legs still largely depends on the collection of wild frogs. In order to meet both international and domestic market demands for frogs, commercial frog aqua culture has been developed in some parts of the world. But frogs have generally been considered difficult to farm as they consume live animals that move and also comparatively slow to reach marketable size and therefore uneconomical. The principal source of frog legs export were India and Bangladesh, however concern about the ecological damage resulting from heavy harvest of frogs in particular of the species Hoplobatrachus and Euphlyctis led to the export bans in India. Millions of frogs and toads were used in the educational sector in India. However University Grants Commission has made some restriction in this regard which remains debatable. In our local markets, generally six species of frogs are sold such as Hoplobatrachus tigerinus (Indian bull frog) 1, Hoplobatrachus crassus (Jerdon’s bull frog) 2, Euphlyctis cyanophlyctis (Skipper frog) 3, Humerana humeralis (Boulenger’s green frog) 4, Fajervarya nepalensis (Warty frog) 5 and Fajervarya teraiensis (Terai warty frog) 6. Apart from this some species, however do not reach market but are collected seasonally for consumption. Overexploitation threatens many species of the world to day and even if a species  is not completely eliminated, the population size may become so low that the species is unable to recover.

There are millions of organisms inhabiting the earth. Of these hardly one and half million have been identified. Yet today, thousands of species become extinct every year. If this trend of depletion continues, many species will become extinct by the end of the century. It is true that as a rule of nature, extinction of species happen all the time. But current extinction rates are much more than the natural rate, rival those of the five greatest mass extinctions in world history. But, while the earlier mass extinctions of species were attributed to their inability to adapt to a changing environment, recent species losses are the direct result of changes resulting from human growth and their activities. In an increasingly human dominated world, we have reduced the populations and habitats of many species to levels where their extinction could be faster. Unfortunately there is yet no stoppage of most of the activities that would accelerate their extinction. To day we talk about vanishing frogs, toads and other biodiversities but if these continues at the present rate then our own survival is at stake in the long run.

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