This article throws light on the ten major causes for the loss of biodiversity, i.e,
(1) Destruction of Habitat, (2) Hunting, (3) Exploitation of Selected Species, (4) Habitat Fragmentation, (5) Collection for Zoo and Research, (6) Introduction of Exotic Species, (7) Pollution, (8) Control of Pests and Predators, (9) Natural Calamities, and (10) Other Factors.
Cause #1 Destruction of Habitat:
The natural habitat may be destroyed by man for his settlement, agriculture, mining, industries, highway construction, dam building etc.
As a consequence, the species must either adapt to the changes in the environment, move elsewhere or may succumb to predation, starvation or disease and eventually die. Several rare butterfly species are facing extinction due to habitat destruction in the Western Ghats. Of the 370 butterfly species available in the Ghats, around 70 are at the brink of extinction.
Cause #2 Hunting:
Wild animals are hunted for the commercial utilization of their products such as hides and skin, tusk, fur, meat, pharmaceuticals, cosmetics, perfumes and decoration purposes. In Africa, in recent years 95% of the black rhino population have been exterminated in Africa by poachers for their horn. Today, rhino horn fetches more than $15,000 in the pharmaceutical market.
In the last one decade, over one-third of Africa’s elephants have been killed to collect 3,000 tonnes of ivory. International regulations have, to a great extent, reduced illegal trading and poaching of African Tuskers. In 1987, the Indian Govt. also banned the trade in Indian ivory. The scarlet macaw, once common throughout South America, has been eliminated from most of its range in Central America.
Several species of spotted cats such as ocelot and Jaguar have been jeopardized by the demand for their fur. In 1962, nearly 70,000 whales were slaughtered. However, international trade in whale products is banned now.
In India, rhino is hunted for its horns, tiger for bones and skin, musk deer for musk (medicinal value), elephant for ivory, Gharial and crocodile for skin and jackal for fur trade in Kashmir. One of the most publicized commercial hunts is that on whale. Convention on International Trade in Endangered Species of Wild Fauna and Flora (CITES) listed 9 Indian animal species which have been severely depleted due to international trade.
These are Fin Whale (Balaenoptera physalus), Himalayan Musk Deer (Moschus moschiferus), Green Turtle (Chelonia mydas), Hawksbill Turtle (Eretmochelya imbricata), Olive Ridley Turtle (Dermochelys olivacea), Salt-water Crocodile (Crocodylus porosus), Desert Monitor Lizard (Varanus griseus), Yellow Monitor Lizard (V. flavesoens) and Bengal Monitor Lizard (V. bengalensis).
Officials of Trade Record Analysis of Flora and Fauna in Commerce (TRAFFIC-India) say poaching of the Indian tiger has risen because of increasing demand from southeast Asian countries and China, where pharmaceutical factories consume the bones of 100 tigers each year. Such demand has decimated the tiger population in China and brought the Russian tiger to the brink of extinction.
As a result, in recent years much of the demand has been met by poachers in India. One kg of tiger bones fetches $ 90 in India and $300 in the international market. Hunting for sport is also a factor for loss of animal biodiversity.
Cause #3 Exploitation of Selected Species:
Exploitation of medicinally important plants has resulted in their disappearance from many of their natural habitat. The pitcher plants, Nepenthes khasiana, Drosera sp., Gnetum sp., Psilotum sp. Isoetes sp. are ruthlessly sought and collected for teaching and laboratory work.
They have already become rare. Medicinal plants like Podophyllum sp., Coptis sp., Aconitum sp., Rouvolfia sp., Saussura lappa, Atropa acuminata, Dioscorea deltoidea etc. are also disappearing rapidly as a consequence of merciless over-collection. Similarly, the natural populations of a number of economically important trees like Pterocarpus santalum, Dysoxylon malabaricum, Santalum album which yield valuable timber are fast dwindling.
In the category of over-exploited plants may also be placed a number of orchids producing world’s most showy flowers. Plants like Paphiopedilum fairieyanum, Cymbidium aloiflium, Aerides crispum etc. are in great demand but their natural populations have almost disappeared.
Today, only nine varieties of wheat occupy more than half of United States wheat fields. Almost 95% of the old strains of wheat grown in Greece before the Second World War (1939-1945) have disappeared. They are replaced by a few new hybrid varieties. Only four varieties provide almost 72% of the entire potato harvest of the United States.
Over 2,000 varieties of apples were under cultivation during the earlier century. Today, three-fourth of entire apple production of France consists of North American varieties of which nearly 70% happens to be the Golden variety. Indonesia has lost nearly 1,500 strains of rice and nearly three fourth of its rice production comes from varieties discussed from a single maternal stock.
Practically all varieties of Sorghum grown in South Africa have disappeared following introduction of high yielding hybrid varieties from Texas. In India, an estimated 50-60 thousand varieties of rice were cultivated before independence, most of which are being dropped in favour of a few high yielding varieties.
All over the world traditional varieties which together constituted a diverse mosaic, are being dropped one by one being replaced by a few high yielding strains. The reduction of genetic diversity among the cultivated species and the disappearance of their wild relatives, drastically limit possibilities of creating new cultivar in the future.
Cause #4 Habitat Fragmentation:
Habitat fragmentation may be defined as an “unnatural detaching or separation of expansive tracts of habitats into spatially segregated fragments” that are too limited to maintain their different species for an infinite future.
This phenomenon was observed as early as 1885 when de Candolle noticed that ‘the breakup of a landmass into smaller units would necessarily lead to the extinction or local extermination of one or more species and the differential preservation of others’.
Habitat fragmentation is one of the most serious causes of erosion of biodiversity. Fragmentation leads to artificially created ‘terrestrial islands’. Such fragments experience microclimatic effects markedly different from those that existed in the large tracks of habitats before fragmentation. Air temperature at the edges of fragments can be significantly higher than that found in the interior; light can penetrate deep into the edge, thereby affecting the growth of existing species. Fragmentation promotes the migration and colonization of alien species. Such substantial and continuous colonization, profoundly affect the survival of native species.
The most serious effect of fragmentation is segregation of larger populations of a species into more than one smaller population. There is considerable evidence that the number of species in a fragmented habitat will decrease over time, although the probable rates at which it will happen are variable. In fact, actual data on rain forests show that forest fragments have lower species richness and fewer populations compared with continuous undisturbed forests.
An example of loss of biodiversity as the result of the fragmentation is that of the Western forest of Ecuador, which were largely undisturbed till 1960, where newly constructed network of roads led to rapid human settlements and clearance of much of the forest area, have been fragmented into small patches of one to few square kilometers.
Such a patch, about 0.8 square kilometers in area at Rio Palenque Biological station now contains only about 1,033 plant species many of which are represented by a single specimen only and are endemic to the locality. Before 1960 the intact forest had thousands of species as found in any other tropical regions of the world.
Cause #5 Collection for Zoo and Research:
Animals and plants are collected throughout the world for zoos and biological laboratories for study and research in science and medicine. For example, primates such as monkeys and chimpanzees are sacrificed for research as they have anatomical, genetic and physiological similarities to human beings.
Cause #6 Introduction of Exotic Species:
Any species which is not a natural inhabitant of the locality but is deliberately or accidentally introduced into the system may be designated as an exotic species. Native species are subjected to competition for food and space due to the introduction of exotic species.
There are many instances when introduction of exotic species has caused extensive damage to natural biotic community of the ecosystem. The introduction of Nile perch from north in Lake Victoria, Africa’s largest lake, has driven almost half of the 400 original fish species of the lake to near extinction.
Both Eucalyptus and Casuarina are plants introduced in India from Australia. The remarkably fast growth of these plants has made them valuable source of rough timber. However, these plants appear to be ecologically harmful as they tend to suppress the original species of the locality.
While economically useful plants are deliberately introduced a large number of exotic weeds are transferred from one locality to another accidentally. The wheat imported to India from the USA under PL-480 scheme were contaminated with seeds of Parthenium hysterophorus, the congress grass and Agrostemma githago, the corn cockle.
Both of these plants have spread throughout India as a pernicious weed in wheat fields. Parthenium was first observed growing on a rubbish heap in Pune in 1960. It is an aggressive plant which matures rapidly and produces thousands of seeds. The native grasses and other herbs are crowded out of existence. Water hyacinth, Eichornia crassipes, was introduced in 1914 in West Bengal.
The first appearance of Alligator weed, Alternanthera philexeroides, was reported near Calcutta airport in 1965, while Salvinia molesta was brought in India by an aquarist. These plants grow vigorously and result in the formation of thick mat on the water surface. They impede run off in streams and promote water logged conditions. A number of useful water plants are displaced by these vigorous but useless plants. There is an overall reduction in biodiversity wherever these exotic weeds migrate.
Cause #7 Pollution:
Pollution alters the natural habitat. Water pollution especially injurious to the biotic components of estuary and coastal ecosystems. Toxic wastes entering the water bodies disturb the food chain and so the aquatic ecosystems. Insecticides, pesticides, sulphur and nitrogen oxides, acid rain, ozone depletion and global warming too, affect adversely the plant and animal species.
The impact of coastal pollution is also very important. It is seen that coral reefs are being threatened by pollution from industrialization, oil transport and offshore mining along the coastal areas.
Noise pollution is also the cause of wildlife extinction. This has been evidenced by the study by the Canadian Wildlife Protection Fund. According to a study, Arctic Whales are seen on the verge of extinction as a result of increasing noise of ships, particularly ice-breakers and tankers.
Cause #8 Control of Pests and Predators:
Predator and pest control measures, generally kill predators that are a component of balanced ecosystem and may also indiscriminately kill non-target species.
Cause #9 Natural Calamities:
Natural calamities, such as floods, draught, forest fires, earth-quakes, volcanic eruptions, epidemics etc. sometimes take a heavy toll of plant and animal life. Floods are frequent in moist tropical regions of the world which inundate much of the ground vegetation, trap a large number of animals while leading away soil nutrients. Failure of monsoon in succession for two or three years dries up ground vegetation and as the subsurface water table recedes trees are also affected. With plant life animals also suffer.
Forest fires in densely wooded localities often reduce to ashes a large number of plant and animal species and so do earthquakes. Volcanic eruptions may at times completely destroy plant and animal life in its surrounding areas. Epidemics sometimes destroy large portions of a natural population. In nature such episodes are usually confined to specific plant or animal populations as the pathogen is often specific to particular species or group of species.
Cause #10 Other Factors:
Other Ecological Factors that may also Contribute to the Extinction of Plant and Animal Diversity are as follows:
(a) Distribution range—The smaller the range of distribution, the greater the threat of extinction,
(b) Degree of specialization— The more specialized an organism is, the more vulnerable it is to extinction,
(c) Position of the organism in the food chain—The higher the organism in food chain, the more susceptible it becomes,
(d) Reproductive rate—Large organisms tend to produce fewer off springs at widely intervals.