Large and diverse communities of biota have thus, occupied distinct climatic zone forming ecosystems. The concept of biodiversity first appeared in 1980. It is, in fact, the shortened form of two words – “ biological” and “ diversity”. It was coined by W.G. Rosen in 1985. Biodiversity ( G.K: bios= life, diversity = forms) or biological diversity can be defined as the vast array of species of micro-organisms, algae, fungi, plants, animals occurring on the earth either in the terrestrial or aquatic habitats and the ecological complexes of which they are a apart. It is so because environmental conditions of the area as well as the range of tolerance of the species determine whether or not a particular species can occur in that area.


      India with about 45,000 species of plants and twice as many species of animals is one of the 12 mega diversity countries of the world. The major area where numerous species are believed to be unknown to science are tropics and coral reefs. Scientists estimate the number of species present in tropics by comparing species richness between tropics and temperate areas. For most groups of organisms, inventories are nearly complete for temperate areas
       On this basis, scientists have calculated that the total number of species in the world is anywhere between 5.50 million. The most intriguing question of biodiversity is that more than 70% of all species are animals while plants accounts for only 22%. Amongst animals, insects are the most numerous  ( about 70%) with present estimate of 7 out of 10 animals. Further the knowledge about protists, archaebacteria and viruses is quite fragmentary.
1.   Genetic diversity
        It is the diversity in the number and type of genes as well as chromosomes present in different species and the variations in the genes and their alleles in the same species. On average a bacteriophage has 100 genes, Drosophila melanogaster 13000 genes and Homo sapiens 30,000 – 40,000 genes.
        Variation in the genes of a species increase with increase in size and environmental parameters of the habitat. Genetic diversity is useful in adaptation to changes in environmental conditions. It helps in speciation or evolution of new species. Lower genetic diversity within a species or variety may be useful for uniformity in yield as well as higher yield. However it is liable to undergo degradation and prone to mass scale destruction at the hands of fungal or insect attacks
2.   Species diversity
        It is the variety in the number and richness of the species of a region.
The number of species per unit area is called species richness. Numbers of individuals of different species represents species evenness or species equitability.
Communities where species are represented more or less number of individual exhibit evenness. Others where one or more species have more individual than other show dominance or unevenness. Species diversity is the product of species richness or evenness or equitability odum et al ( 1960) calculate species diversity as number of species per thousand individuals while Menhinick ( 1964) calculates it as number of species in relation to square root of total number of individuals. Diversity index commonly used in ecological studies is Shannon index.
3. Community and Ecosystem diversity
It has three types
(i)          Alpha diversity ( within community diversity) It is a species diversity.  α – diversity is dependent upon species richness and evenness. There is a lot of competition, adjustments and inter relationships amongst members of the same community. Variations are limited.
(ii)         Beta diversity ( Between communities diversity). It is diversity which appears in a along gradient of habitat within geographical area replacement of species with the change in communities due to different  microhabitat, niches and difference in environmental condition.
(iii)       Gamma diversity. It is diversity presents in ranges of communities as represented by diversity of habitats/ ecosystems over a total landscape geographical area.
Ecosystem diversity is the variety of forms in the ecosystem due to diversity of niches, tropic levels and ecological process like nutrient recycling, food webs, energy flow, role of dominant species and various biotic interactions. Diversity helps in producing more productive stable ecosystems which can tolerate various stresses like prolonged drought
        India has attained a unique distinction as it has been assigned the status of megadiversity nation.
        The country has 10 biogeographical regions namely, Trans Himalaya, Himalaya, Desert, Semi-arid, Western Ghats, Deccan peninsula, Gangetic Plain, Coasts, North-East and Islands.
        India has 89 national parks, 492 wild life sanctuaries, 14 biosphere reserves, 6 wetlands and 5 world heritage sites. The country has also 27 tiger reserves.

The largest biogeographical region is Deccan peninsula and the most biodiversity rich region are Western ghat and north-east.


33% of flowering plants, 10% of mammals, 36% reptiles, 60% of amphibians and 53% fresh water fish are endemic. The richest regions are the Himalayas, Western Ghats. Indian Islands and North-Eastern Hills
                            (i)        Latitudinal gradient
There is little biodiversity at the poles. It increases in temperate areas but reaches the maximum in tropical rain forests. It is because the tropical rain forest have favourable, conditions with no catastrophes. Harsh conditions exist in temperate areas during the cold season only while very harsh conditions prevail for most of the year in arctic regions. Number of vascular species is 118-236/0.1 ha in tropical forests and 21-48 species 0.1 ha in temperate forests. Their number would 10/0.1 ha in arctic regions.
                            (ii)       Altitudinal gradient
A decrease in species diversity occurs as we ascend a high mountains due to drop in temperature and greater seasonal variability.
      German naturalist and geographer Alexander von Humboldt found that within a region the species richness increased with increasing area but upto a certain limit. The relationship between species richness and area turned out to be rectangular hyperbola for a wide variety of taxa whether they are birds, bats, fresh water fishes or flowering plants. On a logarithmic scale it is a straight line.
Here S is species richness, 2 is slope of line or regression coefficient, C is y- intercept while A is area.
       Regression coefficient is generally 0.1-0.2, regardless of taxonomic group or region e.g. plants in Britain birds in California or mollusks in. New York.

However, when the species area relationship is considered for a very large area


like whole continent, retrogression coefficient or slope of the line comes to have Z value of 0.6 – 1.2, e.g. frugivorous birds and mammals of tropical forests of different continents with a steeper line of 1.15.
1.   Source of food
There are over 3000 species of food plants, out of which only 150 species are commercialized, 85% of the food output is produced by less than 20 species. Two third of food is being produced by three carbohydrates rich crops-wheat, corn ( maize) and Rice. Utilisation of more and more food plants has to be made.
2.   Source of fats and oils
The major plants are soyabean, coconut, cotton seed, peanut and sunflower besides a number of others like sesame, safflower, mustard and oil palm. Few species of oil are being investigated e.g. Bitter colocynth, jojoba seed yield high performance lubricants.
3.   Fibres
The major sources are cotton, flax, jute, hemp, sun hemp, rosella, agave and coir. Search for new superior fiber yielding plants is a continuing process
4.   New varieties
Domesticated commercial species are improved for various traits, especially disease resistance by crossing them with wild relatives. Rice was made resistant to four main diseases crossing it with wild species ( Oryza nivara) from India. Similarly, Potato has been made resistant to late blight ( trait from Solanum demissum) Potato Mosaic Virus Y ( trait from solanum stoloniferum), Fusarium and five races of cyst nematodes ( trait from Solanum spegazzini)
5.   Drugs and medicines
A number of drugs are based on plant products. Rosy periwinkle ( charanthus roseus = Vinca rosea) yields alkaloids ( Vicristine and vinblastine) which are useful for treatment of leukaemia. The same are now being synthesized chemically. Some other plant derived drugs are Morphine ( Papaver somniferum for pains), quinine ( from bark of cinchona ledgeriana for malaria), taxol ( from bark of Yew; Taxus brevifolia and Taxus baccata for treating cancers), reserpine ( from
Rauvolfia serpentine for treating blood pressure and schizophrenia) etc.
25% of all drugs are currently being obtained from 120 species of plants. Traditional systems of medicine all over the world uses thousands of local/ wild plants for treating various maladies. Innumerable synthetic products can be manufactured from plant chemical. They are called botanochemicals.
6.   Aesthetic value
Biodiversity has a lot of aesthetic and attraction value. Ecotourism, bird watching, wildlife, pet keeping and gardening are all rewards of aesthetic value of biodiversity.
7.   Cultural benefits
Historically people having linked themselves with certain specific plants and animals. Majority of the Indian homes have specimens of Ocimun sanctum (Tulsi) growning in pots. Trees of Ficus religiosa ( Peepal) and Prosopis cineraria ( Khejri) are held sacred. They are planed and worshipped. Many birds are considered sacred. Snakes are worshipped. Every country and state takes pride in recognizing aparticular plant and particular animal as symbol of national and state pride and cultural heritage.
8.   Ecosystem services
Maintenance and sustainable utilization of useful products and services of various ecosystems as well as individual species require the presence of biodiversity. Forest and oceanic systems control climate and maintain gaseous composition of atmosphere. Amazon rain forest have been called lungs of planet earth since they give out 28% of total oxygen. Biodiversity is essential for natural pest control maintenance of populations of various species, pollination by insects and birds, nutrient cycling, conservation and purification of water, formation and protection of soil etc. The services are valued at 16-54 trillion dollars per year.
The world is facing accelerated rate of species extinction, largely due to human interference. There are four major causes the evil quartet 
i)            Habitat loss and fragmentation: Overpopulation, urbanisation and
industrialization require additional land every year. It can come through destruction or fragmentation of natural habitat through filling wetlands, ploughing grasslands, cutting down trees, burning a forest and clearing some area of vegetation. Animals requiring large territories are badly affected. Migrating animals would go astray and get killed.
ii)           Over –exploitation: Excessive exploitation of a species, whether a plant
or animal reduces size of its population so that it becomes vulnerable to extinction. Dodo, passenger pigeon, three subspecies of Tiger and Stellar sea cow have become extinct in the last 500 years due to overexploitation by humans. Many marine fish populations are declining around the world.
iii)          Alien species invasions

Non native or alien species are often introduced inadvertently for their economic and other uses. They often become invasive and drive away local species. Island ecosystems are most vulnerable due to small size and small number of species

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