Bio-geography is the study of the geographical distribution of plants and animals. Most species of plants and animals of the world are essentially restricted in their distribution either by (natural) barriers of some description or because of the history of their origins and dispersal. The major units of distribution are referred to as bio-geographical regions, which are largely defined by the past and present relations of the continents to each other. These major global units of bio-geography were first recognised by P.L.Sclater (1858, in J. Proc. of Linn. Soc.2), and later elaborated upon and described in modern terms as "realms" by P.J.Darlington (1957,in Zoogeography. John Wiley & Son. New York).

Biological regions or "Bio-geographical Realms" are defined by climate, altitude and terrain (geography) which have acted upon the evolutionary process to give rise to certain distinctive life forms. Bio-geography, therefore, refers to this specific and distinctive community of plants and animals (biological community) occupying a specific geographical region. Each region is then subdivided into faunal or vegetational zones depending on the criteria used.

Six major bio-geographical regions (realms) have been recognised and these are:

1. Ethiopian - Africa except NW corner and parts of southern Arabia

2. Oriental - Tropical Asia & associated continental islands.

3. Palaearctic - Eurasia above the tropics and NW Africa

4. Nearctic - North America except Mexican tropics

5. Neotropic - South & Central America & Mexican tropics

6. Australian - Australia with New Guinea & islands.

The Indian sub-continent occupies a unique position in the Bio-geographical realm of the region. Broadly falling within the Oriental Region of tropical Asia, the Indian sub-region absorbs the Palaearctic Region in the NW Himalayas. The Indo-Chinese sub-region in NE India, together with Andaman island group, forms a part of the Indo-Malayan realm. In the south, the Deccan plateau along with Sri Lanka absorbs the Ethiopian realm.

Biogeographic zones: With the mighty Himalayas forming the northern boundary, the Indian territory is confined within natural features that delineate the sub-continent as a whole. Within this bio-geographical synthesis there is yet another division of bio-geographic sub-regions which are further restricted to bio-climatic and vegetation zones each supporting its own characteristic animal and plant life. The two main Indian bio-geographic sub-regions are the Himalayan sub-region and the Indian Peninsular sub-region and within these are the several zones (Trans Himalayan; West & East Himalayan; North East Inida; Indian Desert; Indo-Gangetic plains; Western & Eastern Ghats; Deccan Plateau; Coasts & Islands).

The Trans-Himalayan plateau at an average altitude of 4500 m consists of the eastern part of Ladakh. It is a windswept region of scanty rainfall, high radiation and intense cold. This is the only region in the world where both desert and arctic conditions prevail. The plants and animals in this area are highly adapted to such conditions.

This habitat is defined by perennially snow-cladpeaks, seasonal marshlands and high-altitude lakes. Except for meandering rivers such as the Indus, most of the marshes and lakes are brackish due to high evaporation regimes. This seemingly inhospitable terrain nevertheless hosts a variety of flora and fauna. The vegetation is dominated by Tibetan Gorse (Caragana sp) on the hill slopes. Marmots, Woolly hare, Tibetan antelope, Tibetan Sheep, Bharal, Wild Yak, Wild Ass, wolf and snow leopard are some of the typical denizens of this zone.

West & East Himalayan: South of the Trans-Himalayan plateau and sharply demarcated by the tree-line is the forested area stretching form west to east Himalayas. This is marked by variations in forest types determined by rainfall regime, altitude and latitude.

In the west Himalayas this zone is dotted with grassy meadows covered with Alpine and sub-Alpine vegetation in Kashmir and the higher reaches of Garhwal. Both floral and faunal elements are influenced by Palaearctic types. The Kashmir Stag, Ibex, Markhor and Urial are some of the examples of the western Palaearctic influence.

Eastwards of Nepal the vegetation in this zone gives way to a contrasting mix of moist to wet-evergreens typical of the east Himalayas.

The true tropical dense forests of the North East start from the lower hills and valleys, also known as "Dun" and "Terai", reaching up to 3000m. This zone is best known for the variety of Indian fauna & flora. Tiger, Elephant, leopard, gaur, sambar, chital, hog deer, swamp deer, buffalo, rhinoceros and many other smaller mammals are found in this belt. Here the faunal element is a mix of Indo-Chinese, Indo-Malayan and Ethiopian.

2. Indian Peninsular sub-region.

The area from the base of the Himalayas to the tip of the peninsula and comprising of Indus-Ganges-Brahmaputra river basins, the entire Deccan plateau, and the Western/Eastern Ghats with adjacent coasts, forms the proper Indian peninsular sub-region. Here the Arid tracts of Rajasthan and northern Gujarat host the elements of the great desert that once stretched from Indus valley all the way to Baluchistan and Iran. The rich alluvial Gangetic Belt with Sal dominated terai forests continue eastwards merging with forested floodplains of Brahmaputra, as both river systems join up to nurture the vast mangrove forests of the Sunderbans.

The triangular Deccan plateau is the true home of Indian mammals, represented by the Spotted deer, Nilgai, Blackbuck, Four-horned antelope and the Sloth bear - found nowhere else in the world. The west-east sloping plateau is dominated by moist deciduous forests of Vindhya and Satpura ranges in the north, bounded by the Western and the Eastern Ghats on either flank and the rest characterised by open stony dry scrub interspersed by innumerable rain-fed waterbodies. Two major rivers originating from the Vindhyas, the Narmada and the Tapti flow westwards while three major rivers originating from the Western Ghats, the Krishna, the Godavari and the Kaveri flow eastwards.

The Desert Region occurs in the extreme west, in Rajasthan and northwestern parts of Gujarat, extending towards Sind, Punjab and Baluchistan. This is the home of the desert cat, the desert fox, the desert hare and several species of gerbilles. The area also forms the home range of the Great Indian Bustard, the Houbara Bustard and the Florican.

The Coasts Areas of the peninsula are dominated by the Western Ghats and Eastern Ghats. The Western Ghats stretching from the Surat Dangs in the north to almost the tip of the peninsula, receive heavy rainfall during the South West monsoon. The vegetation is tropical wet evergreen characterised by tall trees, dense undergrowth, bamboos, sholas and grasslands on the higher elevation and semi-evergreen lowland forests, secondary growth and mangrove stands all along the coastal belt. Major faunal elements are represented here, including tiger, leopard, smaller cats, Gaur, Sambar, spotted deer, Giant squirrels and the unique Nilgiri tahr. The endemic species include, the Lion-tailed macaque, the Nilgiri langur, the Malabar civet, the Brown mongoose, the Stripenecked mongoose and the spiny mouse. Over 500 species of birds occur here of which the Black-and-orange flycatcher and the Nilgiri wood pigeon are endemic.

The offshore Islands of the Andaman and Nicobar group in the Bay of Bengal represent the Indo-Malayan elements. Among the endemics here, the Narcondam hornbill, confined to one small outlying island of the group, has a total population of only about 400 birds!

Contributed by S. A. Hussain