Himalaya houses diverse elevation, climate, rainfalls and soil conditions. So rich is its ecology that each part of the Himalaya shelters unique flora & fauna distinctively marking the uniqueness of Himalaya:
Rainfall determines the vegetation of alluvial Indo-Gangetic plain which has been greatly influenced by Indus and Ganges-Brahmaputra River. Pakistan and the Indian Punjab plains have scrub forests. There are Upper Gangetic plains, moist deciduous forests of Uttar Pradesh and Lower Gangetic plains and moist deciduous forests of Bihar and West Bengal on the east which have monsoon forests, with drought-deciduous trees. Domestic livestock like cattle and yak grazing is a common sight in lowlands.
Terai belt is found above Indo-Gangetic plain and has a seasonally marshy zone of sand and clay soils with higher rainfall than the plains. Monsoon season brings in huge depositions of fertile silt. It has Terai-Duar savanna and grasslands in its central region. Huge water table has been formed in the Terai belt because of the groundwater permeating from bhabhar zone. Indian Rhinoceros are the major attractions of Terai belt.
This belt lies above the Terai belt and has rock-strewn porous soil with debris. As the climate in this belt is subtropical, Bhabhar belt is most richly forested with pine and sal trees.
Siwalik hills, also termed as Churia Hills, lie stretched across Pakistan, India, Nepal and Bhutan. There are many sub-ranges and peaks elevated from 600 to 1,200 meters. When it comes to its slopes, its southern slopes are steeper than northern slopes along Main Frontal Thrust fault zone.
Inner Terai or Dun Valleys
Inner Terai or Dun valleys lie between Siwalik sub ranges. It covers Dehra Dun in India and Chitwan in Nepal.
Mahabharat Lekh or Lesser Himalaya
This section of the Himalaya has ranges from 2,000 to 3,000m along the Main Boundary Thrust fault zone with a steep southern face and gentler northern slopes.
It lies north of Mahabharat Range and its ranges vary from 100 km to about 4,000 meters at the Main Frontal Thrust fault zone.
At the middle elevations of the range, the subtropical forests yield to a belt of temperate broadleaf and mixed forests, with the Western Himalayan broadleaf forests at the western end of the range, and the Eastern Himalayan broadleaf forests in Assam and Arunachal Pradesh. Above the broadleaf forests are the Western and Eastern Himalayan sub alpine conifer forests.
Alpine Shrub and Grasslands
Shila (7026 m) above the Spiti Valley in India
Above the tree line are the Northwestern, Western, and Eastern Himalayan alpine shrub and meadows, which yield to tundra in the higher Himalayan range. The alpine meadows are the summer habitat of the endangered Snow Leopard (Uncia). Alpine vegetation occupies higher parts of the Great Himalayas just below the snow line and includes shrubs, rhododendrons, mosses, lichens, and wildflowers such as blue poppies and edelweiss. These areas are used for grazing in summer by the highland people of the Great Himalayas.
Flora of the region includes elements from tropical Indochina, temperate East Asia, the Palaearctic region and the Deccan Plateau. The low-lying areas along the Brahmaputra River, subject to floods during the monsoon, support mixed evergreen forests. Although most of these semi-evergreen forests have long since been converted into human uses, the vestigial patches—mostly in small protected areas—indicate that these forests were characterized by Syzygium, Cinnamomum, Artocarpus, Terminalia spp. Tetrameles spp. and Stereospermum spp. (Champion and Seth 1968). These forests also contain several Deccan elements, indicative of the geological origins of the region.
The alluvial grasslands and savannas along the foothill valleys are among the tallest in the world. Characteristic species in these highly productive grasslands include Saccharum spontaneum, Phragmitis kharka, Arundo donax, Imperata cylindrica, Erianthus ravennae, Andropogon spp., and Aristida ascensionis (Shrestha and Joshi 1997). Annual silt deposition during monsoon floods rejuvenates these grasslands and promotes rapid regeneration. As the floodwaters recede, grasses such as Saccharum spontaneum and pioneer trees such as Trewia nudiflora and Ehretia laevis begin to colonize the area, and support high densities of a diverse herbivore community.
The grasslands transition into the sal forests that flank the hillsides along the lower reaches of the river valleys, below 1,000 m. The lower hill slopes above 1,000 meters are cooler and less drought-stressed during the spring pre-monsoon season. Here, the subtropical evergreen broadleaf forests are dominated by tree taxa such as Castanopsis and Schima from subtropical East Asia.
The eastern Himalayas temperate forests that grow at elevations where moisture tends to condense and remain in the air during the warm, moist growing season are among the most species-rich temperate forests in the world. They are dominated by evergreen broadleaf trees (e.g. Quercus, Lauraceae) in the lower reaches, from about 2,000-2,500 meters, and mixed conifers (e.g. Tsuga, Taxus) and winter-deciduous broadleaf species (e.g. Acer, Betula, Magnolia) in the upper reaches, from 2,500-3,000 meters. The drier, south-facing slopes support extensive stands of arboreal Rhododendron species that may co-occur with oak (Quercus semecarpifolia) or other ericaceous species such as Lyonia ovalifolia. These temperate forests support a rich epiphytic community, consisting of a variety of dicots, orchids, ferns and mosses. Bamboo (Arundinaria spp.) is dominant in the understory in places, especially where it provides early-successional ground cover following fire.
Further upslope, subalpine conifer forests begin from about 3,000 meters and extend to 4,000 meters. In the eastern Himalayas, Tsuga, Picea or Larix dominate these forests between 3,000 meters to 3,500 meters and Abies dominates above 3,500 meters. Juniperus is widespread along the timberline, and may form dwarf krummoltz formations above 4,700 meters. The dry slopes and inner valleys support Pinus and Cupressus on basic limestone soils.
Above the treeline the vegetation is a moist alpine scrub community of dense juniper and Rhododendron shrubberies that extend to about 4,500 meters. Plant richness in these alpine shrub and meadows is very high, especially on the shady north-facing slopes that are protected from extreme winter cold by an insulating layer of snow. South-facing slopes tend to be dominated by Kobresia sedge and forbs with scattered shrub species of Berberis, Rosa, Lonicera, and Cotoneaster to about 4,500 meters. From 4,500 to 4,700 meters the vegetation consists of alpine meadows with a diverse assemblage of alpine herbs and smaller-statured woody shrubs, such as a variety of dwarf rhododendrons, and numerous alpine herbs such as Potentilla, Ranunculus and the alpine Saussure.
Periglacial and subnival communities occur in the high alpine areas above 4,700 meters, where the short growing season, high winds, and unstable soils allow only specialized plants to survive. Some of these include Androsace, Arenaria and Saxifraga, Meconopsis and Primula. The latter two have their global centers of diversity in the eastern Himalayas. By about 5,500 to 6,000 meters, the nival zone, or permanent ice and bare rock, begins. Even here, at the highest elevations on Earth, microclimates may support small cushion-forming vascular plants, such as Arenaria bryophylla, which was recorded at 6,180 meters by A.F.R. Wollaston (Wollaston 1921, in Polunin and Stainton 1997).
Knowledge of the fauna of the Eastern Himalayas Region is poor. Most of the information available is on the larger vertebrates that are easily observed and inventoried. The smaller mammals, reptiles, amphibians, and fishes have been neglected and the most abundant taxonomic group, the insects, have been virtually ignored. With the exception of a few studies that have documented the Himalayas lepidoptera (Haribal 1992, Mani 1986, Yonzon 1991), little else is available on the insect fauna of the region.
Overall, more than 175 species of mammals and in excess of 500 species of birds are known from the region (WWF and ICIMOD 2001). The mammalian fauna in the lowlands is typically Indo-Malayan, consisting of langurs (Semenopithicus spp.), wild dogs (Cuon alpinus), sloth bear (Melursus ursinus), gaur, and several species of deer, such as muntjacs (Muntiacus muntjak) and sambar (Cervus unicolor). Further up the mountains, the Indo-Malayan fauna transitions into a Palearctic fauna, consisting of snow leopards, Asiatic black bear (Ursus thibetinus) and a diverse ungulate assemblage that includes the blue sheep (Pseudois nayur), takin (Budorcas taxicolor) and Himalayas thar (Hemitragus jemlahicus). The red panda (Ailurus fulgens) is a Himalayan species that lives in old growth subalpine conifer and mixed forests with a bamboo understory.
Because the Himalayas have a relatively recent origin, endemism is low, especially among the better-known higher taxonomic groups. The golden langur (Trachypithecus geei) is restricted to the patch of semi-evergreen and temperate forest on the north bank of the Brahmaputra River, between the Sankosh and Manas rivers that flow south from the mountains. The pygmy hog (Sus salvinus) and hispid hare (Caprolagus hispidus) are restricted to the alluvial grasslands and the Namdapha flying squirrel (Biswamoyopterus biswasi) is restricted to the temperate broadleaf forests of the Eastern Himalayas Region.
Endemism among birds in the region is higher than among mammals. Some species restricted to the region include the Manipur bush quail (Perdicula manipurensis), chestnut-breasted partridge (Arborophila mandelli), Blyth’s tragopan (Tragopan blythii), Temminck’s tragopan (Tragopan temminckii), Sclater’s monal (Lophophorus sclateri), Tibetan eared pheasant (Crossoptilon harmani) and rusty-bellied shortwing (Brachypteryx hyperythra).
But, despite the low overall endemicity, the region harbors several species that are represented by globally significant populations. The foothill grasslands and broadleaf forests harbor important populations of the largest carnivore and herbivores in Asia, notable the tiger (Panthera Tigris), Asian elephant, greater one-horned rhinoceros (Rhinoceros unicornis) and wild water buffalo. The alluvial grasslands, delineated as the Terai-Duar Savanna and Grassland ecoregion (Wikramanayake et al. 2001), support some of the highest densities of tigers in the world (Karanth and Nichols 1998). And the elephant population in the remaining habitat patches along the north bank of the Brahmaputra River in Assam is one of India’s glargest and most important (Sukumar 1992). The greater one-horned rhinoceros, one of three species found in Asia, is restricted to several small, isolated populations contained within protected areas (Dinerstein 2003). The Eastern Himalayas Region is the last bastion for this charismatic mega-herbivore, which once ranged along the length of the Himalayas foothills, from Pakistan to Myanmar. Many other refuge populations of large herbivores—wild water buffalo, swamp deer (Cervus duvaucelii)—restricted to protected areas in southern Nepal and northeastern India—also represent some of the last remaining in the world, and are considered to be of global significance. The Brahmaputra and Ganges rivers that flow along the Himalayas foothills also support globally important populations of the Gangetic dolphin (Platanista gangetica).
Although the snow leopard has a wide distribution across the Himalayas range, and into the Trans-Himalaya, the populations in the Eastern Himalayas Region are important because this high-altitude predator occues at low densities. The populations of vultures, greater and lesser adjutants—some of Asia’s largest birds—in the foothill grasslands and broadleaf forests are globally significant, as are the populations of several of the hornbill species and pheasants, white-winged duck (Cairina scutulata), white-bellied heron (Ardea insignis), black-necked stork (Grus nigricollis) and the Bengal florican (Houbaropsis bengalensis).