By many measures of biodiversity, the Eastern Himalayas Region stands out as being globally important. It has been included among Earth’s biodiversity hotspots (Myers et al. 2000) and includes several Global 200 ecoregions (Olson and Dinerstein 1998), two Endemic Bird Areas (Stattersfield et al. 1998), and several centers for plant diversity (WWF/IUCN 1995). An understanding of why the eastern Himalayas are so exceptionally rich in biodiversity requires a brief overview and analysis of its geological history and ensuing biogeographic patterns.
The Himalayas are geologically young (Xu 1993). More than 200 million years ago, Proto-India detached from the southeastern margin of Africa and began to drift slowly northward until it was intercepted by Eurasia. The Himalayas mountain range rose out of the geologic faulting during this massive collision, which occurred during the latter part of the Tertiary Period as indicated by the fossil record that shows an invasion of India by Eurasian fauna (Molnar 1986). The energy dissipated by this collision was widespread and accounts for some of Asia’s most distinctive geographical features, including the compression of the Tibetan Plateau, the massive distortion of Asia’s southern margin and even the Annamite mountain range in Indochina. Because the Deccan Plate is still inexorably moving northward, both Tibet and the Inner Himalayas continue to be pushed upward even today.
The rugged, and largely inaccessible, landscape makes biological surveys in the Himalayas extremely difficult. Vast areas of intact forests are little or entirely unexplored. Thus, many floral and faunal taxonomic groups are understudied and the true extent of the biodiversity is undoubtedly underestimated. Undescribed species, including some from the higher taxonomic groups such as mammals, reptiles and amphibians are very likely to occur in the more remote, heavily forested regions. But, despite the scant knowledge, what we know of the biodiversity indicates that the Eastern Himalayas Region is amongst the biologically richest areas on Earth.
Several factors contribute to the exceptional biological diversity of the eastern Himalayas. First, the eastern Himalayas has multiple biogeographic origins. Its location at the juncture of two continental plates places it in an ecotone represented by flora and fauna from both. The Indo-Malayan Realm of Southeast Asia contributes many tropical taxa to the eastern Himalayas biota, including trees such as dipterocarpus, shorea and terminalia, and climbing figs, epiphytic orchids, and arums. The monsoon forests below 1,000 meters have a close affinity with the monsoon forests of Indochina, and include dominant trees from the Family Dipterocarpaceae, woody climbers, Phoenix palms, a closed groundcover of grasses and sedges, and laterized red soils infested with termite colonies. Vertebrates include many Indomalayan species such as Asian elephant (
Elephas maximus), wild water buffalo (
Bubalus bubalis), gaur (
Bos gaurus), hornbills, pittas, cobras, and geckoes.
The Palearctic Realm to the north contributes plant species in the higher elevation forests, including conifers such as spruce (Piceae), fir (Abies), and larch (Larix), as well as deciduous broadleaf taxa such as birch (Betula), alder (Alnus), willow (Salix) and numerous alpine forbs such as Potentilla and Pedicularis. The temperate and subtropical East Asian or Sino-Japanese region contributes an ancient biota with high endemism and high biodiversity represented by members of the Fagaceae, Theaceae and Ericaceae. Palearctic mammals include the snow leopard (Uncia uncia), brown bear (Ursus arctos), wolf (Canis lupus) and a diverse assemblage of alpine ungulates.
Second, there is considerable climatic variability associated with the topography and vast reach of the mountains. The moisture-laden monsoon winds that originate from the Bay of Bengal and the South China Sea are funneled inland along the Ganges River valley, and forced up the south-facing mountain slopes. As the winds strike and rise upslope, adiabatic cooling condenses the large volumes of water that fall as rain to flow back into the Indian Ocean along the rivers that drain the mountain range. These monsoon rains deluge the eastern extent of the mountain range, which bears the brunt of the wind. The western extent receives little rainfall by comparison. Consequently, the moister eastern extent of the mountain range is more biodiverse than the western reaches.
Third, because of the complex and steep topography there is large-scale climatic variability across the north-south axis. By acting as a barrier to the monsoon, the southern slopes intercept and receive much more moisture—exceeding 2,000 millimeters per year in many areas—than the northern slopes that face Tibet and Central Asia, which are subject to strong rainshadow and föhn effects. And the topographic complexity also results in meso-scale climatic variability because of localized pockets of high precipitation. Conversely, dry valleys occur where prevailing air movement is catabatic (i.e., downhill), such as in the Kali Gandaki valley of Central Nepal, and Punaka Valley in Bhutan.
Fourth, the scale and complexity of the mountains in the Eastern Himalayas Region contribute to high biological diversity in several ways. The beta (a comparison of of diversity between ecosystems) and gamma (overall diversity across a large region) diversity across the vast landscape increases overall biodiversity, and the extreme vertical relief enhances biological diversity along the north-south axis. The extreme height and steepness of the Himalayas mountain range confers considerable variation to its ecosystems, which are layered along the longitudinal axis as long, narrow ecoregions (WWF and ICIMOD 2001).
The topographic complexity also isolates islands of habitat. Antecedent rivers and streams separated by mountain massifs may support reproductively isolated populations of low-elevation species. And high ridges separated by valleys may isolate high-elevation species. This can contribute to genetic differences among populations, a step toward the evolution of endemic species. On a shorter time scale, historical vicariant events isolate populations by influencing local immigration and extinction. Because the Himalayas are relatively young, levels of endemism are low. However, the stage has been set for speciation.
The 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 (L
ophophorus 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).
The top predators, large herbivores and frugivores, and specialized pollinators that inhabit the Eastern Himalayas Region play critical ecological roles in maintaining the integrity of the ecosystems. Many birds and fishes, and likely many insects, undertake seasonal migrations up and down the mountains. Juvenile and sub-adult tigers disperse from natal areas to establish territories elsewhere, and elephants exhibit seasonal movements along the length of the mountains. Higher up the mountains, blue sheep and takin undertake seasonal migrations from the alpine meadows in the summer to the mixed conifer forests below in the winter. Such ecological phenomenon is also important components of biodiversity that should be included in a conservation strategy.
Protected Areas
Protected areas are, and have been, the cornerstones of biodiversity conservation. South Asia has a long history of biodiversity conservation in protected areas, dating back several centuries. For instance, India established sanctuaries for wildlife conservation by royal decree more than 2,000 years ago (Singh 1986). In the northeastern region of India, many tribal groups have traditionally recognized and protected sacred groves, which have been effective refuges for biodiversity for millennia (Gadgil 1985).
In Assam, Manas and Sonai Rupai were first established as wildlife sanctuaries in 1928 and 1934, respectively, and are among the earliest contemporary protected areas in Asia (IUCN 1990). Several other protected areas were established or extended in northeast India in the 1970s and 1980s to create networks that represent the biodiversity in the region, following recommendations from a comprehensive assessment by Rogers and Panwar (1988). There are more than 70 formally protected areas in the seven northeastern Indian states within the Eastern Himalayas Region, covering more than 15,000 square kilometers. Two of these, Manas Tiger Reserve and Kaziranga National Park in Assam, have been declared World Heritage sites. These harbor globally important populations of large flagship mammals and birds that showcase the region’s fauna, and can also serve as indicators of conservation success.
The protected areas system in Nepal is more recent. Chitwan was established as a national park in 1973—prior to that it was a hunting preserve for the royal family. The park, located in the highly productive Terai, supports an important tiger population and the second largest greater one-horned rhinoceros population. During the same year, Sagarmatha, which includes the world’s tallest mountain, Everest, was established as a national park. Within a decade, the protected areas system had grown from 4,500 square kilometers to more than 13,000 square kilometers with the establishment of three additional reserves along the south and two that covered the northern, montane habitats. By the year 2000, 10 more protected areas had been added to the network, doubling the total area within the protected areas system. Two of these are large conservation areas—Annapurna and Makalu Barun—which have become models for community-based biodiversity management. Both Sagarmatha National Park and Chitwan National Park have since been declared World Heritage sites.
Until 1995, Bhutan’s protected areas system was dominated by the vast Jigme Dorji National Park that effectively protected rock, ice and snow along the inaccessible northern border with China, but did not contribute much to biodiversity conservation. In 1995, a radical revamping of the protected areas system added three protected areas that include the biologically rich temperate forests in the mid-hills. The large Jigme Dorji National Park was also reduced in size and extended southwards to capture biologically important alpine meadows. This new system, which covers about 26 percent of Bhutan, is more representative of the county’s ecosystems and the biodiversity contained in them. In 1999, another significant addition occurred, when a system of corridors that linked the protected areas was recognized to create a conservation landscape that extends across the length and breadth of the country. This landscape, now known as the Bhutan Biological Conservation Complex (B2C2), covers almost 35 percent of the country and consists of five national parks, two wildlife sanctuaries, one strict nature reserve and 12 corridors that cover almost 16,000 square kilometers. In 1999, the system was bequeathed as a Gift to the Earth from the People of Bhutan.
A notable feature of the protected areas systems of Bhutan, Nepal and northeastern India is that several lie adjacent to each other across the national borders and provide opportunities for transboundary conservation. The Kanchandzonga National Park in Sikkim and Kangchenjunga Conservation Area in eastern Nepal, and Manas National Park in Bhutan and Manas Tiger Reserve in Assam are two such complexes. However, there are other opportunities to link protected areas across international boundaries by creating corridors and habitat linkages. Examples of these include Bardia in Nepal and Katerniaghat in India; Sukla Phanta in Nepal and Dudwa in India; and Sakteng in Bhutan and Eagle’s Nest and Sessa Orchid Reserve in India. Some of the priority sites such as Namdapha in Arunachal Pradesh provide opportunities for transboundary conservation with Myanmar and China. These transboundary conservation options are compatible with CEPF goals of partnerships and corridor outcomes.
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