Covering just 0.22% of the geographical area of India, Sikkim shows great biological diversity. The vast altitudinal variation of elevations from around 300 m to 8598 m within very short distances is responsible for the varied ecoregions of the State. This is evident from the presence of Sal (Shorea robusta) forests in the lowland Rangit Valley in the south, to the temperate fir forests in the north, beyond which lie the Trans-Himalayas and the cold desert of the Tibetan plateau.

Broadly speaking, there are five altitudinal zones of vegetation, not clear-cut at their boundaries as they merge into one another

• The Tropical Ecoregion extends roughly from the foothills of the Outer Himalayas to an altitude of about 1,200 m. It contains steep-sided valleys and gorges with well-drained flanking slopes. Various species of orchids, climbers like the robust Aroid Rhaphidophora, wild banana Musa sikkimensis, M. balbisiana, Himalayan Screwpine Pandanus nepalensis, Date Palm Phoenix sylvestris and the rare P. rupicola, the only living fossil tree of Sikkim Cycas pectinata, nettles and giant bamboo are characteristic of the region. In the region of Rangit Valley, Sal Shorea robusta shows a unique association with the Chir Pine Pinus roxburghii. In patches of protected forest, it is possible to see the weak Sal being slowly dominated by the Pine. These patches are relatively poor in bird life. However, the lowland forests of Sikkim are home to several threatened species of birds such as the possibly locally extinct Vulnerable Rufous-necked Hornbill Aceros nipalensis (but visitor from Neora Valley National Park in North Bengal), Great Pied Hornbill Buceros bicornis, locally called ‘Hongraio’, Chestnut-breasted Partridge Arborophila mandelli, even the now uncommon Red Junglefowl Gallus gallus,. Other lowland fauna includes the introduced Peafowl Pavo cristatus, Sikkim’s largest reptile Burmese Python Python bivittatus, house geckos, Himalayan Crestless Porcupine Hystrix brachyura, Assamese Macaque Macaca assamensis, Chinese Pangolin Manis crassicaudata, and Barking Deer Muntiacus muntjak, a variety of hill-stream fish, freshwater and tree frogs, toads and a host of butterflies and other invertebrates. Several species of migratory water birds use the river systems during transit. Six sq km of a representative area of the Kitam Reserve Forests has been declared as Kitam Bird Sanctuary.
• The Sub-Tropical ecoregion extends from about 1800 m to 3000 m. The rainfall in this zone is very heavy and conditions remain humid throughout the year. The upper-storey mainly consists of trees like Castanopsis hystrix, Machilus spp., Rhododendron spp., Symplocos spicata, S. theifolia, Michelia excelsa, Quercus lamellosa, Q. lineata, Q. pachyphylla, Engelhardia spicata, and Leucoceptrum canum. In the understorey are Eurya japonica, Rhododendron arboreum and Viburnum spp. In the middle storey, Symplocos theifolia is the main species and Litsea spp. and Bucklandia populnea are other associates. Dense tall evergreen forests with oaks and Rhododendrons predominate. The undergrowth consists of the bamboo Arundinaria maling, varieties of ferns, epiphytic mosses and orchids. Red Panda Ailurus fulgens, Red Fox Vulpes vulpes, Golden Jackal Canis aureus, Leopard Cat Prionailurus bengalensis, Spotted Linsang Prionodon pardicolor, Common Leopard Panthera pardus, Asiatic Black Bear Selenarctos himalayanus, Palm Civet Paguma larvata, Flying Squirrel Petaurista magnificus, Wild Boar Sus scrofa and Barking Deer Muntiacus muntjac have been recorded here. This area is also rich in forest birds including the Rusty-bellied Shortwing Brachypteryx hyperythra, Lesser Shortwing Brachypteryx leucophrys, Kaleej Pheasant Lophura leucomelanos and Satyr Tragopan Tragopan satyra; reptiles such as Japalura lizards Japalura sp., Cobra Naja naja, Krait and Mountain Pit Viper Ovophis monticola; Himalayan Bullfrog Paa leibigii; butterflies, Atlas moth Attacus atlas, Moon moth Actias selene, jewel beetles and leeches. Fambong Lho and (lower elevations of) Pangolakha Wildlife Sanctuaries in East Sikkim, Barsey in West Sikkim and Maenam Wildlife Sanctuary in South Sikkim as well as lower fringes of the Khangchendzonga National Park and Biosphere Reserve are the protected IBAs in this ecoregion. Lake Khecheopalri in West Sikkim occasionally hosts the Critically Endangered Baer’s Pochard Aythya baeri and once even a Vulnerable Black-necked Crane Grus nigricollis (photographed by a bird-watcher tourist Roger Ahlman in April 2006 in the reed-beds beside the lake).
• The Temperate ecoregion extends from 3000 m to 4000 m, with mixed coniferous forests of Hemlock, Spruce, Pine, Fir and Junipers and with shrubby undergrowth of Rhododendron and Arundinaria as well as the increasingly rare climber Aristolochia griffithii and insectivorous herb Drosera peltata. Red Panda Ailurus fulgens, Common Langur Semnopithecus entellus, Yellow-throated Marten Martes flavigula, the Vulnerable Asiatic Black Bear Ursus thibetanus, Himalayan Goral Naemorhaedus goral, Himalayan Serow Capricornis thar both Near Threatened like Golden Cat Catopuma temminckii, Endangered Wild Dog or Dhole Cuon alpinus, Vulnerable Clouded Leopard Neofelis nebulosa,(all photographed by camera-traps in Khangchendzonga National park), Himalayan Monal Lophophorus impejanus, Fire-tailed Sunbird Aethopyga ignicauda, and some species of reptiles and amphibians are characteristic of this region. The Brown Trout Salmo trutta fario and Rainbow Trout Oncorhynchus mykiss have been introduced in high-altitude lake and river systems over four decades ago. Tender shoots of Cardamine macrophylla, roots of Arisaema spp. and fruit of Seabuckthorn Hippophae salicifolia are collected for food, medicine and dyes.
• The Alpine forests and scrub extend up to 4500 m with small crooked trees and spreading shrubs interspersed with fir and pine. The stunted forest is mainly of Birch Betula spp.and Rhododendron with alpine herbs like various species of colourful Primulas and Potentillas. Dominant wild fauna include the Endangered Alpine Musk Deer Moschus chrysogaster, Near Threatened Himalayan Tahr Hemitragus jemlahicus, Blue Sheep or Bharal Pseudois nayaur, Blood Pheasant Ithaginis cruentus and Ibisbill Ibidorhyncha struthersii. River systems harbour some (introduced) Brown Trout Salmo trutta fario. Most of the flora of this region attracts interest for medicinal purposes. Dwarf rhododendron leaves are used for burning as incense. An important species recorded from this zone is the Caterpillar-fungus Cordyceps sinensis which is aggressively harvested by the local people due to its great commercial potential. This region has a very small resident human population, mainly Bhutias and mostly pastoral, herding livestock such as Yak, Dzo (cow-yak hybrid), a few horses and domestic cattle. The Temperate and Alpine ecoregions are protected in four wildlife sanctuaries at Shingba (North), Kyongnosla (East), Pangolakha (East) and Barsey (West) and one national park namely Khangchendzonga National Park (North and West). They harbour representative biodiversity of these ecoregions. They also harbour many high altitude glacial lakes and tarns which are important stop-over sites for migratory waterfowl and breeding grounds for Brahminy Shelduck Tadorna ferruginea, Common Redshank Tringa totanus and Ibisbill Ibidorhyncha struthersii. Shingba Rhododendron Sanctuary is home to the endemic Rhododendron niveum which has been designated the State Tree of Sikkim, with a small population recently discovered in Khangchendzonga National Park. The Kyongnosla Alpine Sanctuary has sheltered the Takin Budorcas taxicolor, which wandered over probably from Bhutan in 1999 through the recently declared Pangolakha Wildlife Sanctuary, which itself is contiguous with West Bengal’s Neora Valley National Park. One of India’s largest bovids, the Gaur Bos gaurus has been recorded in Pangolakha in the last decade and confirmed from a poached male confiscated by the State Forest Department. The 104 sq km Barsey Rhododendron Sanctuary with its pure Rhododendron stands is contiguous with the Singalila National Park in West Bengal.
• The Trans-Himalayan ecoregion extends from 4,500 m to over 5,500 m with characteristic cold desert vegetation, exclusively restricted to the north of Sikkim. This ecoregion has not yet been included in the protected area network of the State and is perhaps the most threatened. It contains many endangered species such as the Kiang or Tibetan Wild Ass Equus kiang, Near Threatened Nayan or Tibetan Argali Ovis ammon, and Tibetan Gazelle Procapra picticaudata, Endangered Snow Leopard Panthera uncia, Eurasian Lynx Lynx lynx, Near Threatened Pallas’s Cat Otocolobus manul, Tibetan Fox Vulpes ferrilata and Tibetan Wolf Canis lupus chanco. Ocurrence of Brown Bear Ursus arctos can be confirmed from a report of it predating an injured yak in this region. The Tibetan Snowcock Tetraogallus tibetanus, Lamergeier Gypaetus barbatus, Golden Eagle Aquila chrysaetos and Ruddy Shelduck Tadorna ferruginea are also found here. The Vulnerable Black-necked Crane Grus nigricollis has attempted breeding here. The region has a short four-month growing season during which grasses, sedges and medicinal herbs spurt abundantly supporting a host of insect fauna as well as wild and domestic herbivores, Himalayan marmots, pikas, Tibetan and Horned Larks, Tibetan Sandgrouse, Red-billed and Yellow-billed Choughs, Black and Guldenstadt’s Redstarts, pipits, wagtails and Mountain and Snow finches. Mongolian Plover Charadrius mongolus has been seen breeding at a small lake on the border called Bam Tso. There are no permanent settlements. The human population consists of a small number of nomadic Tibetan graziers or ‘Dokpas’ (who herd Yak, sheep and goats) and a large number of Defence personnel, as the area forms the international border with Tibet Autonomous Region of China.