A natural ecosystem is an assemblage of plants and animals which functions as a unit and is capable of maintaining its identity such as forest, grassland, an estuary, human intervention is an example of a natural ecosystem. A natural ecosystem is totally dependent on solar energy. There are two main categories of ecosystems:

• Terrestrial ecosystem: Ecosystems found on land e.g. forest, grasslands, deserts, tundra.
• Aquatic ecosystem: Plants and animal community found in water bodies. These can be further classified into two sub groups:
  • Fresh water ecosystems, such as rivers, lakes and ponds.
  • Marine ecosystems, such as oceans, estuary.

Terrestrial Ecosystem Forests

1. Moist Tropical Evergreen Vegetation

These are the tropical rain forests which are further divided into two sub-types on the basis of their characteristics as under:

The Wet Tropical Evergreen Vegetation:

• It is found in regions of very high annual rainfall exceeding 300 cms. With a very brief dry season. Southern parts of Western Ghats of Kerala and Karnataka are very wet. Northeastern Hills are known for this type of vegetation. It resembles the equatorial vegetation. This type of vegetal cover has been badly depleted due to over cutting of trees. The major characteristics of this type of vegetation are:
  • These forests are dense and have lofty evergreen trees, often as high as 60 metres and above.
  • The number of vegetal species per unit area is too large to exploit them commercially.
  • Mahogony, cinchona, bamboos and palms are typical species of plants found in these forests. Undergrowth is very dense and thick. Grass is almost absent.
  • The wood of these trees is very hard and heavy to work with.

Moist Tropical Semi-evergreen Vegetation:

• It is found between wet evergreen vegetation and moist temperate deciduous vegetation. This type of vegetation is found on the Meghalaya plateau, Sahyadris and Andaman and Nicobar Islands. This vegetation is confined to areas receiving an annual rainfall of about 250 to 300 cms. It’s important characteristics are:
  • The vegetation cover is less dense than the wet evergreen forests.
  • Timber of these forests is fine textured and of good quality.
  • Rosewood, aini and telsur are important trees in Sahyadris, champa. joon and gurjan in Assam and Meghalaya and ironwood, ebony and laurel grew in other regions.
  • Shifting agriculture and over exploitation of forests have depleted this vegetal cover to a great extent.

2. Moist Tropical Deciduous Vegetation

• This is the most wide spread vegetal cover of India. This type of vegetation is found in areas receiving annual rainfall of 100 to 200 cms.
• These include the Sahyadris, the northeastern plateau of the peninsula, the Himalayan foot hills in the Siwaliks, the bhabars and terai. The important characteristics of this vegetation are:
  • The trees shed their leaves once in a year in dry season.
  • This is a typical monsoon vegetation consisting of larger number of commercially important species than the evergreen forests.
  • Teak, sal, sandalwood, shisham, cane and bamboo are important trees of these forests.
  • Large scale cutting of trees for timber has depleted these forests hopelessly.

3. Dry Tropical Vegetation:

This type of vegetation is divided into two groups as under: 

Dry Tropical Deciduous Vegetation

• It is found in regions receiving annual rainfall between 70 to 100 cms. These regions include parts of Uttar Pradesh, northern and western Madhya Pradesh, parts of Gujarat, Maharashtra, Andhra Pradesh, Karnataka and Tamil Nadu.
• These areas experience a long dry season and a moderate rainfall limited at best to four months. The important characteristics of this vegetation are:
  • Stretches of open grass are most common between group of trees. Teak is the dominant tree of this type of vegetation.
  • The trees shed their leaves during the long dry season.
  • Dry Tropical Thorny Vegetation
  • It is found in areas receiving annual rainfall less than 70 cms. These areas include north and north-western parts of India and leeward side of the Sahyadris. The important characteristics of this type of vegetation are:
    • Vast, poor and coarse grasslands are interspersed with widely spaced trees and bushes.
    • Acacia, euphorbias, cactus etc. are true representatives of this type of vegetation. Wild palm and spiny and thorny varieties are also found here and there.

4. Tidal Vegetation

• This type of vegetation grows mainly in the deltaic regions of the Ganga, Mahanadi, Godavari and Krishna which are flooded by tides and high sea waves. Mangrove is the representative of this type of vegetation.
• Sundari is the typical tree of tidal forests. It is found in abundance in the lower Ganga delta of West Bengal. This is the reason why it is popularly known as Sunderban. It is known for its hard and durable timber.

5. The Mountain Vegetation

• Due to the difference in temperature and other weather conditions of northern and peninsular mountain ranges, there exists difference in the vegetal cover of these two groups of mountain ranges. Hence, the mountain vegetation can be classified as the mountain vegetation of peninsular plateau and the mountain vegetation of the Himalayan ranges.

The Mountain Vegetation of Peninsular Plateau:

• The high altitude area of the plateau region include Nilgiri, Annamalai and Palni hills, Mahabaleshwar in Western Ghats, Satpura and Maikal hills. The important characteristics of vegetation of this region are:
  • Stretches of open rolling grass plains with undeveloped forests or bushes are found.
  • The wet temperate forests below 1500 metres are less dense than those found above this height.
  • The forests have thick undergrowth, epiphytes, mosses and ferns.
  • Magnolia, laurel, elm are common trees.
  • Cinchona and eucalyptus have been introduced from outside the country.

The Mountain Vegetation of the Himalayan Ranges:

• In the Himalayan mountain region, the vegetation is different at increasing altitudes. This can be divided into following types:
  • Moist Tropical Deciduous forests are found along the foot hills in the Siwaliks, upto the height of 1000 metres.
  • The Wet Temperate Evergreen forests are found in the areas lying between 1000 to 3000 metres. The important characteristics of these forests are:
    • These are very thick forests of lofty trees.
    • Oak and chestnut are the predominant trees of the eastern Himalayan region while chir and pine are in the western part.
    • Sal is the important tree in lower altitudes.
    • Deodar, silver fir and spruce are predominant trees between the height of 2000 and 3000 metres. These forests are less dense as compared to the forests at lesser elevations.
    • These forests are of great economic importance to the local population.
• Dry Temperate Vegetation
• It is found on the higher hilly slopes of this mountain region which has moderate temperatures and rainfall between 70 cms and 100 cms. Important characteristics of this type of vegetation are:
  • This vegetation resembles the Mediterranean vegetation.
  • Wild olives, acacia are important trees along with hard, coarse and thick savanna grass.
  • Oak and deodar are found here and there.
• Alpine Vegetation 
• It is found between the altitude 3000 and 4000 metres. The important characteristics of these forests are:
  • These are far less dense
  • Silver fir, juniper, birch, pine and rhododendron are important trees of these forests. However, all of them have only a stunted growth.
  • Alpine pastures are found at still higher altitudes.
  • The trees get progressively stunted as they approach the snow line.