4 and 5 Succession; Terrestrial Ecosystem UPSC Notes | EduRev

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4. SUCCESSION

A universal process of directional change in vegetation, on an ecological time scale. Occurs when a series of communities replace one another due to large scale destruction either natural or manmade.

Continuously -one community replacing another community, until a stable, mature community develops.

The first plant to colonise an area is called the pioneer community. The final stage of succession is called the climax community.

The stage leading to the climax community are called successional stages or series. Characterised by the following: increased productivity, the shift of nutrients from' the reservoirs, increased diversity of organisms with increased niche development, and a gradual increase in the complexity of food webs.

Primary Succession 

In primary succession on a terrestrial site the new site is first colonized by a few hardy pioneer species that are often microbes, lichens and mosses.

The pioneers through their death any decay leave patches of organic matter in which small animals can live.

The organic matter produced by these pioneer species produce organic ads during decomposition that dissolve and etch the substratum releasing nutrients to the substratum. Organic debris accumulates in pockets and crevices, providing soil in which seeds can become lodged and grow.

As the community of organisms continues to develop, it becomes more diverse and competition increases, but at the same time new niche opportunities develops.

The pioneer species disappear as the habitat conditions change and invasion of new species progresses, leading to the replacement of the preceding community.

Secondary Succession 

Secondary Succession occurs when plants recognize an area in which the climax community has been disturbed.

Secondary Succession is the sequential development of biotic communities after the complete or partial destruction of the existing community.

This abandoned farmland is first invaded by hardy species of grasses that can survive in bare, sun-baked soil. These grasses may be soon joined by tall grasses and herbaceous plants.

These dominate the ecosystem for some years along with mice, rabbits, insects and seed- eating birds.

Eventually, some trees come up in this area, seeds of which may be brought by wind or animals. And over the years, a forest community develops. Thus an abandoned farmland over a period becomes dominated by trees and is transformed into a forest.

The differences between primary and secondary succession, the secondary succession starts on a well-developed soil already formed at the site. Thus secondary succession is relatively faster as compared to primary succession which may often require hundreds of years.

Autogenic and Allogeneic Succession 

When succession is brought about by living inhabitants of that community itself, the process is called autogenic succession, while change brought about by outside forces is known as allogeneic succession.

Autotrophic and Heterotrophic succession  

Succession in which, initially the green plants are much greater in quantity is known as autotrophic succession; and the ones in which the heterotrophs are greater in quantity is known as heterotrophic succession.

Succession would occur faster in area existing in the middle of the large continent. This is because, here all prop gules or seeds of plants belonging to the different series would reach much faster, establish and ultimately result in climax community.      

 

5. TERRESTRIAL ECOSYSTEMS

The interrelations between organisms and environment on the land constitute "Terrestrial Ecology". The most important limiting factors of the terrestrial ecosystems are moisture and temperature.

 

TUNDRA 

The word tundra means a "barren land" since they are found where environmental conditions are very severe. There are two types of tundra- arctic and alpine.

Distribution: 

Arctic tundra extends as a continuous belt below the polar ice cap and above the tree line in the northern hemisphere. It occupies the northern fringe of Canada, Alaska, European Russia, Siberia and island group of Arctic Ocean. 

On the South Pole, tundra is very small since most of it is covered by ocean.

Alpine tundra occurs at high mountains above the tree line. Since mountains are found at all latitudes therefore alpine tundra shows day and night temperature variations.

Flora and fauna: 

Typical vegetation of arctic tundra is cotton grass, sedges, dwarf heath, willows, birches and lichens.

Animals of tundra are reindeer, musk ox, arctic hare, caribous, lemmings and squirrel.

They are protected from chillness by the presence of thick cuticle and epidermal hair. 

Mammals of the tundra region have large body size and small tail and ear to avoid the loss of heat from the surface. 

The body is covered with fur for insulation. 

 

FOREST ECOSYSTEM

Forest ecosystem includes a complex assemblage of different kinds of biotic communities. Optimum conditions such as temperature and ground moisture are responsible for the establishment of forest communities.

Forests may be evergreen or deciduous which are distinguished on the basis of leaf into broad-leafed or needle leafed coniferous forests in the case of temperate areas.

Classified into three major categories: coniferous forest, temperate forest and tropical forest. 

All these forest biomes are generally arranged on a gradient from north to south latitude or from high to lower altitude

 

Coniferous forest (boreal forest): 

Cold regions with high rainfall, strong seasonal climates with long winters and short summers evergreen plant species such as Spruce, fir and pine trees, etc. and by animals such as the lynx, wolf, bear, red fox, porcupine, squirrel, and amphibians like Hyla, Rana, etc.

Boreal forest soils are characterized by thin podozols and are rather poor. Both because, the weathering of rocks proceeds slowly in cold environments and because the litter derived from conifer needle (leaf is decomposed very slowly and is not rich in nutrients.

These soils are acidic and are mineral deficient. 

This is due to movement of large amount of water through the soil, without a significant counter-upward movement of evaporation, essential soluble nutrients like calcium, nitrogen and potassium which are leached sometimes beyond the reach of roots. This process leaves no alkaline oriented cations to encounter the organic acids of the accumulating litter.

The productivity and community stability of a boreal forest are lower than those of any other forest ecosystem.

Temperate deciduous forest: 

The temperate forests are characterised by a moderate climate and broad-leafed deciduous trees, which shed their leaves in fall, are bare over winter and grow new foliage in the spring. 

The precipitation is fairly uniform throughout.

Soils of temperate forests are podozolic and fairly deep.

Temperate evergreen forest: 

Parts of the world that have Mediterranean type of Climate are characterised by warm, dry summers and cool, moist winters. Low broad leafed evergreen trees.

Fire is an important hazardous factor in this ecosystem and the adaptation of the plants enable them to regenerate quickly after being burnt.

Temperate rain forests: 

Seasonality with regard to temperature and rainfall

Rainfall is high, and fog may be very heavy. It is the important source of water than rainfall itself

The biotic diversity of temperate rain forests is high as compared to other temperate forest.

The diversity of plants and animals is much low as compared to the tropical rainforest.

Tropical rain forests: 

Near the equator.

Among the most diverse and rich communities on the earth.

Both temperature and humidity remain high and more or less uniform.

The annual rainfall exceeds 200 cm and is generally distributed throughout the year.

The flora is highly diversified The extreme dense vegetation of the tropical rain  forests remains vertically stratified with tall trees often covered with vines, creepers, lianas, epiphytic orchids and bromeliads.

The lowest layer is an understory of trees, shrubs, herbs, like ferns and palms.

Soil of tropical rainforests are red latosols, and they are very thick

Tropical seasonal forests: 

Also known as monsoon forest occur in regions where total annual rainfall is very high but segregated into pronounced wet and dry periods.

This kind of forest is found in South East Asia, central and south America, northern Australia, western Africa and tropical islands of the pacific as well as in India.

Subtropical rain forests:

Broad-leaved evergreen subtropical rain forests are found in regions of fairly high rainfall but less temperature differences between winter and summer

Epiphytes are common here.

Animal life of subtropical forest is very similar to that of tropical rainforests. 

 

INDIAN FOREST TYPES 

Forest types in India are classified by Champion and Seth into sixteen types.

      

Tropical Wet evergreen forests 

Are found along the Western Ghats, the Nicobar and Andaman Islands and all along the north-eastern region.

It is characterized by tall, straight evergreen trees.

The trees in this forest form a tier pattern:

Beautiful fern of various colours and different varieties of orchids grow on the trunks of the trees.

Among the following States, which one has the most suitable climatic conditions for the cultivation of a large variety of orchids with minimum cost of production, and can develop

Tropical Semi-evergreen forests 

Found in the Western Ghats, Andaman and Nicobar Islands, and the Eastern Himalayas. 

Such forests have a mixture of the wet evergreen trees and the moist deciduous trees. The forest is dense 

Tropical Moist deciduous forests 

Found throughout India except in the western and the north -western regions. 

The trees are tall, have broad trunks, branching trunks and roots to hold them firmly to the ground. 

These forests are dominated by Sal and teak, along with mango, bamboo, and rosewood.

Littoral and swamp

Found along the Andaman and Nicobar Islands and the delta area of the Ganga and the Brahmaputra. 

They have roots that consist of soft tissue so that the plant can breathe in the water.

Tropical Dry deciduous forest 

The northern part of the country except in the North-East. It is also found in Madhya Pradesh, Gujarat, Andhra Pradesh, Karnataka, and Tamil Nadu. The canopy, of the trees does not normally exceed 25 metres. 

The common trees are the Sal, a variety of acacia, and bamboo.

Tropical Thorn forests 

This type is found in areas with black soil: North, West, Central, and South India. The trees do not grow beyond 10 metres. Spurge, caper, and cactus are typical of this region.

Tropical Dry evergreen forest 

Dry evergreens are found along Tamil Nadu Andhra Pradesh and Karnataka coast. It is mainly hard-leaved evergreen trees with fragrant flowers, along with a few deciduous trees.

Sub-tropical Broad-leaved forests  

Broad-leaved forests are found in the Eastern Himalayas and the Western Ghats, along the Silent Valley. 

There is a marked difference in the form of vegetation in the two areas. 

In the Silent Valley, the poonspar, cinnamon, rhododendron, and fragrant grass are predominant. 

In the Eastern Himalayas, the flora has been badly affected by the shifting cultivation and forest fires. 

There are oak, alder, chestnut, birch, and cherry trees. There are a large variety of orchids, bamboo and creepers.

Sub-tropical Pine forests 

Found in Shivalik Hills, Western and Central Himalayas, Khasi, Naga, and Manipur Hills. 

The trees predominantly found in these areas are the chirr, oak, rhododendron, and   pine as well as Sal, Alma, and laburnum are found in the lower regions.

 

Sub-tropical Dry evergreen forests 

Hot and dry season and a cold winter. It generally has evergreen trees with shining leaves that have a varnished look. 

Found in the Shivalik Hills and foothills of the Himalayas up to a height of 1000 metres.

Montana Wet temperate forests 

In the North, found in the region to the east of Nepal into Arunachal Pradesh, receiving a minimum rainfall of 2000 mm. In the North, there are three layers of    forests: the higher layer has mainly coniferous, the middle layer has deciduous trees such as the oak and the lowest layer is covered by rhododendron and champa.

In the South, it is found in parts of the Niligiri Hills, the higher reaches of Kerala. 

The forests in the northern region are denser than in the South. Rhododendrons and a variety of ground flora can be found here.

Himalayan Moist temperate Forest 

This type spreads from the Western Himalayas to the Eastern Himalayas. The trees found in the western section are broad-leaved oak, brown oak, walnut, Rhododendron, 

Eastern Himalayas, the rainfall is much heavier and therefore the vegetation is also more lush and dense. There are a large variety of broad-leaved trees, ferry, and bamboo. 

Himalayan Dry temperate Forest 

This type is found in Lahul, Kinnaur, Sikkim, and other parts of the Himalayas. 

There are predominantly coniferous trees, along with broad-leaved trees such as the oak, maple, and ash. At higher elevation, fir, juniper, deodar, and chilgoza are found.

 

Sub alpine forest 

Sub alpine forests extend from Kashmir to Arunachal Pradesh between 2900 to 3500 metres. 

In the Western Himalayas, the vegetation consists mainly of juniper, rhododendron, willow, and black currant. 

In the eastern parts, red fir, black juniper, birch, and larch are the common trees. 

Due to heavy rainfall and high humidity the timberline in this part is higher than that in the West. 

Rhododendron of many species covers the hills in these parts.

Moist Alpine scrub 

Moist alpines are found all along the Himalayas and on the higher hills near the Myanmar border. It has a low scrub, dense evergreen forest, consisting mainly of rhododendron and birch. Mosses and ferns cover the ground in patches. This region receives heavy snowfall.

Dry alpine scrub 

Dry alpines are found from about 3000 metres to about 4900 metres. Dwarf plants predominate, mainly the black juniper, the drooping juniper, honeysuckle, and willow.

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