12th Class Biology Organism And Environments Biome


Category : 12th Class

Definition : Each of the major terrestrial ecosystems or distinctive terrestrial areas with their group of climax plants and associated animals constitutes biomes. A biome is the largest terrestrial community. Rainfall, temperature range, nature of soil, barriers, latitude and altitude determine the nature and extent of biomes.

Major biomes of world : Biomes are often classified in seven categories :

(1) Tropical rain forests : The tropical rain forest, a biome occurs in regions of high temperature (average 25°C) and high rainfall (\[200-450\,\,cm\] per year). These tropical rain forests occur in Central America, around Amazon basin in South America, in Africa and in South-East Asia.

(i) This biome is characterized by multistoried vegetation (upto five distinct layers or storeys of vegetation). Further maximum biodiversity on land is shown by this biome and it is estimated that one half to two-thirds of all species of terrestrial plants and insects live in tropical forests.

(ii) Lianas (vascular plants rooted in soil and they only get support of trees for climbing to top) and epiphytes (air plants) are common in this biome due to excess of moisture. Further giant trees of the tropical forest support a rich and diverse community of animals on their branches.

(iii) No one species dominates in this biome.

(iv) The productivity of this biome is maximum.

(v) The trees of this biome possess buttressed trunks and phenomenon of cauliflory (presence of flowers and fruits on main trunk and main branches) is common in this biome.

(2) Savannahs : Like tropical forests, savannahs are found near the equator but in areas having less annual rainfall (90-150 cm/year). Some areas near the equator experience prolonged dry seasons. The heat, periodic dryness and poor soils cannot support a forest but have led to evolution of tropical open grasslands with scattered shrubs and trees.

(i) The vegetation of this biome support large grazing herbivores like buffalo, zebra, etc., which are food for carnivores like lions, tigers, etc. The savannah also supports a large number of plant eating invertebrates like mites, grasshoppers, ants, beetles and termites.

(ii) The termites are one of the most important soil organisms in savannahs.

(iii) Indian tropical grasslands are not true savannahs but these are the result of destruction and modification of tropical deciduous forests by cutting, grazing and fire.

(3) Deserts : These are the biomes that have 25 cm (10 inches) or less of precipitation annually.

(i) Sahara of North Africa, Thar of West Asia and Gobi of Asia are most important deserts.

(ii) Annual plants are abundant in deserts and tide over unfavourable dry season in the form of seeds. Succulent plants are characteristics of deserts. Trees and shrubs present in deserts have deep roots.

(iii) Desert animals have also fascinating adaptations that enable them to adjust with limited water supply.

(iv) Desert plants show phenomenon of Allelopathy, i.e., they secret some chemical substances which inhibit the growth of plants growing in their near vicinity.

(v) Deserts show poor biodiversity and their productivity is minimum.

(4) Temperate grasslands : Temperate grasslands experience a greater amount of rainfall than deserts but a lesser amount than savannahs. They occur at higher latitudes than savannahs but like savannahs are characterized by perennial grasses and herbs of grazing mammals.

Temperate grasslands have different names in different parts of the world, e.g., Prairies of North America, Steppes of Russia, Veldts of South Africa, Pampas of South America, Pusztas of Hungary and Tussocks of New Zealand.

(5) Temperate deciduous forests : Temperate deciduous forests occur in areas having warm summers, cold winters and moderate amount of precipitation (\[75150\,\,cm\]annually). The trees of this forest loose their leaves during autumn and remain dormant throughout winter (term ‘deciduous’ derived from Latin word meaning ‘to fall’). These forests are present in Eastern United States, Canada and extensive region in Eurasia.

(i) In temperate forest biome, there is an upper canopy of dominant trees like beech, oak, birch, maple, etc. followed by lower tree canopy and then a layer of shrubs beneath.

(ii) Animal life in this biome is abundant on the ground as well as on the trees.

(6) Taiga : The taiga or northern coniferous forests or boreal forests consist of evergreen, cone bearing trees like spruce, hemlock and fir and extend across vast areas of Eurasia, and North America.

(i) The taiga is characterized by long, cold winters with little precipitation.

(ii) The harsh climate limits productivity of the taiga community. The cold temperatures, very wet soil during the growing season and acids produced by fallen conifers needles and Sphagnum inhibit full decay of organic matter, due to which thick layers of semidecayed organic material called peat is formed, which acts as energy source.

(7) Tundra : The tundra encircles the top of the world. This biome is characterised by desert like levels of precipitation (less than 25 cm annually), extremely long and cold winters and short warmer summers.

(i) Tundra is uniform in appearance and is dominated by scattered patches of grasses, sedges and lichens. Some small trees do grow but are confined to margins of streams and lakes (In general treeless).

(ii) Tundra is a biome of low diversity and low productivity.

(iii) The precipitation that falls remains unavailable to plants for most of the year because it freezes. During the brief arctic summer, some of the ice melts and permafrost (or permanent ice) found about a meter down from the surface, never melts and is impenetrable to both water and roots. However, the alpine tundra found at high elevation in temperate or tropical regions does not have this layer of permafrost.

Indian biomes : Indian forests are classified into three major types based on temperature are tropical, temperate, alpine.




(1) The marine environment : It is characterized by its high concentration of salt (about 3.5 percent in open sea) and mineral ions (mostly sodium and chloride followed by sulphur, magnesium and calcium).

(i) The vertical zones of the ocean are determined on the basis of availability of light for photosynthesis. The lighted upper 200 metres form the photic or euphotic zone. The next zone upto the depth \[2002000\] metres gets less light which is insufficient for photosynthesis form the aphotic zone. Below 2000 metre is the area of perpetual darkness, the abyssal zone.

(ii) Three major environments may be recognized in the ocean basin     

(a) The littoral zone : The sea floor from the shore to the edge or the continental shelf.

(b)  The benthonic zone : The sea floor along the continental slope and the aphotic and abyssal zone.

(c)   The pelagic zone : Constituting the water of the ocean basin.

  • Marine life : It can be grouped into three main categories :

(i) Plankton : These are passively drifting or floating organisms. Most of these minute organisms, plankton includes photosynthesizing organisms like diatoms (phytoplankton) as well as heterotrophic organisms like small crustaceans (zooplanktons).

(ii) Nektons : These consist of actively moving organisms with well developed locomotory organs.

(iii) Benthonic organisms : These are found along the floor of the sea bed and include creeping, crawling or sessile organisms.

(2) Other (Lakes and Ponds) : Lakes and ponds are stagnant fresh water bodies and are found practically in every biome. Many lakes are direct or indirect result of glaciation. Others are natural or man made depression filled with water. The relatively shallow lakes, called eutrophic lakes, have a rich accumulation of organic products e.g., Dal lake of Kashmir.

Generally deep lakes, often with the steep and rocky sides, are poor in circulating nutrients like phosphates. These are called oligotrophic lakes. Some of the lakes contain a saline or brackish water (Sambhar lake of Rajasthan).

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