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Biosphere is the biologically inhabited part of earth along with its physical environment consisting of lower atmosphere, land and water bodies. Our planet earth is the only one on which life exists. It consists of three components, namely the land (lithosphere), water (hydrosphere) and air (atmosphere).

  • Lithosphere or the outer layer (crust) of earth, hydrosphere or all water sources above and underground, and atmosphere or air (a mixture of gases) comprise abiotic or non-living components of the environment.

Levels of Organisation in the Living System


It is known that the biosphere includes a rich variety of microorganisms, plants and animals including human beings. In the biosphere, we can see a complex arrangement showing a system of interaction between air, light, water, chemical substances and other non-living components of earth with the living organisms.

Following are the levels of organisation in the biosphere:

  1. Species: It is the lowest level of organisation in the biosphere. As we have known already, a species is a group of closely related organisms which can breed among themselves, producing fertile young ones.
  2. Population: It is the term used to describe a collection of individuals of a species that inhabit a given place at any given period of time.
  3. Community: The collection of individuals of different species of organisms inhabiting a given place is known as biotic community.
  4. Ecosystem: The communities of microorganisms, plants and animals found in any habitat will be interacting with each other as well as with the abiotic components in their surrounding environment. Such an interaction forms a functional component of the biosphere and is known as an ecosystem.
  5. Biome: The ecosystems that are spread over large geographical areas having same climatic features constitute a biome.

Ecology or environmental biology encompasses the study of the structural and functional aspects of these components in the biosphere, from the species of the biomes.

The term ‘biome’ is used to describe large ecosystems which have occupied vast geographical areas that show identical climatic conditions. The geographical distribution of biomes is decided on the basis of climatic factors such as rainfall, temperature and availability of sunlight. The biodiversity that we notice in any biome is dependent on the climatic conditions existing there.

Biomes can be broadly classified as follows:

  1. Terrestrial biomes: The total land area which forms about 30% of the earth can be distinguished into five types of biomes on the basis of climatic conditions and biodiversity.
  • Tropical evergreen forests: Rainfall is seen almost throughout the year in this biome. Temperature will be generally high. The flora of this biome is unique, represented by evergreen trees with broad leaves. The fauna of these biomes shows a rich diversity.
  • Deciduous forest: These are distributed in both eastern and southern hemisphere of the earth. Temperature is usually high and varies according to the seasons. The flora of this biome includes strong trees such as oak, sal, myrtle, poplar and elk. The fauna include mammals such as elephants, bison, panthers, moles, bears, squirrels and deer.
  • Grassland biome: In these biomes with an annual rainfall of less than 100 cm, the morning temperature is always high. Thorny shrubs, varieties of grasses, bamboo and other trees are commonly seen growing here.
  • Desert biome: In these biomes where annual rainfall is less than 25 cm, day-times are extremely hot, while the nights are cool. These biomes face a severe scarcity of water. Xerophytic plants are common in these biomes. Sahara of Africa, the great desert of Australia, and Thar desert of India are the common examples.
  • Tundra biomes: These are represented by the grasslands of cold regions where there is snow and ice throughout the year. The annual rainfall is very less. In the wet lands of this biome are found lichens, grasses, small herbs, bryophytes and sedges. The permanently frozen deepest soil layer is called permafrost. It is a characteristic feature of tundra biome.
  1. Aquatic biomes: Large areas of water masses can be called aquatic biomes. Seas and oceans constitute major aquatic biomes. Generally, the life forms in an aquatic biome are classified into planktons, nectons and benthos based on their distribution.
  • Planktons: The microorganisms that are found freely floating on the surface of water are called planktons. They can be classified into phytoplanktons and zooplanktons. The phytoplanktons are represented by members of algae which are capable of photosynthesis. The zooplantktons are consumers which feed on the phytoplanktons.
  • Nectons: These are represented by animals that are found freely swimming in water. They include water crabs, prawns, crustaceans, varieties of fishes, molluscs, snakes, dolphins and whales.
  • Benthos: This group is represented by bottom dwelling animals. Most of them have lost their capacity to move and are found usually attached to some substratum. Others are found slowly crawling on the ground. Sponges, corals, some molluscs, echinoderms such as star fishes and brittle stars and some unique forms of fishes are few examples.

Zonation in the aquatic biomes


  • The part of the sea up to where sunlight can penetrate is called euphotic zone.
  • The part of the sea or ocean where light cannot penetrate is called abyss.

Ecosystems Ecosystem is a functional unit of biosphere composed of biotic and abiotic factors that interact among themselves. There is a continuous flow of energy and matter through the system. Ecosystems vary greatly in size such as a small pond or a large forest.

The term ecosystem was coined by Sir Arthur Tansley. Ecosystem can be recognised as self-regulating and self-sustaining units of landscape. Ecosystem may be aquatic or terrestrial.

  1. Aquatic ecosystem: Any ecosystem where water forms the living medium is called aquatic ecosystem. Aquatic ecosystems where the salt concentration is very less are called freshwater ecosystems. Ponds, pools, ditches and lakes represent lentic ecosystems where water is stationary or stagnant. Rivers, streams and springs represent lotic ecosystems where water is of flowing type. Aquatic ecosystems where salt concentration is high are called marine ecosystems. Seas, oceans and estuaries are examples of marine ecosystems.
  2. Terrestrial ecosystems: Any ecosystem where land forms the medium for living is known as terrestrial ecosystem. It includes several types such as savanna, desert, grassland, forests and tundra.
  3. Artificial ecosystem: Along with the natural ecosystems in the biosphere, we can identify several man-made, artificial ecosystems. Parks, gardens, aquarium, fields and farms, water reservoirs are all examples of artificial ecosystems.
  • Components of an ecosystem: Any ecosystem should have two basic components, namely abiotic and biotic (Figure 10.1).
  • Important structural features of an ecosystem: Include species composition, trophic structures, stratification, standing state and standing crops.
  • Dynamics of an ecosystem: The functional stability of an ecosystem depends on the continuous production, consumption and decomposition activities. The dynamics of an ecosystem can be summarised as follows:
  1. The flow of energy in the ecosystem is unidirectional.
  2. The movement of materials in an ecosystem is cyclic.


Components of an Ecosystem



  • Ten percent law: Any process of energy conversion in an ecosystem is not 100% efficient, since there is always loss of energy in the form of heat at every trophic level. Based on investigations, it can be said that only 10% of the energy available in a trophic level gets transferred to the next trophic level. This idea has been proposed in the form of a law by a biologist by name, Kindermann in 1942. According to this law, only 10% of energy available at a given tropic level gets transferred to the subsequent tropic level while 90% of the energy is lost as heat.

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Ten Percent Law of Energy


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