Ecosystem Class 12 Notes Biology Chapter 14

Chapter at a Glance

  • Ecosystem is the functional unit of nature where living organisms interact with each other and with their environment.
  • Ecosystem can be recognised as self regulating and self sustaining units of landscapes that may be terrestrial or aquatic.
  • Forests, grasslands and deserts are examples of terrestrial ecosystems. The aquatic ecosystems can be either fresh water (ponds, lakes, streams) or salt water (marine estuaries) type.
  • Ecosystem may be natural (forest, sea); if developed under natural conditions or artificial (garden, agriculture) if created by man.
  • An ecosystem has two basic components-abiotic (non-living) and biotic (living organisms).
  • There are three biotic components : producers, consumers and decomposers. Abiotic components may be –
    • Physical or climatic components, like temperature, water, light, etc.
    • Inorganic compounds : Like water, minerals (sulphur, nitrogen, phosphorus etc.) and atmospheric gases (02, CO2, N2, etc.)
    • Organic compounds : These includes the organic substances present in the dead bodies of plants and animals. e.g., proteins, carbohydrates liquids etc.
  • The proper functioning of an ecosystem takes place through the following processes :
    • Productivity
    • Decomposition
    • Relationship of producers and consumers,
    • Flow of energy through different trophic levels,
    • Cycling of nutrients.
  • Standing crop is the amount of living biomass in an ecosystem. It indicates the productivity & luxuriance of growth.
  • The amount of nutrients, e.g., nitrogen, phosphorus & calcium present in the soil at any given time is known as standing state.
  • The rate of biomass production is called productivity. It is of two types – primary productivity and secondary productivity.
  • Primary productivity is the amount of biomass produced per unit area over a time period by plants during photosynthesis.It is expressed in terms of weight (g-2) or energy (kcal m-2). It is of two types:
    • Gross Primary Productivity (GPP) – It is the rate of production of biomass or accumulation of energy by green plants per unit area per unit time.
    • Net Primary Productivity (NPP)- It is the amount of biomass which has been stored by green plants.
    • Net primary productivity = Gross primary productivity – Respiration losses.
    • (or GPP-R = NPP)
  • Secondary productivity is the amount of biomass synthesised by consumers per unit area per unit time.
  • Decomposition is the breakdown of complex organic compounds of dead bodies of plants and animals into simpler inorganic compounds like CO2, water & various nutrients.
  • The various processes involved in decomposition are fragmentation, leaching, catabolism, humification, and mineralisation.
  • The organisms carrying out decomposition are called decomposers. Decomposers include micro-organisms (bacteria and fungi), detritivores (earthworm) and some parasites.
  • The rate of energy transfer between elements of an ecological system is called energy flow.
  • Sun is the ultimate source of energy for all the organisms of an ecosystem. 50% of the total solar radiation that falls on earth is Photosynthetically Active Radiation (PAR).
  • The flow of energy in an ecosystem is unidirectional. That is, it flows from the producer level to the consumer level and never in the reverse direction. Hence energy can be used only once in the ecosystem.
  • Only about 10% of the biomass is transferred from one trophic level to the next one is a food chain. And only about 10% of chemical energy is retained at each trophic level. This is called 10% law of Lindeman (1942).
  • Producers include green plants which are capable of manufacturing their own food. These are able to fix the energy obtained from the sun.
  • Consumers (phagotrophs) cannot make their own food but are directly or indirectly dependent on producers for obtaining food.
  • Consumers may be :
  • Primary consumers or herbivores.
  • Secondary consumers or primary carnivores.
  • Tertiary consumers or secondary carnivores.
  • Quartenery consumers.
  • Primary consumers obtain their food by directly feeding on producers (plants), secondary consumers from primary consumers (herbivores) and tertiary consumers from secondary consumers.
Food Chain and Food Web
  • Food chain is an order or sequence of different organisms which are arranged in a way that the food is passed from one type of organism to other organisms such that the organisms of one order or trophic level are the food of the organisms of next order.
  • Grazing food chain – Begins with plants and ends in carnivores. Eg. Grass  Grasshopper  Lizard  Hawk.
  • Detritus food chain – Begins with dead organic matter and made up of decomposers.
  • Food web refers toa group ofinter- related food chains in a particular community. Under natural conditions, the linear arrangement of food chain hardly occurs & these remain indeed inter-connected with each other through different types of organisms at different trophic level.

The number of individuals present or amount of biomass synthesised or amount of energy stored at successive trophic levels in an ecosystem can be graphically represented in the form of pyramids. These are called ecological or Eltonian pyramids.

Three ecological pyramids which are studied are- pyramid of number, pyramid of biomass and pyramid of energy.

  • Pyramid ofnumber is usually upright.
  • The pyramid of number of a single tree is spindle-shaped.
  • Pyramid of biomass of terrestial ecosystem is upright because at each successive trophic level the biomass tends to decrease, starting from primary producers and ending in top consumers.
  • Pyramid of biomass ofaquatic ecosystem is inverted.
  • Pyramid of energy is always upright because during the flow of energy from one trophic level to the next one, there always occurs a loss of energy.


Cyclic events by which various nutrients which are essential for the living organisms are transferred from one form to other. During these cycles, the nutrients pass from the biotic components to the abiotic components and vice­ versa,; hence these are also called biogeochemical cycles. Two types of nutrient cycles are-

  • Gaseous cycles (nitrogen, carbon cycles)
  • Sedimentary cycles (phosphorus, sulphur cycles)
Carbon cycle
  • Carbon is present as CO2 in atmosphere, as graphite and carbonates in rocks and also in fossil fuels (coal, petroleum).
  • Ocean are big reservoirs of carbon.
  • The cyclic representation of carbon assimilation by green plants (photosynthesis) which then passes into bodies ofanimals (plants are eaten) and finally during respiration of plants & animals & decompositions by microbes, the carbon dioxide is returned back to the atmosphere. Thus carbon is cycled through transfer and transformation between biotic and abiotic components.
Phosphorus cycle
  • Phosphorus is present in
    • biomembranes (as phospholipids)
    • nucleic acids (as phosphoric acid)
    • nucleotides (as AMP, ADP, ATP etc.)
    • bones and teeth (as hydroxyapetite)
  • Phosphorus is released during the decomposition of plant and animal remains.
  • The released phosphorus may reach the deeper layers of soil and gets deposited as phosphate rocks.
  • Phosphorus containing rocks are mined for manufacture of fertilisers, which provide an additional supply of an organic phosphates to the abiotic environment.

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