Ecosystem Class 12 Notes Biology Chapter 14 - CBSE


What are Ecosystem?

Arthur Tansley first defined the term Ecosystem. It is the functional unit of nature, where living organisms nteract among themselves and with the surrounding environment.

Types of Ecosystem Based on Nature


  • Terrestrial: Forests, Grasslands, Deserts etc.
  • Aquatic: Marine- Seas, Oceans, Estuaries etc. Freshwater- Ponds, Lakes, Rivers, Streams etc.

Manmade / Artificial

  • Aquariums, Crop fields, Flowerbeds, etc.

Structure Of Ecosystems

An Ecosystem has two components:

  • Biotic components: bacteria, fungi, plants, consumers
  • Abiotic components: air, water, soil

Functioning Of An Ecosystem


The rate of synthesis of organic matter (biomass) during a given period of time. It is measured as weight (g-2) or as energy (kcalm-2).
  • Primary productivity: Amount of biomass produced per unit area in a given time period by plants during photosynthesis. GPP – R = NPP
  • *GPP-Gross Primary Productivity *NPP-Net Primary Productivity *R- Respiratory loss to the environment
  • Secondary productivity: It is the amount of biomass produced by the consumer at any level in a given period of time.


It is the process of breaking down of dead organic matter into smaller organic or inorganic molecules.

  • Detritus: Dead remains of plants and animals is called detritus.
  • Detritivores: Animals that feed on decaying organic matter(detritus). Examples: earthworms, termites, snails etc.

Energy Flow

  • All living things require chemical energy from food as their primary source of energy. Throughout the food chain, this energy is transferred to various trophic levels.
  • 2- 10 % of PAR is captured by plants.
  • When energy is transferred as food, most part is lost as heat at each stage (10% LAW)
  • Food Chains: Food chains represent energy flow through ecosystems.
  • (i) Grazing Food Chain (GFC) :
  • Primary source of energy - Solar radiations.
  • First trophic level includes - All Herbivores.
  • (ii) Detritus Food Chain (DFC) :
  • Primary source of energy is Detritus.
  • First trophic level includes Detritivores.
  • Food Webs: Interconnected food chains are called as FOOD WEB: :
  • (i) Unlike food chains, food webs are never straight.
  • (ii) Help in ecosystem development and stability.
  • Ten Percent Law (By Lindemann in 1942): When energy is transferred from one trophic level to another, only about 10% is stored at higher levels; remaining 90% is lost in respiration (heat).

Ecological Pyramid

Pyramids represents the relationship between organisms at different trophic levels in terms of their number, biomass or energy.

  • Pyramid of Number: The relationship between numbers of producers and consumers in an ecosystem at a particular trophic level.
  • Pyramid of Biomass: Graphical representation of biomass of organisms present in an ecosystem at different trophic levels.
  • Pyramid of Energy: Graphical representation of amount of energy available at a particular trophic level. It is always upright.

Ecological Succession

Predictable and orderly change in the composition or structure of a community and it can be initiated either by formation of new or unoccupied habitat.

Sere – entire sequence of community that successively change in a given area.

  • Climax Community: Changes that lead finally to a community that is in near equilibrium with the environment. It remains stable as long as the environment remains unchanged.
  • Primary Succession: If the development begins on an area that has not been previously occupied by a community.
  • Secondary Succession: If the community development is proceeding in an area from which a community was removed which was earlier present.

Ways to avoid Self-pollination

  • Release of pollen grains and receptivity of stigma are not synchronized.
  • Stigma and anther are placed at different positions.
  • Self-incompatibility.
  • Production of unisexual flowers (prevents autogamy but not geitonogamy). e.g. castor, maize.
  • Male and Female flowers are produced on different flowers (prevents autogamy & geitonogamy). e.g. papaya.

The Nutrient Cycle/biogeochemical Cycle

  • Environmental factors like soil, moisture, temperature, etc. regulate the rate of release of nutrients into the atmosphere.
  • Standing state – amount of nutrients, such as C, N, P, Ca, etc. present in the soil at any given time.
  • Nutrients are never lost from the ecosystem but are recycled
  • There are two types: Gaseous cycle & Sedimentary cycle

Ecosystem Services

Living things perform a host of ecological services that help maintain life, natural systems, and moderate conditions on our planet.

  • Purify air and water
  • Decomposition of waste materials
  • Cycling of nutrients
  • Pollination of crops