Ecosystem Class 12 | Ecology | Term 2 | Biology | CBSE | NCERT |


Ecosystem Class 12, Ecology, Term 2, Biology


  • An ecosystem is a functional unit of nature, where living organisms interact among themselves and also with the surrounding physical environment.
  • Ecosystem varies greatly in size from a small pond to a large forest or a sea.
  • Many ecologist regard the entire biosphere as a global ecosystem , as a composite of all local ecosystem on earth.

Ecosystem: Types, Structure and Function

  • the various components of ecosystem include biotic and abiotic
  • in an ecosystem , interaction of biotic and abiotic components takes place in a more integrated manner resulting in a physical structure that is characteristic for each type of ecosystem.
  • identification and enumeration of plant and animal species of an ecosystem gives its species composition.
  • vertical distribution of different species occupying different levels is called stratification.
  • example- trees occupy top vertical strata ( layer) of a forest , shrubs the second and the herbs and grasses occupy the bottom layer.

Components of Ecosystem

There are four basic components that function as a unit. these include

  1. Productivity
  2. Decomposition
  3. Energy flow
  4. Nutrient cycling

Types of Ecosystem

a. Terrestrial Ecosystem: Forest , grassland , desert etc.

b. Aquatic Ecosystem: Pond, lake , wetland , river and estuary

c. Man- Made ecosystem : Crop fields and aquarium


A pond is a shallow water body in which all the above mentioned basic structural and functional components are present.
(b) Abiotic components are water with all the dissolved inorganic and organic materials and soil deposited at the bottom.
(c) Autotrophic components are phytoplanktons, some algae and the floating, submerged and marginal plants found at the edges.
(d) Consumers are zooplanktons, which are free swimming and bottom dwellers.
(e) Decomposers are the fungi, bacteria and flagellates found abundantly in the bottom.
(f) Functioning of pond ecosystem occurs in following steps:
* Autotrophs convert inorganic material into organic material with the help of solar energy.
* Heterotrophs consume autotrophs.
* Decomposers decompose dead organic materials and mineralise it to release them back for reuse by the autotrophs.
* The above events are repeated again and again.
Unidirectional movement of energy occur towards the higher trophic levels and lost in the form of heat to the environment.


is the rate of biomass production. It is expressed in g-2 yr-1 or (kcal m-2) yr-1.
(i) The amount of biomass or organic matter produced per unit area over a time period by plants during photosynthesis is called primary production.
(ii) The primary productivity can be divided into:
(a) Gross Primary Productivity (GPP) of an ecosystem is the rate of production of organic matter during photosynthesis. A considerable amount of GPP is utilised by plants in respiration.
(b) Net Primary Productivity (NPP) is gross primary productivity minus respiratory losses (R).
NPP is the available biomass for the consumption to heterotrophs, i.e. herbivores and decomposers.
(iii) Secondary productivity is defined as the rate of formation of new organic matter by . consumers.
(iv) Primary productivity of an ecosystem depends on:
(a) Plant species inhabiting a particular area.
(b) Availability of nutrients.
(c) Photosynthetic capacity of plants.
(d) Variety of environmental factors.
(v) Annual net primary productivity of the whole biosphere is about 170 billion tons (dry weight) of organic matter. Of this, despite of occupying about 70% of the surface of earth, the productivity of the oceans are only 55 billion tons.


Decomposition is the process in which decomposers breakdown complex organic matter into inorganic substances like carbon dioxide, water and nutrients.
(i) The raw materials called detritus are dead plant remains such as leaves, barks, flowers and dead remains of animals, including faecal matter.
(ii) Process of decomposition occurs in the following steps:
(a) Fragmentation is the breakdown of detritus into smaller particles by detritivores, e.g. earthworm.
(b) Leaching is the process by which water soluble inorganic nutrients go down into the soil horizon and get precipitated as unavailable salts.
(c) Catabolism is the process of degradation of detritus into simple inorganic substances by bacterial and fungal enzymes.
(d) Humification is the process of accumulation of a dark coloured amorphous substance called humus. It is highly resistant to microbial action, undergoes decomposition at an extremely slow rate and serves as a reservoir of utrients.
(e) Mineralisation is the process by which humus is further degraded by some microbes to release inorganic substances.

Leaves partially consumed by decomposers such as fungi and bacteria.
They begin to lose form and become litter.
Diagrammatic representation of decomposition cycle in a terrestrial ecosystem
(iii) Decomposition rate depends on
(a) Oxygen availability
(b) Chemical composition of detritus
(c) Climatic factors
(d) Temperature and soil moisture
(iv) Decomposition rate is slower, if detritus is rich in lignin and chitin. It is quicker, if detritus is rich in nitrogen and water soluble substances like sugars.
(v) Warm and moist environment speeds up decomposition, whereas low temperature and anaerobiosis inhibits decomposition and causes the formation of organic materials.

Energy Flow

The chemical energy of food is the main source of energy required by all living organisms. This energy is transmitted to different trophic levels along the food chain. This energy flow is based on two different laws of thermodynamics:

  • First law of thermodynamics, that states that energy can neither be created nor destroyed, it can only change from one form to another.
  • Second law of thermodynamics, that states that as energy is transferred more and more of it is wasted.

Energy Flow in Ecosystem

Sun is the only source of energy for all ecosystems on Earth and except for the deep sea hydro-thermal ecosystem.

Less than 50 per of incident solar radiation is photosynthetically active radiation (PAR).

Plants capture only 2-10 per cent of the PAR.

All organisms are dependent for their food on producers.

Flow of energy in the ecosystem is unidirectional.

The green plant in the ecosystem are called producers. Example-In a terrestrial ecosystem, major producers are herbaceous and woody plants and in aquatic ecosystem phytoplankton, algae and higher plants are producers.

All animals depend on plants for their food needs are called consumers.

Consumers which feed on the producers, they are called primary consumers or herbivores. Example- grass

The animals eat herbivores, they are called secondary consumers or primary carnivores. Example- goat.

The animals eat herbivores, they are called secondary consumers or primary carnivores. Example- goat.

The animals which eat the primary carnivores are called tertiary consumers or secondary carnivores. Example- man.

Based on the source of their nutrition or food, organisms occupy a specific place in the food chain that is known as their trophic level.

Producers belong to the first trophic level, herbivores (primary consumer) to the second and carnivores (secondary consumer) to the third trophic level.

Food chain is the flow of energy from one trophic level to another trophic level by eating and being eaten.

Food chain if is two types-

  • Grazing food chain
  • Detritus food chain

Food chain which starts from producers and ends on carnivores through herbivores is called grazing food chain.

Grass –> Goat –> Man

Food chain which starts from dead organic matter and passes through detrivores to organisms feeding on detrivores is called detritus food chain.

Detrivores are decomposers which meet their energy and nutrient requirements by degrading dead organic matter or detritus, these are also known as saprotrophs.

Decomposers secrete digestive enzymes that breakdown dead and waste materials into simple, inorganic materials.

The interconnected matrix of food chain is called food web. For example- specific herbivore of one food chain may serve as food of carnivores in another food chain.

Each trophic level has a certain mass of living material at a particular time called as the standing crop.

The standing crop is measured as the mass of living organisms (biomass) or the number in a unit area.

A food chain, where grass is eaten by insects and frogs eat the insects which in turn is eaten by the snakes and eagles eat the snakes

 Ecological Pyramid

An ecological pyramid is a graphical representation of various ecological parameters such as the number of individuals present at each trophic level, the amount of energy, or the biomass present at each trophic level. Ecological pyramids represent producers at the base, while the apex represents the top level consumers present in the ecosystem. There are three types of pyramids:

(a) Pyramid of numbers

(b) Pyramid of energy

(c) Pyramid of biomass

Pyramid of Numbers

It is a graphical representation of the number of individuals present at each trophic level in a food chain of an ecosystem. The pyramid of numbers can be upright or inverted depending on the number of producers. For example, in a grassland ecosystem, the pyramid of numbers is upright. In this type of a food chain, the number of producers (plants) is followed by the number of herbivores (mice), which in turn is followed by the number of secondary consumers (snakes) and tertiary carnivores (eagles). Hence, the number of individuals at the producer level will be the maximum, while the number of individuals present at top carnivores will be least.

Grassland Ecosystem (Upright)

Pyramid of Biomass

A pyramid of biomass is a graphical representation of the total amount of living matter present at each trophic level of an ecosystem. It can be upright or inverted. It is upright in grasslands and forest ecosystems as the amount of biomass present at the producer level is higher than at the top carnivore level. The pyramid of biomass is inverted in a pond ecosystem as the biomass of fishes far exceeds the biomass of zooplankton (upon which they feed).

Grassland Ecosystem

Pond Ecosystem

Ecological Succession

The gradual and fairly predictable change in the species composition of a given area is called ecological succession.

The changes lead finally to a community that is in near equilibrium with the environment and that is called a climax community.

During succession some species colonise an area and their populations become more numerous, whereas populations of other species decline and even disappear.

The entire sequence of communities that successively change in a given area are called sere(s).

The individual transitional communities are called seral stages.

Ecological succession is of two types-

  • Primary succession
  • Secondary succession.

Succession that starts where no living organisms are there or these could be areas where no living organisms ever existed.

Primary succession can occur in newly cooled lava, bare rock, newly created pond or reservoir.

Succession that starts in areas where all the living organisms somehow lost that existed there is called secondary succession.

Secondary succession can occur in abandoned farm lands, burned or cut forests, lands that have been flooded.

Secondary succession is faster than primary succession because some soil or sediment is present.

Fig. bare rock, cooled lava, cut forest

Nutrient Cycling

The cyclic movement of the nutrients between biotic and abiotic components of the ecosystem is called nutrient cycle.

Organisms need a constant supply of nutrients to grow, reproduce and regulate various body functions.

The amount of nutrients present in the soil at any given time, is called as the standing state.

Nutrient cycling is also referred as biogeochemical cycle.

Nutrient cycles are of two types-

  • Gaseous
  • Sedimentary

Gaseous cycle is the cycling of carbon, nitrogen etc.

Sedimentary cycle is the cycling of sulphur, phosphorous etc.

The reservoir for gaseous type of nutrient exists in the atmosphere and for the sedimentary cycle the reservoir is located in Earth’s crust.

Carbon Cycle

Carbon constitutes of 49% of dry weight of organisms.

4×1013 kg of carbon is fixed in the biosphere by photosynthesis.

Large amount of carbon returned to the atmosphere as carbon di oxide through respiration of producers and consumers.

Decomposers also return carbon dioxide to reservoir during decomposition process.

Some amount of carbon is lost to sediments and removed from circulation.

Burning wood, forest fire, combustion of organic matter, fossil fuel also release carbon dioxide to atmosphere.

Rapid deforestation and massive burning of fossil fuel for energy and transport have significantly increased the rate of release of carbon dioxide into the atmosphere.

Phosphorus Cycle

Phosphorus is a major constituent of biological membranes, nucleic acids and cellular energy transfer systems.

Many animals also need large quantities of this element to make shells, bones and teeth.

The natural reservoir of phosphorus is rock, which contains phosphorus in the form of phosphates.

When rocks are weathered, minute amounts of these phosphates dissolve in soil solution and are absorbed by the roots of the plants.

Herbivores and other animals obtain phosphorous from plants.

The waste products and the dead organisms are decomposed by phosphate-solubilising bacteria releasing phosphorus.

Difference between carbon and phosphorous cycle

  • There is no respiratory release of phosphorus into atmosphere like carbon.
  • Atmospheric inputs of phosphorus through rainfall are much smaller than carbon inputs.
  • Gaseous exchanges of phosphorus between organism and environment are negligible.

The products of ecosystem processes are named as ecosystem services.

For example, healthy forest ecosystems

  • Purify air and water
  • Mitigate droughts and floods
  • Cycle nutrients
  • Generate fertile soils
  • Provide wildlife habitat
  • Maintain biodiversity
  • Pollinate crops
  • Provide storage site for carbon and also provide aesthetic
  • Cultural and spiritual values

Ecosystem Class 12Pollination

# Ecosystem Class 12

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# Ecosystem Class 12 Biology

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# Ecosystem Class 12 NCERT Solutions

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