CBSE Class 12 Biology Ecosystem Notes

Download CBSE Class 12 Biology Ecosystem Notes Set B in PDF format. All Revision notes for Class 12 Biology have been designed as per the latest syllabus and updated chapters given in your textbook for Biology in Standard 12. Our teachers have designed these concept notes for the benefit of Grade 12 students. You should use these chapter wise notes for revision on daily basis. These study notes can also be used for learning each chapter and its important and difficult topics or revision just before your exams to help you get better scores in upcoming examinations, You can also use Printable notes for Class 12 Biology for faster revision of difficult topics and get higher rank. After reading these notes also refer to MCQ questions for Class 12 Biology given our website

Ecosystem Class 12 Biology Revision Notes

Class 12 Biology students should refer to the following concepts and notes for Ecosystem in standard 12. These exam notes for Grade 12 Biology will be very useful for upcoming class tests and examinations and help you to score good marks

Ecosystem Notes Class 12 Biology

Sir Tansley coined the term `ecosystem’


It is an assemblage of different communities with abiotic factors in an environment.


Terrestrial and aquatic

Terrestrial include Forests, Grasslands, Deserts

Aquatic- Fresh water (lotic & lentic)

- Marine water bodies





STRUCTURE FEATURES ARE            1. Species composition

                                                               2. Stratification

FUNCTIONAL FEATUERS ARE        1. Productivity

                                                             2. Decomposition

                                                            3. Energy Flow

                                                             4. Nutrient cycle


It includes -Primary Productivity (plants)-producers

Has two aspects 1. GPP             2. NPP=GPP-R

- Secondary Productivity involves assimilation and formation of new organic matter by consumers

Primary productivity is high in terrestrial systems rather than in aquatic systems because of BIOMASS.


Detritus is the starting matter.

The steps involved :\

1. Fragmentation- Breaking of detritus into smaller particles.

2. Leaching – Water soluble substances are seeped into the soil.

3. Catabolism- The role of enzymes in the conversion process.

4. Humification- Forming of dark coloured matter called HUMUS.

5. Mineralisation- Humus is degraded into simpler inorganic substances. 


The main source of energy is the light energy obtained from sunlight.

Plants capture 2-10% of the light energy and prepare the food and all other organisms depend on it.



Grazing Food chain (GFC)

Producers trap the sunlight and prepare the food.

Consumers- Herbivores depend on the producers

Carnivores depend on the herbivores

For eg.        Grass---------- Grasshopper--------- Frog------------- Snake--------Hawk

                  Producers        Primary                   Secondary         Tertiary         quaternary

                                        Consumer               .Consumer          consumer     consumer

                 1st Trophic       2nd Trophic              3rd Trophic        4th Trophic    5th Trophic

Detritus Food chain (DFC)

It begins with detritus

A much fraction of energy flows through this food chain as they are connected at any trophic level.

Detritus--------Earthworm-------- Bacteria/fungi-------plants---------animals
                           ↓                                                   ↓                   ↓

                              DECOMPOSITION OCCURS

Both GFC and DFC interconnect to form FOOD WEB.




It comprises the population density (N)

Grass land ecosystem- UPRIGHT 

CBSE Class 12 Biology - Ecosystem notes (1)

Eg., Pond ecosystem-UPRIGHT

A single Tree- INVERTED


It is composition of amount of matter present in the dry state or amount of C/g

Eg., A single tree

EG., Pond ecosystem

CBSE Class 12 Biology - Ecosystem notes (1)

It is always upright because there is transfer of energy from one trophic level to another.

You are familiar of 10% law ( RECALL CLASS X SCIENCE- Our Environment)

There is loss of energy in every state as we move upwards by 10 %.

There is gradual change in the state of species composition from one form to other with relation to time factor in a particular habitat.
The change of species is called seral communities.

A) Primary succession- It is a state of environment where no life exists. It may include dry, bare rock.
In such circumstances, the living state begins very slow because there is no suitable substrata-nutrients, soil. It consumes time duration and depend on factors. (REFER TO CLASS IX-SCIENCE- Environment)
B) Secondary succession-It exists in a place where life exists earlier.
Presence of soil or sediment


CBSE Class 12 Biology - Ecosystem notes (1)

It is found that both the succession lead to mesic community.







Habitat: Environment in which a species normally lives or the location of a living organism.

• Ecology: The Scientific Study of the Interactions between organisms and the environment

• Abiotic: Non-living (ex. Temp, light, water, nutrients)

Biotic: Living organisms (called biota)

Population: A group of individuals of the same species living in a particular geographic area.

Population ecology: Concentrates mainly on factors that affect how many individuals ofa particular species live in an area.

Community: Consists of all the organisms of all the species that inhabit a particular area; it is an assemblage of populations of many different species.

Community ecology: The whole array of interacting species in a community. This area of research focuses on how interactions such as predation, competition, anddisease, as well as abiotic factors such as disturbance, affect community structure andorganization.

Ecosystem: Consists of all the abiotic factors in addition to the entire community of species that exist in a certain area. An ecosystem—a lake, for example—may containmany different communities.

Ecosystem ecology: The emphasis is on energy flow and chemical cycling among thevarious biotic and abiotic components.

•Biome: Any of the world’s major ecosystems, classified according to the predominantvegetation and characterized by adaptations of organisms to that environment. Terrestrial regions inhabited by certain types of life, especially vegetation. eg. Deserts, grassland and forests. • Biological diversity - variety of different species (species diversity), genetic variability among individuals within each species (genetic diversity), and variety of ecosystems (ecological diversity).

Biotic potential - maximum rate at which the population of a given species can increase when there are no limits of any sort on its rate of growth.

Carrying capacity (K) - maximum population of a particular species that a given habitat can support over a given period of time.

Competition - two or more individual organisms of a single species (intraspecificcompetition) or two or more individuals of different species (interspecific competition) attempting to use the same scarce resources in the same ecosystem.

Consumer - organism that cannot synthesize the organic nutrients it needs and gets its organic nutrients by feeding on the tissues of producers or of other consumers; generally divided into primary consumers (herbivores), secondary consumers (carnivores), tertiary (higher-level) consumers, omnivores, and detritivores (decomposers and detritus feeders).
Deforestation - removal of trees from a forested area without adequate replanting.

Desert - biome where evaporation exceeds precipitation and the average amount of precipitation is less than 25 centimeters (10 inches) a year. Such areas have little vegetation or have widely spaced, mostly low vegetation.

Desertification - conversion of rangeland, rain-fed cropland, or irrigated cropland to desert like land, with a drop in agricultural productivity of 10% or more. It is usually caused by a combination of overgrazing, soil erosion, prolonged drought, and climate change.

Ecological diversity - the variety of forests, deserts, grasslands, oceans, streams, lakes, and other biological communities interacting with one another and with their nonliving environment.

Ecological niche - total way of life or role of a species in an ecosystem. It includes all physical, chemical, and biological conditions a species needs to live and reproduce in an ecosystem.
Ecology - study of the interactions of living organisms with one another and with their nonliving environment of matter and energy.Study of the structure and function of nature.

Environmental degradation - depletion or destruction of a potentially renewable resource such as soil, grassland, forest, or wildlife by using it at a faster rate than it is naturally replenished. If such use continues, the resource can become nonrenewable on a human time scale or nonexistent (extinct).

Extinction - complete disappearance of a species from the earth. This happens when a species cannot adapt and successfully reproduce under new environmental conditions or when it evolves into one or more new species (speciation).

Food web - complex network of many interconnected food chains and feeding relationships.

Forest - biome with enough average annual precipitation (at least 76 centimeters,or 30 inches) to support growth of various species of trees and smaller forms of vegetation.

Grassland - biome found in regions where moderate annual average precipitation (25 to 76 centimeters, or 10 to 30 inches) is enough to support the growth of grass and small plants, but not enough to support large stands of trees.

Habitat - place or type of place where an organism or a population of organisms lives.

Immature community - community at an early stage of ecological succession. It usually has a low number of species and ecological niches and cannot capture and use energy and cycle critical nutrients as efficiently as more complex, mature ecosystems.

Keystone species - species that play roles affecting many other organisms in an ecosystem.

Limiting factor - single factor that limits the growth, abundance, or distribution of the population of a species in an ecosystem.

Mature community - fairly stable, self-sustaining community in an advanced stage of ecological succession; usually has a diverse array of species and ecological niches; captures and uses energy and cycles critical chemicals more efficiently than simpler, immature communities.

Native species - species that normally live and thrive in a particular ecosystem.

Pioneer community - first integrated set of plants, animals, and decomposers found in an area undergoing primary ecological succession.

Pioneer species - first hardy species, often microbes, mosses, and lichens, that begin colonizing a site as the first stage of ecological succession.

Primary succession - sequential development of communities in a bare area that has never been occupied by a community of organisms.

Secondary succession - sequential development of communities in an area in which natural vegetation has been removed or destroyed but the soil is not destroyed.

Question. Which element is formed by the weathering of rocks and absorbed by plant from the soil?
(a) Phosphorus
(b) Carbon
(c) Nitrogen
(d) Oxygen
Answer : A

Question. Phosphorus is needed for the production of
(a) DNA and RNA
(b) cellular membranes
(c) bones and teeth
(d) All of these
Answer : D

Question. In the phosphorus cycle, weathering makes phosphate available first to
(a) producers
(b) decomposers
(c) consumers
(d) None of these
Answer : A

Question. The reservoir for the gaseous type of biogeochemical cycle exists in
(a) stratosphere
(b) atmosphere
(c) ionosphere
(d) lithosphere
Answer : B

Question. Which of the following pair is a gaseous type of biogeochemical cycle?
(a) Nitrogen and carbon cycle
(b) Phosphorus and carbon cycle
(c) Nitrogen and sulphur cycle
(d) Sulphur and carbon cycle
Answer : A

Question. In a ……… cycle, the elements returns and is withdrawn from the atmosphere. Most appropriate word to fill the blank is
(a) gaseous
(b) sedimentary
(c) Both (a) and (b)
(d) None of these
Answer : A

Question. The reservoir for the sedimentary cycle exists in
(a) earth’s crust
(b) organic sediments
(c) calcareous sediments
(d) limestone
Answer : A

Question. The total amount of nutrients like carbon, phosphorus, calcium, etc., present in soil at any time is called
(a) standing crop
(b) standing state
(c) nutrient crops
(d) sediment
Answer : B

Question. In an ecosystem, the cycling of nutrient is known as
(a) geological cycle
(b) chemical cycle
(c) geochemical cycle
(d) biogeochemical cycle
Answer : D

Question. In sedimentary nutrient cycling,
(a) the reservoir pool is lithosphere
(b) the sedimentary cycles are less perfect
(c) the withdrawl from reservoir pool is large
(d) All of the above
Answer : D

Question. Which one of the following is not a gaseous biogeochemical cycle in ecosystem?
(a) Oxygen cycle
(b) Phosphorus cycle
(c) Nitrogen cycle
(d) Carbon cycle
Answer : B

Question. What is the reason behind deficit rising in nutrient reservoir?
(a) Due to imbalance in the rate of influx
(b) Due to imbalance in the rate of efflux
(c) Due to imbalance in the rate of influx and efflux
(d) None of the above
Answer : C

Question. Carbon constitutes …… of dry weight of anorgan ism. Most appropriate word to fill the blank is
(a) 49%
(b) 59%
(c) 69%
(d) 39%
Answer : C

Question. …… of the carbon is found dissolved in oceans, which is responsible for its regulation in atmosphere. 
Most appropriate word to fill the blank is
(a) 51%
(b) 81%
(c) 61%
(d) 71%
Answer : A

Question. Which of the following regulates the amount of carbon dioxide in the atmosphere
(a) respiration in animals
(b) respiration in plants
(c) photosynthesis activity of plants
(d) oceanic resesvoir of carbon
Answer : D

Question. What is themediumbywhich carbon cycle takes place?
(a) Through atmosphere
(b) Through ocean
(c) Through living and dead organisms
(d) All of the above
Answer : D

Question. What human activities are responsible to increase the amount of CO 2 in the atmosphere?
(a) Deforestation
(b) Massive burning of fossil fuels
(c) Vehicle used for transport
(d) All of the above
Answer : D

Question. Which of the following factor is contributing to an overload of the carbon cycle?
(a) Photosynthesis
(b) Cellular respiration
(c) Deforestation
(d) Afforestation
Answer : C

Question. The exchange pool in the carbon cycle is
(a) fossil fuels
(b) sedimentary rock
(c) water
(d) atmosphere
Answer : D

Question. Select the incorrect match. AIIMS 2018
I. Sedimentary nutrient cycle–Nitrogen cycle
II. Pioneer species–Lichens
III. Secondary succession–Burned forests
IV. Pyramid of biomass of aquatic ecosystem–Upright
(a) I and IV
(b) I, II and III
(c) I and III
(d) III and IV
Answer : A

Question. Fill up the blank.
I. Theproductsof ecosystemprocesses are called…A….
II. …B…are the major source of ecosystem services.
III. …C…and his colleagues tried to put price tags on nature’s life support services, which came up toUS …D…a year.
Choose the correct option for A, B, C and D.
(a) A–Ecosystem services, B–Plants, C–Robert Brown,D–31 trillion
(b) A–Ecology services, B–Plants, C–Robert Constanza,D–32 trillion
(c) A–Ecosystem services, B–Forests, C–Robert Constanza, D–33 trillion
(d) A–Ecology services, B–Ponds, C–Robert Brown, D–34 trillion
Answer : C

I. Assertion and Reason
n Direction (Q. Nos. 160-168) In each of the following questions, a statement of Assertion (A) is given by corresponding statement of Reason (R). Of the statements, mark the correct answer as
(a) If both A and R are true and R is the correct explanation of A
(b) If both A and R are true, but R is not the correct explanation of A
(c) If A is true, but R is false
(d) If A is false, but R is true

Question. Assertion (A) An ecosystem is an interaction between biotic and abiotic components.
Reason (R) AG Tansley coined the term ecosystem.
Answer : B

Question. Assertion (A) Bacteria and fungi are microconsumers.
Reason (R) Bacteria and fungi use a very little part of living plant and animals.
Answer : C

Question. Assertion (A) Herbivores are also called as first order consumers.
Reason (R) These obtain their food directly from plants.
Answer : A

Question. Assertion (A) A network of food chains existing together in an ecosystem is known as a food web.
Reason (R) An animal like kite cannot be a part of a food web.
Answer : C

Question. Assertion (A) The pyramid of energy is always upright.
Reason (R) The flow of energy is unidirectional.
Answer : A

Question. Assertion (A) The pyramid of number of pond ecosystem is upright.
Reason (R) Phytoplanktons are maximum and secondary consumers are lesser in number.
Answer : A

Question. Assertion (A) Ecological succession can turn a lake into a dryland forest with time.
Reason (R) A bare rock can become a forest through ecological succession.
Answer : B

Question. Assertion (A) Nutrient cycle means the cycling of glucose or reserved food material within the plant body.
Reason (R) Transfer of biogenetic nutrients between living and non-living components is called biogeochemical cycle.
Answer : D

Question. Assertion (A) Oceans act as the global sink for CO 2.
Reason (R) Human activities are increasing CO2 concentration in the air.
Answer : B

II. Statement Based Questions

Question. Consider the following statements.
I. Forest, glassland and desert are examples of terrestrial ecosystem.
II. Pond, lake, wetland, river and estuary are examples of aquatic ecosystem.
(a) Statement I is true, but II is false
(b) Statement I is false, but II is true
(c) Both statements I and II are true
(d) Both statements I and II are false
Answer : C

Question. Consider the following statements.
I. The components of ecosystem do not include decomposition.
II. Decomposers are saprophytic organisms like fungi,bacteria and flagellates especially abundant in the bottom of the pond.
Choose the correct option.
(a) I is true, but II is false
(b) I is false, but II is true
(c) Both I and II are true
(d) Both I and II are false
Answer : B

Question. Consider the following statements.
I. Producers are also called as transducers because they are able to change radiant energy into chemical form.
II. Consumers are animals, which feed on other organisms or their parts.
III. Decomposers are saprotrophs, which feed on dead bodies of organisms.
Which of the statements given above are correct?
(a) I, II and III
(b) I and II
(c) I and III
(d) II and III
Answer : A

Question. Select the true statements.
I. Productivity can be divided into gross primary productivity and net primary productivity.
II. Net primary productivity is the available biomass for the consumption to heterotrophs.
III. Net primary productivity is equal to gross primary productivity minus respiration.
IV. There is unidirectional movement of energy towards higher trophic levels and its dissipation and loss as heat to the environment.
Choose the correct option.
(a) I, II and III
(b) I and IV
(c) II and III
(d) I, II, III and IV
Answer : D




Question. What are decomposers? Write their function.
Answer : a)Saprotrophs feed on dead bodies of organisms, b) Decomposition and mineralization.

Question. What is the difference between gaseous and sedimentary cycle?
Answer : a) Gaseous-Reservoir in atmosphere ,Nitrogen cycle b) Sedimentary-Soil,eg-phosphorus.

Question. Why is the length of a food chain in an ecosystem generally limited to 3-4 trophic levels?
Answer : As 90% energy is lost in the form of heat from one trophic level to another, residual energy decreases drastically within 2-3 trophic levels.

Question. What are the differences between detritus and grazing food chains?
Answer : a) Begins with Detritus-dead and decaying organic matter. b) Grazing-Begins with living green plants.

Question. What are the two basic catagories of ecosystem? Give example.
Answer : a) Terrestrial-Forest, grassland, desert. b) Aquatic-Pond, lake, sea, ocean

Question. Mention two factors by which productivity is limited in an aquatic ecosystem.
Answer : a) Light-decreases with increasing water depth. b) Nutrient –Limiting factor in Deep Ocean

Question. What is food chain? Give an example.
Answer : a) Food and feeding relation among organisms makes a chain like structure b) Grass—Deer—Lion

Question. Expand PAR, How much PAR is used in gross primary productivity?
Answer :.Photosynthetically Active Radiation. 2-10%.



Question. Briefly describe the process and products of decomposition.
Answer : Breakdown of complex organic matter by decomposers.a)Process-i)fragmentation ii)leaching iii)catabolism. Humification and mineralization –humification leads to accumulation of dark colour substance called humus. Mineralisation result in release of inorgranic substances.

Question. Give account of factors affecting the rate of decomposition.
Answer : a) climatic factor – i)temp ii) soil b) chemical quality of detritus . Higher temp and moist 
condition – high rate of decomposition .Dry soil , High temp – Low rate

Question. What are ecological pyramids ? Mention its limitations .
Answer : a) Arrangement of trophic levels from producers to top carnivores forms pyramid like  structure 3 types – i) Pyramid of number ii) Biomass iii) Energy
Limitations – i) Assumes simple food chain ii) Single species may operate at two or more trophic levels.

Question. Explain carbon cycle with ray diagram .
Answer : Given in text.

Question. Describe pond as an ecosystem .
Answer : Pond has biotic and abiotic components
a) Biotic – Phytoplankton ,Zooplankton , small fishes , large fishes , frogs , snake ,etc.
b) Abiotic - water , dissolved organic and inorganic substances ,sunlight , temp .
Phytoplankton (microscopic plants) – producers . Zooplankton (microscopic animals) –
primary consumers .Small fishes - secondary consumers .Large fishes , frog, snails – tertiary consumers.


[5 Marks Questions]

Question. Describe the major components of ecosystems.
Answer : a) Biotic-i) Producer-green plants. ii) Consumers-primary, secondary,tertiary and decomposers. b) Abiotic-i) Physical and climatic factors-soil, temperature,light, humidity. ii) Chemical factors-inorganic chemical substances (sodium, potassium, nitrogen etc.) organic substances-(humus, protein, fat etc.)

Question. Give an account of energy flow in an ecosystem.
Answer : Rate of energy transfer between the organisms of different trophic levels is called energy flow. Energy flow is unidirectional, 10% loss of energy in each trophic levels. 2-10% PAR captured by green plants.Energy flow diagram from the text.

Question. What is xerosere?
Answer : Describe the process of succession on a bare rock. Answer : a) Succession on bare rock. b) Steps in Xerosere i) Lichens-Pioneer Community. ii) Mosses iii) Herbs iv) Shrubs v) Trees-Climax community.



BIOGEOCHEMICAL CYCLE :- The movement of nutrient elements through various components of an ecosystem( between living organisms, soil, air and water).

BIOMASS :- The amount of organic matter present in an organism/ a trophic level / ane cosystem.

DETRITIVORES :- Organisms which feed on the detritus and break it down into smaller particles.

DETRITUS :- Dead remains of plants and animals or their wastes.

ECOLOGICAL PYRAMIDS :- Representation of trophic structure ( Number, Biomass . or Energy at various trophic levels) of a food chain.

ECOLOGICAL SUCCESSION :- Phenomenon in which structure and composition of a community changes in an orderly and sequential manner leading to the climax community.

ECOSYSTEM: - Functional unit of nature where living organism interact among themselves and also with physical environment.

ECOSYSTEM SERVICES: - The products of ecosystems processes are termed as ecosystem services.

FOOD WEB: - A network formed by interconnected the food chain. 

HUMIFICATION: - Process of changing Detritus into a dark coloured amorphous matter called humus.

HYDRARCH: - Ecological succession in water bodies like lakes resulting in climax community.

MINERALISATION :- Process of Degradation of Humus by some microbes into Inorganic nutrients.

PIONEER SPECIES:-The species which invade a bare area and initiate the ecological succession.

PRIMARY PRODUCTIVITY: - Amount of biomass or organic matter produced per unit area over a time period by the plants.

PRIMARY SUCCESSSION: - Ecological succession on previously sterile area such as bare rocks or lake forming climax community.

PRODUCTIVITY:-Rate of production of biomass.

SECONDARY PRODUCTIVITY:- The rate of assimilation and formation of new organic matter by consumer.

SECONDARY SUCCESSION: - Ecological succession in an area where previously established community is destroyed due to fire or floods etc.

SERE :- The sequence in which one community is replaced in an area by another resulting in a climax community.

STANDING CROP :- The amount of living matter(biomass) present at a trophic level.

STANDING STATE :- The amount of nutrients present in the soil at any given time.

STRATIFICATION :-Vertical distribution of different species occupying different levels in an ecosystems.

TROPHIC LEVEL :- Every step or link of a food chain.

XERARCH :- Ecological succession on bare rocks or sand resulting in climax community. 


CBSE Class 12 Biology Ecosystem Mind Map




Chapter 10 Microbes In Human Welfare
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Chapter 11 Biotechnology Principles and Processes
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Chapter 12 Biotechnology and Its Application
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Chapter 13 Organisms and Populations
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Chapter 15 Biodiversity and Conservation
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Chapter 16 Environmental Issues
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