CBSE Class 12 Biology Ecosystem Assignment Set B

Read and download free pdf of CBSE Class 12 Biology Ecosystem Assignment Set B. Get printable school Assignments for Class 12 Biology. Standard 12 students should practise questions and answers given here for Biology in Grade 12 which will help them to strengthen their understanding of all important topics. Students should also download free pdf of Printable Worksheets for Class 12 Biology prepared as per the latest books and syllabus issued by NCERT, CBSE, KVS and do problems daily to score better marks in tests and examinations

Ecosystem Class 12 Biology Assignment Pdf

Class 12 Biology students should refer to the following printable assignment in Pdf for Ecosystem in standard 12. This test paper with questions and answers for Grade 12 Biology will be very useful for exams and help you to score good marks

Class 12 Biology Assignment for Ecosystem


Startification : Vertical distribution of different species occupying different levels in an ecosystem.
Primary Production : Amount of biomas or organic matter produced per unit area over a time period by plants during photosynthesis.
Productivity : Rate of biomass production. Its unit is g/m2/year.
Gross Primary Productivity : Rate of production of organic matter during photosynthesis.
Net Primary Productivity : Gross primary productivity minus the respiration losses.
Ecosystem : Relationship between living organisms and their abiotic surroundings.
Secondary Productivity : Rate of formation of new organic matter by consumers.
Detritus : Dead leaves, twigs, animal remains etc. constitute detritus.
Detrivore : Organisms who break down detritus into smaller particles. e.g., earthworm.
Ecological succession : The successive and orderly replacement of one community by the other community in an area, over a period of time.
Ecological Pyramids : The sequential graphic representation of an ecological parameter (number/ biomass/energy) depicting different trophic levels in a food chain.
Climax community : The stable and final biotic community that develops at the end of ecological succession and is in perfect harmony with its physical environment.
Pioneer species : The species that invade a bare area at the onset of ecological succession.


Short Answer Type Questions

Question. How do you distinguish between humification and mineralisation?
Answer : Humification is the process of decomposition of soil that leads to accumulation of a dark-coloured amorphous substance called humus. Humus are highly resistant to microbial action and under goes dicomposition at a very slow rate. Mineralisation is the process by which the humus is further degraded by microbes and inorganic nutrients or minerals are released back into the substratum.

Question. Poaching of tiger is a burning issue in today’s world. What implication would this activity have on the functioning of the ecosystem of which the tigers are an integral part?
Answer : Tiger represents an important part of the food web and helps in maintaining the ecological stability. As a carnivore, it keeps a check on the unlimited growth of herbivoresing the and also removes sick or old animlas from the population. It also acts as an indicator of the forest’s health. Saving the tiger means we save the forest. Since, tiger (top carnivore of the food chain) cannot live in places where trees or herbivores, that it hunts, have vanished and in turn secure food and water for all.

Question. In relation to energy transfer in ecosystem, explain the statement ‘‘10kg of deer’s meat is equivalent to 1 kg of lion’s flesh’’.
Answer : In an ecosystem, flow (transfer) of energy is unidirectional. As energy trapped in Ist tropic level, only 10% of energy is transferred to next trophic level.

Question. Primary productivity varies from ecosystem to ecosystem. Explain?
Answer : Primary productivity is the rate at which primary producers (plants) capture and store solar radiation to form chemical energy. Primary production depends upon producer (green plant) which are variable in different ecosystem. So, primary productivity varies from ecosystem to ecosystem.

Question. Organisms at a higher trophic level have less energy available. Comment.
Answer : Energy flow in the ecosystem follows the 10% energy flow law, proposed by Lindman. According to this law only 10% of the energy available at each trophic level, gets transferred to the next trophic level, the rest is lost in the environment as heat. As we move to higher trophic levels, the energy available to organisms keeps on decreasing.Thus, the top carnivore gains the lesst energy in a food chain. Heat energy last during Respiration.

Question. The number of trophic levels in an ecosystem are limited. Comment.
Answer : The number of trophic level in an ecosystem are each limited and are not more than 4-5. because the amount of energy flow decreases with successive trophic level as only 10% of energy is transferred from one trophic level to the next successive level. So rest of the energy is lost in the form of respiration and other vital activities to maintain life. If more trophic levels are present, the residual energy will be limited and decreases to such an extent that it cannot further support any trophic level by the flow of energy. So, the food chain is generally limited to 3-4 trophic levels only.

Question. Sometimes due to biotic/abiotic factor the climax remain in a particular seral stage (pre climax) without reaching climax. Do you agree with this statement. If yes give a suitable example.
Answer : Sometimes climax remains in a particular seral stage without reaching to the climax because during ecological succession any change in abiotic and biotic component may affect the particular seral stage, leading to preclimax stage before the climax is achieved. This type of condition occurs presence of seeds and other propagules. This secondarily based area may be invaded by moss or exotic weeds thus exhibiting succession seriously and the climax community is never regenerated. in the case of natural calamities like-fire landslide, floods, change in soil texture.

Question. What is an incomplete ecosystem? Explain with the help of suitable example.
Answer : An ecosystem comprises with biotic and a biotic component. A biotic component include light, air, water, temperature, humidity etc, while biotic factor comprises all living organism. Absence or limited availability of any component (either abiotic or biotic) makes an ecosystem incomplete like the profundal and benthic zone in an aquatic ecosystem.


Long Answer Type Questions

Question. “The energy flow in the ecosystem follows the second law of thermodynamics.” Explain.
Answer : According to second law of thermodynamics every activity involving energy transformation (According to first law - energy can be transferred and transformed) is accompanied by dissipation of energy as heat and only a part of it is used in building up tissues in an organisms. This trapped energy as biomass is transferred to next trophic level. According to Lindman law only 10% of the stored energy is passed from one trophic level to successive trophic level.

Question. What will happen to an ecosystem if
(a) All producers are removed
(b) All organisms of herbivore level are eliminated and
(c) All top carnivore population is removed
Answer : (a) Removal of all producer reduce primary production in the ecosystem. Hence, no biomass will be available to the successive/higher trophic level or heterotrophic organisms. (b) Elimination of all organisms of herbivore level results into an increase in primary productivity and biomass of producer and carnivorous animal will not survive due to inavailability of food herbivores. (c) Removal of top carnivores also disturb the ecosystem as it will result in huge increase in number of herbivores which will finish plants (producers) creating desertification.

Question. A farmer harvests his crop and expresses his harvest in three different ways.
(a) I have harvested 10 quintals of wheat.
(b) I have harvested 10 quintals of wheat today in one acre of land.
(c) I have harvested 10 quintals of wheat in one acre of land, 6 months after sowing. Do the above statements mean one and the same thing. If your answer is ‘yes’, give reasons. And if your answer is ‘no’ explain the meaning of each expression.
Answer : (a) Farmer's expression for his crop harvestation
(a) he has harvested 10 quintal of wheat,
(b); He has harvested 10 quintals of wheat in one acre of land
(c), six, months after sowing mean one and the same thing. Because crop an artificial ecosystem can be prepared with inclusion of bitoic and abiotic component in a given area. Here abiotic components like water, is given by the farmer, while climatic factor like light, humidity, air is supplied naturally. The living component is wheat plant, which are obtained on harvestation by farmer.

Question. Justify the following statement in terms of ecosystem dynamics. “Nature tends to increase the gross primary productivity, while man tends to increase the net primary productivity”.
Answer : In term of ecsosystem dynamics, flow of energy takes place from one trophic level to the next trophic level and occured in unidirectional way. About 50% of solar energy incident over earth is present in Photosynthetic Active Radiation (PAR) and only 2-10% of this PAR is utilised by green plants to form chemical energy (Gross Primary Productivity) (GPP). Out of 90% of gross primary productivity is lost maximum during respiration and other vital activities. GPP utilised by plants in respiration minus respiration losses is the net primary productivity and is available to the organism of next trophic level (herbivore and decomposers) for consumption. Gross primary productivity of an ecosystem is the rate of production of organic matter during photosynthesis. So nature tend to increase gross primary productivity through supporting the large number of plant (producers) in an ecosystem. Net primary productivity is the available biomass for the consumption to heterotrophs (human and animals) man tries to increase net primary productivity by cultivating food and other crops which the depend on to fullfil their needs. Formula to genrate NPP is NPP1 GPP – R

Question. Give two examples of artificial or man made ecosystems. List the salient features by which they differ from natural ecosystems.
Answer : Aquarium and farm house, are artificial or man made ecosystem. In artificial ecosystem biotic and abiotic component are maintained artificially like cleaning, feeding and supply of oxygen to fishes in aquarium and irrigation in crop or farm house. While abiotic and biotic component of natural ecosystem are maintained naturally like nutrient cycle, self sutainability, prevention of soil erosion, pollutant absorption and reduction of threat to global warming (ecological servicer), etc.

Question. The biodiversity increases when one moves from the pioneer to the climax stage. What could be the explanation?
Answer : During ecological succession biodiversity increase or changes from pioneer to climax stages. Following are the effects of ecological succession (a) It leads to changes in vegetation that affects food and shelter for various types of animals. (b) As succession proceeds, the numbers and types of plants, animals and decomposers also change. (c) At any time during primary or secondary succession, natural or human induced disturbances (fire, deforestation, etc.) can convert a particular seral stage of succession to an earlier stage. Also such disturbances can create new conditions that encourage some species and discourage or eliminate other species of producer, consumers and decomposers. (d) Over the time, they are succeeded by bigger plants and, ultimately a stable climax forest community is attained. (e) The climax community remains stable if the environment remains unchanged. (f) With time, the xerophytic habitat may gets converted into a mesophytic one.



1. Decomposition is faster if deteritus is rich in nitrogen and water soluble substance like sugars. When is the decomposition process slower?

2. If we count the number of insects on a tree and number of small birds depending on those insects as also the number of larger birds eating the smaller, what kind of pyramid of number would we get?

3. Differentiate between Sere and Seral communities.

4. Who are generally the pioneer species in a Xerarch succession and in a Hyararch succession?

5. Which metabolic process causes a reduction in the Gross Primary Productivity?

6. What percentage of photosynthetically active radiation is captured by plants?

7. Name the pioners of primary succession in water.

8. What is the shape of pyramid of biomass in sea? Why?

9. Give an example of an ecological pyramid which is always upright. Justify your answer.

10. Differentiate between primary succession and secondary succession. Which one occurs faster?

11. Gaseous nutrient cycle and sedimentary nutrient cycles have their reservoir. Name them. Why is a reservoir necessary?

12. Fill up the missing links depicted as A, B, C and D in the given model of primary succession.

Important Questions for NCERT Class 12 Biology Ecosystem


Question. In an ecosystem based on production of nearly 6 million plants, how many top consumers can be supported ?
(a) 708000
(b) 354000
(c) 3
(d) 30000

Answer : C

Question. In an ecosystem if dry weight of producers is 809 kgm-2. Then what will be the biomass of tertiary consumers
(a) 37 dry weight (Kg m-2)
(b) 11 (Kg m-2)
(c) 15 kg m-2
(d) 1.5 kgm-2

Answer : D

Question. Ecological pyramids show diagramatic representation of ecological parameters like number, biomass and energy. Which is / are limitation of ecological pyramids ?
(a) It does not take into account the same species belonging to two or more trophic levels
(b) It does not accomodate a food web
(c) Saprophytes are not given any place in pyramids
(d) All the above

Answer : D

Question. The gradual and fairly predictable changes in the species composition of a given area is called :-
(a) Bioprospecting
(b) Biofortification
(c) Ecological succession
(d) Ecological assessment

Answer : C

Question. All successions whether taking place in water or on land proceeds to which climax community :-
(a) Hydric
(b) Xeric
(c) Mesic
(d) Halophytic

Answer : C

Question. How much amount of carbon is fixed in biosphere through photosynthesis annually
(a) 2 x 1013 kg
(b) 4 x 1013 kg
(c) 5 x 1013 kg
(d) 6 x 1013 kg

Answer : B

Ques. The term ecosystem was coined by
(a) E. Haeckel
(b) E.Warming
(c) E.P. Odum
(d) A. G. Tansley.

Answer: D

Ques. Which one of the following is a characteristic feature of cropland ecosystem?
(a) Absence of weeds
(b) Ecological succession
(c) Absence of soil organisms
(d) Least genetic diversity 

Answer: D

Ques. Vertical distribution of different species occupyin different levels in a biotic community is known as
(a) zonation
(b) pyramid
(c) divergence
(d) stratification.

Answer: D

Ques. Which one of the following is not a functional uni of an ecosystem?
(a) Energy flow
(b) Decomposition
(c) Productivity
(d) Stratification 

Answer: D

Ques. Which one of the following is one of the characteristics of a biological community?
(a) Stratification
(b) Natality
(c) Mortality
(d) Sex-ratio 

Answer: A

Ques. Which of the following is the most stable ecosystem?
(a) Mountain
(b) Ocean
(c) Forest
(d) Desert 

Answer: B

Ques. In relation to Gross primary productivity and Net primary productivity of an ecosystem, which one of the following statements is correct?
(a) Gross primary productivity is always less than Net primary productivity.
(b) Gross primary productivity is always more than Net primary productivity.
(c) Gross primary productivity and Net primary productivity are one and same.
(d) There is no relationship between Gross primary productivity and Net primary productivity.

Answer: B

Ques. The mass of living material at a trophic level at a particular time is called
(a) net primary productivity
(b) standing crop
(c) gross primary productivity
(d) standing state. 

Answer: B

Ques. In an ecosystem the rate of production of organic matter during photosynthesis is termed as
(a) secondary productivity
(b) net productivity
(c) net primary productivity
(d) gross primary productivity. 

Answer: D

Ques. Secondary productivity is rate of formation of new organic matter by
(a) consumers
(b) decomposers
(c) producers
(d) parasites.

Answer: A

Ques. The rate of formation of new organic matter by rabbit in a grassland, is called
(a) net productivity
(b) secondary productivity
(c) net primary productivity
(d) gross primary productivity. 

Answer: B

Ques. Mass of living matter at a trophic level in an area at any time is called
(a) standing crop
(b) detritus
(c) humus
(d) standing state. 

Answer: A

Ques. The biomass available for consumption by the herbivores and the decomposers is called
(a) net primary productivity
(b) secondary productivity
(c) standing crop
(d) gross primary productivity. 

Answer: A

Ques. Which one of the following ecosystem types has the highest annual net primary productivity?
(a) Tropical deciduous forest
(b) Temperate evergreen forest
(c) Temperate deciduous forest
(d) Tropical rainforest 

Answer: D

Ques. Which of the following is expected to have the highest value (gm/m2/yr) in a grassland ecosystem?
(a) Secondary production
(b) Tertiary production
(c) Gross production (GP)
(d) Net production (NP) 

Answer: C

Ques. The rate at which light energy is converted into chemical energy of organic molecules is the ecosystem’s
(a) net secondary productivity
(b) gross primary productivity
(c) net primary productivity
(d) gross secondary productivity. 

Answer: B

Ques. Which of the following ecosystem has the highest gross primary productivity?
(a) Mangroves
(b) Rainforest
(c) Grassland
(d) Coral reef 

Answer: B

Ques. Maximum solar energy is trapped by
(a) planting trees
(b) cultivating crops
(c) growing algae in tanks
(d) growing grasses. 

Answer: C

Ques. A very efficient converter of solar energy with net productivity of 204 kg/m2 or more is the crop
(a) wheat
(b) sugarcane
(c) rice
(d) bajra. 

Answer: B


Please refer to the link below for CBSE Class 12 Biology Ecosystem Assignment Set B

Chapter 2 Sexual Reproduction In Flowering Plants
CBSE Class 12 Biology Sexual Reproduction in Flowering Plants Set A

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