Important Questions for CBSE Class 12 Biology Chapter 12 - Ecosystem 2024-25

Very Short Answer Questions.  (1 Mark)

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

Ans. The process of decomposition is slower when detritus is rich in lignin and chitin. 

2. If we count the number of insects on a tree and the number of small birds depending on those insects as also the number of larger birds eating the smaller, what kind of pyramid of20 number would we get? 

Ans. Inverted Pyramid of Number.

3. Differentiate between Sere and Seral communities. 

Ans. The difference between sere and seral communities are:

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

Ans. Lichens are the pioneer species in Xerarch succession, while small phytoplanktons are the pioneer species in Hydrarch succession.

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

Ans. Respiration is a metabolic process that causes a reduction in Gross Primary Productivity.

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

Ans. Plants capture $\text{2-10  }\!\!%\!\!\text{ }$ of the photosynthetically active radiation.

7. Name the pioneers of primary succession in water. 

Ans. Phytoplankton is the pioneer of primary succession in water.

8. Name any two man-made ecosystems? 

Ans. Aquarium and Garden are two man-made ecosystems.

9. Define stratification? 

Ans. In an ecosystem, stratification refers to the vertical distribution of distinct species at different levels.

10. Name the ecological pyramid that is always upright? 

Ans. The pyramid of energy is an ecological pyramid that is always upright.

11. Name the trophic level occupied by secondary consumers and tertiary consumers? 

Ans. The third trophic level is occupied by secondary consumers, while the fourth trophic level is occupied by tertiary consumers respectively.

12. Define standing crop? 

Ans. Standing crop refers to the amount of living matter or biomass present at each trophic level.

13. Name the ecological pyramid that is inverted in the tree ecosystem? 

Ans. The pyramid of number is an ecological pyramid that is inverted in a tree ecosystem. 

14. What are the products of decomposition? 

Ans. The products of decomposition are $\text{C}{{\text{O}}_{\text{2}}}$, ${{\text{H}}_{\text{2}}}\text{O}$ and nutrients. 

15. What is $\text{10  }\!\!%\!\!\text{ }$ law? 

Ans. At each trophic level, $\text{90 }\!\!%\!\!\text{ }$ of energy is degraded into heat and only $\text{10  }\!\!%\!\!\text{ }$ is transferred to the next trophic level. This rule is known as the $\text{10  }\!\!%\!\!\text{ }$ law.

16. Mention one similarity between Hydrarch and Xerarch succession? 

Ans. Both hydrarch and xerarch result in the establishment of similar mesic conditions.

17. What is the approximate value of the net primary productivity of the biosphere? 

Ans. The approximate value of the net primary productivity of the biosphere is 170 billion tons. 

18. Name two climatic factors that regulate decomposition? 

Ans. The two climatic factors that regulate decomposition are temperature and soil moisture.

19. What is sere? 

Ans. Sere refers to an entire sequence of communities that change one after the other in a given area.

20. Name the primary consumers in the aquatic ecosystem?

Ans. Zooplanktons are the primary consumers in the aquatic ecosystem.

21. Name the pioneer species in the primary succession on the rock? 

Ans. Lichens are the pioneer species in the primary succession on the rock.

Short Answer Questions (2 Marks)

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

Ans. The shape of the pyramid of biomass in the sea is inverted. This is because the amount of biomass is maximum at the apex of the pyramid and least at the bottom of the pyramid.

2. Give an example of an ecological pyramid that is always upright. Justify your answer. 

Ans. The pyramid of energy is always upright and can never be inverted because some energy is always lost as heat as energy flows from one trophic level to the next.

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

Ans. The difference between primary succession and secondary succession are: 

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

Ans. The atmosphere serves as a reservoir for the gaseous nutrient cycle, while the Earth's crust serves as a reservoir for the sedimentary nutrition cycle. The reservoir is required to meet the deficit caused by an imbalance in the influx and efflux. 

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

Ans. 

A: - Plant stage 

B: - Reed Swamp Stage 

C: - Scrub stage 

D: - Forest stage

6. In the model of the phosphorus cycle given below, what does A, B, C and D refer to?

Ans. 

A: - Detritus 

B: - Decomposition 

C: - Weathering 

D: - Producers

7. Differentiate between Hydrarch and Xerarch succession. 

Ans. The difference between Hydrarch and Xerarch succession are:

8. What is the effect on decomposition rate if: – 

a. Detritus is rich in lignin and chitin

b. Detritus is rich in nitrogen and sugars?

Ans. The effect on decomposition rate: -

a. If the detritus is rich in lignin and chitin, the decomposition rate is slower.

b. If the detritus is rich in nitrogen and sugars, the decomposition rate is faster.

9. What are the limitations of ecological pyramids? 

Ans. The following are the limitations of ecological pyramids: 

i. Does not account for the equivalent species belonging to two or more trophic levels.

ii. Assumes a simple food chain and does not account for a food web.

iii. Saprophytes have been excluded from ecological pyramids

10. Name any four ecosystem services. Who gave the price tags on nature’s life support services? Which is the most important ecosystem service provider? 

Ans. Four ecosystem services provided by the natural ecosystem are: - 

i. Drought and flood mitigation.

ii. Offer aesthetic, cultural, and spiritual values.

iii. Cycling nutrients and generating fertile soil.

iv. Providing habitat for wildlife, pollinating flowers, and maintaining Biodiversity.

Robert Constanza gave price tags to nature's life support services. Soil formation is the most important ecosystem services provider.

11. Study the table given below and fill the blanks from ‘A’ to ‘F’.

Ans.

12. In the pyramid of biomass drawn below, name the two crops (i) one which is supported (ii) one which supports. In which ecosystem are such types of pyramids found?

Ans. 

i. Supported trophic level is formed by zooplanktons (Primary consumers).

ii. Supporting trophic level is formed by phytoplankton (Secondary consumers). such types of pyramids are found in aquatic ecosystems.

13. Why is secondary succession faster than primary succession? 

Ans. Secondary succession is the development of a community on a site that was previously occupied by well-developed communities where the environment is both organic and inorganic. Secondary succession is faster than primary succession because these bare areas have suitable soil for proper growth.

14. Distinguish between upright and inverted pyramids? 

Ans. The difference between upright and inverted pyramids are:

15. Explain with an example, why is the length of a food chain in an ecosystem generally limited to $\text{3-4}$trophic level? 

Ans. The length of a food chain in an ecosystem is generally limited to $\text{3-4}$ trophic levels because the energy transferred for $\text{3-4}$ trophic levels is sufficient to do work and then the energy is insufficient to do work because only $\text{10 }\!\!%\!\!\text{ }$ of energy is transferred to the next level. This is because according to the $\text{10 }\!\!%\!\!\text{ }$ Law of transfer of energy, in a food chain at each trophic level, about $\text{90 }\!\!%\!\!\text{ }$ of energy is degraded into heat and only $\text{10 }\!\!%\!\!\text{ }$ of energy is transferred to the next trophic level.

16. What is meant by ecological succession? Describe the different stages in which succession occurs? 

Ans. Ecological succession is a community–controlled phenomenon in which the structure and composition of the community change in a systematic and orderly manner, eventually leading to the establishment of a climax community.

Stages of Succession: - 

1. Invasion: - Invasion is the arrival of propagating organs such as seeds, spores, bulbils, etc. on a bare area of primary or secondary succession. Those for which conditions are favorable germinate in a new area and some of them grow into mature plants. These new arrivals from outside are called pioneers.

2. Establishment: - The process by which migrants adjust themselves to new areas after the migration is called ecesic. It consists of three essential processes i.e., germination, growth, and reproduction. 

3. Aggregation: - The coming together of individuals of various species in an area are called aggregation. 

4. Competition: - The species which have similar requirements of nutrition are known as competitive species e.g., those with the different requirements are complementary species. 

5. Reaction: - It is the changes caused by colonizers within the habitat. The influence of vegetation on the site is referred to as a reaction until a stable community develops in that area.

17. What is meant by ecological pyramid? With the help of one example each, show that the pyramid of numbers can be both upright as well as inverted. 

Ans. An ecological pyramid is a graphical representation in the form of a pyramid that depicts the feeding relationship of groups of organisms. It is frequently depicted as follows: the producers are at the bottom, followed by the various trophic levels, with the highest at the top.

A pyramid of numbers is a graphical representation of the number of organisms at each trophic level.

In an upright pyramid of numbers, the more abundant species form the first trophic level i.e., the base of the pyramid, and less abundant species remain near the top. Example: - Grassland ecosystem. 

\[\text{Green plants }\xrightarrow{{}}\text{Rabbit}\xrightarrow{{}}\text{ Fox}\xrightarrow{{}}\text{ Lion}\]

In a grassland ecosystem, producers (Green plants) are more in number, and herbivores (Rabbit) are more than carnivores (Fox and Lion). 

In an inverted pyramid of numbers, the most abundant species occurs at the top while the less abundant species form the base of the pyramid. Example: - Tree ecosystem.

\[\text{Tree }\xrightarrow{{}}\text{Birds}\xrightarrow{{}}\text{ Parasite}\]

A big tree provides food to several herbivores (birds) which in turn provides food to few carnivores.

18. Describe the components of an ecosystem? 

Ans. An ecosystem is a functional unit of nature consisting of biotic and abiotic factors where the living organisms interact among themselves and with the physical environment. The ecosystem consists of two components: -

1. Abiotic Components: - 

1. Inorganic substances e.g., phosphorus, Sulphur, carbon, nitrogen hydrogen, etc.

2. Organic substances e.g., carbohydrates, proteins, lipids. 

3. Climatic regime e.g., light, humidity, rainfall, temperature. 

2. Biotic Components: - 

1. Producers: - The organisms which produce food for themselves and all living organisms from inorganic raw material and solar radiation are called producers. 

2. Consumers: - These are living heterotrophic members of the ecosystem that consume the food synthesized lay producers. They are broadly classified as: 

1. Primary Consumers: - They are directly dependent on producers called herbivores. Example: - Rat, deer, cow, goat. 

2. Secondary Consumers: - The organisms that use primary consumers as their food are called carnivores. Example: - Fox cats, lions. 

3. Tertiary Consumers: - These are top carnivores that prey upon other carnivores and herbivores. Example: - Crow, man. 

3. Decomposers: - Organisms that break up the dead bodies of plants, animals, and related waste products are called decomposers. Example: - Bacteria, Fungi etc.

19. “Energy flow in an ecosystem is always unidirectional”. Justify the statement. 

Ans. Energy flow in an ecosystem is always unidirectional, which means that it is always transferred from one trophic level to the next and is never reverted. Example: - Energy from the sun is captured by producers, which is then consumed by the primary consumer, and energy is transferred to the next trophic level.

20. Differentiate between Production and decomposition? 

Ans. The difference between production and decomposition are:

21. Explain why the pyramid of energy of an ecosystem is always upright and never inverted?

Ans. The pyramid of energy represents the total amount of energy utilized by the different trophic level organisms in a unit area. The pyramid of energy of the ecosystem is always upright and can never be inverted. This is because when energy flows from one trophic level to the next, some energy is always degraded as heat at each step. 

22. 

i. Name the compound whose cycle is depicted. 

ii. In what way do vehicles add this compound to the atmosphere? 

iii. What adverse effect does its excess have on the environment? 

iv. Cite an event that depicts this effect in modern times. 

v. Suggest two ways of depleting this effect.

Ans. 

i. Carbon cycle

ii. By burning fossil fuel. Example: - Diesel or petroleum. They introduce $\text{C}{{\text{O}}_{\text{2}}}$ in the atmosphere.

iii. Environmental pollution

iv. Ozone layer depletion

v. Two ways of depleting this effect are: - 

a) By cutting down the use of fossil fuels.

b) By planting more and more trees.

23. What do you mean by “productivity of an ecosystem? What types of productivity also mention the factors on which productivity of an ecosystem depends? 

Ans. Productivity of an ecosystem is the rate at which solar radiation energy is fixed by the vegetation of an ecosystem per unit area & per unit time. It is generally expressed in terms of a unit of energy $\text{(cal)}$ produced in a unit area $\text{(}{{\text{m}}^{\text{2}}}\text{)}$ per unit time (year). 

Productivity can be of two types: - 

1. Primary Productivity: - It is defined as the amount of biomass or organic matter produced per unit area by plants during photosynthesis over a given period. Primary productivity is of following two types: - 

1. Gross primary productivity (GPP): - It refers to the total amount of food formed by the producers during photosynthesis.

2. Net primary productivity (NPP): - It refers to gross primary productivity minus loss by respiration (R). 

                      $\text{NPP = GPP - R}$

2. Secondary Productivity: - Secondary productivity is the rate of storage at the consumer level. It is defined as the rate at which organic matter is synthesized by consumers over a given period.

Primary productivity is determined by: - 

1. Several environmental factors.

2. Availability of nutrients. 

3. Photosynthetic capacity of plants.

24. What is decomposition? Describe the different processes involved in decomposition?

Ans. Decomposers (bacteria, fungi) helps in the breakdown of complex organic matter into simple inorganic substances like $\text{C}{{\text{O}}_{\text{2}}}$, ${{\text{H}}_{\text{2}}}\text{O}$, minerals. This process is called decomposition. Detritus is made up of dead plant remains such as leaves, bark, and flowers, as well as remains of dead animals along with fecal matter. The important processes involved in decomposition are: -

1. Fragmentation: - Detritivores are organisms that break down detritus into smaller particles. 

2. Leaching: - By leaching, water-soluble inorganic nutrients penetrate the soil horizon and precipitate as unavailable salts.

3. Catabolism: - The enzymes of bacteria and fungi degrade detritus into simple inorganic substances.

4. Humification: - Humification results in the accumulation of humus, a dark-colored amorphous substance. This is highly resistant to microbial action and undergoes decomposition at an extremely slower rate. 

5. Mineralization: - Some microbes further degrade the humus, resulting in the release of inorganic nutrients via the mineralization process.

25. Why is the productivity of coral reefs maximum? 

Ans. The productivity of coral reefs is maximum because of the availability of good light, enough warmth, water, and abundant nutrients.

26. In the pyramid of biomass, drawn below, name the two crops: - 

1. one which is supported and the one which supports

2. In which ecosystem is such a pyramid found.

Ans. 

1. Supported trophic level is formed by zooplanktons (Primary consumers) and supporting trophic level is formed by phytoplankton (Secondary consumers).

2. Such types of pyramids are found in aquatic ecosystems (Pond).

27. Differentiate between primary productivity and secondary productivity? 

Ans. The difference between primary productivity and secondary productivity are:

28. What ecological principles are derived from the study of food chains?

Ans. The ecological principles are derived by studying food chains are: -

1. Each food chain is complete and self–sufficient.

2. Every food chain must start with photosynthesis and end with decay.

3. Shorter food chains are more efficient because the greater the number of steps, the more energy is wasted. 

4. The members of food chains are large in size but fewer in number.

29. List the factors on which pioneer species depend during secondary succession? 

Ans. In secondary succession, the factors on which pioneer species depends are: -

1. Soil conditions, 

2. Water availability, 

3. Environmental conditions, and 

4. The presence of seeds or other propagules.

30. The productivity of the ecosystem increases from polar regions towards tropics. Why? 

Ans. The productivity of the ecosystem increases from the polar region towards the tropics because of the suitable temperature and increasing sunlight.

31. Mention some of the ecological services provided by forests? 

Ans. Some of the ecological services provided by forests are: - 

1. Drought and flood mitigation.

2. Offer aesthetic, cultural, and spiritual values.

3. Cycling nutrients and generating fertile soil.

4. Providing habitat for wildlife, pollinating flowers, and maintaining Biodiversity.

32. Differentiate between food chain & food web? 

Ans. The difference between the food chain and the food web is:

Short Answer Question (3 Marks)

1. With the help of a diagram, represent the energy flow through the different trophic levels. 

Ans. In an ecosystem, energy flows through the different trophic levels by the food chain. It consists of two steps: - 

1. Trapping solar energy: - Primary source of energy is the sun. The only \[\text{48  }\!\!%\!\!\text{ }\] of the sun’s energy reaches the surface of the earth and only a part of it is used by plants for photosynthesis. The chemical energy produced by plants is stored in plant tissues. The photosynthetic organism uses a part of this chemical energy and transfers the rest to the organism at the next trophic level.

2. Path and flow of energy: - The primary consumers, therefore, take chemical potential energy in the form of food. Most of it dissipates as heat during respiration and is lost out of the ecosystem. The same process is repeated at secondary consumers. At each trophic level, \[\text{90  }\!\!%\!\!\text{ }\] of energy is degraded into heat and only \[\text{10  }\!\!%\!\!\text{ }\] is then transferred to the next trophic level. This rule of energy flow is called the ten percent law.

2. What is a pyramid of biomass? Represent the pyramid of biomass in 

1. Grassland ecosystem 

2. Aquatic ecosystem.

Ans. The total weight of dry matter is referred to as biomass. The pyramid of biomass represents the weight of dry matter at different trophic levels of the ecosystem at one time in a food chain or food web. 

1. Grassland ecosystem: - In a grassland ecosystem, the pyramid of biomass is upright i.e., the pyramid of biomass shows a gradual reduction in biomass at each trophic level from base to apex.

\[\text{Green plants }\xrightarrow{{}}\text{Rabbit}\xrightarrow{{}}\text{ Fox}\xrightarrow{{}}\text{ Lion}\]

2. Aquatic ecosystem: - In the pond or aquatic ecosystem, the biomass of consumers is always greater than the biomass of producers. As a result, it takes the form of an inverted pyramid.

$\text{Phytoplanktons }\xrightarrow{{}}\text{ Zooplanktons }\xrightarrow{{}}\text{ Small fishes }\xrightarrow{{}}\text{Large fishes}$

Long Answer Questions (5 Marks)

1. Represent schematically and describe the phosphorus cycle in an ecosystem?

Ans. Phosphorus Cycle: -

1. The major reservoir of phosphorus is sedimentary rocks which are only available to basic cycle in small amounts as a result of weathering.

2. This phosphorus is weathered and later transported to the soil by wind and water, where they exist as inorganic dissolved phosphates.

3. The basic phosphorus cycle begins with dissolved phosphates which are absorbed by plants for making their issue. 

4. Plants are then eaten by animals. Decaying bacteria break the tissue of dead animals and these products return phosphates to the soil. 

5. The water-soluble phosphates are lost to the deep rudiments of the ocean through run-off. 

6. The major pathway of returning phosphorus to land is the uplifting of marine sediments. Some amount of phosphorus is returned to absorb inorganic phosphate, when they die, most of the absorbed phosphate is recycled back into ambient matter. This sort of cycling is called the biological cycle or metabolic cycle.

2. Represent schematically and describe the carbon cycle in the ecosystem? 

Ans. Carbon is derived from the atmosphere and water. Carbon is present in the atmosphere predominantly in the form of carbon dioxide. It is essential for the production of carbohydrates via photosynthesis. It is incorporated into tissues of green plants e.g., carbohydrates, proteins, and lipids. 

$\text{6 C}{{\text{O}}_{\text{2}}}\text{+ 12 }{{\text{H}}_{\text{2}}}\text{O }\xrightarrow[\text{Chlorophyll}]{\text{hv}}\text{ }{{\text{C}}_{\text{6}}}{{\text{H}}_{\text{12}}}{{\text{O}}_{\text{6}}}\text{ + 6 }{{\text{H}}_{\text{2}}}\text{O + 6 }{{\text{O}}_{\text{2}}}$

The $\text{C}{{\text{O}}_{\text{2}}}$ dissolved in seawater is utilized by marine animals like protozoans, corals, molluscs etc. for their life. In these animals, $\text{C}{{\text{O}}_{\text{2}}}$ is converted into calcium carbonate which is used for the construction of shells. 

$\text{C}{{\text{O}}_{\text{2}}}\text{ + }{{\text{H}}_{\text{2}}}\text{O }\xrightarrow{{}}\text{ }{{\text{H}}_{\text{2}}}\text{C}{{\text{O}}_{\text{3}}}\xrightarrow{{}}\text{ CaC}{{\text{O}}_{\text{3}}}\text{ + }{{\text{H}}^{\text{+}}}$

After the death of marine animals, \[\text{CaC}{{\text{O}}_{\text{3}}}\] store in shells is either deposited as sedimentary rock or dissolved in water to release $\text{C}{{\text{O}}_{\text{2}}}$. A certain amount of carbon is deposited in the form of coal carbon. Coal returns to the air in the form of $\text{C}{{\text{O}}_{\text{2}}}$ through combustion and weathering. Thus, carbon from the atmospheric pool moves to green plants, then to animals, and finally to bacteria, fungi, etc. They return it to the atmosphere through decomposition.

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Conclusion

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