CBSE Class 12 Biology 2024-25: Chapter 12 Ecosystem Competency-Based Questions with Answers; Download Free PDF

Get ready for your CBSE Class 12 Biology exams with our easy guide on Chapter 12: Ecosystem. This chapter is very important, especially with the new exam pattern starting in the 2024-25 school year. The CBSE will include 50% more competency-based questions. These questions ask you to use what you've learned in real-life situations.

In this article, we focus on key questions from Chapter 12: Ecosystem. These questions help you think and solve problems. You’ll find different types of questions, such as multiple-choice, true-false, gap-filling, and short-answer questions. Each question has clear answers to help you understand the concepts better.

To help you study, you can download a free PDF that has all the competency-based questions from Chapter 12: Ecosystem along with their answer keys.

CBSE 12 Biology Chapter 12 : Ecosystem Competency-Based Questions

Q.1 Two statements are given below - one labelled Assertion (A) and the other labelled Reason (R).

Assertion (A): The energy utilised by an organism for growth is not transferred to the next trophic level.

Reason (R): Growth causes an increase in biomass. Which of the following is correct?

A. Both A and R are true, and R is the correct explanation for A.
B. Both A and R are true, but R is not the correct explanation for A.
C. A is true, but R is false.
D. A is false, but R is true.

Answer. D. A is false, but R is true.

Q.2 Two statements are given below - one labelled Assertion (A) and the other labelled Reason (R).

Assertion (A): Humus is organic in nature.

Reason (R): Steps occurring before humification ensure the removal of most inorganic substances from the detritus.

Which of the following is correct?

A. Both A and R are true, and R is the correct explanation for A.
B. Both A and R are true, but R is not the correct explanation for A.
C. A is true, but R is false.
D. A is false, but R is true.

Answer. A. Both A and R are true, and R is the correct explanation for A.

Q.3 If X is the amount of energy produced by the producers, which of the following is the correct amount of energy received by humans from the producers in a food web?

A. only 10%
B. either 10% or 1%
C. either 10%, 1% or 0.1%
D. Humans are not part of a food web.

Answer. C. either 10%, 1% or 0.1%

Q.4 Two statements are given below - one labelled Assertion (A) and the other labelled Reason (R).

Assertion (A): Plants do not constitute the first trophic level in all food chains.

Reason (R): Dead plants or animals is the first trophic level for detritivores.

Which of the following is correct?

A. Both A and R are true, and R is the correct explanation for A.
B. Both A and R are true, but R is not the correct explanation for A.
C. A is true, but R is false.
D. A is false, but R is true.

Answer. A. Both A and R are true, and R is the correct explanation for A.

Q.5 If the energy produced by the producers of a food web is 20,810 kcal, what is the minimum amount of energy that will be available to snakes in this food web?

A. 383 kcal
B. 2081 kcal
C. 10000 kcal
D. Cannot say without knowing the level that is occupied by snakes in this chain.

Answer. A. 383 kcal

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Q.6 A carbon sink is any environment that can store carbon and remove it from the atmosphere. Which of the following will lead to a DECLINE in carbon sink?

A. Deforestation
B. Population growth
C. Burning of fossil fuels
D. Increase in industrial activity

Answer. A. Deforestation

Q.7 A pioneer species is a species that invades a bare area. Such bare areas are usually xerophytic. The pioneer species is gradually replaced by other species, till a climax species/ community remains stable in the environment.

(a) Why is the pioneer species replaced by the climax species/community?
(b) What is the environment type usually required by the climax species?

Answer. (a) because the climax species makes the environment non-conducive for the pioneer species
(b) mesophytic

Q.8 State THREE points of difference between a natural terrestrial ecosystem and a man-made ecosystem such as zoo.

Answer. 1 mark each for any three of the following:

- Natural ecosystems are self-sustaining whereas man-made ecosystems require the assistance of humans.
- The nutrient cycle starts and ends in the same ecosystem in natural ecosystems whereas in man-made ecosystems the nutrient cycle may begin in one place and end in another.
- Individuals of a species may be scattered throughout the geography in natural ecosystems whereas species are present in close proximity in manmade ecosystems.
- Interactions are spontaneous in natural ecosystems whereas they are controlled by humans in man-made ecosystems.

[Accept any other valid points.]

Q.9 The Tundra desert's gross primary productivity (GPP) is 800 kilocalories/m2 and respiration losses are about 200 kilocalories.

(a) What is the net primary productivity of the desert? Show calculations.
(b) Why do deserts have the least NPP across most ecosystems?

Answer. (a) 0.5 marks for the correct formula and 0.5 marks for the correct answer:

NPP = GPP - R
NPP = 800 - 200 = 600 kilocalories/m2

(b) The low water availability and other conditions are not unfavourable for rapid photosynthesis.

Q.10 Fish farming involves the commercial breeding of fish either in fish tanks or in artificial enclosures such as fish ponds.

Is a fish farm an example of an ecosystem? Justify your answer.

Answer. 1 mark for each of the following:

- Yes, it is.
- In a fish farm, fish interact with each other as well as the physical environment created for them making it an ecosystem.

[No marks to be allotted if correct justification is not provided.]

Q.11 Give a reason why:

(a) Mass lost in faeces is considered available biomass for the next trophic level.
(b) Secondary productivity of herbivores is lower than primary productivity. (
c) The ocean is not a productive ecosystem.

Answer. 1 mark for each of the following:

(a) Faeces are available to decomposers and so are not completely removed from the food web.
(b) Not all biomass produced by producers is consumed by herbivores, a lot is lost and is available to decomposers.
(c) Sunlight is a largely limiting factor, especially in deep ocean layers, making photosynthesis and therefore productivity low.

[Accept any other valid answer.]

Q.12 The first law of thermodynamics states that energy can neither be created nor destroyed, only altered in form.

Does the energy flow in an ecosystem follow this rule? Justify

Answer. - Yes it does.

- Energy from the Sun is converted to chemical energy by photosynthesis which is then transferred through various trophic levels as chemical energy or utilised to produce heat energy.

[No marks to be awarded if justification is not provided.]

Q.13 Identify if the statements given below are true or false. Justify your answer.

(a) In an inverted food pyramid, the energy is the lowest at the first trophic level.
(b) The biomass of zooplankton is greater than phytoplankton as they reproduce fast and have a longer life span.
(c) In the long run, an inverted food pyramid is unstable

Answer. 0.5 marks for identifying if true or false and 0.5 marks for the justification:

(a)
- False.
- Energy is always highest in the first trophic level and reduces as we move up the pyramid.

(b)
- True.
- Since the biomass of zooplankton is greater it is likely that more individuals are present per unit area than phytoplankton which is possible when organisms do not die fast but reproduce fast.

(c)
- True.
- Since a lower amount of food will be available to the higher trophic levels, it is likely to be unstable over time.

[Accept any other valid justification. No marks are to be awarded for writing true/false if the correct justification is not provided.]

Q.14 Justify the following statements:

(a) A xeric environment turns into a mesic environment over time.
(b) A mesic environment is preferred over a hydric or xeric environment

Answer. 1 mark for each of the following:

(a) As pioneer species and then seral plant communities grow in a xeric environment, they increase the moisture content in the environment as they perform their physiological processes making the environment mesic.
(b) A mesic environment provides an ambient environment with the right moisture content for the growth of most species.

[Accept any other valid justification.]

Q.15 State ONE quality that a pioneer species must have.

How can the process ofsuccession be interrupted?

Answer. (a) They should be able to derive energy and produce organic matter from inorganic and non-living substances.

[Accept any other valid answer]

(b) Succession can be interrupted in the event of disturbances such as fire that convert a seral stage to an earlier stage or even lead to the elimination of species.

[Accept any other valid answer]

👉 CBSE Class 12 Study Materials

CBSE Class 12 Syllabus 2024-25	CBSE Class 12 Previous Year Papers
NCERT Books For Class 12 Books	NCERT Class 12 Solutions
CBSE Class 12 Full Study Material	CBSE Class 12 Sample Paper 2024-25