CBSE Class 12 Biology Chapter 14: Ecosystem NCERT Solutions

Reviewed by NCERT Help Editorial Team · Updated on 29 Jul 2026

NCERT Solutions PDF Class 12 PDF

This resource provides detailed NCERT Solutions for Class 12 Biology, Chapter 14 on Ecosystem. It covers essential ecological concepts including the roles of producers, consumers, and decomposers, the process of decomposition, and the measurement of productivity. The solutions explain ecological pyramids, focusing on why biomass pyramids can be inverted in certain ecosystems like marine environments. It also clarifies which organisms function as producers and identifies the most productive ecosystems in terms of net primary production. These solutions are designed to help students grasp complex ecological principles and prepare effectively for their board examinations by offering clear explanations and step-by-step problem-solving.

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Board	CBSE
Class	Class 12
Subject	Biology Exemplar
Session	2026
Language	English
Type	NCERT Solutions
Chapter	Chapter 14

Chapter summary

Chapter 14 of the CBSE Class 12 Biology Exemplar focuses on Ecosystems. The NCERT Solutions provided here cover key topics such as the classification of organisms based on nutrition (autotrophs, heterotrophs, saprotrophs), the stages and significance of decomposition including mineralization, and the definition and measurement of productivity. It also delves into the graphical representations of ecological relationships through pyramids of numbers, biomass, and energy, explaining the conditions under which these pyramids can be inverted. The chapter highlights examples of producers and compares the net primary productivity across different ecosystems.

Learning outcomes

Understand the different types of organisms based on their mode of nutrition.
Explain the process of decomposition and its role in nutrient cycling.
Define and express productivity in an ecosystem.
Analyze the structure and types of ecological pyramids.
Identify producers and compare net primary productivity across various ecosystems.

Topics covered

Paper topics

Ecosystem structure
Producers
Consumers
Decomposers
Decomposition process
Mineralization
Humification
Productivity (NPP, GPP)
Ecological pyramids
Pyramid of numbers
Pyramid of biomass
Pyramid of energy

Important topics

Roles of Producers, Consumers, and Decomposers
The Process of Decomposition and Mineralization
Understanding Productivity and its Measurement
Analysis of Ecological Pyramids (Upright vs. Inverted)
Comparison of Net Primary Productivity across Ecosystems

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Questions and Solutions

Q. 1

Decomposers like fungi and bacteria are:

autotrophs
heterotrophs
saprotrophs
chemo-autotrophs

Choose the correct answer from the following options:

(i) and (iii)
(i) and (iv)
(ii) and (iii)
(i) and (ii)

Solution: Fungi and bacteria are decomposers because they obtain their nutrition by breaking down dead organic matter from plants and animals. Organisms that obtain nourishment from dead organic matter are known as saprotrophs. Autotrophs, on the other hand, synthesize their own food. Chemo-autotrophs are a type of autotroph that uses chemical energy. Therefore, decomposers fit the description of heterotrophs (as they cannot produce their own food) and specifically saprotrophs. The correct option combines these classifications.

Answer: (c) (ii) and (iii)

Q. 2

The process of mineralization by microorganisms helps in the release of:

inorganic nutrients from humus
both organic and inorganic nutrients from detritus
organic nutrients from humus
inorganic nutrients from detritus and formation of humus.

Solution: Decomposition is a complex process involving several steps, including fragmentation, leaching, catabolism, humification, and mineralization. Mineralization is the final stage where decomposers break down complex organic matter into simpler inorganic substances. This process releases essential inorganic nutrients (like K+, Mg++, Ca++, NH4+) back into the soil, making them available for plants. Humus formation (humification) is a separate process that leads to the accumulation of stable organic matter.

Answer: (a) inorganic nutrients from humus

Q. 3

Productivity is the rate of production of biomass expressed in terms of:

$(kcal m^{-3}) yr^{-1}$
$g m^{-2} yr^{-1}$
$g m^{-1} yr^{-1}$
( $kcal m^{-2}$ ) $yr^{-1}$

Choose the correct option:

(i)
(ii)
(ii) and (iv)
(i) and (iii)

Solution: Productivity refers to the rate at which biomass or organic matter is produced by any trophic level in an ecosystem. It is quantified per unit area over a specific period. This rate can be expressed in terms of mass per unit area per unit time (e.g., grams per square meter per year,

g m^{-2} yr^{-1}

) or in terms of energy per unit area per unit time (e.g., kilocalories per square meter per year,

(kcal m^{-2}) yr^{-1}

). The units

g m^{-1} yr^{-1}

and

(kcal m^{-3}) yr^{-1}

are not standard for ecosystem productivity.

Answer: (c) (ii) and (iv)

Q. 4

An inverted pyramid of biomass can be found in which ecosystem?

Forest
Marine
Grass land
Tundra

Solution: Ecological pyramids represent the relationship between different trophic levels. While pyramids of numbers and biomass are usually upright, they can be inverted under certain conditions. A pyramid of biomass is inverted when the total biomass of organisms at a lower trophic level is less than the total biomass at a higher trophic level. This commonly occurs in aquatic ecosystems, such as marine environments, where producers (like phytoplankton) have a short lifespan and rapid reproductive rate, resulting in a smaller standing crop biomass compared to the larger, longer-lived consumers (like fish). In contrast, forest, grassland, and tundra ecosystems typically exhibit upright pyramids of biomass.

Answer: (b) Marine

Q. 5

Which of the following is not a producer?

Spirogyra
Agaricus
Volvox
Nostoc

Solution: Producers are organisms capable of synthesizing their own food, typically through photosynthesis. Spirogyra is a type of green algae, Volvox is a colonial green alga, and Nostoc is a cyanobacterium (blue-green alga). All of these are photosynthetic and thus are producers. Agaricus, commonly known as mushroom, belongs to the fungi kingdom. Fungi are heterotrophs, specifically saprotrophs, as they obtain nutrients by decomposing dead organic matter. Therefore, Agaricus is not a producer.

Answer: (b) Agaricus

Q. 6

Which of the following ecosystems is most productive in terms of net primary production?

Deserts
Tropical rain forests
Oceans
Estuaries

Solution: Net primary production (NPP) is the rate at which producers accumulate biomass after accounting for respiration. Tropical rain forests are known for their exceptionally high NPP due to abundant sunlight, warm temperatures, and high rainfall, which support dense vegetation growth. While coral reefs and estuaries also have high productivity, tropical rain forests generally rank the highest. Deserts and open oceans have significantly lower NPP due to limiting factors like water scarcity or nutrient availability.

Answer: (b) Tropical rain forests

Q. 7

Pyramid of numbers is:

Always upright
Always inverted
Either upright or inverted
Neither upright nor inverted

Solution: Ecological pyramids are graphical representations of the relationships between different trophic levels in an ecosystem. The pyramid of numbers depicts the number of individual organisms at each trophic level. This pyramid can be upright, as seen in most terrestrial ecosystems where the number of producers is greater than the number of primary consumers, and so on. However, it can be inverted in cases where a single large producer (like a large tree) supports a large population of primary consumers (like insects). Therefore, the pyramid of numbers can be either upright or inverted, unlike the pyramid of energy which is always upright.

Answer: (c) Either upright or inverted

Common mistakes

Confusing decomposers (fungi, bacteria) with other heterotrophs.
Misunderstanding the units and expressions for productivity.
Incorrectly assuming ecological pyramids are always upright.
Failing to identify organisms that are not producers (e.g., fungi).

Revision tips

Review the definitions of autotrophs, heterotrophs, and saprotrophs.
Understand the five stages of decomposition and the significance of mineralization.
Memorize the units used to express productivity.
Focus on the conditions that lead to inverted ecological pyramids, especially in aquatic ecosystems.
Practice identifying producers versus consumers in different ecological contexts.

Practice MCQs

Q1. Decomposers, such as fungi and bacteria, are primarily classified based on their mode of nutrition as:

Q2. The process of mineralization by microorganisms is crucial for:

Q3. Productivity in an ecosystem is measured in terms of the rate of biomass production, typically expressed as:

Q4. An inverted pyramid of biomass is characteristic of which type of ecosystem?

Q5. Which of the following organisms is NOT considered a producer?

Q6. Which ecosystem exhibits the highest net primary productivity?

Q7. The pyramid of numbers in an ecosystem can be:

Frequently asked questions

What are the main components of an ecosystem discussed in these solutions?

These solutions discuss the roles of producers (autotrophs), consumers (heterotrophs), and decomposers (saprotrophs) within an ecosystem, along with processes like decomposition and productivity.

How is productivity measured in an ecosystem?

Productivity is measured as the rate of biomass production, typically expressed in units of mass per unit area per unit time (e.g., g/m²/yr) or energy per unit area per unit time (e.g., kcal/m²/yr).

Under what conditions can a pyramid of biomass be inverted?

A pyramid of biomass can be inverted in aquatic ecosystems, such as marine environments, where the biomass of consumers may exceed the biomass of producers due to factors like rapid turnover rates of producers.

What is the difference between humification and mineralization?

Humification is the process of forming humus (a stable, dark organic matter) from detritus. Mineralization is the subsequent step where microorganisms break down humus and other organic matter to release inorganic nutrients.

Which organisms are considered producers?

Producers are organisms that can synthesize their own food, primarily through photosynthesis. Examples include plants, algae (like Spirogyra, Volvox), and cyanobacteria (like Nostoc).

Why are these NCERT solutions helpful for Class 12 Biology students?

These solutions provide clear, rewritten explanations for each question in Chapter 14, helping students understand complex ecological concepts and prepare effectively for their CBSE board exams.

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