Class 12 Biology Ch 14 Ecosystem NCERT Solution

Class 12 Biology Ch 14 NCERT Solutions

Question 1

Fill in the blanks.
(a) Plants are called as————- because they fix carbon dioxide.
(b) In an ecosystem dominated by trees, the pyramid (of numbers) is————- type.
(c) In aquatic ecosystems, the limiting factor for the productivity is————-.
(d) Common detritivores in our ecosystem are————-.
(e) The major reservoir of carbon on earth is————-.

Solution: (a) Autotrophs
(b) Spindle
(c) Sunlight
(d) Earthworms, bacteria & fungi of decay, and vulture
(e) Oceans

Question 2

Which one of the following has the largest population in a food chain?
(a) Producers
(b) Primary consumers
(c) Secondary consumers.
(d) Decomposers

Solution: Option (d) Decomposers

Question 3

The second trophic level in a lake is-
(a) Phytoplankton
(b) Zooplankton
(c) Benthos
(d) Fishes

Solution: Option (b) Zooplanktons

Question 4

Secondary producers are
(a) Herbivores
(b) Producers
(c) Carnivores
(d) None of the above

Solution: Option (d) Non of these

Question 5

5. What is the percentage of photosynthetically act., radiation (PAR), in the incident solar radiation?
(a) 100%
(b) 50 %

(c) 1-5%
(d) 2-10%

Solution: Option (b) 50%

Question 6

Distinguish between
(a) Grazing food chain and detritus food chain
(b) Production and decomposition
(c) ‘Upright and inverted pyramid
(d) Food chain and food web
(e) Litter and detritus
(f) Primary and secondary productivity

Solution: (a)

Responsive Table Example
Grazing Food Chains Detritus Food Chains
Begins with green plants as producers Begins with dead organic matter as a source of energy
Herbivores are the primary consumers Decomposers are the primary consumers
Carnivores are the secondary consumers Secondary consumers may include scavengers or predators of decomposers
Food chain is relatively short and straightforward Food chain can be longer and more complex due to the involvement of decomposers
More energy is transferred from one trophic level to the next, resulting in higher biomass at each level Less energy is transferred from one trophic level to the next, resulting in lower biomass at each level
Examples include grass → grasshopper → mouse → snake → hawk Examples include dead leaves → detritivores → decomposers → soil organisms → plants


Production vs Decomposition Table
Production Decomposition
Conversion of sunlight energy to organic matter through photosynthesis Breakdown of organic matter into simpler inorganic compounds by decomposers
Performed by autotrophs, such as green plants and algae Performed by heterotrophs, such as bacteria, fungi, and detritivores
Results in the creation of new biomass and energy Results in the release of stored energy and nutrients back into the ecosystem
Net primary production (NPP) is the amount of energy captured by autotrophs after respiration Decomposition rates are influenced by temperature, moisture, and the quality and quantity of organic matter
Contributes to the flow of energy and nutrients through food webs Recycles nutrients and helps to maintain soil fertility
Can be limited by factors such as light, water, and nutrient availability


Upright vs Inverted Pyramid Table
Upright Pyramid Inverted Pyramid
Represents a stable ecosystem with a broad base and a narrow top Represents an unstable ecosystem with a narrow base and a broad top
Energy flows from the base of the pyramid (producers) to the top (apex predators) Energy flows from the top of the pyramid (decomposers) to the base (producers)
Top predators have a lower biomass but a higher energy content Producers have a higher biomass but a lower energy content
Indicates a healthy ecosystem with high biodiversity and energy transfer efficiency Indicates an ecosystem with low biodiversity and inefficient energy transfer
Commonly observed in terrestrial ecosystems such as grasslands and forests Commonly observed in aquatic ecosystems such as oceans and lakes
Exhibits a trophic structure that follows the 10% rule, with only 10% of energy transferred from one trophic level to the next May exhibit a trophic structure that does not follow the 10% rule, with energy loss at each trophic level


Food Chain vs Food Web Table
Food Chain Food Web
Linear representation of energy flow from one organism to another Complex representation of energy flow from multiple organisms in an ecosystem
Shows a single pathway of energy flow through the ecosystem Shows multiple interconnected pathways of energy flow in the ecosystem
Consists of only one trophic level for each organism Consists of multiple trophic levels for each organism, with some organisms occupying more than one level
Usually involves a predator-prey relationship Involves multiple feeding relationships, including predator-prey, herbivore-plant, and omnivore relationships
Less complex and less diverse than a food web More complex and more diverse than a food chain
Does not show the entire energy flow in the ecosystem Shows the entire energy flow in the ecosystem
Shows the transfer of energy and nutrients in a simplified manner Shows the transfer of energy and nutrients in a more detailed manner


Characteristics Litter Detritus
Definition Organic matter on the soil surface Dead organic matter in the soil
Source Freshly fallen leaves, twigs, fruits, seeds, etc. Decomposed organic matter, such as humus, decomposed plant and animal matter
Size Usually larger in size Smaller in size
Decomposition Slow decomposition rate Fast decomposition rate
Nutrient content High nutrient content Lower nutrient content
Function Provides a habitat for soil organisms and protects the soil from erosion Provides nutrients for plants and soil organisms
Location Found on the soil surface Found within the soil
Importance Plays a crucial role in nutrient cycling and soil health Provides nutrients for plant growth and supports soil structure
Examples Fallen leaves, twigs, branches, acorns, cones, and fruits Humus, dead plant roots, animal remains, and feces


Question 7

Describe the components of an ecosystem.

Solution: There are three components of the ecosystem:-

(a) Autotrophs or producers which have the capacity to manufacture their own food or which can fix the radiant energy of the sun into chemical energy, e.g., green plants and photosynthetic bacteria.

(b) Heterotrophs or consumers who are unable to manufacture their own food and depend upon other organisms for their food. These are of the following types:

  • Primary consumers or herbivores depend upon producers or green plants for their food.
  • Secondary consumers or carnivores live upon herbivores.
  • Top consumers or top carnivores live upon secondary consumers.

(c) Decomposers or micro consumers decompose dead organic substances of producers and consumers into simple substances and thus continue mineral cycles, e.g., bacteria, fungi, actinomycetes, etc.

Question 8

Define ecological pyramids and describe with examples, pyramids of number and biomass.

Solution: An ecological pyramid is a graphic representation of an ecological parameter, as a number of individuals present in various trophic levels of a food chain with producers forming the base and top carnivores at the tip. Ecological pyramids were developed by Charles Elton (1927) and are, therefore, also called Eltonian pyramids.

There are three types of ecological pyramids – (1) Pyramid of numbers (2) Pyramid of biomass (3)Pyramid of energy

Pyramid of numbers: It is a graphic representation of the number of individuals per unit area of various trophic levels stepwise with producers at the base and top carnivores at the tip. In a grassland, the producers, which are mainly grasses, are always the maximum in number. This number then shows a decrease towards the apex, as the primary consumers (herbivores) like rabbits, mice, etc. are lesser in number than the grasses; the secondary consumers, snakes, and lizards are lesser in number than the rabbits and mice. Finally, the top (tertiary) consumers hawks or other birds, are the least in number. Thus, the pyramid becomes upright.

Pyramid of biomass: The amount of living organic matter (fresh and dry weight) is called biomass. Here, the different trophic levels of the ecosystem are arranged according to the biomass of the organisms. In grassland and forest, there is generally a gradual decrease in the biomass of organisms at successive levels from the producers to the top carnivores. Thus these pyramids are upright. But in the pond ecosystem, it is inverted because the biomass gradually increases from the producers to carnivores.

Question 9

What is primary productivity? Give a brief description of factors that affect primary productivity.

Solution:  Primary productivity of an ecosystem is the amount of energy fixed or biomass synthesized by primary producers or green plants per unit area per unit time during photosynthesis. Factors affecting primary productivity are –
-Plant species inhabiting a particular area
-Soil water
lit deserts, sunlight is abundant but water is scarce or nutrients are lacking. Therefore, in such areas, water & nutrient supply become the limiting factors.

Question 10

Define decomposition and describe the processes and products of decomposition.

Solution: Decomposition is the breakdown of dead or waste organic matter by microorganisms. Decomposition is both physical and chemical in nature. Processes involved in decomposition are – fragmentation, catabolism & leaching.

Fragmentation – The process primarily due to the action of detritus feeding invertebrates (detritivores) causes it to break into smaller particles. The detritus gets pulverized when passing through the digestive tracts of animals. Due to fragmentation, the surface area of detritus particles is greatly increased.

Catabolism – Enzyme degradation of detritus into simpler organic substances by bacteria and fungi.

Leaching – The process by which nutrients, chemicals, or contaminants are dissolved & carried away by water, or are moved into a lower layer of soil.

Various inorganic and organic substances are obtained by decomposition. Inorganic substances are obtained in the process of mineralization while organic substances are obtained in humification. A dark-coloured amorphous substance called humus is formed by decomposition. Humus is highly resistant to microbial action & undergoes extremely slow decomposition. It serves as a reservoir of nutrients.

Question 11

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