NCERT Solutions for Class 12 Biology Chapter 12 Ecosystem

NCERT
SOLUTIONS
CLASS - 12th

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Class : 12th
Subject : Biology
Chapter : 14
Chapter Name : Ecosystem

Q1 Fill in the blanks.
(a) Plants are called as _________ because they x 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 _________

Answer. (a) Plants are called as autotrophs because they x carbon dioxide.
(b) In an ecosystem dominated by trees, the pyramid (of numbers) is inverted type.
(c) In aquatic ecosystems, the limiting factor for the productivity is light.
(d) Common detritivores in our ecosystem are earthworms .
(e) The major reservoir of carbon on earth is oceans.

Page : 256 , Block Name : Exercise

Q2 Which one of the following has the largest population in a food chain?
(a) Producers
(b) Primary consumers
(c) Secondary consumers
(d) Decomposers

Answer. (d) Decomposers : Decomposers include micro-organisms such as bacteria and fungi.
They form the largest population in a food chain and obtain nutrients by breaking down the
remains of dead plants and animals.

Q3 The second trophic level in a lake is-
(a) Phytoplankton
(b) Zooplankton
(c) Benthos
(d) Fishes

Answer. (b) Zooplankton : Zooplankton are primary consumers in aquatic food chains that feed
upon phytoplankton. Therefore, they are present at the second trophic level in a lake.

Q4 Secondary producers are
(a) Herbivores
(b) Producers
(c) Carnivores
(d) None of the above

Answer. (d) None of the above : Plants are the only producers. Thus, they are called primary
producers. There are no other producers in a food chain.

Q5 What is the percentage of photosynthetically active radiation (PAR), in the incident solar
radiation.
(a) 100%
(b) 50 %
(c) 1-5%
(d) 2-10%

Answer. (b) 50% : Out of total incident solar radiation, about fty percent of it forms
photosynthetically active radiation or PAR.

Q6 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

Answer. (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

Q7 Describe the components of an ecosystem.

Answer. An ecosystem is de ned as an interacting unit that includes both the biological
community as well as the non-living components of an area. The living and the non- living

components of an ecosystem interact amongst themselves and function as a unit, which gets
evident during the processes of nutrient cycling, energy ow, decomposition, and productivity.
There are many ecosystems such as ponds, forests, grasslands, etc. The two components of an
ecosystem are:
(a) Biotic component: It is the living component of an ecosystem that includes biotic factors such
as producers, consumers, decomposers, etc. Producers include plants and algae. They contain
chlorophyll pigment, which helps them carry out the process of photosynthesis in the presence of
light. Thus, they are also called converters or transducers. Consumers or heterotrophs are
organisms that are directly (primary consumers) or indirectly (secondary and tertiary consumers)
dependent on producers for their food. Decomposers include micro-organisms such as bacteria
and fungi. They form the largest population in a food chain and obtain nutrients by breaking down
the remains of dead plants and animals.
(b) Abiotic component: They are the non-living component of an ecosystem such as light,
temperature, water, soil, air, inorganic nutrients, etc.

Q8 De ne ecological pyramids and describe with examples, pyramids of number and biomass.

Answer. An ecological pyramid is a graphical representation of various ecological parameters such
as the number of individuals present at each trophic level, the amount of energy, or the biomass
present at each trophic level. Ecological pyramids represent producers at the base, while the apex
represents the top level consumers present in the ecosystem. There are three types of pyramids:
(a) Pyramid of numbers
(b) Pyramid of energy
(c) Pyramid of biomass
Pyramid of numbers:lt is a graphical representation of the number of individuals present at each
trophic level in a food chain of an ecosystem. The pyramid of numbers can be upright or inverted
depending on the number of producers. For example, in a grassland ecosystem, the pyramid of
numbers is upright. In this type of a food chain, the number of producers (plants) is followed by
the number of herbivores (mice), which in turn is followed by the number of secondary consumers
(snakes) and tertiary carnivores (eagles). Hence, the number of individuals at the producer level
will be the maximum, while the number of individuals present at top carnivores will be least.

On the other hand, in a parasitic food chain, the pyramid of numbers is inverted. In this type of a

food chain, a single tree (producer) provides food to several fruit eating birds, which in turn
support several insect species. Pyramid of biomass A pyramid of biomass is a graphical
representation of the total amount of living matter present at each trophic level of an ecosystem.
It can be upright or inverted. It is upright in grasslands and forest ecosystems as the amount of
biomass present at the producer level is higher than at the top carnivore level. The pyramid of
biomass is inverted in a pond ecosystem as the biomass of shes far exceeds the biomass of
zooplankton (upon which they feed).

Q9 What is primary productivity? Give brief description of factors that affect primary productivity.

Answer. It is de ned as the amount of organic matter or biomass produced by producers per unit
area over a period of time. Primary productivity of an ecosystem depends on the variety of
environmental factors such as light, temperature, water, precipitation, etc. It also depends on the
availability of nutrients and the availability of plants to carry out photosynthesis.

Q10 De ne decomposition and describe the processes and products of decomposition.

Answer. Decomposition is the process that involves the breakdown of complex organic matter or
biomass from the body of dead plants and animals with the help of decomposers into inorganic
raw materials such as carbon dioxide, water, and other nutrients. The various processes involved
in decomposition are as follows:
(1) Fragmentation: It is the rst step in the process of decomposition. It involves the breakdown of

detritus into smaller pieces by the action of detritivores such as earthworms.
(2) Leaching: It is a process where the water soluble nutrients go down into the soil layers and get
locked as unavailable salts.
(3) Catabolism: It is a process in which bacteria and fungi degrade detritus through various
enzymes into smaller pieces.
(4) Humi cation: The next step is humi cation which leads to the formation of a dark- coloured
colloidal substance called humus, which acts as reservoir of nutrients for plants.
(5) Mineralization: The humus is further degraded by the action of microbes, which nally leads to
the release of inorganic nutrients into the soil. This process of releasing inorganic nutrients from
the humus is known as mineralization. Decomposition produces a dark coloured, nutrient-rich
substance called humus. Humus nally degrades and releases inorganic raw materials such as CO₂,
water, and other nutrient in the soil.

Q11 Give an account of energy ow in an ecosystem.

Answer. Energy enters an ecosystem from the Sun. Solar radiations pass through the atmosphere
and are absorbed by the Earth's surface. These radiations help plants in carrying out the process of
photosynthesis. Also, they help maintain the Earth's temperature for the survival of living
organisms. Some solar radiations are re ected by the Earth's surface. Only 2-10 percent of solar
energy is captured by green plants (producers) during photosynthesis to be converted into food.
The rate at which the biomass is produced by plants during photosynthesis is termed as 'gross
primary productivity'. When these green plants are consumed by herbivores, only 10% of the
stored energy from producers is transferred to herbivores. The remaining 90 % of this energy is
used by plants for various processes such as respiration, growth, and reproduction. Similarly, only
10% of the energy of herbivores is ransferred to carnivores. This is known as ten percent law of
energy ow.

Q12 Write important features of a sedimentary cycle in an ecosystem.

Answer. Sedimentary cycles have their reservoirs in the Earth's crust or rocks. Nutrient elements
are found in the sediments of the Earth. Elements such as sulphur, phosphorus, potassium, and
calcium have sedimentary cycles. Sedimentary cycles are very slow. They take a long time to
complete their circulation and are considered as less perfect cycles. This is because during
recycling, nutrient elements may get locked in the reservoir pool, thereby taking a very long time
to come out and continue circulation. Thus, it usually goes out of circulation for a long time.

Q13 Outline salient features of carbon cycling in an ecosystem

Answer.

The carbon cycle is an important gaseous cycle which has its reservoir pool in the atmosphere. All
living organisms contain carbon as a major body constituent. Carbon is a fundamental element
found in all living forms. All biomolecules such as carbohydrates, lipids, and proteins required for
life processes are made of carbon. Carbon is incorporated into living forms through a fundamental
process called 'photosynthesis'. Photosynthesis uses sunlight and atmospheric carbon dioxide to
produce a carbon compound called 'glucose'. This glucose molecule is utilized by other living
organisms. Thus, atmospheric carbon is incorporated in living forms. Now, it is necessary to
recycle this absorbed carbon dioxide back into the atmosphere to complete the cycle. There are
various processes by which carbon is recycled back into the atmosphere in the form of carbon
dioxide gas. The process of respiration breaks down glucose molecules to produce carbon dioxide
gas. The process of decomposition also releases carbon dioxide from dead bodies of plants and
animals into the atmosphere. Combustion of fuels, industrialization, deforestation, volcanic
eruptions, and forest res act as other major sources of carbon dioxide.