The VIDYANKER team has thoughtfully prepared the NCERT Solutions for Class 12 Biology Chapter 14, "Ecosystem" These solutions are designed to help you tackle the NCERT textbook questions with ease. We recommend going through the chapter's theory before diving into the solutions for a deeper understanding. Feel free to share these NCERT Solutions for Class 12 Biology with others—learning is always better when shared!
QUESTIONS FROM TEXTBOOK SOLVED
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————- .
Ans: (a) Autotrophs
(b) Spindle
(c) Sunlight
(d) Earthworm, bacteria & fungi of decay and vulture
(e) Oceans
2. Which one of the following has the largest population in a food chain?
(a) Producers
(b) Primary consumers
(c) Secondary consumers.
(d) Decomposers
Ans: (d)
3. The second trophic level in a lake is-
(a) Phytoplankton
(b) Zooplankton
(c) Benthos
(d) Fishes
Ans: (b)
4. Secondary producers are
(a) Herbivores
(b) Producers
(c) Carnivores
(d) None of the above
Ans: (d)
5. What is the percentage of photo synthetically act., radiation (PAR), in the incident solar radiation?
(a) 100%
(b) 50 %
(c) 1-5%
(d) 2-10%
Ans: (b)
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
Ans: (a) Grazing Food Chain (GFC) vs. Detritus Food Chain (DFC)
Grazing Food Chain (GFC):
Begins with producers (green plants), which are consumed by herbivores, and then carnivores.
It depends on the direct consumption of living plants or plant parts.
Energy flow is primarily from the sun through the producers.
Detritus Food Chain (DFC):
Begins with detritus (dead organic matter), which is consumed by decomposers (like bacteria, fungi, earthworms).
It involves the breakdown of dead material into simpler compounds.
Energy flow is based on the breakdown of dead organisms, independent of direct sunlight.
(b) Production vs. Decomposition
Production:
Refers to the creation of organic matter (biomass) by producers (plants) during photosynthesis.
It is measured in terms of primary productivity, which is the amount of biomass produced per unit area over time.
Decomposition:
Refers to the breakdown of organic matter into simpler inorganic substances by decomposers (bacteria, fungi).
It releases nutrients back into the ecosystem, making them available for producers.
(c) Upright Pyramid vs. Inverted Pyramid
Upright Pyramid:
In most ecosystems, biomass or energy decreases from producers to top carnivores.
For example, in a grassland ecosystem, there are many plants (producers) and fewer carnivores, forming an upright pyramid of biomass.
Inverted Pyramid:
In some ecosystems, such as an aquatic ecosystem, the pyramid of biomass can be inverted because the biomass of consumers (like fish) can exceed the biomass of producers (like phytoplankton).
The pyramid of numbers can also be inverted if there are fewer producers and a larger number of herbivores or carnivores.
(d) Food Chain vs. Food Web
Food Chain:
A linear sequence in which energy flows from one organism to another.
It consists of a single pathway, e.g., grass → grasshopper → frog → snake → hawk.
Food Web:
A network of interconnected food chains, showing multiple feeding relationships among organisms.
It represents how various organisms in an ecosystem are interdependent for food.
(e) Litter vs. Detritus
Litter:
Refers to freshly fallen leaves, twigs, bark, and other organic material on the surface of the soil.
It is the initial stage of organic material before it begins to decompose.
Detritus:
Refers to dead and decaying organic matter, including decomposed plant and animal material.
It is a later stage of organic matter, consisting of more broken-down material.
(f) Primary Productivity vs. Secondary Productivity
Primary Productivity:
Refers to the rate at which producers (like plants) convert solar energy into organic substances through photosynthesis.
It measures the amount of biomass produced by producers over time.
Secondary Productivity:
Refers to the rate at which consumers (herbivores, carnivores) produce biomass by consuming primary producers or other organisms.
It focuses on the energy flow through consumers.
These concepts are fundamental to understanding energy flow and nutrient cycling in ecosystems.
7. Describe the components of an ecosystem.
Ans: Ecosystem This can be defined 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 flow, decomposition, and productivity. There are many ecosystems such as ponds, forests, grasslands, etc.
Two components of an ecosystem:
Biotic component: Living part of an ecosystem consists of biotic factors, and it includes producers, consumers, decomposers, etc. Producers consist of plants and algae. They have the chlorophyll pigment, which enables them to perform the photosynthesis process in the presence of light. Thus, they are also called converters or transducers. These are heterotrophs consumers. Consumers obtain food either directly or indirectly from the producers. Heterotrophs are primary consumers but usually obtain food indirectly in secondary and tertiary consumers. Decomposers include micro-organisms such as bacteria and fungi. Decomposers represent the largest population in a food chain and break down dead remains of plants and animals for nutrient uptake.
Abiotic component: These are the non-living components of an ecosystem like light, temperature, water, soil, air, inorganic nutrients, etc.
8. Define’ecological pyramids and describe with examples, pyramids of number and biomass.
Ans: Ecological pyramids are graphical representations that depict the relationship between different trophic levels in an ecosystem. They show the number of organisms, biomass, or energy content at each level, moving from producers at the base to top carnivores at the apex. These pyramids can be of three types:
- Pyramid of Numbers
- Pyramid of Biomass
- Pyramid of Energy
- Producers (plants): In a grassland, there are numerous grasses that form the base of the pyramid.
- Primary consumers (herbivores): Fewer in number, such as grasshoppers, feed on the grasses.
- Secondary consumers (carnivores): Fewer still, like frogs, which eat grasshoppers.
- Tertiary consumers (top carnivores): Very few hawks or eagles that prey on the frogs.
- Shape: The pyramid is upright because the number of organisms decreases as you move to higher trophic levels.
- Producers (trees): Few large trees form the base of the pyramid.
- Primary consumers (herbivores): A larger number of insects feed on the trees.
- Secondary consumers (parasites): Even more parasites, such as lice or ticks, live on the insects.
- Tertiary consumers (hyperparasites): The highest number of organisms at the top, like bacteria feeding on the parasites.
- Shape: The pyramid is inverted because the number of organisms increases as you move to higher levels.
- Producers: In a forest, trees and other plants at the base have the largest biomass.
- Primary consumers: Herbivores like deer have less biomass than the trees they feed on.
- Secondary consumers: Carnivores like foxes have even less biomass than herbivores.
- Tertiary consumers: Top predators like tigers have the least biomass.
- Shape: The pyramid is upright as the biomass decreases from producers to top consumers.
- Producers: In a pond, phytoplankton (microscopic plants) form the base, but they have less biomass than the organisms at higher levels.
- Primary consumers: Zooplankton (microscopic animals) that eat phytoplankton have more biomass.
- Secondary consumers: Small fish eating zooplankton have even more biomass.
- Tertiary consumers: Large fish at the top of the food chain have the most biomass.
- Shape: The pyramid is inverted because the biomass of producers is lower than that of consumers.
9. What is primary productivity? Give brief description of factors that affect primary productivity.
Ans: It is defined as the quantity of organic matter or biomass produced by producers per unit area within a specified time. The primary productivity of an ecosystem depends upon the variety of environmental factors, such as light, temperature, water, precipitation, etc. It also depends upon the availability of nutrients and the availability of plants to carry out photosynthesis.
10. Define decomposition and describe the processes and products of decomposition.
Ans: Decomposition is the process by which decomposers break down the complex organic remains like dead plants, animal remains and excretions into inorganic substances like carbon dioxide, water and nutrients. The important steps in the process of decomposition are fragmentation, leaching, catabolism, humification and mineralisation. Detritivores, such as earthworm, break down detritus into smaller particles. This process is called fragmentation.
By the process of leaching, the inorganic water-soluble nutrients move downward into the soil horizon and get precipitated as unavailable salts .
Detritus gets broken into simpler inorganic substance through bacterial and fungal enzymes. The process is called catabolism.
All the above steps of decomposition act together to work on detritus. Humification and mineralisation occur during the process of decomposition in the soil.
Humification results in the accumulation of a dark coloured amorphous substance known as humus which is quite resistant to microbial action and it decomposes at a very low rate. It is colloidal in nature and hence it acts as a reservoir of nutrients.
The humus is further degraded by some microbes and release of inorganic nutrients occurs by a process known as mineralization.
11. Give an account of energy flow in an ecosystem.
Ans: The energy flow in an ecosystem is unidirectional. Since the ultimate source of energy is sun, this energy can be captured by the green plants and utilized in synthesizing their own food. The energy fixed by the green plants is transferred to herbivores that feed on them. The energy is then transferred to higher trophic levels such as carnivores. At every step, a major amount of energy is lost. The 10% law is one that only 10% of the total energy retained within a trophic level is passed on to the next trophic level of a food chain.
12. Write important features of a sedimentary cycle in an ecosystem.
Ans: The sedimentary cycles have their reservoirs in the Earth's crust or rocks. Nutrient elements can be found in the sediments of the Earth. Other elements having sedimentary cycles include sulphur, phosphorus, potassium, and calcium.
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.
13. Outline salient features of carbon cycling in an ecosystem.
Ans: The carbon cycle is one of the critical gaseous cycles which have 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 needed for life processes are constituted 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'. The glucose molecule is consumed by other living organisms. Therefore, carbon of the atmosphere gets absorbed in living forms. It is now essential that this absorbed carbon dioxide must be recycled back into the atmosphere so that it could form a cycle. There are different processes due to which carbon is recycled back into the atmosphere in the form of carbon dioxide gas. In the course of respiration, the glucose molecules are broken down to produce carbon dioxide gas. The process of decomposition also releases carbon dioxide from dead bodies of plants and animals into the atmosphere. Other important sources of carbon dioxide are combustion of fuels, industrialization, deforestation, volcanic eruptions, and forest fires.
