NCERT Solutions for Class 12 Biology Chapter 14 Ecosystem

NCERT Solutions for Class 12 Biology Chapter 14 Ecosystem

These Solutions are part of NCERT Solutions for Class 12 Biology. Here we have given NCERT Solutions for Class 12 Biology Chapter 14 Ecosystem

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 ecosystem, 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) producers
(b) inverted or spindle
(c) light
(d) saprotrophs
(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) Decomposer’s
Solution:
(d) decomposer’s

Question 3.
The second trophic level in a lake is
(a) phytoplankton
(b) zooplankton
(c) benthos
(d) fishes.
Solution:
(b) zooplankton

Question 4.
Secondary producers are
(a) herbivores
(b) producers
(c) carnivores
(d) none of these
Solution:
(a) herbivores

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

Question 6.
Distinguish between
(a) Grazing food chain and detritus food chain
(b) Upright and inverted pyramid
(c) Litter and detritus
(d) Production and decomposition
(e) Food chain and food web
(f) Primary and secondary productivity
Solution:
(a) Differences between grazing food chain and detritus food chain are as follows

(b) Differences between upright and inverted pyramids are as follows :

(c) Differences between litter and detritus are as follows :

(d) Differences between production and decomposition are as follows :

(e) Differences between food chain and food web are as follows:

(f) Differences between primary productivity and secondary productivity are as follows :

Question 7.
Describe the components of an ecosystem.
Solution:
The components of an ecosystem can be divided into two categories: biotic and abiotic.
(i) Biotic components:
(i) Producers – Green plants which can synthesize their own food.
(ii) Consumers – They do not synthesize their food. They may be
-Primary consumer/herbivores – consuming plants as food.
-Secondary and tertiary consumers or carnivores – They feed on either herbivores or carnivores.
(iii) Decomposers – These organisms breakdown the dead bodies or waste products of plants and animals into simpler inorganic compounds.
(2) Abiotic components:
(i) Climatic components
•Light
•Temperature
•Wind
•Atmospheric gases
•Rain
•Atmospheric humidity
(ii) Soil factors
•Organic materials
•Minerals
•Soil, water
•Soil air
(iii) Topographic factors
•Altitude
•Direction and steepness slope

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, like number of individuals present in various trophic levels of a food chain with producers forming the base and top carnivores the tip. Ecological pyramids were developed by

Charles Elton (1927) and are, therefore, also called Eltonian pyramids.

There are three types of ecological pyramids, namely,

• Pyramid of numbers
• Pyramid of biomass
• 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 maximum in number. This number then shows a decrease towards 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 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, different trophic level of the ecosystem are arranged according to the biomass of the organisms. In grassland and forest, there is generally a gradual decrease in biomass of organisms at successive levels from the producers to the top carnivores. Thus these pyramids are upright. But in pond ecosystem, it is inverted because the biomass gradually increases from the producers to carnivores.

Question 9.
What is primary productivity? Give brief description of factors that affect primary productivity.
Solution:
Primary productivity is the amount of biomass or organic matter produced per unit area over a time period by plants during photosynthesis. It is expressed in terms of weight (g/m²/yr) or energy (kcal/m²/yr). It is of two types : gross primary productivity and net primary productivity. Gross primary productivity of an ecosystem is the rate of production for organic matter during photosynthesis. Gross primary productivity minus respiration losses (R), is the net primary productivity (NPP).

Factors affecting primary productivity are as follows:

(i) Solar radiation : Maximum light is available in tropics. Poles receive minimum light. Due to this, photosynthesis is maximum and net primary productivity (NPP) is highest (> 20 t ha^-1 year^-1) in tropics against (8 t ha^-1 year^-1) in temperate forests.
(ii) Temperature : Temperate forests have lesser productivity (about 8 t ha^-1 year^-1) than tropical rain forests (20 t ha^-1 year^-1) due to cold climate.
(iii) Moisture : Rain and humidity increase productivity of the ecosystem.
(iv) Nutrients : Nutrients are essential for producers growth. Desert soils are deficient in nutrients and therefore, are less productive.
(v) Photosynthetic efficiency of producers: C₄ plants are more productive than C₃ plants.

Question 10.
Define decomposition and describe the processes and products of decomposition.
Solution:
Decomposition is the breakdown of dead or waste organic matter by micro-organisms. Decomposition is both physical and chemical in nature.Process involved in decomposition are – fragmentation, catabolism & leaching.
•Fragmentation – The process primarily due to the action of detritus feeding invertebrate (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.
Give an account of energy flow in an ecosystem.
Solution:
Ecosystem require a constant input of energy as every component of an ecosystem is regularly dissipating energy.

Two laws of thermodynamics govern this flow of energy. According to first law of thermodynamics energy can be transferred as well as transformed but is neither created nor destroyed. According to second law of thermodynamics every activity involving energy transformation is accompanied by dissipation of energy. Except for deep hydrothermal ecosystems, the source of energy in all ecosystems is solar energy. 50% of the solar energy incident over earth is present in PAR (photosynthetically active radiation).

Energy flow in an ecosystem is always unidirectional or one way, i.e., solar radiations —> producers —> herbivores —> carnivores. It cannot pass in the reverse direction. There is decrease in the content and flow of energy with the rise in trophic level. Only 10% of energy is transferred from one trophic level to the next.
Producer biomass (1000 K cal) —> Herbivore biomass (100 K cal) —> Carnivore I biomass (10 K cal) Carnivore II biomass (1 K cal)

Question 12.
Write important features of a sedimentary cycle in an ecosystem
Solution:
In sedimentary cycle materials involved in circulation between biotic and abiotic components of biosphere are non-gaseous and the lithosphere is the reservoir pool, e.g., phosphorus, calcium and magnesium. Sulphur has both sedimentary and gaseous phases. Sedimentary cycles are slow and less perfect as compared to gaseous cycles.

Question 13.
Outline salient features of carbon cycling in an ecosystem.
Solution:
Carbon constitutes 49 percent of dry weight of organisms and is next only to water. 71 percent carbon is found dissolved in oceans. This ocean reservoir regulates the amount of carbon dioxide in the atmosphere. Fossil fuels also represent a reservoir of carbon. Carbon cycling occurs through atmosphere, ocean and through living and dead organisms. 4 x 10¹³ kg of carbon is fixed in the biosphere through photosynthesis annually. A considerable amount of carbon returns to the atmosphere as Co₂ through respiratory activities of the producers and consumers. Decomposers also contribute substantially to Co₂ pool by their processing of waste materials and dead organic matter of land or oceans. Some amount of fixed carbon is lost to sediments and removed from circulation. Burning of wood, forest fire and combustion of organic matter, fossil fuels, volcanic activity are additional sources for releasing Co₂ in the atmosphere.

Human activities have significantly influenced the carbon cycle. Rapid deforestation and massive burning of fossil fuels for energy and transport have significantly increased the rate of release of carbon dioxide into the atmosphere.

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