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NCERT Class 12 Biology Solutions: Chapter 14 – Ecosystem

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) Plants are called as autotrophs because they fix carbon dioxide. 

 

(b) In an ecosystem dominated by trees, the pyramid (of numbers) is of inverted type. 

(c) In aquatic ecosystems, the limiting factor for productivity is light

(d) Common detritivores in our ecosystem are earthworms.

(e) A major reservoir of carbon on Earth is oceans.

Which one of the following has the largest population in a food chain?

(a) Producers                                          (b) Primary consumers 

(c) Secondary consumers                       (d) Decomposers

Solution: (a) Producers has the largest population in a food chain.

 

The second trophic level in a lake is

(a) Phytoplankton                      (b) Zooplankton 

(c) Benthos                                (d) Fishes

Solution: (b) Zooplankton are present at the second trophic level in a lake.

 

Secondary producers are

(a) Herbivores                (b) Producers 

(c) Carnivores                (d) None of the above

Solution: (c) None of the above 

Plants are the only producers.

 

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% 

Out of total incident solar radiation, about fifty percent of it forms photosynthetically active radiation or PAR.

 

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) Grazing food chain and detritus food chain

 

 S no: Grazing food chain   S no: Detritus food chain
1. In this food chain, sun is the source of energy. 1. In this food chain, energy comes from organic matter (or detritus) generated in trophic levels of the grazing food chain.
2. It begins with producers, present at the first trophic level. The plant biomass is then eaten by herbivores, which in turn are consumed by carnivores. 2. It begins with detritus such as dead bodies of animals or fallen leaves, which are then eaten by decomposers or detritivores. These detritivores are in turn consumed by their predators.
3. This food chain is usually large. 3. It is usually smaller as compared to the grazing food chain.

 

(b) Production and decomposition

 S no:  Production   S no: Decomposition
1. It is the rate of producing organic matter (food) by producers. 1. It is the process of breaking down of complex organic matter or biomass into organic raw material such as CO2, H2O, and other nutrients by decomposers.
2. It depends on the photosynthetic capacity of the producers. 2. It occurs with the help of decomposers.
3. Sunlight is required by plants for primary production. 3. Sunlight is not required for decomposition by decomposers

 

(c) Upright and inverted pyramid

 S no: Upright pyramid   S no:  Inverted pyramid
1. The pyramid of energy is always upright. 1. The pyramid of biomass and the pyramid of numbers can be inverted.
2. In the upright pyramid, the number and biomass of organisms in the producer level of an ecosystem is the highest, which keeps decreasing at each trophic level in a food chain. 2. In an inverted pyramid, the number and biomass of organisms in the producer level of an ecosystem are the lowest, which keeps increasing at each trophic level.

 

(d) Food chain and Food web

 S no:  Food chain   S no: Food web
1. It is a single linear sequence of organisms showing who eats whom. 1. It contains a number of interconnected food chains which appears like a web.
2. Members present at higher trophic levels feed on single types of organisms. 2. One organism has alternate food sources.

 

(e) Litter and detritus

 S no:   Litter   S no: Detritus
1. Litter contains all kinds of wastes generated above the ground. 1. Detritus is composed of the remains of dead plants and animals.
2. Litter contains both biodegradable and non-biodegradable matter. 2. Detritus contains only biodegradable matter.

 

(f) Primary and secondary productivity

 S no: Primary productivity   S no: Secondary productivity
1. It is defined as the amount of organic matter produced by producers per unit area over a period of time. 1. It is defined as the rate of production of organic matter by consumers over a period of time.

 

Describe the components of an ecosystem.

Solution: The two components of an ecosystem are: 

  • 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 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 break down the remains of dead plants and animals into simpler ones. 

  • Abiotic component: They are the non-living component of an ecosystem such as light, temperature, water, soil, air, inorganic nutrients, etc.

 

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

Solution:

Definition: 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. In ecological pyramid producers are at the base, while the apex represents the top level consumers.

There are three types of pyramids: 

  • Pyramid of numbers
  • Pyramid of energy  
  • Pyramid of biomass

Pyramid of numbers:

  • It is a graphical representation of the number of individuals present at each trophic level in a food chain of an ecosystem.
  • It can be upright or inverted.
  • 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).

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

 

 

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.
  • It is inverted in a pond ecosystem as the biomass of fishes far exceeds the biomass of zooplankton (upon which they feed).

 

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

Solution:

Primary productivity is defined 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 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.

Define decomposition and describe the processes and products of decomposition.

Solution:

Decomposition is the process that involves the breakdown of complex organic matter or biomass into inorganic raw materials such as carbon dioxide, water, and other nutrients from the body of dead plants and animals with the help of decomposers.

The various processes involved in decomposition are as follows:

  • Fragmentation: It is the first step in the process of decomposition. It involves the breakdown of detritus into smaller pieces by the action of detritivores such as earthworms.
  • Leaching: It is a process where the water soluble nutrients go down into the soil layers and get locked as unavailable salts.
  • Catabolism: It is a process in which bacteria and fungi degrade detritus through various enzymes into smaller pieces.
  • Humification: The next step is humification which leads to the formation of a dark coloured colloidal substance called humus, which acts as reservoir of nutrients for plants.
  • Mineralization: The humus is further degraded by the action of microbes, which finally leads to the release of inorganic nutrients into the soil.

Give an account of energy flow in an ecosystem.

Solution:

Energy enters an ecosystem from the Sun. These radiations help plants in carrying out the process of photosynthesis. Only 2-10 percent of solar energy is captured by green plants (producers) during photosynthesis to be converted into food. 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 transferred to carnivores. This is known as ten percent law of energy flow.

 

Write important features of a sedimentary cycle in an ecosystem.

Solution:

  • 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.
  • These cycles are very slow.
  • These 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.

Outline salient features of carbon cycling in an ecosystem.

Solution:

  • The carbon cycle is an important gaseous cycle which has its reservoir pool in the atmosphere.
  • Carbon is incorporated into living forms through a fundamental process called ‘photosynthesis’. The glucose molecule produced during photosynthesis is utilized by other living organisms. Thus, atmospheric carbon is incorporated in living forms
  • 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 fires act as other major sources of carbon dioxide

A detailed process of carbon cycle is shown below:

 

Related: NCERT Class 12 Biology solutions – Chapter 13 – Organism and Population

 

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