NCERT Solutions for class 12th Biology Chapter 14 Ecosystem

Exercise

Question 1. Fill in the blanks.

  1. Plants are called as         because they fix carbon dioxide.
  2. In an ecosystem dominated by trees, the pyramid (of numbers) is            type.
  3. In aquatic ecosystems, the limiting factor for the productivity is  __
  4. Common detritivores in our ecosystem are  ___
  5. The major reservoir of carbon on earth is _____

Sol.

  1. autotrophs (or producers)
  2. inverted
  3. sunlight
  4. earthworm, bacteria and fungi
  5. 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

Sol. (d) Decomposers are those organisms, especially a soil bacterium, fungus, or invertebrate that decomposes organic material. They form the largest population in a food chain because they are present at the top trophic level of the food chain and break down dead organic material to water, carbon dioxide, minerals and other simple chemicals which can again be consumed by green plants. Dead organic material occurs in form of leaf litter, dead wood, animal carcasses and faeces.

Note: If decomposers weren’t in the ecosystem,the plants would not get essential nutrients, and dead matter and waste would pile up.

Question 3. The second trophic level in a lake is:

(a) Phytoplankton                          (b) Zooplankton

(c)  Benthos                                       (d) Fishes

Sol. (b) Primary producers such as phytoplankton and algae form the lowest trophic level of food chain in a lake. These primary producers are eaten byprimary consumers such as zooplanktons. Primary consumers occupy the second trophic level and are eaten by secondary consumers such as small fish. Tertiary consumers such as large fish eat the secondary consumers and are eaten by top level predators which include fish-eating birds.

Question 4. Secondary producers are:

(a) Herbivores                                (b) Producers

(c)  Carnivores                                 (d) None of these

Sol. (a) A secondary producer is a herbivore, an animal that eats plant matter and, in turn, is food for a predator. Although herbivores are consumers, they are a source of food for carnivores. Since they eat plants, they are eating the primary producers, making them primary consumers. But when carnivores eat them, they turn into secondary producers because they are producing the energy for the meat – eaters since they ate the plants first. So this means that technically, secondary producers are also primary consumers.

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%

Sol. (b) Photosynthetically active radiation (PAR) is light of wavelength between 400 and 700 nm and is the portion of the light spectrum utilised by plants for photosynthesis. Of the incident solar radiation, less than 50 per cent of it is photosynthetically active radiation (PAR).

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

Sol. (a) Grazing food chain and detritus food chain

Question 7. Describe the components of an ecosystem.

Sol. Ecosystems represent the interconnected nature of living organisms and their world. The components of an ecosystem can be divided into two categories : biotic and abiotic.

  • Biotic components: These are the living components of an ecosystem that have a direct or indirect influence on other organisms in an environment. The life forms of an ecosystem help in the transfer and cycle of energy. They are grouped in terms of the means they use to get energy. Based on the source of their energy, biotic components can be further classified into the following:
  • Producers: They can synthesise their own food, e.g., green plants.
  • Consumers: They do not synthesise their food. They depends on others for their food and may be classified into the following:
    • Primary consumer/herbivores: They feed on plants, e.g., insects, rats, squirrels, etc.
    • Secondary consumers or primary carnivores : Secondary consumers are those organisms that eat primary consumers for energy. Primary consumers are always herbivores, or organisms that only eat autotrophic plants. However, secondary consumers can either be carnivores or omnivores. E.g., frogs, birds, etc.
    • Tertiary consumers or large carnivores: They feed on secondary consumers such as owl, fox, snakes, etc.
    • Quaternary consumers: They are at the top of the food chain and eats tertiary consumers. They are also known as apex predators.
  • Decomposers: These organisms breakdown the dead bodies or waste products of plants and animals into simpler inorganic compounds. E.g., bacteria, fungi, earthworms.
  • Abiotic components: These are the non-living or physical and chemical factors of an ecosystem. These are classified into following three categories:
    • Physical climatic factors: It includes light, temperature, wind, rain.
    • Inorganic compounds: Like water, minerals and atmospheric gases, etc.
    • Organic compounds: These includes organic substance present in the dead bodies of plant & animals.

Question 8. Define ecological pyramids and describe with examples, pyramids of number and biomass.

Sol. Ecological pyramid is a graphical representation showing relationship among organisms of a food chain occupying different trophic levels expressed in terms of number, biomass or energy. In a pyramid, the base represents the producers or the first trophic level while the apex represents tertiary or top level consumer. Depending on the parameter used to represent an ecological pyramid, three types of pyramids are, pyramid of energy, pyramid of biomass and pyramid of number.

Pyramid of number: In this pyramid, the number of individuals of species of a food chain occupying different trophic levels is expressed. For example, the number of individuals in a food chain on grassland ecosystem is shown in the following pyramid of number:

From the pyramid, it is observed that as we move up the levels of the pyramid, the number of individuals in each level decreases. The producers form the largest number and hence are at the bottom of the pyramid. Such a pyramid of number is upright.

Inverted pyramid of number is obtained when the number of top carnivores is

greater than the number of producers. For example, food chain in a forest ecosystem represented below results in an inverted pyramid of number.

Tree —> Insects—>  Small birds—>  Large birds

Pyramid of biomass: In this pyramid, amount of biomass (total weight) present in a unit area of various trophic levels is expressed. For example, biomass of individuals of different trophic levels in a food chain on grassland ecosystem is shown in the following pyramid of biomass:

This pyramid indicates decrease of biomass in each tropical level from base to apex. The pyramid of biomass in sea is inverted because the biomass of fishes (primary consumer) far exceeds that of phytoplankton (primary producer).

Note: The concept of ecological pyramid was first introduced by Charles Elton, British Ecologist thus, ecological pyramids are also called Eltonian pyramids.

Question 9. What is primary productivity? Give brief description of factors that affect primary productivity.

Sol. Primary productivity is the rate of capture of solar energy or biomass production of the producers. It is expressed in terms of gm-2 yr-1 or (kcal m-2) yr-1 to compare the productivity of different ecosystems. It is divided into two types: gross primary productivity (GPP) and net primary productivity (NPP). GPP is the rate of capture of solar energy or total production of organic matter and NPP is the remaining biomass or the energy left after utilisation of producers.

Factors affecting primary productivity are:

  • Availability of nutrients which varies in different types of ecosystem.
  • Photosynthetic capacity of plants.
  • The plant species inhabiting a particular area.
  • Environmental factor.

Question 10. Define decomposition and describe the processes and products of decomposition.

Sol.

  • Decomposition is the process of breakdown of dead or waste complex organic matter into inorganic substances such as carbon dioxide, water and nutrients by decomposers. Decomposition is both physical and chemical in nature.
  • Processes involved in decomposition are as follows:
    • Fragmentation: It is the first process during which detritivores such as earthworm break down detritus into smaller particles. Due to this process, the surface area of detritus is greatly increased.
    • Leaching: During this process, water-soluble inorganic nutrients present in fragmented detritus go down into the soil horizon along with percolating water and get precipitated as unavailable salts.
    • Catabolism: In this process, bacteria and fungi secrete extracellular enzymes that degrade detritus into simpler inorganic substances.
    • Humification: It is the process of accumulation of a dark coloured amorphous substance called humus which serves as a reservoir of nutrients. It is highly resistant to microbial action and undergoes decomposition at an extremely slow rate.
    • Mineralisation: During this process, inorganic nutrients are released in the soil by further degradation of humus by some microbes.
  • Products of decomposition are water, carbon dioxide and nutrients such as nitrogen, phosphorous, calcium, potassium, sulphur, etc.

Question 11. Give an account of energy flow in an ecosystem.

Sol. Energy flow refers to the flow of energy in an ecosystem through a series of organisms of a food chain or web. It flows through different organisms in the following manner:

  • Sun is the only source of energy for all ecosystems on Earth. Solar energy enters the ecosystem through the process of photosynthesis. Plants and photosynthetic bacteria (autotrophs) called primary producers capture Sun’s radiant energy to make food from simple inorganic materials. All animals depend on plants for their food needs directly or indirectly.
  • No energy that is trapped into an organism remains in it forever. The energy trapped by the producer, hence, is either passed on to a consumer through the beginning of grazing food chain or on to decomposers in case of death of the plant thus, beginning a detritus food chain or web.
  • The energy stored in plants is passed on to the primary consumers (herbivores) in the food chain when they consume the plants as food. When these herbivores are consumed by carnivores of the first order (secondary consumers), energy is transfer occurs from herbivores to carnivores. Finally, when tertiary consumers consume the carnivores, again energy will be transferred. Thus, there is unidirectional flow of energy from the sun to producers and then to consumers.
  • Based on the source of their nutrition or food, organisms occupy a specific place in the food chain called trophic level. Producers belong to the first trophic level, herbivores (primary consumer) to the second and carnivores (secondary consumer) to the third.
  • The transfer of energy follows 10 per cent law according to which only 10 per cent of the energy is transferred to each trophic level from the lower trophic level i.e., the amount of energy decreases at successive trophic levels due to loss of energy in the form of heat.

Note: The Ten percent law for the transfer of energy from one trophic level to the next was introduced by Lindemann (1942). According to this law, during the transfer of energy from organic food from one trophic level to the next, only about ten percent of energy from organic matter is stored as flesh. The remaining is lost during transfer, broken down in respiration, or lost to incomplete digestion by higher trophic levels.

Question 12. Write important features of a sedimentary cycle in an ecosystem.

Sol. Sedimentary cycle is the movement of non-gaseous nutrient elements such as phosphorous, calcium and sulphur through the various components of an ecosystem.

The important features of such cycles are as follows:

  • They have Earth’s crust as their reservoir.
  • These cycles consist of two phases, solution and rock phases. In the solution phase, minerals are released from Earth’s crust in the form of salts due to weathering and some of these salts dissolve in water, pass through a series of organisms, and ultimately reach the deep seas, where they move out of circulation indefinitely. In the rock phase, other salts deposit out as sediment and rock in shallow seas, eventually to be weathered and recycled.
  • These cycles are slower than gaseous cycles.
  • Such cycles are less perfect system because elements get trapped in the reservoir pool and go out of circulation for long periods.

Question 13. Outline salient features of carbon cycling in an ecosystem.

Sol. Carbon cycle is a gaseous biogeochemical cycle with its reservoir in ocean, atmosphere and fossil fuels.

Salient features of carbon cycle are:

  • It occurs through atmosphere, ocean and through living and dead organisms.
  • Producers use CO2 through photosynthesis in the presence of sunlight to form their food in the form of carbohydrate which is transferred to other organisms through food chains. Thus, it is fixed in the biosphere.
  • It returns to the atmosphere as CO2 through respiratory activities of the producers and consumers and processing of waste materials and dead organic matter of land or oceans by decomposers.
  • Some amount of the fixed carbon is lost to sediments and removed from circulation.
  • Burning of wood, forest fire and combustion of organic matter, fossil fuel and volcanic activity also release CO2 in the atmosphere.

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