Biology 12 NCERT Ecosystem Solutions – Chapter 14 Exercise

Q 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 _______.

Ans 1.

  1. Autotrophs
  2. Upright
  3. Light
  4. Bacteria, fungi of decay and earthworms
  5. Oceans

Q 2. Which one of the following has the largest population in a food chain?

  1. Producers
  2. Primary consumers
  3. Secondary consumers
Ans 2. Decomposers.

Q 3. Second trophic level in a lake is

  1. Phytoplankton
  2. Zooplankton
  3. Benthos
  4. Fishes
Ans 3. Zooplankton.

Q 4. Secondary producers are

  1. Herbivores
  2. Producers
  3. Carnivores
  4. None of these.
Ans 4. None of these.

Q 5. What is the percentage of photosynthetically active radiation (PAR), if incident solar radiation is considered 100%?

  1. 100%.
  2. 50%
  3. 1-5%
  4. 2-10%
Ans 5. 50%

Q 6. Distinguish between the following:

  1. Grazing food chain and detritus food chain
  2. Production and decomposition
  3. Upright and inverted pyramid
  4. Food chain and food web
  5. Litter and detritus
  6. Primary and secondary productivity

Ans 6.

  1. Grazing food chain: The common examples of grazing food chain are those of grassland ecosystem starting from Grasses-Herbivores-Carnivores (Primary, secondary, tertiary) and aquatic ecosystems starting from Phytoplanktons-Zooplanktons-small fishes-large fishes.

In grazing food chain (e.g., grassland ecosystem), green plants (producers) constitute the first step. The plant biomass, in turn, is eaten by the herbivores (e.g., insects, rats and mice, grazing cattle) which are subsequently consumed by a veriety of carnivors. The grazing food chains are directly dependent on an influx of solar radiations. Green plants, with the help of solar radiations, manufacture food (plant biomass) by the process of photosynthesis. These are longer food chains and these always end at decomposer level. From energy point of view, these grazing of food chains are very important.
Detritus food chain: Unlike grazing food chains, the detritus food chains depend upon the dead organic matter either in the form of fallen leaves etc. or dead bodies. Hence, these are not directly dependent on solar energy.
the starting point, in a detritus food chain, is the dead bodies of animals or fallen leaves etc. commonly termed as detritus. This dead organic matter is first eaten by decomposers, e.g., bacteria, fungi and protozoans etc. these decomposing organisms have saprotrophic mode of feeding and are designated as primary consumers in this type of chain. These are also termed as detritivores (detritus eating). The detritivores, in turn, are eaten by their predators like insect larvae, nematodes etc. The main function of the detritivores is to release the inorganic elements into the ecosystem by decomposing the dead organic matter. The recycling of inorganic elements in ecosystem helps in keeping them fit.

  1. Production: refers to the process of synthesis of organic compounds from inorganic substances such as carbon dioxide, water and minerals utilizing generally the sunlight. It is mainly done by plants.
    Decomposition, on the other hand, is the process of breaking down a substance into its constituent parts. It is brought about by reducer organisms (bacteria, fungi of decay).
  2. When the number of producer organism of their biomass is maximum in an ecosystem and these decrease progressively at each trophic level in food chain, we get upright pyramids. On the contrary, when number of individuals or their biomass at producer level is minimum and it increases progressively at each trophic level in a food chain, when we get inverted pyramid.
  3. Food Chain: The sequential inter-linking of organisms involving the transfer of food energy from the producers, through a series of organisms with repeated eating and being eaten is referred to as the food chain.
    Food web: Food web is a network of food chains which become inter-connected at various trophic level so as to form a number of feeding connections amongst different organism of a biotic community.
  4. dead remains of plants and animals constitute detritus. It is differentiated into litter fall (above ground detritus) and below ground detritus.

Litter, on the other hand, is above ground detritus. The dead remains of plants (leaves, bark, flowers etc.) and dead remains of animals, their faecal matter that fall on the surface of earth in terrestrial ecosystem is litter (above ground detritus).

  1. Primary productivity refers to the rate of production of biomass or organics matter per unit area over a time period by the producers in an ecosystem. On the other hand, secondary productivity refers to rate of production of biomass or organic over a time period by the consumer.
Q 7. Describe the components of an ecosystem.
Ans 7. Biotic Components: The living organisms present in an ecosystem form the biotic component. They are interconnected through food. Food is a substance which provides materials for building, growth and repair of the body, and energy to sustain life. Thus, matter and energy are transferred in the living world as food. Regarding the mode of obtaining food, the organisms occurring in an ecosystem are classified into three main categories. Plants, animals and microorganisms (bacteria and fungi). These are respectively called producers, consumers and reducers or decomposers.
Abiotic Components: these include the non-living Physico-chemical factor of the environment. These components not only affect the distribution and structure of organisms but also their behavior and inter-relationships. Abiotic factors include-
i. Inorganic substances
ii. Organic compounds
iii. Climatic factors and edaphic factors.
Q 8. Define ecological pyramids and describe with examples pyramids of numbers and biomass.
Ans 8. Ecological Pyramid: An ecological pyramid may be defined as a graphic representation of the numbers or biomass or status of accumulated energy at different trophic levels in a food chain in an ecosystem.
food web
Pyramid of Numbers: Pyramids of numbers is the graphic representation depicting the arrangement of number of individuals of different trophic levels in a food chain in an ecosystem.ecosystem pyramid of numbers

Pyramid of Biomass: Pyramid of biomass is a graphic representation of biomass (total amount of living or organic matter in an ecosystem at any time) present per unit area in different trophic levels.
pyramid of biomass

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

Ans 9. Primary productivity refers to the rate of production of biomass or organic matter per unit area over a time period by the producers in an ecosystem.

Various factor which affect primary productivity include light, temperature, water, nutrients etc. in deserts, sunlight is abundant but water is scarce or nutrients are lacking. Therefore, in such areas, water in nutrient supply become the limiting factor. Productivity increases from polar regions towards the tropics because of the increasing sunlight and temperature.

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

Ans 10. Decomposition is the process of breaking down a substance into its constituent parts It is a complex, enzymatic process that involves stepwise degradation of detritus.

It involves three steps: Fragmentation of detritus, leaching and catabolism involving humification and mineralization. Humification results in formation of humus in soil. Mineralization results in the release of inorganic substances (CO2, H2O) and nutrients in the soil.

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

Ans 11. Each organism needs energy for carrying on vital activities and for building up and repairing the body tissues. The ultimate source of entire energy used by living things is sunlight. Solar energy received by an ecosystem depends on the latitude, slope, cloud cover, air pollutions, etc. only about 1% of the light energy striking the green parts of the plants is changed by photosynthesis into the potential energy of the organic substances, the rest dissipates as heat. The loss of energy is not due to the inefficiency of the photosynthetic mechanism of the plants but to the operation of the second law of thermodynamics. This law states that “whenever energy is transformed from one form into another, there is a decrease in the amount of useful energy; some energy is degraded into heat and dissipated.”

energy flow in an ecosystem

Q 12. Write important features of a sedimentary cycle in an ecosystem.
Ans 12. In the sedimentary cycles, the reservoir for the nutrient elements is in the sediments of the earth. Elements such as phosphorus, Sulphur, potassium and calcium have sedimentary cycles. These cycles are slow and less perfect systems in that the elements may get locked in the reservoir pool and go out of circulation for long periods.
Q 13. Outline salient features of carbon cycling in an ecosystem.

Ans 13. Salient features of carbon cycling are as follows

  1. Carbon cycling occurs through atmosphere, ocean and through the living and dead organisms.
  2. Most of the carbon is fixed by plants during the process of photosynthesis and returns to the atmosphere in the form of CO2 during respiration.
  3. Burning of wood, forest fire and combustion of organic matter, fossil fuel, and volcanic activity are some other sources of releasing CO2 in the atmosphere.

carbon cycle

Updated: October 6, 2020 — 12:15 pm

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