NCERT Solutions for Class 12 Biology Chapter 14: Ecosystem

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    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/Producers
    (b) Invested
    (c) Sunlight
    (d) Earthworm
    (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) Decomposers
    Explanation: Decomposers are the integral part of food chain and it includes many types of micro organisms such as fungi, bacteria. Thus the decomposers are the largest population in food chain.

    3. The second trophic level in a lake is:
    (a) Phytoplankton
    (b) Zooplankton
    (c) Benthos
    (d) Fishes

    Ans. (b) Zooplankton
    Explanation: Zooplankton is the primary consumer the second tropic level in the aquatic food chain that feeds upon phytoplankton.

    4. Secondary producers are
    (a) Herbivores
    (b) Producers
    (c) Carnivores
    (d) None of the above

    Ans. (d) None of the above
    Explanation: Plants are only the producers. Thus they are called primary producers. There are no other producers in food chain.

    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%

    Ans. (b) 50%
    Explanation: Around 50% of total radiation from sunlight falls under photosynthetically. 

    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 Detritus food chain
    1. In this food chain, energy is obtained from sunlight.
    In this food chain, energy is obtained from dead/decay organic matter.
    1. It starts with the plants, producers present at the first trophic level. The plant Biomass is eaten by the herbivorous, which are consumed by carnivores.
      Ex: Grass → Frog → Snake → Eagle
    It starts with decay or death bodies, which are eaten by decompose. These decomposers are in turn consumed by their predators.
    Ex: Dead organic wastes → micro-organic → detritivore organisms.


    Production Decomposition
    1. It is a process of production of organic biomass through photosynthesis.
    It is a process of breaking down of complex organic matter from the dead bodies of animal and plants with the help of decomposers into simpler organic compound such as CO2 and H2O.
    1. It fixes energy
    It releases energy and nutrient.


    Upright pyramid Inverted pyramid
    1. Pyramid is upright when the number of produces or their biomass is maximum in food chain.
    Pyramid is inverted when the number of produces or their biomass in minimum and increase in each trophic level in food chain.
    1. The pyramid of energy is always upright.
    The pyramid of energy is always inverted.


    Food chain Food web
    1. It is a linear sequence of organisms where nutrients and energy is transferred from one organisms to the other.
    Several interconnected food chain from a food web.
    1. Only number of one tropic level complete for some food.
      Ex: Grass → Grasshopper → mice → Snake → Eagle
    Members of different species complete for different food. Ex:


    Litter Detritus
    1. It contains all kinds of wastes generated.
    It contain only dead bodies of plants and animals.
    1. It contains both biodegradable and non-biodegradable matter.
    It contains only biodegradable matter.


    Primary Productivity Secondary Productivity
    1. It is defined as the amount of organic matter produced by producers per unit area over a period of time.
    It is defined as rate of production of organic matter by consumers over a period of time.
    1. It is comparatively larger.
    It is comparatively smaller.

    7. Describe the components of an ecosystem.

    Ans. Their are two components of an ecosystem are as follows :

    (i) Abiotic components or non living components: These include inorganic substances or minerals (standing state or standing quality), organic substances and different climatic conditions like temperature, pH, light, etc.

    (ii) Biotic components or living components: Biotic components can be classified into there categories:

    (a) Autotrophs or producers: Which have capacity to manufacture their own food or which can fix radiant energy of sun into chemical energy, e.g., green plants and green algae.

    (b) Heterotrophs or consumers: Which are unable to manufacture their own food and depend upon other organisms for their food. These are of following types:

    • Primary consumers or herbivores which depend upon producers or green plants for their food.
    • Secondary consumers or carnivores which live upon herbivores.
    • Omnivores which live upon secondary consumers.

    (c) Decomposers or microconsumers: Decompose dead organic substances of producers and consumers into simple substances and thus continue mineral cycles, e.g., bacteria, fungi, actinomycetes etc.

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

    Ans. Ecological pyramid: Ecological pyramid is a graphical method to show the number of organisms or biomass or amount of energy present at different trophic levels.
    Pyramid of number: Number of individuals at each trophic level is shown in pyramid. Example:
    Oak plant → caterpillar → blue tit → sparrow hawk.
    A caterpillar feeds on many oak leaves, the blue tit feeds on many caterpillars and finally sparrow hawk feeds on many blue tit. Hence, the number of organisms decreases in each trophic level.
    Pyramid of biomass: Pyramid of biomass is graphic representation of amount of biomass per unit area sequence wise in rising trophic levels with producers at the base and top carnivores at the apex. The total biomass of organisms at each stage of food chain decreases as the number of organisms decreases at each trophic level.

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

    Ans. Primary productivity: It is defined as amount of organic matter or biomass produced by producers per unit area over a period of time.
    (i) Plant species inhabiting a particular area
    (ii) Sunlight
    (iii) Temperature
    (iv) Soil water
    (v) Nutrients
    lit deserts, sunlight is abundant but water is scarce or nutrients are lacking. Therefore, in such areas, water and nutrients supply become the limiting factors.

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

    Ans. The process by which decomposers break down complex organic remains (dead plants, animal remains and excretions) into inorganic substances like carbon dioxide, water and nutrients is called decomposition. The important steps in the process of decomposition are fragmentation, leaching, catabolism, humification and mineralisation. Detritivores (e.g., earthworm) break down detritus into smaller particles. This process is called fragmentation.
    Leaching: By the process of leaching, water-soluble inorganic nutrients go down into the soil horizon and get precipitated as unavailable salts.
    Catabolism: Bacterial and fungal enzymes degrade detritus into simpler inorganic substances. The process is called as catabolism.
    Decomposition: All the above steps in decomposition operate simultaneously on the detritus. Humification and mineralisation occur during decomposition in the soil.
    Humification: Humification leads to accumulation of a dark coloured amorphous substance called humus that is highly resistant to microbial action and undergoes decomposition at an extremely slow rate. Being colloidal in nature it serves as a reservoir of nutrients.
    The humus is further degraded by some microbes and release of inorganic nutrients occur by the process known as mineralisation.

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

    Ans. Flow of energy in an ecosystem is unidirectional. The ultimate source of energy is sun. The solar energy is captured by the green plants which utilize it in synthesizing their own food by photosynthesis. The energy fixed by the green plants is transferred to herbivores which feed on them. The energy is then transferred to higher trophic levels (carnivores). At every step, considerable amount of energy is lost. According to 10% law, only 10% of total energy stored in a trophic level is transferred to the next trophic level of a food chain.

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

    Ans. Sedimentary cycle is the cycle of minerals found in earth crust e.g., phosphorus, sulphur etc.
    Important features of sedimentary cycle in an ecosystem:
    (i) It is the major reservoir for nutrients elements in the lithosphere and the elements are released by weathering.
    (ii) It is very slow and take more time.
    Example: Phosphorus cycle, sulphur cycle, lodine cycle.

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

    Ans. Carbon is an important constituent of living matter. Green plants take it in the form of CO2 from atmosphere and fix it as carbohydrates. Carbon which is also present in proteins, fats etc. is transferred to the organisms of other trophic levels. Apart from being released in atmosphere as CO2 during respiration, carbon is also released in atmosphere through burning of wood, fossil fuel and decomposition of organic matter by microbes.

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