NCERT Exemplar Problems Class 12 Biology Chapter 14 Ecosystem

Multiple Choice Questions
Single Correct Answer Type
1. Decomposers like fungi and bacteria are
i. Autotrophs ii. Heterotrophs
iii. Saprotrophs iv. Chemo-autptrophs
Choose the correct answer:
(a) i and iii (b) i and iv (c) ii and iii (d) i and ii
Answer. (c) Decomposers like fungi and bacteria are heterotrophs and saprotrophs.

2. The process of mineralisation by micro organisms helps in the release of
(a) Inorganic nutrients from humus
(b) Both organic and inorganic nutrients from detritus
(c) Organic nutrients from humus
(d) Inorganic nutrients from detritus and formation of humus.
Answer. (a)

• The process of mineralisation by microorganisms helps in the release of inorganic nutrients from humus.
• The humus is further degraded by some microbes and release of inorganic nutrients occur by the process called mineralisation.

3. Productivity is the rate of production of biomass expressed in terms of
i. (kcalm^-3 )yr^-1 ii. g^-2 y^-1 r
iii. g^-1yr^-1 iy. (kcal m^-2) yr^-1
(a) ii (b) iii (c) iiandiv (d) i and iii
Answer. (c) Productivity is the rate of production of biomass expressed in terms of g^-1yr^-1 and (heal m^-2) yr^-1.

4. An inverted pyramid of biomass can be found in which ecosystem?
(a) Forest (b) Marine (c) Grassland (d) Tundra
Answer. (b) In terrestrial ecosystem pyramid of biomass is generally upright. The pyramid of biomass in sea or lake is generally inverted because the biomass of fishes for exceeds that of phytoplankton.

5. Which of the following is not a producer?
(a) Spirogyra (b) Agaricus (c) Volvox (d) Nostoc
Answer. (b) Spirogyra, Volvox and Nostoc are producers. Agaricus is a fungus.

6. Which of the following ecosystems is most productive in terms of net primary . production?
(a) Deserts (b) Tropical rain forests
(c) Oceans (d) Estuaries
Answer. (b) Tropical rain forests ecosystems is most productive in terms of net primary production.

7. Pyramid of numbers is
(a) Always upright (b) Always inverted
(c) Ether upright or inverted (d) Neither upright nor inverted
Answer. (c) Pyramid of number in grassland or forest or pond ecosystem is upright. In parasitic food chain, the pyramid of number is inverted. In an ecosystem dominated by trees, the pyramid of number is spindle shaped/inverted.

8. Approximately how much of the solar energy that falls on the leaves of a plant is converted to chemical energy by photosynthesis?
(a) Less than 1% (b) 2-10% (c) 30% (d) 50%
Answer. (b) Approximately how much of the solar energy that falls on the leaves of a plant is converted to chemical energy by photosynthesis 2-10%.

9. Among the following, where do you think the process of decomposition would be the fastest?
(a) Tropical rain forest (b) Antarctic
(c) Dry arid region (d) Alpine region
Answer. (a)

• Warm and moist environment favour decomposition. Decomposition is largely an oxygen requiring process. Low temperature and anaerobiosis inhibit decomposition resulting in build up of organic materials.
• The process of decomposition would be the fastest is tropical rain forest.

10. How much of the net primary productivity of a terrestrial ecosystem is eaten and digested by herbivores?
(a) 1% (b) 10% (c) 40% (d) 90%
Answer. (b) 10% of the net primary productivity of a terrestrial ecosystem is eaten and digested by herbivores.

11. During the process of ecological .succession the changes that take place in communities are
(a) Orderly and sequential
(b) Random
(c) Very quick
(d) Not influenced by the physical environment
Answer. (a) The gradual and fairly predictable change in the species composition of a given area is called ecological succession. An important characteristic of all the communities is that their composition and structure constantly change in response to the changing environmental conditions. These changes are orderly and sequential parallel with the changes in the physical environment.

12. Climax community is in a state of
(a) Non-equilibrium (b) Equilibrium
(c) Disorder (d) Constant change
Answer. (b) Climax community is in a state of equilibrium.

13. Among the following bio-geo-chemical cycles which one does not have losses due to respiration?
(a) Phosphorus (b) Nitrogen
(c) Sulphur (d) All of the above
Answer. (d) Phosphorus, nitrogen and sulphur bio-geo-chemical cycles does not have losses due to respiration.

14. The sequence of communities of primary succession in water is
(a) Phytoplankton, sedges, free-floating hydrophytes, rooted hydrophytes, grasses and trees
(b) Phytoplankton, free-floating hydrophytes, rooted hydrophytes, sedges, grasses and trees
(c) Free-floating hydrophytes, sedges, phytoplankton, rooted hydrophytes, grasses and trees
(d) Phytoplankton, rooted submerged hydrophytes, floating hydrophytes, reed swamp, sedges, meadow and trees
Answer. (b) The sequence of communities of primary succession in water is phytoplankton, free-floating hydrophytes, rooted hydrophytes, sedges, grasses and trees

15. The reservoir for the gaseous type of bio-geo chemical cycle exists in
(a) Stratosphere (b) Atmosphere (c) Ionosphere (d) Lithosphere
Answer. (b) The reservoir for the gaseous- type of bio-geo-chemical cycle exists in atmosphere.

16. Which of the following type of ecosystem is expected in an area where evaporation exceeds precipitation, and mean annual rainfall is below 100 mm
(a) Grassland (b) Shrubby forest
(c) Desert (d) Mangrove
Answer. (c) Desert ecosystem is expected in an area where evaporation exceeds precipitation, and mean annual rainfall is below 100 mm.

17. The zone at the edge of a. lake or ocean which is alternatively exposed to air and immersed in water is called
(a) Pelagic zone (b) Benthic zone
(c) Lentic one (d) Littoral zone
Answer. (d) The zone at the edge of a lake or ocean which is alternatively exposed to air and immersed in water is called littoral zone.

18. If the carbon atoms fixed by producers already have passed through three species, the trophic level of the last species would be
(a) Scavenger (b) Tertiary producer
(c) Tertiary consumer (d) Secondary consumer
Answer. (c)

If the carbon atoms fixed by producers already have passed through three species, the trophic level of the last species would be tertiary consumer.

19. Edaphic factor refers to
(a) Water (b) Soil
(c) Relative humidity (d) Altitude
Answer. (b) Edaphic factor refers to soil.

20. Which of the following is <an ecosystem service provided by a natural ecosystem?
(a) Cycling of nutrients
(b) Prevention of soil erosion
(c) Pollutant absorption and reduction of the threat of global warming
(d) All of the above
Answer. (d) Cycling of nutrients, prevention of soil erosion and pollutant absorption and reduction of the threat of global warming is an ecosystem service provided by a natural ecosystem.

Very Short Answer Type Questions
1. Name an organism found as secondary carnivore in an aquatic ecosystem.
Answer. Catfish/water snake etc.

2. What does the base tier of the ecological pyramid represent?
Answer. Producers

3. Under what conditions would a particular stage in the process of succession revert back to an earlier stage?
Answer. Natural or human induced disturbances like fire, deforestation etc.

4. Arrange the following as observed in vertical stratification of a forest: Grass, Shrubby plants, Teak, Amaranths
Answer. Grass, Amaranths, Shrubby plants, Teak

5. Name an omnivore which occurs in both grazing food chain and the decomposer food chain.
Answer. Sparrow/crow

6. Justify the pitcher plant as a producer.
Answer. It is chlorophyllous and is thus capable of photosynthesis.

7. Name any two organisms which can occupy more than one trophic level in an ecosystem.
Answer. Man and sparrow ete.

8. In the North East region of India, during the process of Jhum cultivation, forests are cleared by burning and left for regrowth after a year of cultivation. How would you explain the regrowth of forest in ecological term?
Answer. The regrowth of forest is called secondary succession.

9. Climax stage is achieved quickly in secondary succession as compared to primary succession. Why?
Answer. The rate of succession is much faster in secondary succession as the substratum (soil) is already present as compared to primary succession where the process starts from a bare area (rock).

10. Among bryophytes, lichens and fern which one is a pioneer species in a xeric succession?
Answer. Crustose lichens.

11. What is the ultimate source of energy for the ecosystems?
Answer. Sunlight (solar radiation)

12. Is the common edible mushroom an autotroph or a heterotroph?
Answer. Heterotroph (as it does not have chlorophyll)

13. Why are oceans least productive?
Answer. Oceans are least productive because they do not receive sufficient sunlight which is necessary for 1° productivity.

14. Why is the rate of assimilation of energy at the herbivore level called secondary productivity?
Answer. It is because the biomass available to the consumer for consumption is a resultant of the primary productivity from plants.

15. Why are nutrient cycles in nature called biogeochemical cycles?
Answer. Nutrient cycles in nature called biogeochemical cycles because the cycling of nutrients occurs between living organisms (bio) and non-living things-geo- (rocks, air and water, etc.)

16. Give any two examples of xerarch succession.
Answer. (a) Bare rocks (xerosere)
(b) Sandy areas (psammosere)

17. Define self sustainability.
Answer. The degree at which the system can sustain itself without external support is called self sustainability.

18. Given below is a figure of an ecosystem. Answer the following questions.

(i) What type of ecosystem is shown in the figure?
(ii) Name any plant that is characteristic of such ecosystem.
Answer. (i) Desert ecosystem
(ii) Opuntia, Cacti and Acacia

19. What is common to earthworm, mushroom, soil mites and dung beetle in an ecosystem.
Answer. They are all detritivores i.e., decomposing organisms which feed on dead remains of plants and animals.

Short Answer Type Questions
1. Organisms at a higher trophic level have less energy available. Comment.
Answer. The transfer of energy follows 10 per cent law – only 10 per cent of the
energy is transferred to each trophic level from the lower trophic level. So, higher trophic level have less energy as- compared to lower trophic level.

2. The number of trophic levels in an ecosystem are limited. Comment.
Answer. In a food chain, only 10% of the total amount of energy is passed on to the next trophic level from the previous trophic level. So, there is a decrease in the amount of energy available at the successive trophic levels. As we move higher up in the food chain the amount of energy diminishes to a level at which it cannot sustain any trophic level, thereby limiting the number of trophic levels.

3. Is an aquarium a complete ecosystem?
Answer. Aquarium has all the biotic and abiotic components of an ecosystem, so, aquarium is a complete ecosystem.

4. What could be the reason for the faster rate of decomposition in the tropics?
Answer. The rate of decomposition is regulated by climatic factors like temperature and soil moisture as they have an effect on the activities of soil microbes. The
tropics with its hot and humid.climaticcondition provides an environment which is ideal for the microbes to speed up the process of decomposition.

5. Human activities interfere with carbon cycle. List any two such activities.
Answer. Human activities have significantly influenced the carbon cycle. Rapid
deforestation and massive burning of fossil fuel for energy and transport have significantly increased the rate of release of carbon dioxide into the atmosphere.

6. Flow of energy through various trophic levels in an ecosystem is unidirectional and non-cyclic. Explain.
Answer. The energy from the sun reaches the food chain through the primary producers (plants). This energy is passed on through successive trophic levels in the food chain. The energy transfer in the food chain follows the 10 percent law where in only 10% of the energy is transferred from one trophic level to the next successively. So, the movement of energy is only in one direction from lower to higher trophic level.

7. Apart from plants and animals, microbes form a permanent biotic component in an ecosystem. While plants have been referred to as autotrophs and animals as heterotrophs, what are microbes referred to as? How do the microbes fulfil their energy requirements?
Answer. Microbes are referred to as heterotrophs and saprotrophs. They fulfil their energy requirement by feeding on dead remains of plants and animals through the process of decomsposition.

8. Poaching of tiger is a burning issue in today’s world. What implication would this activity have on the functioning of the ecosystem of which the tigers are an integral part?
Answer. Poaching (hunting) of tiger disturbs the ecological balance in ecosystem. Tigers are important predator of forest. Predator keeps prey population under control. Due to hunting of tiger its population decreases, hence the population of deer (prey) increases and interspecific competition occurs between them. Due to increase in prey population (that feeds on vegetation/grass), vegetation destroyed rapidly which causes ecological disturbance.

9. In relation to energy transfer in ecosystem, explain the statement “10 kg of deer’s meat is equivalent to 1 kg of lion’s flesh”.
Answer. Transfer of energy in an ecosystem follow 10% law, i.e., only 10% energy is transferred from one trophic level to higher trophic level. So, if lion feeds on deer than 10 kg of deer’s meat will form 1 kg of lion’s flesh.

10. Primary productivity varies from ecosystem to ecosystem. Explain.
Answer. Primary productivity varies from ecosystem to ecosystem because it depends on the plant species inhabiting the area and their photosynthetic activity. It also depend on various environmental factors.

11. Sometimes due to biotic/abiotic factor the climax remain in a particular serai stage (pre climax) without reaching climax. Do you agree with this statement? If yes, give a suitable example.
Answer. It is true that any change in the. abiotie/biotic factor will arrest a particular serai stage leading to a pre-climax condition before the climax stage is achieved. This can happen in cases of forest fires, landslides, changes in soil characteristics, increase in herbivore population leading to overgrazing.

12. What is an incomplete ecosystem? Explain with the help of suitable example.
Answer. An ecosystem is a functional unit with biotic and abiotic factors interacting
with one another resulting in a physical structure. Absence of any component will make an ecosystem incomplete as it will hinder the functioning of the ecosystem. Examples of such an ecosystem can be a fish tank or deep aphotic zone of the oceans where producers are absent.

13. What are the shortcomings of ecological pyramids in the study of ecosystem?
Answer. The ecological pyramid assumes a simple food chain and does not
accommodate food webs. Thereby,’ it does not take irlto account the fact that species may belong to two or more trophic levels at a time. Also saprophytes despite their vital rolg in ecosystem are given no place in the ecological pyramids.

14. How do you distinguish between humification and mineralisation?
Answer. 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.

15. Fill in the trophic levels (1, 2, 3 and 4) in the boxes provided in the figure.

Answer.

16. The rate of decomposition of detritus is affected by the abiotic factors like availability of oxygen, pH of the soil substratum, temperature etc. Discuss.
Answer. The decomposition of detritus is due to activities of micro-organisms. The rate of growth of microbes is affected by temperature. The pH of substratum affects the composition of microbes (acidophiles / basophiles) which degrade the dead organic matter. If oxygen is present, aerobic degradation occurs. In the absence of oxygen anaerobiosis sets in and there will be incomplete degradation. Also, the degradation is due to activity of exo enzymes secreted by the microbes and the activity of enzyme is affected by factors such as temperature etc.

Long Answer Type Questions
1. A farmer harvests his crop and expresses his harvest in three different ways.
(a) I have harvested 10 quintals of wheat.
(b) I have harvested 10 quintals of wheat today in one acre of land.
(c) I have harvested 10 quintals of wheat in one acre of land, 6 months after sowing.
Do the above statements mean one and the same thing. If your answer is yes, give reasons. And if your answer is ‘no’ explain the meaning of each expression.
Answer. No, the meaning of above statements is different
(a) In this statement farmer gives information of only the quantity of the wheat but area and time period is not given.
(b) In this statement farmer gives information of primary production that is quantity and area but not the time period.
(c) In this statement farmer gives information of primary productivity that is quantity, area and time period all are given.

2. Justify the following statement in terms of ecosystem dynamics. “Nature tends to increase the gross primary productivity, while man tends to increase the net primary productivity”.
Answer. Gross primary productivity of an ecosystem is the rate of production of organic matter during photosynthesis. A considerable amount of GPP is utilised by plants in respiration. Gross primary productivity minus respiration losses (R), is the net primary productivity (NPP). Net primary productivity is the available biomass for the consumption to heterotrophs (herbivores and decomposers).
• Nature tends to increase the gross primary productivity so that it can utilise much of the solar energy and convert it into food.
• Man tends to increase the net primary productivity so that sufficient food is provided to the growing human population.

3. Which of the following ecosystems will be more productive in terms of primary productivity? Justify your answer. A young forest, a natural old forest, a shallow polluted lake, alpine meadow.
Answer. Among these a young forest will be more productive because primary productivity depends on the plant species inhabiting a particular area. A young forest grows more rapidly than old forest hence it has more primary productivity.
A shallow polluted lake and alpine meadow has lower number of producers hence shows lower primary productivity.

4. What are. the three types of ecological pyramids? What information is conveyed by each pyramid with regard to structure, function and energy in the ecosystem?
Answer. The three ecological pyramids that are usually studied are (a) pyramid of number (b) pyramid of biortiass and (c) pyramid of energy.
• Any calculations of energy content, biomass, or numbers has to include all organisms at that trophic level. No generalisations we make will be true if we take only a few individuals at any trophic level into account. Also a given organism may occupy more than one trophic level simultaneously. One must remember that the trophic level represents a functional level, not a species as such. A given species may occupy more than one trophic level in the same ecosystem at the same time; for example, a sparrow is a primary consumer when it eats seeds, fruits, peas, and a secondary consumer when it eats insects and worms.
• In most ecosystems, all the pyramids, of number, of energy and biomass are upright, i.e., producers are more in number and biomass than the herbivores, and herbivores are more in number and biomass than the carnivores. Also energy at a lower trophic level is always more than at a higher level.

5. Write a short note on pyramid of numbers and pyramid of biomass.
Answer. Pyramid of Number
• Pyramid of number deals with number of individuals in a trophic level.

• In Grassland ecosystem only 3 top carnivores are supported in an ecosystem based on production of nearly 6 million plants.
• Pyramid of number in grassland or forest or pond ecosystem is upright.

• In parasitic food chain, the pyramid of number is inverted.
• In an ecosystem dominated by trees the pyramid of number shaped/inverted.

• Among the grassland, pond and forest ecosystem, the number of primary producers per unit area would be maximum in pond ecosystem.
Pyramid of Biomass
• Total amount of living material at various trophic level of a food chain is depicted by pyramid of biomass.

• In most of the ecosystem, the pyramids of number and biomass are upright.
• In terrestrial ecosystem pyramid of biomass is generally upright.
• The pyramid of biomass in sea or lake is generally inverted because the biomass of fishes for exceeds that of phytoplankton.

6. Given below is a list of autotrophs and heterotrophs. With your knowledge about food chain, establish various linkages between the organisms on the principle of ‘eating and being eaten’. What is this inter-linkage established known as?
Algae, Hydrilla, grasshopper, rat, squirrel, crow, maize plant, deer, rabbit, lizard, wolf, snake, peacock, phytoplankton, crustaceans, whale, tiger, lion, sparrow, duck, crane, cockroach, spider, toad, fish, leopard, elephant, goat, Nymphaea, Spirogyra.
Answer. Primary Producer’s (First trophic level): Algae, Hydrilla, maize plant, Phytoplankton, Nymphaea, Spirogyra.
Primary Consumer (Second trophic level): Crustaceans, grasshopper, deer, mouse, squirrel, rabbit, elephant, goat.
Secondly Consumer (Third trophic level): Spider, cockroach, lizard, wolf, snake, toad, fish, crane.
Top carnivore (Fourth trophic level): Lion, Tiger.

7. “The energy flow in the ecosystem follows the second law of thermodynamics.” Explain.
Answer. Ecosystems need a constant supply of energy to synthesise the molecules they require, to counteract the universal tendency toward increasing disorderliness. Hence, ecosystems are not exempt from the Second Law of thermodynamics.

8. What will happen to an ecosystem if:
(a) All producers are removed;
(b) All organisms of herbivore level are eliminated; and
(c) All top carnivore population is removed
Answer. (a) Reduction in primary productivity. No biomass available for consumption by higher trophic levels / heterotrophs
(b) Increase in primary productivity and biomass of producers.
Carnivores population will subsequently dwindle due to food shortage.
(c) Increase in number of herbivores
• Overgrazing by herbivores
• Desertification

9. Give two examples of artificial or mail made ecosystems. List the salient features by which they differ from natural ecosystems.
Answer. Crop fields and an aquarium considered as man-made ecosystems. In artificial ecosystems the biotic and abiotic components of ecosytems are maintained artificially by regular feeding and clearing of aquarium and regular irrigation and sowing of seeds in crop field ecosystem. Natural ecosystem self sustains biotic and abiotic components of an ecosystem.

10. The biodiversity increases when one moves from the pioneer to the climax stage. What could be the explanation?
Answer. In the successive serai stages there is a change in the diversity of species of organisms, increase in the number of species and organisms as well as an • increase in the total biomass. Description of ecological succession usually focuses on changes in vegetation. However, these vegetational changes in turn affect food and shelter for various types of animals. Thus, as succession proceeds, the numbers and types of animals and decomposers also change.

11. What is a biogeochemical cycle? What is the role of the reservoir in a biogeochemical cycle? Give an example of a sedimentary cycle with reservoir located in earth’s crust.
Answer. The movement of nutrient elements through the various components of an ecosystem is called nutrient cycling. Another name of nutrient cycling is biogeochemical cycles (bio: living organism, geo: rocks, air, water). Nutrient cycles are of two types: (a) gaseous and (b) sedimentary.
The function of the reservoir is to meet with the deficit which occurs due to imbalance in the rate of influx and efflux. For the sedimentary cycle (e.g., sulphur and phosphorus cycle), the reservoir is located in Earth’s crust.

12. What will be the P/R ratio of a climax community and a pioneer community? What explanation could you offer for the changes seen in P/R ratio of a pioneer community and the climax community?
Answer. P/R (Production/Respiration) ratio shows the relationship between gross production and respiration.
In climax community the P/R ratio will be 1. In pioneer community the P/R ratio will be more than one.