Ecosystem

What to remember

• An ecosystem has biotic components (producers, consumers, decomposers) and abiotic components. Its four functions are productivity, decomposition, energy flow and nutrient cycling.
• Net primary productivity: NPP = GPP − R. The annual NPP of the whole biosphere is about 170 billion tonnes (dry weight) of organic matter, and the oceans contribute only about 55 billion tonnes.
• Energy flows in one direction only. Plants capture about 2–10% of the photosynthetically active radiation (PAR), and only about 10% of the energy passes from one trophic level to the next (the 10 percent law).
• Pyramids of energy are always upright. Pyramids of number and biomass can be inverted, for example number in a tree ecosystem and biomass in the sea.

Ecosystem: structure and components

An ecosystem is a functional unit of nature where living organisms interact among themselves and with the physical surroundings. It can be as small as a pond or as large as a forest or an ocean. The whole biosphere is a global ecosystem made of all the local ecosystems.

Types of ecosystems:

• Terrestrial: forest, grassland and desert.
• Aquatic: pond, lake, wetland, river and estuary.
• Artificial (man-made): crop fields and an aquarium.

Structure. The components are of two kinds.

Component	Parts	Example in a pond
Biotic	Producers, consumers, decomposers	Phytoplankton, zooplankton, fish, bacteria
Abiotic	Temperature, light, water, soil, inorganic substances (such as C, N, P, Ca), organic compounds	Sunlight, dissolved oxygen, water, minerals

Stratification is the vertical distribution of different species occupying different levels. In a forest, the trees occupy the top vertical strata (canopy), shrubs the second, and herbs and grasses the bottom layer. In a pond, the producers, consumers and decomposers are arranged in layers according to light and depth.

Pond as an example. A pond is a shallow water body and a good example of a simple aquatic ecosystem. The abiotic part includes water, dissolved oxygen, light, temperature and minerals. The producers are the phytoplankton, algae and some floating, submerged and marginal plants. The consumers are the zooplankton, insects, fishes and frogs. The decomposers are bacteria and fungi, mostly at the bottom. Energy enters the pond as sunlight and leaves as heat. Nutrients are used again and again within the pond, so they move in cycles.

Four basic components of the ecosystem function:

• 1. Productivity
• 2. Decomposition
• 3. Energy flow
• 4. Nutrient cycling

Productivity

A constant input of solar energy is the basic requirement for any ecosystem to function. The amount of biomass (organic matter) produced per unit area over a time period by plants during photosynthesis is called primary production. It is expressed as weight (g m⁻²) or energy (kcal m⁻²). The rate of biomass production is called productivity, expressed as g m⁻² yr⁻¹ or kcal m⁻² yr⁻¹. This allows comparison of different ecosystems.

• Gross primary productivity (GPP) of an ecosystem is the rate of production of organic matter during photosynthesis.
• A considerable amount of GPP is used by plants in respiration (R).
• Net primary productivity (NPP) = GPP − R.

NPP is the available biomass for the consumption of heterotrophs (herbivores and decomposers).

Secondary productivity is the rate of formation of new organic matter by consumers.

Primary productivity depends on:

• The plant species inhabiting the area.
• Environmental factors such as light, temperature and water.
• The availability of nutrients.
• The photosynthetic capacity of plants.

So it varies in different types of ecosystems.

Quantity	Approximate value
Annual NPP of the whole biosphere	About 170 billion tonnes (dry weight) of organic matter
Annual NPP of the oceans	About 55 billion tonnes
Area of the biosphere covered by oceans	About 70 per cent

Despite occupying about 70 per cent of the surface, the oceans have a low productivity compared to land because of the limited light and nutrients available to producers. So the productivity of the oceans is only about one third of the total.

Decomposition

Decomposers break down complex organic matter into inorganic substances such as carbon dioxide, water and nutrients. The process is called decomposition. The raw material for decomposition is detritus: dead plant remains (leaves, bark, flowers) and dead remains of animals, including faecal matter. Detritus is the raw material for decomposition.

The important steps in decomposition are:

Step	What happens	Agent
Fragmentation	Detritivores break detritus into smaller particles	Earthworms (detritivores)
Leaching	Water-soluble inorganic nutrients go down into the soil horizon and are precipitated as unavailable salts	Water
Catabolism	Bacterial and fungal enzymes degrade detritus into simpler inorganic substances	Bacteria and fungi
Humification	Formation of humus	Microbes
Mineralisation	Humus is degraded by some microbes and inorganic nutrients are released	Microbes

The steps of decomposition operate simultaneously on the detritus.

Humus is a dark-coloured, amorphous substance that is highly resistant to microbial action and decomposes at an extremely slow rate. Being colloidal in nature, it serves as a reservoir of nutrients.

Factors that affect the rate of decomposition:

• Chemical composition of detritus. Decomposition is slower if the detritus is rich in lignin and chitin. It is quicker if the detritus is rich in nitrogen and water-soluble substances such as sugars.
• Climatic factors: temperature and soil moisture. Warm and moist conditions favour decomposition. Low temperature and anaerobiosis (lack of oxygen) inhibit decomposition, which results in the build-up of organic materials.

Decomposition is largely an oxygen-requiring process.

Energy flow

The sun is the only source of energy for all ecosystems on earth (except deep-sea hydrothermal ecosystems). Of the incident solar radiation, less than 50 per cent is photosynthetically active radiation (PAR). Plants capture only about 2–10 per cent of the PAR. This small amount sustains the entire living world.

Producers (green plants, algae, cyanobacteria) fix solar energy. All organisms depend on the producers directly or indirectly. Energy flows in one direction: from the sun to producers and then to consumers. It is unidirectional. It is lost as heat at each step and cannot be reused.

Consumers

• Primary consumers: herbivores that feed on producers.
• Secondary consumers: primary carnivores that feed on herbivores.
• Tertiary consumers: secondary carnivores.

Food chains

• Grazing food chain (GFC) starts with producers: producers → herbivores → carnivores. Example: grass → goat → human. It is the major conduit for energy flow in aquatic ecosystems.
• Detritus food chain (DFC) starts with dead organic matter (detritus): detritus → detritivore → predator. The decomposers are the primary consumers of detritus. In terrestrial ecosystems, a much larger fraction of energy flows through the detritus food chain than through the grazing food chain.
• The two chains are connected. Some animals of the GFC are food for the animals of DFC (for example, a bird eating an earthworm).

A food web is a network of food chains where different chains are connected at various trophic levels. Natural ecosystems have food webs and not simple chains.

Trophic level. Each step of food or energy transfer is a trophic level. Producers are the first trophic level (T₁). Herbivores are the second (T₂), primary carnivores the third (T₃) and secondary carnivores the fourth (T₄). An organism can occupy more than one trophic level; for example, a sparrow feeding on both seeds and insects is both a primary and a secondary consumer.

The standing crop is the mass of living organisms at each trophic level at a given time. It is measured as mass per unit area. The amount of nutrients in the soil at a given time is the standing state.

The 10 percent law (Lindeman): when energy flows from one trophic level to the next, only about 10 per cent of the energy is transferred to the next level. The rest is lost as heat in respiration or in other activities and in wastes. So the number of trophic levels in a food chain is limited, usually about four or five.

Trophic level	Organisms	Energy (example, 10% law)
T₁	Producers	1,000 units
T₂	Primary consumers	100 units
T₃	Secondary consumers	10 units
T₄	Tertiary consumers	1 unit

Ecological pyramids

A graphical representation of trophic structure and function is an ecological pyramid. The base represents the producers (first trophic level). The tip (apex) represents the top consumer. The three types are pyramids of number, biomass and energy.

Pyramid	Based on	Usual shape	Exceptions (inverted)
Number	Number of individuals at each level	Upright in a grassland	Tree ecosystem (one large tree supports many insects), and a parasitic food chain
Biomass	Dry weight of organisms at each level	Upright in most land ecosystems	Sea: the biomass of fishes exceeds the biomass of phytoplankton
Energy	Energy at each level per unit area per unit time	Always upright	None

• In a grassland the number of grass plants is the largest, then herbivores, then carnivores. So the pyramid of number is upright.
• In a tree ecosystem the pyramid of number is inverted or spindle shaped because only one tree supports many birds and insects.
• In the sea the pyramid of biomass is generally inverted. The biomass of fishes is far more than that of phytoplankton, because the producers (phytoplankton) have a very short life span and a very high turnover.
• The energy pyramid is always upright because energy at a lower trophic level is always more than at a higher level (energy is lost as heat at each transfer).

Limitations of ecological pyramids:

• They do not take into account the same species belonging to two or more trophic levels.
• They assume a simple food chain, which almost never exists in nature.
• They do not accommodate a food web.
• Saprophytes (decomposers) are not given any place in a pyramid even though they play a vital role in the ecosystem.

NEET traps

• 1. GPP vs NPP: NPP = GPP − R. NPP is what remains for consumers, not the total fixed.
• 2. Primary vs secondary productivity: primary is by producers; secondary is by consumers.
• 3. Ocean share: oceans are about 70% of the surface but contribute only about 55 of about 170 billion tonnes of annual NPP.
• 4. Leaching vs catabolism: leaching is the movement of water-soluble nutrients downward; catabolism is enzymatic breakdown by microbes.
• 5. Humification vs mineralisation: humification forms humus; mineralisation breaks humus down and releases inorganic nutrients.
• 6. Lignin and chitin vs nitrogen and sugars: lignin and chitin slow decomposition; nitrogen and water-soluble sugars speed it up.
• 7. PAR vs the percentage captured: PAR is less than 50% of incident solar radiation; plants capture only about 2–10% of PAR.
• 8. GFC vs DFC: GFC is the main route of energy flow in aquatic ecosystems; DFC carries more energy in terrestrial ecosystems.
• 9. Number vs biomass inversion: number is inverted in a tree ecosystem; biomass is inverted in the sea.
• 10. Energy pyramid: never inverted, even when the number or biomass pyramid of the same ecosystem is inverted.

One-liners

• 1. An ecosystem is a functional unit of nature made of biotic and abiotic components.
• 2. Pond, lake, wetland, river and estuary are aquatic ecosystems.
• 3. Stratification means vertical layering of species in an ecosystem.
• 4. Primary productivity is the rate of organic matter production by producers.
• 5. NPP = GPP − R.
• 6. Secondary productivity is the rate of new organic matter formation by consumers.
• 7. The annual NPP of the biosphere is about 170 billion tonnes (dry weight).
• 8. The oceans contribute only about 55 billion tonnes of the annual NPP.
• 9. Detritus is the raw material for decomposition.
• 10. Humus is dark, colloidal and decomposes very slowly.
• 11. Earthworms are detritivores that carry out fragmentation.
• 12. Warm and moist conditions speed up decomposition.
• 13. Plants capture only about 2–10% of PAR.
• 14. Energy flow is unidirectional and follows the 10 percent law.
• 15. The energy pyramid is always upright.

Practice questions

1. Net primary productivity (NPP) is equal to

   1. GPP + R
   2. GPP − R
   3. R − GPP
   4. GPP × R
   Answer

   B. GPP − R

   NPP is the gross primary productivity minus the respiratory loss.

2. The annual net primary productivity of the whole biosphere is about

   1. 1,700 billion tonnes
   2. 17 billion tonnes
   3. 170 billion tonnes (dry weight) of organic matter
   4. 55 billion tonnes
   Answer

   C. 170 billion tonnes (dry weight) of organic matter

   The annual NPP of the biosphere is about 170 billion tonnes of dry organic matter.

3. The oceans contribute about how much of the annual NPP of the biosphere?

   1. 115 billion tonnes
   2. 5.5 billion tonnes
   3. 170 billion tonnes
   4. 55 billion tonnes
   Answer

   D. 55 billion tonnes

   Although oceans cover about 70% of the surface, they contribute only about 55 billion tonnes.

4. The raw material for decomposition is

   1. detritus
   2. standing crop
   3. humus
   4. PAR
   Answer

   A. detritus

   Detritus is dead plant remains and animal remains including faecal matter.

5. The step in which water-soluble nutrients move down into the soil and may be precipitated as unavailable salts is

   1. fragmentation
   2. humification
   3. leaching
   4. catabolism
   Answer

   C. leaching

   Leaching carries water-soluble inorganic nutrients down into the soil horizon.

6. Which statement about humus is correct?

   1. It decomposes very rapidly
   2. It is light coloured and crystalline
   3. It contains only water-soluble sugars
   4. It is dark coloured, amorphous and highly resistant to microbial action
   Answer

   D. It is dark coloured, amorphous and highly resistant to microbial action

   Humus decomposes extremely slowly and, being colloidal, acts as a reservoir of nutrients.

7. Which of the following is a detritivore that carries out fragmentation?

   1. Earthworm
   2. Phytoplankton
   3. Grass
   4. Cyanobacterium
   Answer

   A. Earthworm

   Detritivores such as earthworms break detritus into smaller particles.

8. Decomposition is faster when the detritus is rich in

   1. lignin
   2. nitrogen and water-soluble sugars
   3. lignin and chitin
   4. chitin
   Answer

   B. nitrogen and water-soluble sugars

   Lignin and chitin slow decomposition; nitrogen and sugars speed it up.

9. Plants capture about what percentage of the photosynthetically active radiation (PAR)?

   1. 0.1–0.5%
   2. 50–60%
   3. 2–10%
   4. 20–30%
   Answer

   C. 2–10%

   Only about 2–10% of PAR is captured by plants.

10. Energy flow in an ecosystem is

    1. unidirectional
    2. random
    3. cyclic
    4. bidirectional
    Answer

    A. unidirectional

    Energy flows from the sun to producers to consumers and is lost as heat; it cannot be reused.

11. In an aquatic ecosystem the major conduit for energy flow is the

    1. detritus food chain
    2. saprophytic food chain
    3. parasitic food chain
    4. grazing food chain
    Answer

    D. grazing food chain

    In aquatic ecosystems the grazing food chain is the major route of energy flow.

12. Which pyramid is always upright?

    1. Pyramid of biomass
    2. Pyramid of energy
    3. Pyramid of number
    4. Pyramid of number in a tree ecosystem
    Answer

    B. Pyramid of energy

    Energy at a lower trophic level is always more than at a higher level.

13. In which ecosystem is the pyramid of biomass generally inverted?

    1. Forest
    2. Grassland
    3. Sea
    4. Crop field
    Answer

    C. Sea

    In the sea the biomass of fishes exceeds that of phytoplankton.

14. Producers form the

    1. third trophic level
    2. second trophic level
    3. last trophic level
    4. first trophic level
    Answer

    D. first trophic level

    Producers are the first trophic level.

15. In a tree ecosystem, the pyramid of number is

    1. inverted (spindle shaped)
    2. absent
    3. upright
    4. always a straight line
    Answer

    A. inverted (spindle shaped)

    One large tree supports many insects and birds, so the base is small.

16. In terrestrial ecosystems, a much larger fraction of energy flows through the

    1. pyramid of number
    2. detritus food chain
    3. standing state
    4. grazing food chain
    Answer

    B. detritus food chain

    In terrestrial ecosystems the detritus food chain carries more energy than the grazing food chain.

17. Given below are two statements: Statement I: Net primary productivity is gross primary productivity minus respiratory loss. Statement II: Secondary productivity is the rate of formation of new organic matter by producers. Choose the correct answer.

    1. Both Statement I and Statement II are correct
    2. Both Statement I and Statement II are incorrect
    3. Statement I is correct but Statement II is incorrect
    4. Statement I is incorrect but Statement II is correct
    Answer

    C. Statement I is correct but Statement II is incorrect

    Secondary productivity is formed by consumers.

18. Given below are two statements: Statement I: The oceans contribute more than half of the annual NPP of the biosphere. Statement II: The oceans contribute about 55 billion tonnes of the annual NPP of about 170 billion tonnes. Choose the correct answer.

    1. Both Statement I and Statement II are correct
    2. Both Statement I and Statement II are incorrect
    3. Statement I is correct but Statement II is incorrect
    4. Statement I is incorrect but Statement II is correct
    Answer

    D. Statement I is incorrect but Statement II is correct

    The share of the oceans is only about one third.

19. Given below are two statements: Statement I: Humus is highly resistant to microbial action. Statement II: Mineralisation releases inorganic nutrients from humus. Choose the correct answer.

    1. Both Statement I and Statement II are correct
    2. Both Statement I and Statement II are incorrect
    3. Statement I is correct but Statement II is incorrect
    4. Statement I is incorrect but Statement II is correct
    Answer

    A. Both Statement I and Statement II are correct

    Both are correct; humus is broken down slowly by microbes in mineralisation.

20. Given below are two statements: Statement I: Decomposition is faster at low temperature and without oxygen. Statement II: Detritus rich in lignin decomposes faster than detritus rich in sugars. Choose the correct answer.

    1. Both Statement I and Statement II are correct
    2. Both Statement I and Statement II are incorrect
    3. Statement I is correct but Statement II is incorrect
    4. Statement I is incorrect but Statement II is correct
    Answer

    B. Both Statement I and Statement II are incorrect

    Warm, moist and oxygen-rich conditions and sugar-rich detritus favour faster decomposition.

21. Given below are two statements: Statement I: Energy flow in an ecosystem is bidirectional. Statement II: Plants capture only about 2–10 per cent of the PAR. Choose the correct answer.

    1. Both Statement I and Statement II are correct
    2. Both Statement I and Statement II are incorrect
    3. Statement I is correct but Statement II is incorrect
    4. Statement I is incorrect but Statement II is correct
    Answer

    D. Statement I is incorrect but Statement II is correct

    Energy flow is unidirectional.

22. Given below are two statements: Statement I: The grazing food chain begins with producers. Statement II: In terrestrial ecosystems, more energy flows through the grazing food chain than through the detritus food chain. Choose the correct answer.

    1. Both Statement I and Statement II are correct
    2. Both Statement I and Statement II are incorrect
    3. Statement I is correct but Statement II is incorrect
    4. Statement I is incorrect but Statement II is correct
    Answer

    C. Statement I is correct but Statement II is incorrect

    In terrestrial ecosystems the detritus food chain carries more energy.

23. Given below are two statements: Statement I: The pyramid of energy is always upright. Statement II: The pyramid of biomass in the sea is generally inverted. Choose the correct answer.

    1. Both Statement I and Statement II are correct
    2. Both Statement I and Statement II are incorrect
    3. Statement I is correct but Statement II is incorrect
    4. Statement I is incorrect but Statement II is correct
    Answer

    A. Both Statement I and Statement II are correct

    Both are correct.

24. Given below are two statements: Statement I: A sparrow feeding on seeds and insects occupies more than one trophic level. Statement II: Ecological pyramids accommodate food webs well. Choose the correct answer.

    1. Both Statement I and Statement II are correct
    2. Both Statement I and Statement II are incorrect
    3. Statement I is correct but Statement II is incorrect
    4. Statement I is incorrect but Statement II is correct
    Answer

    C. Statement I is correct but Statement II is incorrect

    Pyramids assume a simple food chain and do not accommodate food webs.

25. Which of the following statements about decomposition are correct? 1. Detritus includes dead plant parts and faecal matter. 2. Humification leads to the formation of humus. 3. Catabolism is carried out by earthworms.

    1. 2 and 3 only
    2. 1 and 2 only
    3. 1 and 3 only
    4. 1, 2 and 3
    Answer

    B. 1 and 2 only

    Catabolism is by bacterial and fungal enzymes; earthworms do fragmentation.

26. Which of the following statements about trophic levels are correct? 1. Producers form the second trophic level. 2. Herbivores are primary consumers. 3. The 10 percent law concerns energy transfer between trophic levels.

    1. 1 and 2 only
    2. 1 and 3 only
    3. 1, 2 and 3
    4. 2 and 3 only
    Answer

    D. 2 and 3 only

    Producers are the first trophic level, so statement 1 is wrong.

27. Assertion (A): The number of trophic levels in a food chain is limited. Reason (R): Only about 10 per cent of energy passes to the next trophic level.

    1. Both A and R are true and R is the correct explanation of A
    2. Both A and R are true but R is not the correct explanation of A
    3. A is true but R is false
    4. A is false but R is true
    Answer

    A. Both A and R are true and R is the correct explanation of A

    Energy falls steeply at each step, so a chain can support only a few levels.

28. Assertion (A): The pyramid of number in a tree ecosystem is inverted. Reason (R): The pyramid of energy is always upright.

    1. Both A and R are true and R is the correct explanation of A
    2. Both A and R are true but R is not the correct explanation of A
    3. A is true but R is false
    4. A is false but R is true
    Answer

    B. Both A and R are true but R is not the correct explanation of A

    Both are true, but the second does not explain the first.

29. Assertion (A): Decomposition is largely an oxygen-requiring process. Reason (R): Anaerobic conditions speed up decomposition.

    1. Both A and R are true and R is the correct explanation of A
    2. Both A and R are true but R is not the correct explanation of A
    3. A is true but R is false
    4. A is false but R is true
    Answer

    C. A is true but R is false

    Lack of oxygen inhibits decomposition, so R is false.

30. Assertion (A): Humus is rapidly decomposed in the soil. Reason (R): Humus is a reservoir of nutrients.

    1. Both A and R are true and R is the correct explanation of A
    2. Both A and R are true but R is not the correct explanation of A
    3. A is true but R is false
    4. A is false but R is true
    Answer

    D. A is false but R is true

    Humus decomposes extremely slowly.

31. Assertion (A): Energy flow in an ecosystem is unidirectional. Reason (R): Energy lost as heat at each step cannot be reused by the producers.

    1. Both A and R are true and R is the correct explanation of A
    2. Both A and R are true but R is not the correct explanation of A
    3. A is true but R is false
    4. A is false but R is true
    Answer

    A. Both A and R are true and R is the correct explanation of A

    Heat is lost and cannot be fixed again, so energy does not return.

32. Assertion (A): The sun is the source of energy for an ecosystem. Reason (R): Plants capture almost all of the PAR.

    1. Both A and R are true and R is the correct explanation of A
    2. Both A and R are true but R is not the correct explanation of A
    3. A is true but R is false
    4. A is false but R is true
    Answer

    C. A is true but R is false

    Plants capture only about 2–10% of PAR.

33. Match List I with List II. List I: a. Fragmentation b. Leaching c. Catabolism d. Humification List II: i. Formation of dark amorphous humus ii. Detritivores break detritus into small particles iii. Movement of soluble nutrients down into the soil iv. Enzymatic degradation by microbes Choose the correct answer.

    1. a-ii, b-i, c-iv, d-iii
    2. a-ii, b-iii, c-iv, d-i
    3. a-i, b-ii, c-iv, d-iii
    4. a-ii, b-iv, c-iii, d-i
    Answer

    B. a-ii, b-iii, c-iv, d-i

    Fragmentation by detritivores; leaching down the soil; catabolism by enzymes; humification forms humus.

34. Match List I with List II. List I: a. GPP b. NPP c. Secondary productivity d. Standing crop List II: i. Rate of new organic matter formation by consumers ii. GPP − R iii. Rate of production of organic matter in photosynthesis iv. Mass of living organisms at a trophic level at a given time Choose the correct answer.

    1. a-ii, b-iii, c-i, d-iv
    2. a-iii, b-ii, c-i, d-iv
    3. a-iii, b-ii, c-iv, d-i
    4. a-i, b-ii, c-iv, d-iii
    Answer

    B. a-iii, b-ii, c-i, d-iv

    Each term matches its definition.

35. Match List I with List II. List I: a. Producers b. Herbivores c. Primary carnivores d. Secondary carnivores List II: i. Third trophic level ii. Fourth trophic level iii. First trophic level iv. Second trophic level Choose the correct answer.

    1. a-iii, b-iv, c-i, d-ii
    2. a-i, b-iii, c-iv, d-ii
    3. a-ii, b-iv, c-i, d-iii
    4. a-iii, b-ii, c-i, d-iv
    Answer

    A. a-iii, b-iv, c-i, d-ii

    Producers T1, herbivores T2, primary carnivores T3, secondary carnivores T4.

36. Match List I with List II. List I: a. Lignin and chitin in detritus b. Nitrogen and sugars in detritus c. Low temperature and lack of oxygen d. Warm and moist conditions List II: i. Rapid decomposition because of chemical composition ii. Favour decomposition iii. Build-up of organic matter iv. Slow decomposition because of chemical composition Choose the correct answer.

    1. a-iv, b-i, c-ii, d-iii
    2. a-i, b-ii, c-iii, d-iv
    3. a-ii, b-i, c-iii, d-iv
    4. a-iv, b-i, c-iii, d-ii
    Answer

    D. a-iv, b-i, c-iii, d-ii

    Lignin and chitin slow decomposition; nitrogen and sugars speed it; cold anaerobic conditions inhibit it; warm moist conditions favour it.

37. Match List I with List II. List I: a. Pyramid of energy b. Pyramid of biomass in the sea c. Pyramid of number in a grassland d. Pyramid of number in a tree ecosystem List II: i. Inverted; fish biomass exceeds phytoplankton biomass ii. Always upright iii. Upright; grass plants are the most numerous iv. Inverted; one tree supports many insects Choose the correct answer.

    1. a-i, b-iii, c-ii, d-iv
    2. a-ii, b-i, c-iii, d-iv
    3. a-ii, b-i, c-iv, d-iii
    4. a-ii, b-iv, c-iii, d-i
    Answer

    B. a-ii, b-i, c-iii, d-iv

    Energy pyramid is always upright; sea biomass is inverted; grassland number is upright; tree number is inverted.

38. Match List I with List II. List I: a. Grazing food chain b. Detritus food chain c. Food web d. Trophic level List II: i. Network of interconnected food chains ii. Begins with dead organic matter iii. Begins with producers iv. A step of food or energy transfer Choose the correct answer.

    1. a-i, b-ii, c-iv, d-iii
    2. a-iii, b-ii, c-iv, d-i
    3. a-ii, b-iii, c-i, d-iv
    4. a-iii, b-ii, c-i, d-iv
    Answer

    D. a-iii, b-ii, c-i, d-iv

    Each term matches its description.

39. In an ecosystem GPP is 2,000 kcal m⁻² yr⁻¹ and plants lose 800 kcal m⁻² yr⁻¹ in respiration. The NPP is

    1. 1,600 kcal m⁻² yr⁻¹
    2. 2,400 kcal m⁻² yr⁻¹
    3. 1,200 kcal m⁻² yr⁻¹
    4. 2,800 kcal m⁻² yr⁻¹
    Answer

    C. 1,200 kcal m⁻² yr⁻¹

    NPP = GPP − R = 2,000 − 800 = 1,200.

40. Producers fix 20,000 J of energy. By the 10 percent law, the energy available to the secondary consumers is about

    1. 200 J
    2. 20 J
    3. 2,000 J
    4. 1,000 J
    Answer

    A. 200 J

    20,000 → 2,000 in primary consumers → 200 in secondary consumers.

41. Dead leaves pile up for years in a cold, waterlogged soil. The most likely reason is that

    1. low temperature and lack of oxygen inhibit decomposers
    2. earthworms are too active
    3. high temperature speeds up leaching
    4. the leaves are rich in nitrogen and sugars
    Answer

    A. low temperature and lack of oxygen inhibit decomposers

    Low temperature and anaerobiosis inhibit decomposition, so organic material builds up.

42. At the time of sampling in the sea, the fish biomass is more than the phytoplankton biomass. Which statement is correct?

    1. Both pyramids are inverted
    2. The biomass pyramid is inverted but the energy pyramid is upright
    3. Both pyramids are upright
    4. The biomass pyramid is upright but the energy pyramid is inverted
    Answer

    B. The biomass pyramid is inverted but the energy pyramid is upright

    Phytoplankton have a short life span and fast turnover, but energy pyramids are never inverted.

43. A fungus decomposing a dead log cannot be placed in an ecological pyramid. This shows that pyramids

    1. show the energy lost as heat
    2. count only the producers
    3. are always inverted
    4. do not give a place to saprophytes
    Answer

    D. do not give a place to saprophytes

    One limitation of pyramids is that saprophytes find no place in them.

44. Taking the annual NPP of the biosphere as about 170 billion tonnes and that of the oceans as about 55 billion tonnes, the land contributes about

    1. 170 billion tonnes
    2. 55 billion tonnes
    3. 115 billion tonnes
    4. 225 billion tonnes
    Answer

    C. 115 billion tonnes

    170 − 55 = 115 billion tonnes.