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NCERT Solutions · Class 12 Biology Ecosystem

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Exercises 12.1–12.11

  1. Exercise 12.1

    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_________.

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    (a) autotrophs (producers) — they fix carbon dioxide into organic matter.
    (b) inverted — a few large trees support a much larger number of insects and birds, so the numbers widen upward.
    (c) light — a constant input of solar energy is the basic requirement for any ecosystem, and in water it is what runs short.
    (d) earthworms — the farmer's 'friend', which break detritus into smaller particles.
    (e) oceans — they hold about $\displaystyle 71$ per cent of the earth's carbon, dissolved.

  2. Exercise 12.2

    Which one of the following has the largest population in a food chain?
    (a)
    Producers
    (b)
    Primary consumers
    (c)
    Secondary consumers
    (d)
    Decomposers

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    (d) Decomposers.
    Decomposers are fungi and bacteria, which are microscopic and enormously numerous — the chapter notes they are "especially abundant" at the bottom of a pond.
    They are not confined to one trophic level: every organism, at every level, becomes detritus when it dies, so decomposers draw on the dead matter of the whole chain.
    This is also why the chapter lists it as a limitation of ecological pyramids that saprophytes are given no place in them, despite their vital role.

  3. Exercise 12.3

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

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    (b) Zooplankton.
    In an aquatic ecosystem the producers are phytoplankton, algae and higher plants — they form the first trophic level.
    Zooplankton feed on the phytoplankton, so they are the herbivores or primary consumers, and herbivores belong to the second trophic level.
    Fishes that eat zooplankton are the carnivores (third trophic level); benthos are the bottom-dwelling forms.

  4. Exercise 12.4

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

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    (a) Herbivores.
    The chapter defines secondary productivity as the rate of formation of new organic matter by consumers, not by plants.
    Herbivores are the primary consumers — the first group to convert plant biomass into animal biomass — so the organic matter they build up is the "secondary" production of the ecosystem.
    Producers (green plants) are responsible for primary, not secondary, production.

  5. Exercise 12.5

    What is the percentage of photosynthetically active radiation (PAR) in the incident solar radiation?
    (a)
    $\displaystyle 100$%
    (b)
    $\displaystyle 50$ %
    (c)
    $\displaystyle 1$-$\displaystyle 5$%
    (d)
    $\displaystyle 2$-$\displaystyle 10$%

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    (b) $\displaystyle 50$% — the chapter states that of the incident solar radiation, less than $\displaystyle 50$ per cent is photosynthetically active radiation (PAR).
    Do not confuse this with option (d): $\displaystyle 2$–$\displaystyle 10$ per cent is the fraction of the PAR that plants actually capture, and that small amount sustains the entire living world.

  6. Exercise 12.6

    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

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    (a) Grazing food chain (GFC) vs detritus food chain (DFC)
    GFC starts with living green plants (producers); DFC starts with dead organic matter, i.e. detritus.
    GFC runs producer → herbivore → carnivore; DFC is made up of decomposers (saprotrophs), mainly fungi and bacteria, which degrade dead organic matter.
    GFC gets its energy directly from the Sun through photosynthesis; DFC gets its energy from the dead biomass of other trophic levels.
    GFC is the major conduit for energy flow in an aquatic ecosystem; in a terrestrial ecosystem a much larger fraction of energy flows through the DFC.
    (b) Production vs decomposition
    Production is the building of organic matter: the amount of biomass produced per unit area over a time period by plants during photosynthesis.
    Decomposition is the breaking down of complex organic matter into inorganic substances — carbon dioxide, water and nutrients.
    Production is carried out by producers (autotrophs); decomposition by decomposers (fungi and bacteria).
    Production stores solar energy in organic compounds; decomposition releases the locked-up nutrients back for reuse by the autotrophs.
    (c) Upright vs inverted pyramid
    An upright pyramid has a broad base that narrows towards the apex: producers exceed herbivores, and herbivores exceed carnivores, in number, biomass or energy.
    An inverted pyramid has a narrow base and widens upwards, so a lower trophic level is smaller than the one it supports.
    Example of upright — the pyramid of numbers in a grassland, where nearly $\displaystyle 6$ million plants support only three top carnivores.
    Example of inverted — the pyramid of biomass in the sea, where a small standing crop of phytoplankton supports a much larger standing crop of zooplankton and fishes.
    The pyramid of energy is always upright and can never be inverted, because some energy is lost as heat at every step.
    (d) Food chain vs food web
    A food chain is a single, straight-line sequence of who eats whom, e.g. Grass (producer) → Goat (primary consumer) → Man (secondary consumer).
    A food web is the natural interconnection of many such food chains.
    In a chain, an organism has one source of food and one consumer; in a web, an organism may be eaten by several others and may itself feed at more than one level.
    Webs arise because omnivores such as cockroaches and crows link the detritus food chain to the grazing food chain; a straight chain almost never exists in nature.
    (e) Litter vs detritus
    Detritus is defined in the chapter as dead plant remains — leaves, bark, flowers — together with the dead remains of animals, including faecal matter; it is the raw material for decomposition.
    Litter is the surface layer of freshly fallen, still largely undecomposed plant material (leaves, twigs) lying on the soil, so it is one part of the detritus.
    (f) Primary vs secondary productivity
    Primary productivity is the rate of production of biomass or organic matter by plants during photosynthesis, per unit area per unit time.
    Secondary productivity is the rate of formation of new organic matter by consumers.
    Primary productivity is subdivided into gross primary productivity (GPP) and net primary productivity (NPP), where \(\displaystyle GPP - R = NPP \); secondary productivity has no such division.
    Primary productivity depends on the plant species present, nutrient availability and the photosynthetic capacity of the plants; secondary productivity depends on the NPP made available to the heterotrophs.

  7. Exercise 12.7

    Describe the components of an ecosystem.

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    An ecosystem is a functional unit of nature in which living organisms interact among themselves and with the surrounding physical environment; its components are abiotic and biotic.
    Abiotic components
    The non-living, inorganic part: air, water and soil, together with all the dissolved inorganic and organic substances.
    Solar input, the cycle of temperature, day-length and other climatic conditions regulate the rate at which the whole system functions.
    Biotic components
    Producers (autotrophs) — green plants and photosynthetic bacteria that fix the Sun's radiant energy and make food from simple inorganic materials. In a terrestrial ecosystem these are herbaceous and woody plants; in an aquatic one, phytoplankton, algae and higher plants.
    Consumers (heterotrophs) — animals that depend on plants directly or indirectly. Primary consumers are herbivores, secondary consumers are the primary carnivores that eat them, and tertiary consumers feed on those.
    Decomposers (saprotrophs) — mainly fungi and bacteria, which secrete digestive enzymes that break dead and waste material into simple inorganic substances and then absorb them.
    Structural features
    Species composition — the identification and enumeration of the plant and animal species of the ecosystem.
    Stratification — the vertical distribution of species at different levels, e.g. trees occupy the top layer of a forest, shrubs the second, and herbs and grasses the bottom.
    Functional aspects
    The components work as a unit through four processes: productivity, decomposition, energy flow and nutrient cycling.
    A pond as an example
    Abiotic: the water with its dissolved substances and the rich soil deposit at the bottom.
    Autotrophs: phytoplankton, some algae, and the floating, submerged and marginal plants at the edges.
    Consumers: the zooplankton and the free-swimming and bottom-dwelling forms.
    Decomposers: fungi, bacteria and flagellates, especially abundant at the pond bottom.

  8. Exercise 12.8

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

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    Definition
    An ecological pyramid is a graphic representation of the relationship between the organisms at the different trophic levels of an ecosystem, expressed in terms of number, biomass or energy.
    The base of the pyramid represents the producers (first trophic level) and the apex represents the tertiary or top-level consumer.
    The three types usually studied are the pyramid of number, the pyramid of biomass and the pyramid of energy.
    Any calculation must include all the organisms at that trophic level; a trophic level is a functional level, not a species, and one species may occupy more than one level at the same time — a sparrow is a primary consumer when it eats seeds and a secondary consumer when it eats insects.
    Pyramid of number
    Each bar gives the number of individuals at that trophic level.
    Usually upright — in a grassland, nearly $\displaystyle 6$ million plants support a much smaller number of herbivores, which support only three top carnivores at the apex.
    Can be inverted — a single big tree supports a very large number of insects, which support a smaller number of small birds, which in turn support fewer large birds; here the base (one tree) is the narrowest bar.
    Pyramid of biomass
    Each bar gives the standing crop, i.e. the mass of living material at that trophic level, measured as fresh or (more accurately) dry weight.
    Usually upright on land, showing a sharp decrease in biomass at the higher trophic levels.
    Inverted in the sea — the small standing crop of phytoplankton supports a much larger standing crop of zooplankton and fishes, because the phytoplankton are short-lived and replace themselves very rapidly.
    Limitations
    Ecological pyramids do not allow for the same species belonging to two or more trophic levels.
    They assume a simple food chain, which almost never exists in nature, and cannot accommodate a food web.
    Saprophytes are given no place in them, although they play a vital role in the ecosystem.

  9. Exercise 12.9

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

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    Primary productivity
    It is the rate at which biomass or organic matter is produced per unit area by plants during photosynthesis.
    Primary production is the amount of biomass produced per unit area over a time period, expressed in weight \(\displaystyle (g\,m^{-2}) \) or energy \(\displaystyle (kcal\,m^{-2}) \); productivity, being a rate, is expressed as \(\displaystyle g\,m^{-2}\,yr^{-1} \) or \(\displaystyle kcal\,m^{-2}\,yr^{-1} \) so that different ecosystems can be compared.
    It is of two kinds: gross primary productivity (GPP), the rate of production of organic matter during photosynthesis, and net primary productivity (NPP), what is left after respiration losses — \(\displaystyle GPP - R = NPP \). NPP is the biomass available to the heterotrophs (herbivores and decomposers).
    Factors affecting it
    Plant species inhabiting the area — productivity depends on which plants are growing there.
    Photosynthetic capacity of the plants — plants capture only $\displaystyle 2$–$\displaystyle 10$ per cent of the photosynthetically active radiation reaching them.
    Availability of nutrients — a shortage limits how much organic matter can be built up.
    Environmental factors — solar radiation, temperature, water and other climatic conditions, which is why productivity varies from one type of ecosystem to another.
    Scale
    The annual net primary productivity of the whole biosphere is about $\displaystyle 170$ billion tons (dry weight) of organic matter.
    Of this the oceans contribute only $\displaystyle 55$ billion tons, even though they occupy about $\displaystyle 70$ per cent of the Earth's surface; the rest is produced on land.

  10. Exercise 12.10

    Define decomposition and describe the processes and products of decomposition.

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    Definition
    Decomposition is the process by which decomposers break down complex organic matter into inorganic substances such as carbon dioxide, water and nutrients.
    The raw material is detritus — dead plant remains such as leaves, bark and flowers, and the dead remains of animals, including faecal matter.
    Processes
    Fragmentation — detritivores such as the earthworm break the detritus into smaller particles.
    Leaching — water-soluble inorganic nutrients pass down into the soil horizon and get precipitated as unavailable salts.
    Catabolism — bacterial and fungal enzymes degrade the detritus into simpler inorganic substances.
    Humification — the accumulation of a dark-coloured amorphous substance called humus, which is highly resistant to microbial action, decomposes extremely slowly and, being colloidal, acts as a reservoir of nutrients.
    Mineralisation — some microbes degrade the humus further, releasing the inorganic nutrients.
    All these steps operate simultaneously on the detritus; humification and mineralisation occur in the soil.
    Products
    Carbon dioxide, water and inorganic nutrients released back to the soil for reuse by the autotrophs, plus humus.
    Rate of decomposition
    Decomposition is largely an oxygen-requiring process.
    Chemical composition of detritus: slower if the detritus is rich in lignin and chitin, quicker if it is rich in nitrogen and water-soluble substances such as sugars.
    Climatic factors: temperature and soil moisture are the most important, acting through the activity of soil microbes. A warm and moist environment favours decomposition, while low temperature and anaerobiosis inhibit it and organic material builds up.

  11. Exercise 12.11

    Give an account of energy flow in an ecosystem.

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    The source and the input
    Except for the deep-sea hydrothermal ecosystem, the Sun is the only source of energy for all ecosystems on Earth.
    Less than $\displaystyle 50$ per cent of the incident solar radiation is photosynthetically active radiation (PAR), and plants capture only $\displaystyle 2$–$\displaystyle 10$ per cent of that PAR — this small amount sustains the entire living world.
    Producers (green plants and photosynthetic bacteria) fix this radiant energy and make food from simple inorganic materials.
    The direction of flow
    Energy flow is unidirectional: Sun → producers → consumers. It is never returned to the Sun.
    Ecosystems are not exempt from the Second Law of thermodynamics — they need a constant supply of energy to build the molecules they require and to counteract the universal tendency towards disorder.
    The route: trophic levels
    Organisms occupy a place in the food chain according to their source of nutrition; that place is their trophic level.
    Producers are the first trophic level, herbivores (primary consumers) the second, and carnivores (secondary consumers) the third; tertiary consumers may follow.
    Each level holds a certain mass of living material at a given time, the standing crop, measured as biomass (fresh or, more accurately, dry weight) or as number per unit area.
    Two channels
    The grazing food chain (GFC) begins with living producers: Grass → Goat → Man. It is the major conduit for energy flow in aquatic ecosystems.
    The detritus food chain (DFC) begins with dead organic matter and runs through decomposers or saprotrophs, mainly fungi and bacteria, which secrete enzymes that break dead material into simple inorganic substances and absorb them. In a terrestrial ecosystem a much larger fraction of energy flows through the DFC than through the GFC.
    The two are connected — some DFC organisms are prey to GFC animals, and omnivores such as cockroaches and crows feed in both — so the chains become a food web.
    The loss at every step
    The amount of energy decreases at each successive trophic level, because some is always lost as heat.
    Transfer follows the $\displaystyle 10$ per cent law — only $\displaystyle 10$ per cent of the energy passes from one trophic level to the next — which is why the number of trophic levels in a grazing food chain is restricted.
    For the same reason the pyramid of energy is always upright and can never be inverted.
    When any organism dies it becomes detritus, an energy source for the decomposers, and the energy finally leaves the system as heat.