Ecosystem  Important Questions for CBSE Class 12 Biology Energy Flow and Ecological Succession

1.Energy Flow
(i)Sun is the only source of energy for all the ecosystems on earth, except for deep sea hydrothermal ecosystem.
(ii)Of the total incident solar radiation, less than 50% is Photosynthetically Active Radiation (PAR).
(iii)Plants capture only 2-10% of the PAR and this small amount of energy sustains the entire living world.
(iv)There is unidirectional flow of energy from the sun to producers and then to consumers. This indicates the first law of thermodynamics.
(v)Ecosystem needs a constant supply of energy to synthesise the molecules they require, to counteract the universal tendency towards increasing disorderliness. This indicates the second law of thermodynamics.
(vi) In an ecosystem, major producers are green plants, i.e. herbaceous and woody plants. In aquatic ecosystem, the producers constitute phytoplanktons, algae and higher plants.
(vii) There are various types of food chains that operate in an ecosystem:
(a)Grazing Food Chain (GFC) starts with green plants called producer at first trophic level.

  • In an aquatic ecosystem, the GFC is the major channel for energy flow.
  • Energy for food chain comes from sun.

A simple Grazing Food Chain (GFC) is depicted below:

Grass ———- > Goat —————— > Men

(Producer)                         (Primary consumer) (Secondary consumer)
(b)Detritus Food Chain (DFC) starts with dead organic matter with decomposers called saprophytes at first trophic level, e.g. bacteria and fungi.

  • A much large fraction of energy flows through this type of food chain.
  • Energy for the food chain comes from organic remain or detritus.
  • Detritus food chain may be connected with the grazing food chain at some levels, forming a food web.

(viii) All animals are consumers or heterotrophs. If they feed on the producers they are called primary consumers, e.g. insects, birds, mammals and molluscs.
(ix)The consumers that feed on herbivores (primary consumers) are called primary carnivores or secondary consumers. The animals that eat primary carnivores are called secondary carnivores.
(x)Food web forms in an ecosystem, when a number of food chains interconnect with each other forming a web-like pattern.

  • In a food web, one organism can hold more than one position.
  • The flow of energy is very difficult to calculate.
  • It is a series of branching lines.

(xi)Trophic level is the specific place occupied by an organism in the food chain in an ecosystem.
There are many trophic levels in nature:
(a) Producer (first trophic level)
(b)Herbivore (second trophic level)
(c)Carnivore (third trophic level)
(d) Top carnivore (fourth trophic level)
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  • Amount of energy decreases at successive trophic levels.
  • Organisms at each trophic level depend on those at the lower trophic level for their energy demands

(xii)Flow of energy through different trophic levels, i.e. from producers to top carnivores can be easily depicted on the basis of the diagram given below:
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  • Each trophic level has a certain mass of living material at a particular time called  the standing crop.
  • Standing crop is measured as the mass of living organism (biomass) or the number in a unit area. ‘
  • The number of trophic levels in the grazing food chain is restricted as the transfer of energy follows 10% law, i.e. only 10% energy is transferred to euch trophic level from  he lower tn hie level

2.Ecological pyramid is the graphical representation of relation between producers and consumers in the form of pyramid.
(i) In pyramid structure, the base (broader) always represents the producers or the first trophic level and the apex (narrower) represents top level consumer or last trophic level.
(ii) Types of ecological pyramids:
(a)Pyramid of number represents the relationship in terms of numbers, between the producers and consumers in an ecosystem. It can be:

  • Upright, e.g. in grassland ecosystem
  • Inverted, e.g. in tree ecosystem

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(b)Pyramid of biomass represents the relationship between producers and consumers in terms of biomass. It can be:

  • Upright, e.g. in grasslands ecosystem
  • Inverted, e.g. in pond ecosystem

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(c)Pyramid of energy states the relationship between the producers and consumers in an ecosystem in terms of flow of energy. It is always upright because the energy is lost as heat at each step
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(iii)A given species may occupy more than one trophic level in the same ecosystem at the same time, e.g. a sparrow is a primary consumer when it eats seeds, fruit, peas and a secondary consumer when it eats insects and worms.
(iv)Ecological pyramids also have certain limitations.

  • It does not take into account the same species belonging to two or more trophic levels.
  • It assumes a simple food chain, which never exists in nature.
  • Saprophytes/decomposers are not given any position in ecological pyramids.

3.Ecological succession is a sequential, gradual and predictable change in the species composition in an area over a period of time.
(i)Sere is the entire sequence of communities that successively change in a given area.
(ii)Serai communities are the individual transitional communities of serai stages.
(iii) Climax community is the community that is in near equilibrium with the  environment.
(iv)Pioneer species is the species that invade a bare area.
(v)To reach a climax community, there are following changes that occur in successive serai stages:

  • Changes in the diversity of species of organisms.
  • Increase in the number of species and organisms.
  • Increase in the total biomass.

(vi)Types of ecological succession:

  • Primary succession begins in an area, where no living organisms are present. It is a very slow process taking thousands of years for climax to be reached.
  • Secondary succession begins in an area, where natural biotic communities have been destroyed, e.g. abandoned from lands, buried or cut forests. It is a faster process as soil is available.

(vii)Succession of plants is of following types:

  • Hydrarch succession occurs in wet areas or water and the successional series progress from hydric to the mesic conditions.
  • Xerarch succession takes place in dry areas and the series progress from xeric to mesic conditions.

Primary Succession in Water

  • The pioneer species are the small phytoplanktons.
  • The phytoplanktons are replaced with time by free-floating angiosperms, then the rooted hydrophytes, sedges, grasses and finally the trees.
  • At last, stable climax forest is formed.
  • In this way, an aquatic habitat is converted into mesic habitat.

Primary Succession on Rocks

  • Lichens are the pioneer species.
  • Lichens secrete acids to dissolve rock, help in weathering and soil formation.
  • Later, small plants like bryophytes hold in small amount of soil.
  • Bryophytes, are succeeded’by bigger plants.
  • After several more stages of succession, ultimately a stable climax forest community is formed.
  • In this way, xerophytic habitat gets converted into a mesophytic one.

The important fact is that all succession whether taking place in water or on land, proceeds to a similar mesic climax community.
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Previous Year  Examination Questions

1 Mark Questions

1.State what does standing crop of a trophic level represent. [Delhi 2013]
Ans.Standing crop represents the total mass of  living material or energy content of all the organisms of a trophic level at a particular time and location

2.What is a detritus food chain made  up of? How do they meet their energy and nutritional requirements? [All India 2013]
Ans.It is made up of decomposers, i.e. some  bacteria and fungi. They meet their energy and nutrient requirements by degrading the dead organic matter of detritus

3.Mention the role of pioneer species in primary succession on rocks.  [Foreign 2012]
Ans.Lichens are the pioneer species in the  succession on rocks. They secrete acids to dissolve the rock and help in weathering and soil formation and pave way to small  plant like bryophytes.

4.List any two ways of measuring the standing crop of a trophic level.[Foreign 2010]
Ans. Standing crop can be measured as:
(i) Biomass of living organism in a unit area.
(ii)Number in a unit area

2 Marks Questions

5.A part from being a part of food chain, predators play other important roles. Mention any two such roles supported by examples.[Delhi 2014]
Ans.Apart from being a part of food chain, predators play important role in:
(i) Maintaining the prey population and also regulates intraspecies competition.
(ii)Indicating ecological disturbances as predators are small in number and highly sensitive to ecological changes owing to their excessive adaptation.

6.It is possible that a species may  occupy more than one trophic level in the same ecosystem at the same time. Explain with the help of one example.[All India 2013,2008 C]
Ans.Yes, the trophic level of species represents the functional level of species in the energy ^ flow. The preference of food habit depends  on the available food and worms.               A single species may occupy more than one trophic level at the same time in ecosystem. For example, sparrow is primary consumer when it eats seeds and secondary consumer when feeds on insects and worms.

7.Name the type of food chains  responsible for the flow of larger fraction  of energy in an aquatic and a terrestrial ecosystem respectively. Mention one difference between the two food chains.    [Delhi 2010]
Ans.Food chain in aquatic ecosystem is the grazing food chain. In terrestrial ecosystem, it is detritus food chain.  Differences between grazing and detritus food chain are:
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 8.Explain standing crop in an  ecosystem. Draw a pyramid of biomass when a small standing crop of phytoplanktons supports a large standing crop of zooplankton in the sea. [Delhi 2010]
Ans.Standing crop represents the total mass of  living material or energy content of all the organisms of a trophic level at a particular time and location
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Inverted pyramid of biomass small standing crop of phytoplankton supports large standing crop of zooplankton

9.Construct a grazing food chain and detritus food chain using the following five links each. Earthworm, bird, snake, vulture, grass, grasshopper, frog, decaying plant matter.           [Foreign 2008]
Ans.Grazir ood chain
Grass — >Grasshopper— > Bird— >Snake -> Vulture
Detritus food chain
Dead and decaying plant matter— >Earthworm— >Bird — >Snake Vulture

3 Marks Questions

10.Differentiate between two different types of pyramids of biomass with the help of an example. [Delhi 2013]
Ans.Pyramid of’ biomass refers to the relationship between producers and consumers in terms of biomass.It can be:

  • Upright, e.g. in grasslands ecosystem
  • Inverted, e.g. in pond ecosystem

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11.Name the pioneer species on a bare rock. How do they help in establishing the next type of vegetation? Mention the type of climax community that will ultimately get established.[Delhi 2009]
Ans.(i)Lichens are the pioneer species on a bare  rock.
(ii) The lichens secrete some acid to dissolve rock and help in weathering and soil formation.
(iii) Later, some small bryophytes invade and hold the small amount of soil.
(iv)The bryophytes are succeeded by herbs, shrubs and ultimately big trees.
(v)At last, a stable climax forest is formed.
(vi)Mesophytic climax community will be established from xerophytic habitat.

12.Construct an ideal pyramid of energy, when 1000000 J of sunlight is available. Label all its levels.[Delhi 2009]
Ans.
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13.Construct a pyramid of biomass  starting with phytoplanktons, label three trophic levels. Is this upright or inverted? Why?                   [All India 2009]
Ans.The pyramid is inverted because the biomass of fishes is much more than that of the phytoplanktons.
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14.Define a climax community. How  does a sere differ from a serai community?     [Delhi 2008 C]
Ans.Climax community can be defined as a community which gets established at the terminal stage of succession and remains in equilibrium with the environment,
Difference between sere and serai  community
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5 Marks Questions

15.(i) Differentiate between primary  and secondary ecological successions.
(ii)Explain the different steps of xerarch succession occurring in  nature.   [Foreign 2014]
Ans.(i)The differences between primary and secondary ecological succession can be summarised as:
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(ii)The different succession in  occurring in xerarch re can be summarised  as:

  • The pioneer species lichens grows on hare rocks
  • They secrete some acids that dissolve rock, help in weathering and soil formation.
  • Allows small plants, e.g. bryophytes to invade and hold some soil.
  • These are succeeded by bigger plants  in order as herbs, shrubs and finally big trees.

16.(i) With suitable examples, explain  the energy flow through different trophic levels. What does each bar in this pyramid represent?
(ii) Write any two limitations of  ecological pyramids. [Delhi 2014 c]
Ans.(i)The energy flows unidirectionally from  the first trophic level (producers) to last trophic level (consumers), and as the energy flows from one trophic level to another, some energy is always lost as heat into the surrounding environment. So, the amount of energy flowing decreases at each successive trophic levels.
This can be explained with the help of a diagram of a grazing food chain.
The pyramid of energy is always upright and each bar in the pyramid indicates the amount of energy the present at each trophic level in a given time or per unit area:
(ii) The limitations of ecological pyramids are:

  • It does not consider the same single species operating at two or more trophic levels.
  • It assumes simple food chains that do not exist in nature and do not accomodate food web.
  • Saprophytes, detritivores and  decomposers are not given any place in pyramids, despite their vital role in ecosystem (any two).

17.Explain how does a primary succession start on a bare rock and reach a climax community?[Delhi 2012]
Ans.Primary succession rocks The species of  organisms that first invade a bare area are called pioneer species. The pioneer species on a bare rock are usually lichens. Lichens secrete acids which dissolve rocks, thereby leading to weathering and soil formation. This paves the way for small plants or bryophytes which hold the soil. They are succeeded by bigger plants and ultimately an entire forest gets established. Forests represent the climax community in this succession.

18.(i) Draw a pyramid of numbers of a  situation, where a large population of insects feed upon a very big tree. The insects in turn, are eaten by small birds which in turn are fed upon by big birds.
(ii) Differentiate giving reasons, between the pyramid of biomass of the above situation and the pyramid of numbers that you have drawn.   [Delhi 2012]
Ans.(i) Pyramid of numbers   showing  interaction between trees, insects., birds and big birds.
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(ii) (a) Pyramid of number is spindle-shaped as the number of insects is maximum. The number of trees and birds are less than the insects. The number is gradually decreasing at each trophic level.
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(b)The pyramid of biomass in this ecosystem is erect because the biomass decreases at each trophic level.

19.(i) Explain the significance of  ecological pyramids with the help of  an example.
(ii) Why are the pyramids referred to as upright or inverted? Explain.[All India 2012]
Ans.(i)Significance of ecological pyramids
They graphically represent the relation between producers and consumers. To calculate energy. content, biomass or numbers, organisms of that trophic level needs to be calculated. A trophic level represents only 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. The ecological pyramids provide an over all idea of the trophic levels occupied by an organism in an ecosystem.
Example A sparrow is a .primary consumer, when it eats seeds, fruits, peas and a secondary consumer when it eats insects and worms.
(ii)Upright pyramids When producers are more in number ar d biomass than the herbivo-es and herbivores are more in  number and bionrss than the carnivores. Energy at a lower trophic level is always more than at a higher trophic level. Pyramid of energy is ah rays upright.
Inverted pyramids When the numbers of producers are less and consumers increase and become largest in top consumer level.Pyramid pf number and biomass may be inverted

20.(i) Draw the pyramids of biomass in  a sea and in a forest. Explain giving reason, why the two pyramids different.
(ii) Pyramid of energy is always upright. Explain.[HOTS; Foreign 2010]
Ans.(i) (a) The pyramid of biomass in a ;ea ecosystem is inverted. Because, the sum total of the weight of phytoplankton (producer) is far less than a few fishes feeding at higher trophic levels.
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(b)Pyramid of biomass in a forest ecosystem is upright because producers are more in  biomass than primary consumers. Primary consumers are more than secondary consumers and secondary consumers are more than tertiary consumers (top).
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(ii)Pyramid of energy is neve- inverted. Because, .when energy flows from a particular trophic level to the nex. trophic level, some energy is a I wry s lost as heat at each step. Each bar in the energy pyramid indicates the amount of energy present at each trophic level in a given time.

21.(i) Explain how a hydrarch  succession progresses from hydric to mesic condition from a stable community?
(ii) Why is the rate of secondary succession faster than that of primary succession? [Delhi 2010c]
Ans.(i) Hydrarch succession occurs in wet areas,or water, leading to successional series that progress from hydric to the mesic condition.In hydrarch succession, pioneers are phytoplanktons. These phytoplanktons with time are replaced by free floating angiosperms, followed by rooted hydrophytes, sedges, grasses and finally the trees, leading to mesic condition.
(ii) The secondary succession is faster than primary succession because it begins in areas where natural biotic communities have been destroyed such as abondaned farm lands, burned or cut forests. Since, some soil is present succession is faster than primary succession. At any time during primary or secondary succession, natural and human induced disturbance can convert a particular serai stage of succession to an earlier stage.

22.Explain how xerarch succession  progresses from xeric to mesic condition and form a stable climax community. You may use a flow Chart.[All India 2010 C]
Ans.Xerarch succession occurs in dry areas and  the series progress from xeric to mesic condition. The climax community remains stable as long as environment remains unchanged. With time the xerophytic habitat gets converted into a mesophytic one.
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It occurs in following steps as shown in  flow chart:
(i) Lichens secrete some acids which dissolve rocks and help in weathering and soil formation,
(ii) Bryophytes invade and hold some soil.
(iii) Bryophytes are succeeded by herbs, shrubs and ultimately big trees.
(iv) At last, a stable climax forest is formed.
(v) Finally, a xerarch condition is converted into a mesic one.

23.All successions proceed to a similar  climax community, the mesic. Explain. [HOTS; All India 2008]
Ans.All succession proceed to a similar climax community, i.e. the mesic.
(i) Succession of plants occurs as hydrarch or xerarch.
(ii) Hydrarch succession occurs in water bodies and the successional series progress from hydric to mesic conditions.
(iii) Xerarch succession takes place in dry areas and the series progress from xeric to mesic conditions.
(iv) Hence, both xerarch and hydrarch conditions lead to medium water conditions (mesic).
So, it can be said that all successions proceed to a similar complex community.
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