VBQs Class 12 Biology Ecosystem

Very Short Answer Type Questions

Question. Define Standing Crop. 
Answer :
Each trophic level has a certain mass of living material at a particular time called as the standing crop. 

Question. What is detritus food chain made up of ? How do they meet their energy and nutritional requirements ?  
Answer :
Detritus food chain is made up of decomposer organisms, which are mainly the microorganisms like bacteria and fungi. They meet their energy and nutritional requirements by degrading dead organic matter or detritus. 

Question. State what does ‘standing crop’ of a trophic level represent.
Answer :
It is the total amount of living matter or organic matter present in an ecosystem in a unit area and unit time. It is measured as the mass of living organisms i.e. biomass or their number in unit area.
The biomass of a species is expressed in terms of dry or fresh weight. 

Question. What is the starting point of a detritus food chain.
Answer :
Detritus food chain (DFC) start from detritus or dead and decaying organic matter i.e. dead plants and animals waste and animal faeces. Detrivores and decomposers feed over it. 

Question. Write the basis on which an organism occupies a space in its community/natural surroundings.
Answer :
Feeding relationship with other organisms. 

Short Answer Type Questions

Question. Why is it difficult to get rid of ‘water hyacinth’ from a water body ? Name one abiotic component and one biotic component of the ecosystem that get affected by its spread in the water body.
Answer :
Water hyacinth have caused havoc by their excessive growth. They grow abundantly in eutrophic water bodies and lead to an imbalance in the ecosystem dynamics of the water body.
Abiotic component : Water hyacinth impacts water flow and blocks oxygen.
Biotic component : This plant blocks sunlight from reaching native aquatic plants and poses threats to them. 

Question. List the features that make a stable biological community. 
Answer :
Following features make the biological community stable :
(i) The community is an equilibrium with the biotic and abiotic factors.
(ii) In a community there is a large number of small and larger sized organisms.
(iii) The organisms in a stable community have longer life span. 

Question. In a botanical garden of a city there is a huge banyan tree growing on which hundreds of birds and thousands of insects live. Draw the pyramids of numbers and also biomass represented by this community. Comment giving reasons on the two different pyramids drawn.
Answer :
Pyramid of Numbers – Inverted, number is increasing as one moves from producers to secondary consumers.

VBQs Class 12 Biology Ecosystem

(Shape, labeling, reason) 
Pyramids of Biomass – upright – a huge tree with maximum biomass supports several birds with lesser biomass and insects with even lesser biomass.

VBQs Class 12 Biology Ecosystem

(Shape, labeling, reason)

Question. Differentiate between primary and secondary succession. Provide one example of each.
Answer :

S. No. Primary SuccessionSecondary Succession
(i) It takes place in area where no living organisms ever existed.It takes place in areas which have lost all life forms due to any destructions like floods.
(ii) Process of establishment of biotic community is very slow, as there is no fertile soil present.Since, some soil or sediment is present, it is faster than primary succession.
(iii)Example : Succession on newly cooled lava and bare rock.Example : Succession in abandoned farm lands and burnt forest.

Question. Explain successions of plants in xerophytic habitat until it reaches climax community.
Answer :
Lichens on bare rock, secrete acids to dissolve rock (weathering of soil), Bryophytes to hold soil, water, grass, small plants/shrubs, tree-forest
Detailed Answer:
The species that invade a bare area are called pioneer species. In primary succession on rocks, lichens are the pioneer species which secrete acids to dissolve rock, helping in weathering and soil formation. These result in some very small plants like bryophytes, which take hold in the small amount of soil. With the passage of time, these are succeeded by bigger plants and after several more stages, ultimately a stable climax forest community is formed. The climax community remains stable as long as the environment remains unchanged.

Question. How does a detritivore differ from a decomposer ?
Explain each with an example.
Answer :

S. No. DetritivoreDecomposer 
(i)Feeds on waste dead plant and animal remains including faecal matter.Degrades dead organic matter.
(ii)Breaks feeding material into fragments.Secretes enzymes into dead organic matter for decomposition.
(iii)e.g., Earthworm. e.g., Bacteria / Fungi.

Question. Differentiate between two different types of pyramids of biomass with the help of one example of each. 
Answer :
Pyramids of biomass are of both types, upright and inverted. Upright pyramid of biomass can be found in forest and grassland ecosystems, while inverted pyramid of biomass is seen in lake and ocean ecosystem. Biomass on next trophic level is higher than previous trophic level in inverted pyramid.

VBQs Class 12 Biology Ecosystem
VBQs Class 12 Biology Ecosystem

Question. Match the column A with any two of the two column B.

VBQs Class 12 Biology Ecosystem

Answer :

VBQs Class 12 Biology Ecosystem

Long Answer Type Questions

Question. (i) Colonization of a rocky terrain is a natural process. Mention the group of organisms which invade this area first. Give an example.
(ii) Over the years, it has been observed that some of the lakes are disappearing due to urbanization. In absence of human interference, depict by making a flow chart, how do the successional series progress from hydric to mesic condition.
(iii) Identify the climax community of hydrarch and xerarch succession. 
Answer :
(i) Pioneer species, lichen 
(ii) Phytoplankton-hydric 

Submerged plant stage

Submerged free floating plant stage

Reed swamp stage

Marsh-meadow stage

Shrub stage

Forest stage-Mesic
(iii) Forest 

Question. Explain the different steps of xerarch succession occurring in nature.
Answer : Xerarch Succession :
(a) Takes place in dry area hence progress from xeric to mesic condition.
(b) Pioneer species such as lichens secrete acids to break rocks, initiate soil formation.
(c) lichens pave way to bryophytes.
(d) which are succeeded by bigger plants.
Ultimately, stable mesic community is formed. 

Question. (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. 
Answer :
(i) Significance of ecological pyramids:
(a) Helps in comparing different ecosystems.
(b) Helps in studying seasonal variations and changes in ecosystem.
(c) Helps in studying amount of energy transfer, biomass production, number of organisms at each trophic level in ecosystems.
For e.g. pyramid of number in grassland: In this type of ecological pyramid, the number of producers is followed by number of herbivores, which in turn are followed by number of secondary and tertiary carnivores. Hence, the number of individuals at the producer level will be maximum while the number of individuals present at top carnivores will be least.

VBQs Class 12 Biology Ecosystem

(ii) Pyramids can be upright or inverted. For e.g. Pyramid of energy is always upright as only 10% energy is transferred from one trophic level to the next while pyramid of biomass is inverted in pond ecosystem. Inverted pyramid shows less number / biomass of producers when compared to primary consumers. 

Question. (i) Draw an ideal pyramid of energy upto four trophic levels where 10,000 J are available from sunlight to the primary producer. Indicate the amount of end product available at each trophic level.
(ii) Why is pyramid of energy always upright? Explain.
(iii) Mention the limitations of an ecological pyramid.
Answer : (i) (a) Please note: In fig. 14.4

VBQs Class 12 Biology Ecosystem

the energy from sunlight mentioned is 1000,000 J. However, in question it is 10,000 J. Hence, there will be change in the energy transferred at different tropic levels. Energy Levels will start from100, 10 and 1
(b) Figure 14.4 shows an ideal pyramid of energy.
Observe that primary producers convert only 1% of the energy in the sunlight available to them into NPP.
(ii) Pyramid of energy is always upright, can never be inverted, because when energy flows from a particular trophic level to the next tropic level, some energy is always 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 or annually per unit area.
(iii) There are certain limitations of ecological pyramids such as :
(a) It does not take into account the same species belonging to two or more trophic levels.
(b) It assumes a simple food chain, something that almost never exists in nature.
(c) It does not accommodate a food web. 
(d) Moreover, saprophytes are not given any place in ecological pyramids even though they play a vital role in the ecosystem. 

Question. (i) Comment on the pattern in which all communities undergo a change in composition and structure with changing environmental conditions.
(ii) Explain ‘Climax community‘ and ‘sere‘.
(iii) Differentiate between primary and secondary succession with examples.
Answer :
(i) Orderly and sequential changes, parallel with changes in physical environment 
(ii) Climax community – changes finally, lead to a community that is in equilibrium with environment. 
Sere – the entire sequence of communities that successively change in a given area. 
(iii) Primary succession
(i) Occurs in newly cooled lava / bare rock / newly created pond.
(ii) Slow process.
Secondary succession
(i) Occurs in abandoned / destroyed forest.
(ii) Fast process. 

Question. Draw the pyramids of biomass in sea and in a forest. Explain giving reasons why are the two pyramids different ?
Answer : Pyramid of biomass in Sea :

VBQs Class 12 Biology Ecosystem

Pyramid of biomass in a Forest :

VBQs Class 12 Biology Ecosystem

Sea – Inverted , because biomass of fish /other aquatic animals exceeds that of phytoplanktons / small standing crop of phytoplankton supports large standing crop of zooplankton. 
Forest – Upright, because biomass of producers exceeds that of herbivores / carnivores / allows the sharp decrease in biomass at higher trophic levels.

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