CBSE Class 12 Biology Ecosystem Worksheet Set B

 QUESTION

1 Differentiate between a detritivore and a decomposer giving an example of each.

2 Expand PAR

3 Define climax community.

4 Identify the pioneer species of xerarch and hydrarch succession.

5 List the steps involved in decomposition process.

6 What I ecological succession? Write different stages of xerarch succession.

7 hat is primary productivity? How is it different from Net Primary Productivity.

9 Name the pioneer and the climax species in a water body. Mention the changes observed in the biomass and the biodiversity of the successive seral communities developing in the water body.

10 Name the pioneer species on a barren rock. How do they help in establishing the next type of vegetation? Mention the type of climax community that will ultimately get established?

11 Construct an ideal pyramid of energy when 1,00,000 joules of sunlight is available. Label all its trophic levels.

12 Construct a pyramid of biomass starting with phytoplanktons. Label 3 trophic levels. Is the pyramid upright or inverted? Why?

13 Draw and complete the following model of carbon cycle filling a, b, c, d, e and f

Important Questions for NCERT Class 12 Biology Ecosystem

Question. Which of the following regulate the rate of release of nutrients into atmosphere
(a) Soil
(b) Moisture
(c) pH and Temperature
(d) All the above

Answer : D

Question. In net primary productivity of whole biosphere what is the contribution of oceans in billion ton ?
(a) 50
(b) 70
(c) 42
(d) 55

Answer : D

Question. What percentage of carbon constitute the dry weight of organisms
(a) 49 %
(b) 71 %
(c) 30 %
(d) 20 %

Answer : A

Question. Leaching is one of the important step of decomposition. During leaching, which of the following nutrient go down into the soil horizon ?
(a) Water soluble inorganic substance
(b) Water insoluble inorganic substances
(c) Water soluble organic substances
(d) Both water soluble organic substances and inorganic substances

Answer : D

Question. Which of the following chemical will not reduce the rate of decomposition of detritus
(a) Lignin
(b) Chitin
(c) Cutin
(d) Sugars

Answer : D

Question. How much amount of incident solar radiation and PAR is capture as GPP respectively :-
(a) 1-5% and 2-10%
(b) 2-10% and 1-5%
(c) 1-4% and 2-8%
(d) 0.4-4% and 0.8-8%

Answer : A

Question. The components of ecosystem are seen to function as an unit, when we consider which of the folloiwng aspect
(a) Productivity and decomposition
(b) Decomposition and energy flow
(c) Productivity and energy flow
(d) Productivity, decomposition, energy flow and nutrient cycling

Answer : D

Ques. Energy transfer from one trophic level to other, in a food chain, is
(a) 10%
(b) 20%
(c) 1%
(d) 2%.

Answer: A

Ques. In a terrestrial ecosystem such as forest, maximum energy is in which trophic level?
(a) T3
(b) T4
(c) T1
(d) T2 

Answer: C

Ques. The 10% energy transfer law of food chain was given by
(a) Lindemann
(b) Tansley
(c) Stanley
(d) Weismann. 

Answer: A

Ques. In a biotic community, the primary consumers are
(a) detritivores
(b) herbivores
(c) carnivores
(d) omnivores.

Answer: B

Ques. The dominant second trophic level, in a lake ecosystem, is
(a) phytoplankton
(b) zooplankton
(c) benthos
(d) plankton. 

Answer: B

Ques. Food chain in which microorganisms breakdown the food formed by primary producers is
(a) parasitic food chain
(b) detritus food chain
(c) consumer food chain
(d) predator food chain. 

Answer: B

Ques. Pick up the correct food chain.
(a) Grass → Chameleon → Insect → Bird
(b) Grass → Fox → Rabbit → Bird
(c) Phytoplankton → Zooplankton → Fish
(d) Fallen leaves → Bacteria → Insect larvae

Answer: C

Ques. Upper part of sea/aquatic ecosystem contains
(a) plankton
(b) nekton
(c) plankton and nekton
(d) benthos. 

Answer: A

Ques. What is true of ecosystem?
(a) Primary consumers are least dependent upon producers.
(b) Primary consumers out-number producers.
(c) Producers are more than primary consumers.
(d) Secondary consumers are the largest and most powerful. 

Answer: C

Ques. In an ecosystem, which one shows one-way passage?
(a) Free energy
(b) Carbon
(c) Nitrogen
(d) Potassium 

Answer: A

Ques. Match the trophic levels with their correct species examples in grassland ecosystem.
(A) Fourth trophic level (i) Crow
(B) Second trophic level (ii) Vulture
(C) First trophic level (iii) Rabbit
(D) Third trophic level (iv) Grass
Select the correct option.
     (A) (B) (C) (D)
(a) (ii) (iii) (iv) (i)
(b) (iii) (ii) (i) (iv)
(c) (iv) (iii) (ii) (i)
(d) (i) (ii) (iii) (iv) 

Answer: A

Ques. Which of the following ecological pyramids is generally inverted?
(a) Pyramid of biomass in a sea
(b) Pyramid of numbers in grassland
(c) Pyramid of energy
(d) Pyramid of biomass in a forest 

Answer: A

Ques. What type of ecological pyramid would be obtained with the following data?
Secondary consumer : 120 g
Primary consumer : 60 g
Primary producer : 10 g
(a) Inverted pyramid of biomass
(b) Pyramid of energy
(c) Upright pyramid of numbers
(d) Upright pyramid of biomass 

Answer: A

Ques. Which ecosystem has the maximum biomass?
(a) Grassland ecosystem
(b) Pond ecosystem
(c) Lake ecosystem
(d) Forest ecosystem

Answer: D

Ques. The upright pyramid of number is absent in
(a) pond
(b) forest
(c) lake
(d) grassland. 

Answer: B

Ques. Which one of the following statements for the pyramid of energy is incorrect ?
(a) Its base is broad.
(b) It shows energy content of different trophic level organisms.
(c) It is inverted in shape.
(d) It is upright in shape.

Answer: C

Ques. Which one of the following is not used for construction of ecological pyramids?
(a) Fresh weight
(b) Dry weight
(c) Number of individuals
(d) Rate of energy flow 

Answer: A

Question. -------------and his colleges have very recently tried to put price tags on Nature‘s life support services.
(a) Robert Constanza
(b) Robert May
(c) Paul Erlich
(d) Alexander Von Humbolt

Answer: A

Question. Pyramid of -------------- is always upright, can never be inverted.
(a) Biomass
(b) Numbers
(c) Energy
(d) None of these

Answer: C

Question. In a particular climatic condition, decomposition rate is slower if detritus is rich in -----------
(a) Nitrogen and sugar
(b) Lignin and chitin
(c) Sugar and chitin
(d) None of these

Answer: B

Question. Based on the source of their nutrition or food, organisms occupy a specific place in the food chain is known as ---------.
(a) Trophic level
(b) Biomass
(c) Ecological pyramid
(d) None of these

Answer: A

Question. If we completely remove the -----------from an ecosystem, their functioning will be adversely affected because Mineral movement will be blocked.
(a) Grass
(b) carnivores
(c) Decomposers
(d) Herbivores

Answer: C

Question. Which of the following is a man-made ecosystem
(a) Forest
(b) Aquarium
(c) River
(d) Desert

Answer: B

Question. Gross primary productivity of an ecosystem is the rate of production of organic matter by ---------------
(a) Respiration
(b) Decomposition
(c) Photosynthesis
(d) None of these

Answer: C

Question. Vertical distribution of different species occupying different levels represents:
(a) Productivity
(b) Standing crop
(c) Stratification,
(d) Trophic level

Answer: C

Question. 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) 30 %
(c) 2-10 %
(d) 50%

Answer: C

Question.------------- are not given any place in ecological pyramids even though they play an important role in the ecosystem.
(a) Saprophytes
(b) Producers
(c) Consumers
(d) Abiotic factors

Answer: A

Assertion - Reason Type Questions:

A. If both Assertion and Reason are true and Reason is the correct explanation of Assertion.
B. If both Assertion and Reason are true but Reason is not the correct explanation of Assertion.
C. If Assertion is true but Reason is false.
D. If both Assertion and Reason are false.

Question. Assertion: Most food chains and webs have only about four or five trophic levels.
Reason: Trophic efficiencies are generally only about 10% in different ecosystems.

Answer: A

Question. Assertion: Some aquatic ecosystems have inverted biomass pyramids.
Reason: The pyramid of energy is also inverted in such cases.

Answer: C

Question. Assertion: Stratification helps in accommodation of a large number and types of plants in the same area.
Reason: It is the occurrence of horizontal zonation in the ecosystem.

Answer: C

Question. Assertion: Most of an ecosystem‘s production is eventually consumed by detritivores and decomposers.
Reason: Energy is said to flow through, not cycle within the ecosystem.

Answer: B

Question. Assertion: There is no limitation of the number of trophic levels in a detritus food chain.
Reason: The transfer of energy between successive trophic levels in a detritus food chain does not follow 10% rule.

Answer: D

Question. Assertion: Net primary productivity is higher in warm climates than cold ones.
Reason: The limit on a community‘s productivity is determined ultimately by the amount of sunlight it receives.

Answer: A

Question. Assertion: Strongly vertically stratified habitats are very stable ecosystems.
Reason: Through the formation of different layers a given habitat is better utilized.

Answer: A

Question. Assertion: Ecological pyramids of biomass are generally inverted in sea.
Reason: Biomass of fishes far exceeds that of phytoplankton.

Answer: A

Question. Assertion: Saprophytes play a vital role in ecosystem.
Reason: Saprophytes are accorded the highest trophic levels in a food chain or food web.

Answer: C

Question. Assertion: Both agriculture and aquaculture are man – maintained ecosystems.
Reason: All biotic and abiotic factors are managed by humans in these ecosystems.

Answer: A

Question. Assertion: Pyramid of biomass is always upright for tree ecosystem.
Reason: Total biomass of a tree in a specific area is more than that of herbivores.

Answer: A

Short Answer Type Questions:

Question. Write a difference between net primary productivity and gross productivity.
Answer: Gross productivity – Rate of production of organic matter during photosynthesis.
Net primary productivity – Available biomass for the consumption of heterotrophs.
GPP – R = NPP. 
Detailed Answer:
The rate of production of organic matter during photosynthesis represents the gross primary productivity (GPP) while the net primary productivity is the available biomass for the consumption by the heterotrophs.
GPP—R = NPP.
(Gross primary productivity–Respiration = Net primary productivity)

Question. Why is an earthworm called detrivore ?
Answer: Earth worms are called detrivores because they break down the detritus i.e. the dead plant and animal remains including faecal matter into smaller particles. 

Question. Why are green algae not likely to be found in the deepest strata of the ocean ?
OR
Why are green plants not found beyond certain depth in ocean? 
Answer: Green algae are not found beyond a certain depth in ocean because only about 1 percent of sunlight penetrates at this depth which is not sufficient for the plants to photosynthesize and thus grow and survive. 

Question. How is ‘stratification’ represented in a forest ecosystem ? 
Answer: Stratification is the formation of different layers (strata where vegetation is dense such as tropical rain forest). It represents vertical distribution of species at different levels.
There may be many strata such as long trees, medium trees, short tress, shrubs, herbs and ground flora etc. Trees occupy vertical strata, shrubs the second layer and herbs / grasses occupy the bottom layers.

Question. What does secondary productivity in an ecosystem indicate?
Answer: The rate of assimilation and formation of new organic matter by consumers is called secondary productivity.

Question. How does the chemical composition of detritus affect the rate of decomposition in a particular climatic condition?
Answer: The rate of decomposition is slower if the detritus is rich in lignin and chitin and quicker if the detritus is rich in nitrogen and water-soluble substances like sugar.

Question. Citing Lake as an example of a simple aquatic ecosystem, interpret how various functions of this ecosystem are carried out.
Answer: i) productivity: conversion of inorganic into organic material with the help of solar energy by the autotrophs.
ii) energy flow: unidirectional moment of energy towards higher trophic level.
iii) decomposition: fragmentation, leaching, catabolism, humification, mineralisation by bacteria fungi.

Question. What do you mean by standing crop?
Answer: The standing crop is measured as the mass of living organisms or the number in a unit area.

Question. In a food chain a trophic level represents a functional level not a species. Explain.
Answer: A given species may occupy more than one trophic level in the same ecosystem at a given time, if the function of the mode of nutrition of species changes it position shall change in the trophic levels. The same species can be at primary consumer level in one food chain and at secondary consumer level in another food chain in the same ecosystem at the given time.

Question. Why is the length of a food chain in an ecosystem generally limited to 3 -- 4 trophic levels? Explain with an example.
Answer: The amount of energy flow decreases with successive trophic levels as only 10% of energy is transferred from one trophic level to the next successive trophic level. The energy is lost in the form of respiration and other vital activities to maintain life. If more trophic levels are present the residual energy will be limited and decreased to such an extent that it cannot further support any trophic level by the flow of energy. so, the food chain is generally limited to 3-- 4 trophic levels only.

Question. Primary productivity varies in different types of an ecosystem why?
Answer: Primary productivity depends on the plant species in habiting a particular area, it also depends on the variety of environment factors, availability of nutrients and photosynthetic capacity of a plants. Therefore, it varies from one ecosystem to another ecosystem.

Question. What are the limitations 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 consider the fact that species may belong to two or more tropic levels at a time. Also, saprophytes despite their vital role in ecosystem are given no place in the ecological pyramid.

Question. 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.
Answer: For example, sparrow is an omnivore. When it eats seeds fruits or any other plant product it occupies a primary trophic level where as when it eats worms and any other insect it occupies the secondary trophic level thus it occupies more than one trophic level in the same ecosystem.

Question. State the difference between the first trophic levels of the detritus food chain and grazing food chain.
Answer: 
First Trophic Level of Detritus Food Chain
(i) Decomposers are the first trophic level.
(ii) They break down the complex organic matter into simpler form by secreting enzymes.
First Trophic Level of Grazing Food Chain
(i) Producers are the first trophic level.
(ii) They prepare complex organic molecules from simple inorganic material with the help of sunlight.

Question. Explain the steps of decomposition which operates simultaneously on the detritus.
Answer: The steps of decomposition which operate simultaneously on detritus are fragmentation, leaching and catabolism.
(a) Fragmentation: the process of breaking down of the detritus in the smaller particles is called fragmentation.
(b) Leaching: the process by which water soluble in organic nutrients go down into the soil and get precipitated as unavailable salt.
(c) Catabolism: the enzymatic process by which degraded detritus is converted into simple inorganic substance is called catabolism.

Question. (i) What is primary productivity ? Why does it vary in different types of ecosystems ?
(ii) State the relation between gross and net primary productivity. 
Answer: (i) Primary productivity : Production of biomass / produced energy per unit area in a certain time period (per year) by plants during photosynthesis.
It depends upon – plant species inhabiting a particular area, environmental factors, availability of nutrients, photosynthetic capacity of plants.
(ii) GPP – R = NPP
where,
NPP = Net Primary Productivity
GPP = Gross Primary Productivity
R = Respiration Losses

Question. What does secondary productivity in an ecosystem indicate ? List any two factors by which productivity is limited in an aquatic ecosystem.
Answer: Secondary productivity indicates the organic matter synthesized by the consumers specially
by the primary consumers or herbivores. The two factors by which the productivity is limited in an aquatic ecosystem are light and nutrient supply. 

Question. Write about ‘humification’ and the ‘mineralisation’ occurring during the process of decomposition.
Answer: Humification leads to break down of complex organic matter and accumulation of humus in the form of a dark amorphous substance in soil.
Mineralisation is a process in which humus is degraded by microbes to release inorganic nutrients. 

Question. Why is earthworm considered a farmer’s friend?
Explain humification and mineralisation occurring in decomposition cycle.
Answer: Earthworms are called farmer’s friends because they bring about the fragmentation of detritus and loosening of soil.
Humification is the process of formation of humus from detritus. Mineralization is the process of release of inorganic substances as both non-mineral and minerals from organic matter. 

Question. How does the dead organic matter get decomposed in nature ? Explain.
Answer: In nature, the dead organic matter gets decomposed by the micro-organisms like bacteria and fungi. These microorganisms called decomposers break down the complex organic matter into simpler inorganic substances like CO₂, water and nutrients. 

Long Answer Type Questions:

Question. What is primary productivity? Give brief description of factors that affect primary productivity.
Answer: Primary productivity is the rate of synthesis of biomass by producers per unit time per unit area through the process of photosynthesis.
Factors affecting primary productivity:
This varies from ecosystem to ecosystem. This is due to various factors that listed below:
i)The plant species inhabiting a particular area.
ii)Various environmental factors also contribute in affecting primary productivity. They are,
2 Light: Sunlight is the ultimate source of energy. Due to the less availability of light in an aquatic ecosystem, here productivity is less than terrestrial ecosystem. Maximum light is available in tropics and poles receives the minimum light. Therefore, productivity is comparatively low on poles.
3 Temperature: It regulates the activity of an enzyme, so optimum temperature is required for proper functioning of an enzyme.
Moisture: Rain/humidity increases the productivity of the ecosystem, but it tends to decrease with the scarcity of water. So, deserts have the lowest primary productivity as soil is deficient in moisture.
4 Nutrients availability: Nutrients are essential for the growth of producers. Thus, higher the nutrients, greater the primary productivity.

Question. What is an ecological pyramid? Compare the pyramid of energy, pyramid of biomass and pyramid of number.
Answer: The relation between producers and consumers in an ecosystem can be graphically represented in the form of a pyramid is called ecological pyramid.
Ecological pyramids are of 3 kinds
(c) Pyramid of number
(d) Pyramid of biomass
(c) Pyramid of energy.
Pyramid of number: The relationship between producers and consumers in an ecosystem can be represented in the form of a pyramid in terms of number is called pyramid of number.
Pyramid of biomass: The relationship between producers and consumers in an ecosystem can be represented in the form of pyramid in terms of biomass is called pyramid of biomass. It can be upright or inverted.
Pyramid of energy: The relationship between producers and consumers in an ecosystem can be represented in the form of pyramid in terms of flow of energy called pyramid of energy. It is always upright as energy is lost as heat at each step.

Question. a) Taking an example of a small pond, explain how the four components of an ecosystem function as unit?
b) Name the type of food chain that exists in a pond.
Answer: a) Pond ecosystem: Pond is an example of an aquatic ecosystem, it is fairly a self-sustainable unit. A pond is a shallow water body in which all the four basic components of an ecosystem are well exhibited.
α The abiotic component is the water with all the dissolved inorganic and organic substances and the rich soil deposits at the bottom of the pond. The solar input, the cycle of temperature, day-length and other climatic conditions regulate the rate of function of the entire pond.
β The autotrophic biotic components include the phytoplankton, some algae and the floating submerged and marginal plants found at the edges.
χ The consumers are represented by the zooplankton, the free swimming and bottom dwelling forms.
δ The decomposers are the fungi, bacteria and flagellates especially abundant in the bottom of the pond.
This system performs all the functions of any ecosystem and of the biosphere as a whole. ie, conversion of inorganic into organic material with the help of radiant energy of the sun by the autotrophs, consumption of the autotrophs by heterotrophs, decomposition and mineralization of the dead matter to release them back or reuse by the autotrophs etc These events are repeated over and over again. There is unidirectional movement of energy towards higher trophic levels and its dissipation and loss as heat to the environment.
b) Grazing food chain.

Question. Taking a small pond as an example of an ecosystem, list the four components of this ecosystem. How do these components function as a unit in a small pond ? 
Answer: Four components of small pond ecosystem are:
Abiotic components – air, water, soil, light, temperature, other climatic conditions.
Biotic components – producers-phytoplanktons, some algae, floating and submerged plants.
Consumers of various order – Zooplanktons
Decomposers – Fungi, bacteria and other flagellates.
Components work as a unit for the following functions :
Productivity : Conversion of inorganic into organic material through photosynthesis with the help of solar energy and consumption of autotrophs by heterotrophs i.e. secondary productivity.
Decomposition : Decomposition and mineralisation of the dead matter.
Nutrient Cycling : To release the nutrients / elements back for use by autotrophs.
Energy Flow : Unidirectional flow of energy and its subsequent dissipation as one moves towards higher trophic levels.

Question. (i) Explain primary productivity and the factors that influence it.
(ii) Describe how oxygen and chemical composition of detritus control decomposition.
Answer: (i) Primary productivity : Amount of biomass / organic matter produced per unit area over a time period by the plant during photosynthesis.
Factors : availability of nutrients / quality of light available / availability of water / temperature of the given place / type of plant species of the area / photosynthetic capacity of the plants.
(ii) Oxygen increases rate of decomposition.
Decomposition is an oxygen consuming process. It takes place under aerobic conditions i.e. in presence of oxygen.
Chemical decomposition is slow when chitin and lignin are present and fast when cellulose and water soluble substances are present.

Question. (i) Taking an example of a small pond, explain how the four components of an ecosystem function as a unit.
(ii) Name the type of food chain that exists in a pond.
Answer: (i) Productivity : Conversion of inorganic substances into organic material with the help of radiant energy / sunlight by the autotrophs / producers (phytoplankton, algae, floating, submerged plants). 
Decomposition : Decomposers (fungi, bacteria, flagellates) breakdown dead decayed organic matter into simpler compounds. 
Energy flow : Unidirectional movement of energy towards higher trophic levels (producer to consumer) and its dissipation and loss as heat to the environment. 
Nutrient cycle : Mineralisation of dead matter to release them back for reuse of autotrophs. 
(ii) Grazing food chain / detritus food chain. 
Detailed Answer :
(i) The various aspects taken into consideration to study the functioning of ecosystem are :
Productivity, decomposition, energy flow and nutrient cycling. The four components of ecosystem (viz : abiotic, biotic, consumers and decomposers) function as a unit as follows :
(a) Productivity : If we take the example of pond ecosystem, the phytoplanktons are the organisms that capture the solar energy to perform photosynthesis. Thus, they contribute to primary productivity.
(b) Energy flow : They are then consumed by the zooplanktons. Zooplanktons are eaten by small fishes, which are in turn eaten by big fishes.
Hence, there is constant unidirectional flow of energy between different trophic levels from producer to consumers.
(c) Decomposition and Nutrient cycling : When any organisms dies at any trophic level, the various microbes present in the pond water decompose the dead remains. The nutrients that are released in the process of decomposition are again available to the producers for the primary productivity.
(ii) Grazing detritus type of food chain is present in a pond ecosystem.

Diagram Based & Case Based Questions:

1. Organisms in an ecosystem acquire energy in a variety of ways, which is transferred between trophic levels as the energy flows from the bottom to the top of the food web, with energy being lost at each transfer. The efficiency of these transfers is important for understanding the different behaviours and eating habits of warmblooded versus cold-blooded animals. Modelling of ecosystem energy is best done with ecological pyramids of energy, although other ecological pyramids provide other vital information about ecosystem structure. 

Question. The essence of second law of thermodynamics is ‗no form of energy can fully convert into another form of energy‘. Analyse this statement with reference to energy flow.
Answer: During energy flow some amount of energy is lost in the form of heat.

Question. 'Pyramid of energy is always upright‘. Substantiate this statement.
Answer: Pyramid of energy is always upright, can never be inverted, because when energy flows from a particular trophic level to the next trophic level, some energy is always lost as heat at each step.

2. Observe the diagram and answer the following questions 

Question. How temperature and moisture content influence decomposition?
Answer: Warm and moist environment favour decomposition whereas low temperature and anaerobiosis inhibit decomposition resulting in build-up of organic materials.

Question. List out the stages in decomposition.
Answer: The stages in decomposition are, a. Fragmentation b. Leaching c. Catabolism d. Humification e. Mineralisation.

Question. Shell of a prawn and a piece of wood are undergoing decomposition. Compare their rate of decomposition.
Answer: Shell of prawn is composed of chitin but wood predominantly contains cellulose. So, rate of decomposition is lower i shell of prawn.

3. Analyse the graph and answer the following questions 

Question. Despite occupying about 70 percent of the surface, the productivity of the oceans are only 55 billion tons. 
Answer: The various reasons include, Non-uniformity in the availability of sunlight.

Question. How GPP and NPP is related with respiration?
Answer: Non-availability of sufficient minerals.
Size of producers.

Question. List out the reasons for the variations in productivity in different ecosystems.
Answer: Primary productivity depends on, 
a. The plant species inhabiting a particular area.
b. Environmental factors.
c. Availability of nutrients.
d. Photosynthetic capacity of plants.