CBSE Class 12 Biology Ecosystem Worksheet Set B

Read and download free pdf of CBSE Class 12 Biology Ecosystem Worksheet Set B. Download printable Biology Class 12 Worksheets in pdf format, CBSE Class 12 Biology Chapter 14 Ecosystem Worksheet has been prepared as per the latest syllabus and exam pattern issued by CBSE, NCERT and KVS. Also download free pdf Biology Class 12 Assignments and practice them daily to get better marks in tests and exams for Class 12. Free chapter wise worksheets with answers have been designed by Class 12 teachers as per latest examination pattern

Chapter 14 Ecosystem Biology Worksheet for Class 12

Class 12 Biology students should refer to the following printable worksheet in Pdf in Class 12. This test paper with questions and solutions for Class 12 Biology will be very useful for tests and exams and help you to score better marks

Class 12 Biology Chapter 14 Ecosystem Worksheet Pdf

 

 ECOSYSTEM

An ecosystem is a functional unit of nature, where living organisms interact among themselves and also with the surrounding physical environment.
 
Types of ecosystems
 
• Terrestrial ecosystem: Forest, grassland, desert etc
• Aquatic ecosystem: Pond, lake, wetland, river & estuary
• Man-made ecosystem: Crop fields and aquarium
The entire biosphere can be regarded as a global ecosystem.
 
ECOSYSTEM – STRUCTURE & FUNCTION
 
- In an ecosystem, interaction of biotic and abiotic components occurs. These components function as a unit.
- Vertical distribution of different species occupying different levels is called stratification. E.g. trees occupy top vertical strata (layer) of a forest, shrubs the second and herbs and grasses occupy the bottom layers.
4 basic components of functioning of an ecosystem:
i. Productivity ii. Decomposition
iii. Energy flow iv. Nutrient cycling Pond (Aquatic ecosystem)
- A pond is a shallow, simple, self-sustainable water body
that exhibits all basic components of an ecosystem.
- Abiotic components in pond: water and the rich soil deposit at the bottom. Climatic conditions: The solar input, the cycle of temperature, day-length etc.
- Autotrophic components: phytoplankton, some algae and the floating, submerged and marginal plants.
- Consumers (heterotrophs): zooplankton, free swimming and bottom dwelling forms.
- Decomposers: fungi, bacteria and flagellates.
- Pond performs all the functions of an ecosystem such as o Conversion of inorganic into organic material with the help of the radiant energy of the sun by the autotrophs.
 
o Consumption of the autotrophs by heterotrophs.
o Decomposition and mineralization of the dead matter to release them back for reuse by the autotrophs.
 
PRODUCTIVITY
 
- A constant input of solar energy is the basic requirement for any ecosystem to function and sustain.
- The amount of biomass or organic matter produced per unit area over a time period by plants during photosynthesis is called primary production. It is expressed in terms of weight (g–2) or energy (kcal m–2).
- The rate of biomass production is called productivity. It is expressed in terms of g–2 yr–1 or (kcal m–2) yr–1.
- It can be divided into gross primary productivity (GPP) and net primary productivity (NPP).
- Gross primary productivity: It is the rate of production of organic matter during photosynthesis. A considerable amount of GPP is utilized by plants in respiration.
- Gross primary productivity minus respiration losses (R) is the net primary productivity (NPP), i.e. NPP is the available biomass for the consumption to heterotrophs (herbivores and decomposers). NPP = GPP – R
- Secondary productivity: It is the rate of formation of new organic matter by consumers.
- Primary productivity depends on
 
o The plant species inhabiting a particular area
o Environmental factors
o Availability of nutrients
o Photosynthetic capacity of plants
Therefore, it varies in different types of ecosystems.
 
- The annual net primary productivity of the whole biosphere is approximately 170 billion tons (dry weight) of organic matter. Of this, despite occupying about 70 % of the surface, the productivity of the oceans is only 55 billion tons. Rest of course, is on land.
 
DECOMPOSITION
 
- It is the breakdown of complex organic matter by decomposers into inorganic substances like carbon dioxide, water and nutrients.
- It is largely an oxygen-requiring process.
- Detritus (dead plant remains such as leaves, bark, flowers and dead remains of animals, including fecal matter) is the raw material for decomposition.
 
Steps of decomposition
a. Fragmentation: It is the breakdown of detritus into smaller particles by detritivores (e.g. earthworm).
b. Leaching: By this process, water soluble inorganicnutrients go down into the soil horizon and get precipitated as unavailable salts.
c. Catabolism: Degradation of detritus into simpler inorganic substances by bacterial and fungal enzymes. Fragmentation, leaching and catabolism operate simultaneously on the detritus.
d. Humification: Accumulation of humus (dark amorphous substance) in soil. Humus is resistant to microbial action and so decomposes very slowly. Being colloidal in nature it serves as a reservoir of nutrients.
e. Mineralization: It is the release of inorganic nutrients due to the degradation of humus some microbes.
 
Factors influencing decomposition
• Chemical composition of detritus: Decomposition rate is slower if detritus is rich in lignin & chitin, and quicker, if detritus is rich in nitrogen and water-soluble substances like sugars.
• Climatic factors like temperature and soil moisture: Warm and moist environment favour decomposition whereas low temperature and anaerobiosis inhibit decomposition resulting in buildup of organic materials.
 
ENERGY FLOW
 
- Sun is the only source of energy for all ecosystems (except deep sea hydro-thermal ecosystem).
- Of the incident solar radiation less than 50% of it is photosynthetically active radiation (PAR).
- Plants and photosynthetic & chemosynthetic bacteria (autotrophs), fix solar radiant energy to make food.
- Plants capture only 2-10% of the PAR and this small amount of energy sustains the entire living world. So, it is very important to know how the solar energy captured by plants flows through different organisms of an ecosystem.
- Ecosystems obey 2nd Law of thermodynamics. They need a constant supply of energy to synthesise the molecules they require, to counteract the universal tendency toward increasing disorderliness.
- Producers: All organisms are dependent for their food on producers (green plants), either directly or indirectly. In a terrestrial ecosystem, major producers are herbaceous and woody plants. Primary producers in an aquatic ecosystem are phytoplankton, algae and higher plants.
- The energy trapped by the producer is either passed on to a consumer or the organism dies. Death of organism is the beginning of the detritus food chain/web.
- Consumers (heterotrophs): These are all animals that depend on plants (directly or indirectly) for their food.
 
They include:
o Primary consumers (herbivores- feed on plants). E.g. insects, birds and mammals in terrestrial ecosystem and molluscs in aquatic ecosystem.
o Secondary consumers (primary carnivores- feed on herbivores). E.g. frog, fox, man etc.
o Tertiary consumers (secondary carnivores- feed on primary carnivores).
 
- A simple grazing food chain (GFC) is depicted below:
- Detritus food chain (DFC) begins with dead organic matter. It is made up of decomposers (saprotrophs) which are heterotrophic organisms. E.g. fungi & bacteria. They meet their energy and nutrient requirements by
degrading dead organic matter or detritus.
- Decomposers secrete digestive enzymes that breakdown dead and waste materials into simple, inorganic materials, which are subsequently absorbed by them.
- In an aquatic ecosystem, GFC is the major conduit for energy flow.
- In a terrestrial ecosystem, a much larger fraction of energy flows through the DFC than through the GFC.
- DFC may be connected with GFC at some levels: some of the organisms of DFC are prey to the GFC animals. Some animals (cockroaches, crows etc.) are omnivores. These interconnections of food chains make a food web.
- Organisms occupy a place in the natural surroundings or in a community according to their feeding relationship. A specific place of organisms in the food chain is known as their trophic level. Producers belong to the first trophic level, herbivores to the second and carnivores to the third.
 
MULTIPLE CHOICE QUESTIONS

Question. Trophic levels are formed by : 
(a) plants
(b) animals
(c) organisms linked in food chain
(d) carnivores
Answer. C

Question. Desert can be converted into green land by planting
(a) oxylophytes
(b) psammophytes
(c) halophytes
(d) trees
Answer. B

Question. A scorpion stalks, kills, and then eats a spider.
Based on its behavior, which ecological terms describe the scorpion?
(a) producer, herbivore, decomposer
(b) producer, carnivore, heterotroph
(c) predator, carnivore, consumer
(d) predator, autotroph, herbivore
Answer. A

Question. In the vast marine ecosystem, certain sea develop red colouration. This red colour is due to the presence of large population of which one of the following organisms? 
(a) Trichodesmium erythrium
(b) Physarium
(c) Dinoflagellates
(d) Diatoms and members of red algae
Answer. D

Question. The food chain in which microbes breakdown energy rich compounds synthesized by producers is called 
(a) ecosystem
(b) parasitic food chain
(c) detritus level chain
(d) predator food chain
Answer. C

Question. Mr. X is eating curd/yoghurt. For this food intake in a food chain Mr. X should be considered as occupying 
(a) first trophic level
(b) second trophic level
(c) third trophic level
(d) fourth trophic level
Answer. C

Question. Given below is one of the types of ecological pyramids. This type represents
CBSE Class 12 Biology Ecosystem Worksheet

(a) pyramid of numbers in a grassland
(b) pyramid of biomass in a fallow land
(c) pyramid of biomass in a lake
(d) energy pyramid in a spring
Answer. C

Question. Whale is 
(a) Primary producer
(b) Carnivorous, secondary consumer
(c) A decomposer
(d) Herbivorous
Answer. D

Question. Which one of the following is not a function of an ecosystem? 
(a) Energy flow
(b) Decomposition
(c) Productivity
(d) Stratification
Answer. D

Question. Food chain starts with 
(a) autotrophs
(b) herbivores
(c) carnivores
(d) decomposers
Answer. A


ASSERTION REASON QUESTIONS

Directions : These questions consist of two statements, each printed as Assertion and Reason.While answering these questions, you are required to choose any one of the following five responses.
(a) If both Assertion and Reason are correct and the Reason is a correct explanation of the Assertion.
(b) If both Assertion and Reason are correct but Reason is not a correct explanation of the Assertion.
(c) If the Assertion is correct but Reason is incorrect.
(d) If both the Assertion and Reason are incorrect.
(e) If the Assertion is incorrect but the Reason is correct.

Question. Assertion: Insectivorous habitat of plants is to cope up O2 deficiency.
Reason: Insectivorous plants are partly autotrophic and partly heterotrophic.
Answer. E


Very Short Answer Questions

Question. How is ‘stratification’ represented in a forest ecosystem?
Answer. Stratification in the vertical distribution of species at different levels. Trees occupy top vertical strata, shrubs the second layer and herbs/ grasses occupy the bottom layers.

Question. Give an account of energy flow in an ecosystem.
Answer. Energy Flow
 The sun is the only source of energy for all ecosystems on earth.
 Out of the total incident solar radiation, only 50 per cent of it is photosynthetically active radiation (PAR).
 Plants capture only 2–10 per cent of the PAR and this small amount of energy sustains the entire living world. So, there is unidirectional flow of energy from the sun to producers and then to consumers.

Question. What is secondary productivity?
Answer. Secondary productivity is defined as the rate of formation of new organic matter by consumers.

Question. What is a detritus food chain made up of? How do they meet their energy and nutritional requirements?
Answer. Dead plant and animal remains and their faecal matter constitute Detritus. DFC includes decomposers which obtain energy by decomposing the dead materials.

Question. State what does ‘standing crop’ of a trophic level represent.
Answer. Standing crop represents the mass of living material (biomass) at a particular time.

Question. Arrange the following as you observe in vertical stratification of a forest—Grass, Shrubby plants, Teak, Amaranthus. 
Answer. Grass, Amaranthus, Shrubby plants, Teak.

Question. List any two ways of measuring the standing crop of a trophic level.
Answer. Standing crop is measured as the biomass or the number of plant in a unit area.

Question. What is meant by humification?
Answer. The process of formation of humus from detritus or dead organic matter is called humification.

Question. Why is a food web formed in nature?
Answer. Many organisms occupy positions in different food chains and several food chains become interconnected to form a food web.

Short Answer Questions

Question. Differentiate between two different types of pyramids of biomass with the help of one example of each.
Answer. Table 14.8: Differences between upright and inverted pyramids of biomass

S.No. Upright pyramid of biomass Inverted pyramid of biomass
(i) The biomass of producers is more than that of consumers. The biomass of the producers (phytoplankton) is less than that of consumers (fish).
(ii) For example, forest ecosystem. For example, aquatic ecosystem.

Question. State the difference between the first trophic levels of detritus food chain and grazing food chain.
Answer. Table 14.9: Differences between first trophic level of detritus food chain and grazing food chain

S.No. First trophic level of detritus food chain First trophic level of grazing food chain
(i) Decomposers are the first trophic level. Producers are the first trophic level.
(ii) They break down the complex organic matter into simpler form by secreting enzymes. They prepare complex organic molecules from simple inorganic material with the help of sunlight.

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 (in different food chains) at the given time. If the function of the mode of nutrition of species changes, its 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. Identify the type of the given ecological pyramid and give one example each of pyramid of number and pyramid of biomass in such cases. 

Answer. The given ecological pyramid is the inverted pyramid.
Inverted pyramid of biomass in a lake: Phytoplankton → Zooplankton → fishes.
Inverted pyramid of number: Tree → insects → birds.

Question. Apart from plants and animals, microbes form a permanent biotic component in an ecosystem.
While plants have been referred to as autotrophs and animals as heterotrophs, what are microbes referred to as? How do these microbes fulfil their energy requirements?
Answer. Microbes are referred to as heterotrophs and saprotrophs. They fulfil their energy requirement by feeding on dead remains of plants and animals through the process of decomposition.

Question. “Decomposition is an oxygen requiring process” Comment.
Answer. Detritus is rich in nitrogen and sugars. For oxidation of nitrogen and sugars oxygen is required by a class of aerobic microbes.

Long Answer Questions

Question. (a) Differentiate between primary and secondary ecological successions.
(b) Explain the different steps of xerarch succession occurring in nature. 
Answer. (a) Primary succession: It begins in areas where no living organisms ever existed. Therefore, the establishment of a biotic community is very slow, e.g., newly cooled lava, bare rock, newly created pond or reservoir.
Secondary succession: It begins in areas where natural biotic communities have been destroyed, e.g., abandoned farm lands, buried or cut forests. Since soil is available, it is a faster process.
(b) Xerarch Succession in Nature:
OU Lichens are the pioneer species on a bare area.
OU The lichen secretes some acids to dissolve rocks and help in weathering and soil formation.
OU Later, some small bryophytes invade and hold the small amount of soil.
OU The bryophytes are succeeded by herbs, shrubs and ultimately big trees.
OU At last, a stable climax forest is formed.
OU The xerophytic habitat gets converted into a mesophytic one.

Question. (a) Explain primary productivity and the factors that influence it.
(b) Describe how do oxygen and chemical composition of detritus control decomposition.
Answer. (a) Refer to Basic Concept Points 5 and 6.
(b) Decomposition of detritus is slow if it contains lignin, chitin, tannins and cellulose, whereas it is quicker if detritus is made up of nitrogenous compounds and water-soluble substances like sugars. This is because the latter are easy to degrade.
Oxygen is required for the activity of decomposers and detritivores. Therefore, a reduced oxygen amount will slow down the process of decomposition.

Question. (a) What is a trophic level in an ecosystem? What is ‘standing crop’ with reference to it?
(b) Explain the role of the ‘first trophic level’ in an ecosystem.
(c) How is the detritus food chain connected with the grazing food chain in a natural ecosystem?
Answer. (a) The specific places occupied by an organism in the food chain is called trophic level.
Each trophic level has a certain mass of living material at a particular time which is called as the standing crop.
(b) The first trophic level is comprised by the producers which trap solar energy to convert it into chemical bond energy of food. They serve as food for subsequent trophic levels.
(c) The organisms of the detritus food chain (DFC) are the prey to the grazing food chain (GFC) organisms. The dead remains of GFC are decomposed into simple inorganic materials. These materials are then absorbed by DFC organisms.

Question. “It is often said that the pyramid of energy is always upright. On the other hand, the pyramid of biomass can be both upright and inverted.” Explain with the help of examples and sketches.
Answer. Pyramid of biomass: The relationship between producers and consumers in an ecosystem can be represented in the form of a pyramid in terms of biomass called pyramid of biomass. It can be (a) Upright, e.g., in case of grassland ecosystem; or
(b) Inverted, e.g., in case of pond ecosystem as biomass of fishes for exceeds that of phytoplanktons.

Pyramid of energy: The relationship between producers and consumers in an ecosystem can be represented in the form of a pyramid, in terms of flow of energy called pyramid of energy. It is always upright because energy is always lost as heat at each step and as it follow 10% law.

Question. Name the kind of organisms which constitute the pioneer community of xerarch and hydrarch succession, respectively.
Answer. Xerarch succession—Lichens and Hydrarch succession—Phytoplanktons

Question. Citing lake as an example of a simple aquatic ecosystem, interpret how various functions of this ecosystem are carried out. Make a food chain that is functional in this ecosystem.
Answer. (i) Productivity: Conversion of inorganic into organic material with the help of solar energy by the autotrophs.
(ii) Energy flow: Unidirectional movement of energy towards higher trophic level (and its dissipation and loss as heat to the environment).
(iii) Decomposition: Fragmentation, leaching, catabolism, humification, mineralization by bacteria, fungi and flagellates (abundant at the bottom of lake).
(iv) Nutrient cycling: Decomposition of dead matter to release the nutrients back to be re-used by the autotrophs.
Food chain in aquatic ecosystem (lake):
Phytoplanktons → Zooplanktons → Small fish → Big fish (Any other appropriate example).

Question. Explain with the help of two examples, how the pyramid of number and the pyramid of biomass can look inverted. 
Answer. Pyramid of number is inverted in parasitic food chain. Here, a single large sized producer like tree provides nourishment to several herbivores (e.g.,
fruit-eating birds) and each herbivore in turn may support a still larger population of parasities. Thus, from the producer towards consumers, the number of organisms gradually shows an increase, making the pyramid inverted in shape.
Pyramid of biomass is inverted in shape in aquatic habitats where the biomass of a trophic level depends upon reproductive potential and longevity of its members. In a pond, the producers are small organisms, so their biomass is least and this value gradually shows an increase towards the apex of the pyramid, thus making the pyramid inverted in shape.

Question. Explain ‘standing crop’ in an ecosystem. Draw a pyramid of biomass when a small standing crop of phytoplanktons supports and large standing crop of zooplanktons in the sea. 
Answer. Standing crop refers to mass of living material at a particular trophic level at a particular time. It is measured as biomass or the number in a unit area. 

Question. It is stated that the pyramid of energy is always upright. Justify. 
Answer. The pyramid of energy is always upright in shape as there is always a gradual decrease in the energy content at successive trophic levels from producers to various consumers. This is because some energy is used at each trophic level for various metabolic activities and some energy is lost as heat, so only 10% of the energy is available to the next trophic level (Lindeman’s 10% law).

Question. “In a food chain, a trophic level represents a functional level, not a species”. Explain.
Answer. In a food chain, each trophic level represents a functional level not a species because:
(i) At a particular time, a trophic level is never occupied by a single species.
(ii) At a particular time, all organisms of a trophic level are considered together for calculating biomass, number and energy not a single species.
(iii) A particular species may occupy more than one trophic level, e.g., omnivorous species may occupy more than one trophic level in the same ecosystem at the same time.
(iv) If we consider few species then no generalisation can be made for studying ecosystem.

Question. Differentiate between two different types of pyramids of biomass with the help of one example of each.
Answer. Pyramid of biomass is a graphic representation of biomass present sequence-wise per unit area of different trophic levels with producers at the base and top carnivores at the tip. Pyramid of biomass may be upright or inverted.

Upright pyramid of
biomass
Inverted pyramid
of biomass
The biomass of
producers is more than
that of consumers.
The biomass of
producers is less than
that of consumers.
Pyramid of biomass
in grassland is always
upright.
Pyramid of biomass
in aquatic ecosystem
is always inverted.



Question. Construct a pyramid of biomass starting with phytoplanktons. Label 3 trophic levels. Is the pyramid upright or inverted. Why? 
Answer. Pyramid of biomass starting with phyto- planktons will be for an aquatic ecosystem. 
The pyramid of biomass in aquatic ecosystem is generally inverted because the biomass of fishes far exceeds that of phytoplanktons.

Question. “It is often said that the pyramid of energy is always upright. On the other hand, the pyramid of biomass can be both upright and inverted.” Explain with the help of examples and sketches. 
Answer. Pyramid of energy is a graphic representation of amount of energy trapped per unit time and area at different trophic levels of a food chain with producers forming the base and top carnivores the tip. The energy content is expressed as kcal/m2/yr. Pyramid of energy is always upright, because there is always a gradual decrease in the energy content at successive trophic levels from the producers to various consumers. As the energy passes onto higher trophic levels, its amount decreases because of its dissipation as heat, its use in overcoming entropy as well as for performing various body activities. 
Pyramid of biomass is a graphic representation of biomass present per unit area at different trophic levels with producers at the base and top carnivores kept at the tip. Pyramid of biomass is upright for terrestrial habitats. However, inverted pyramids are obtained in aquatic habitats. It is because the biomass of a trophic level depends upon reproductive potential and longevity of its members. Thus, the biomass of phytoplankton is smaller than that of zooplankton and that of the latter less than of primary carnivores. (a) Inverted pyramid of biomass in an aquatic ecosystem.

Question. (a) With suitable examples, explain the energy flow through different trophic levels. What does each bar in this pyramid represent?
(b) Write any two limitations of ecological pyramids.
Answer. (a) Ecosystem requires a constant input of energy. Energy flow in the ecosystem is very important as it is the basis of life. Food provides both matter and energy. Flow of energy determines the diversity of organisms. It also determines the developmental and functional status of the ecosystem. Energy flow in an ecosystem is always unidirectional or one way, i.e., solar radiations → producers → herbivores → carnivores. It cannot pass in the reverse direction as there is always decrease in the content and flow of energy with the rise in trophic level. A part of energy captured by producers (gross primary productivity) is used for maintenance (lost in respiration) and as food to herbivores.
Only 10% of the gross productivity of producers is entrapped by herbivores for their body building. Herbivores are eaten by primary carnivores. Herbivores not preyed by carnivores die a natural death and energy trapped in their body is transferred to decomposers. Only 10% of the herbivores productivity is utilised for raising productivity of primary carnivores. The rest is consumed in ingestion, respiration, maintenance of body heat and other activities. Higher carnivores similarly are able to retain only 10% of energy present in primary carnivores.
Therefore, the pyramid of energy is always upright with base representing the producers and apex occupied by top carnivores. Each bar of the pyramid represents one trophic level.
(b) The limitations of ecological pyramids are :
(i) Ecological pyramids assume that food chains are simple. Simple food chains do not occur in nature. Instead, food webs are present.
(ii) A single species may operate at two or more trophic levels. Ecological pyramids have no method of accomodating such cases.
(iii) Ecological pyramids have no place for detrivores and decomposers though they play a vital role in ecosystem.

Question. (a) 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.
(b) Differentiate giving reason, between the pyramid of biomass of the above situation and the pyramid of numbers that you have drawn.
Answer. Pyramid of biomass in the above situation will be upright as tree has the maximum biomass which reduces with successive trophic levels resulting in an upright pyramid whereas the pyramid of number in above case is spindle-shaped where producers which are mainly large sized trees are lesser in number and form the base of the pyramid. The insects are more in number than producers. Then there is gradual decrease in the number of successive carnivores, thus making the pyramid upright again.  

Question. (a) Explain the significance of ecological pyramid with the help of an example.
(b) Why are the pyramids referred to as ‘upright’ or ‘inverted’? Explain. 
Answer. (a) An ecological pyramid is a graphic representation of an ecological parameter present in various trophic levels of a food chain with producers forming the base and top carnivores the tip.
It expresses the relationship between the organisms at different trophic levels with reference to their number, energy and biomass. For example, pyramid of numbers is a graphic representation of the number of individuals per unit area of various trophic levels stepwise with producers at the base and top carnivores at the tip.
Pyramid of biomass shows the total amount of
living matter (fresh weight or dry weight) present in each level and pyramid of energy is a graphic representation of amount of energy trapped per unit time and area in different trophic levels of a food chain with producers forming the base and top carnivores the tip.
(b) Usually producers exceed higher trophic levels in biomass, number and energy. Hence a structure with broad base, gradually tapering body and narrow apex is obtained which resembles an upright pyramid.
In some cases e.g., for biomass in pond ecosystem, for number of individuals in a parasitic food chain the relationship changes. Here producers show the least biomass or number and the top consumers show the highest biomass or number. As a result a structure resembling an ‘inverted’ pyramid is obtained.

Question. What is ecological succession? Where and why would the rate of succession be faster, in newly created pond or a forest destroyed by a forest fire? 
Answer. Ecololgical succession refers to natural development of series of biotic communities at the same site, one after the other till a climax community develops. Succession will be faster in a forest destroyed by fire because soil, organic matter and vegetative propagules will be present there.

Question. Explain how does a primary succession start on a bare rock and reach a climax community?
Answer. Xerarch succession occurs on bare rocks. The habitat lacks soil, has intense light, fluctuations of temperatures and winds. The species that invade a bare area are called pioneer species. In primary succession on rocks lichens are usually the pioneer species which are able to secrete acids to dissolve rocks, helping in weathering and soil formation. These pave way for some small plants like bryophytes, which are able to take hold in the small amount of soil. They are, with time, succeeded by herbs stage, then shrub stage and then bigger plants, and ultimately a stable climax community, i.e., forest is formed. The climax community remains stable as long as the environment remains unchanged. With time, the xerophytic habitat gets converted into a mesophytic one.
Lichens (Pioneer community)
             ↓
Bryophytes
             ↓
Herbaceous plants
             ↓
Shrubs
             ↓
Trees (Climax community)

Question. Explain the differences and the similarities between hydrach and xerach successsion of plants. 
Answer. Differences between hydrarch and xerarch succession are as follows :

Xerarch succession Hydrarch succession
It begins with
lichens or blue green
algae.
It begins with
phytoplankons.
Initial succession is a
slow process.
Initial succession is
quite fast.
Succession is seen all
over the area.
Succession is observed
in area where water is
not very deep.
e whole of the
area is involved in
formation of climax
community.
Climax community
develops on the edge
only.
Succession converts
xeric environment
into mesic
environment.

It reduces bare land
area and converts it
into fertile forested
area.
It converts aquatic
environment into
mesic environment.


It fills up water body
and changes it into
forested land.


Similarities between hydrarch and xerarch succession are as follows:
(i) Both start with pioneer communities and end in a climax one.
(ii) Both end in a mesic environment.
(iii) Both lead from low to high diversity of life.
(iv) Both result in gradual increase of biomass.
(v) Both involve considerable niche specialisation.

Question. Differentiate between primary and secondary succession. Provide one example of each.
Answer. Differences between primary succession and secondary succession are as follows:

Primary
succession
Secondary
succession
It occurs in an
area which has
been bare from the
beginning.
It occurs in an area
which has been
denuded recently.
Soil is absent at the
time of beginning
of primary
succession.
Soil is present in
the area where
secondary succesion
begins.
There is no humus
in the beginning.
Humus is present
from the very
beginning.
Reproductive
structures of
any previous
community are
absent.
Reproductive
structures of the
previous occupants
are present in the
area.
Pioneer
community comes
from outside.
Pioneer community
develops partly from
previous occupants
and partly from
migrants.
In the beginning
the environment is
very hostile.
e environment is
favourable from the
beginning.
Seral communities
are many.
Seral communities
are a few.
Primary succession
takes a long time
for completion,
1000 years or
more.
Secondary
succession takes
lesser time for
completion, 50-200
years.
Example :
Succession on a
newly exposed sea
floor.
Example : Succession
on an area destroyed
by ood.

 

Question. Define climax community. How does a sere differ from a seral community.
Answer. Climax community is the stable, self perpetuating and final biotic community that develops at the end of biotic succession and is in perfect harmony with the physical environment. Seral communities, also termed transitional communities are the various biotic communities that develop during biotic succession. On the other hand, the name sere is given to the entire sequence of developmental stages of biotic succession from pioneer to a climax community.

Question. (a) Explain how a hydrarch succession progresses from hydric to mesic condition and forms a stable climax community.
(b) Why is the rate of secondary succession faster than that of primary succession?
Answer. (a) Hydrarch succession refers to series of biotic communities that develop one after the other in a newly formed pond or lake. Various successive stages of hydrosere are :
(i) Plankton stage : It is the pioneer stage of hydrosere. The autotrophic organisms called phytoplanktons, (e.g., diatoms), zooplanktons (which feed on phytoplankton) and decomposers constitute this stage. Death and decomposition of plankton produce organic matter which mixes up with clay and settles at the bottom to form soft mud favourable for growth of next seral stage.
(ii) Submerged stage : The bottom lined by soft mud having organic mater is favourable for growth of submerged plants like Hydrilla, Potamogeton and Najas. They are rooted in the mud and form dense growth. On decaying, the older plants and buried parts of other plants form humus which enriches the newly built up bottom and makes it favourable for growth of next stage.
(iii) Floating stage : Floating leaved anchored plants (e.g., Nymphaea, Nelumbo, etc.) appear when water becomes shallow. The plants make the water rich in mineral and organic matter. It becomes suitable for growth of free floating plants like Lemna, Spirodela etc. Rapid growth of floating stage further builds up bottom so that water becomes shallow.
(iv) Reed swamp stage : Amphibious plants e.g., Phragmites, Typha etc. grow when the water body becomes shallow. Their tangled growth accumulates slit.
(v) Sedge or marsh meadow stage : Reed swamp stage are invaded by Carex (Sedge), grasses like Themeda and Dichanthium and herbs like Campanula etc. The plants transpire rapidly and add abundant humus. Therefore, soil is build up for the next stage. (vi) Woodland stage : Sedge meadow stage is invaded by some rhizome bearing plants which can tolerate bright sunlight as well as water logged conditions, e.g., Cornus (Bogwood) etc. They invite invasion by trees capable of bearing bright sunlight and water logging, e.g., Populus (Cottonwood), Alnus (Alder). They built up more soil.
(vii) Climax forest : New trees invade the area. These trees grow to greater heights with time a hydrophytic habitat gets converted to mesophytic one and forms a stable community.
(b) Rate of secondary succession is faster as a secondarily barred area has a built in soil organic matter. It is biologically fertile so succession is completed quickly. Underground parts, like vegetative propagules and seeds etc., of plants and invaders quickly give rise to a new community as soon as conditions become favourable. It has few seral changes and quickly gives rise to climax community.

Question. Explain how xerarch succession progresses from xeric to mesic condition and forms stable climax community. You may use a flow chart.
Answer. Xerarch succession is the sequence of successional stages that occur on bare rocks. The various stages and their component plant species appearing on a rock are as follows :
Pioneer stage : The first vegetation to occupy the rocky habitat is of crustose lichens, e.g., species of Rhizocarpon, Rhinodina and Lecanora. They produce some acids which bring about weathering of rocks which invites foliose lichen. The CO2 released during the respiration of these lichens react with water and forms acid. Thus, the process of breaking up of rock becomes more rapid.
             ↓
Moss stage : The bryophytes and particularly the mosses develop and spread very rapidly on the rocks. They accumulate more soil and organic matter which makes the surface suitable for next stage.
             ↓
Herb stage : This stage is constituted by shallow rooted grasses as Eleusine, Poa, Aristida, etc. These accumulate more soil and more minerals.
             ↓
Shrub stage : Due to much accumulation of soil, the habitat becomes suitable for shrub vegetation. This habitat includes the species of Zizyphus, Caparis, etc. These, in turn, are finally replaced by trees, the climax community.
             ↓
Forest stage : Some xerophytic tree species invade the area. Further weathering of rocks and increasing humus content of the soil favour the arrival of more trees, and vegetation finally becomes mesophytic. Thus, there develops finally a forest community.

Question. “All successions proceed to a similar climax community–the mesic.” Explain. 
Answer. Based on the nature of the habitat, succession of plants is of two types-hydrarch and xerarch. Hydrosere or hydrarch starts in regions where water is in plenty and progresses from hydric to mesic conditions (adequate moisture conditions). On the other hand, xerosere or xerarch starts in regions where moisture is present in minimal or negligible amounts such as dry deserts, rocks etc. and it progresses from xeric to mesic conditions. Hence, both hydrarch and xerach successions lead to medium water conditions. 

Question. Explain the function of ‘reservoir’ in a nutrient cycle. List the two types of nutrient cycles in nature.
Answer. The function of reservoir is to meet the deficiency of nutrient which occurs due to imbalance in the rate of influx and effiux. Two types of nutrient cycles are gaseous and sedimentary.

Question. How does phosphorus cycle differ from carbon cycle? 
Answer. Differences between carbon cycle and phosphorus cycle are as follows:

Carbon cycle Phosphorus cycle
Its major component
is gaseous.
Its major component
is non-gaseous.
There is respiratory
release of carbon as
CO2.
There is no
respiratory release of
phosphorus.
Its cycling pool is present
in hydrosphere
and atmosphere.
Its cycling pool is
lithosphere.
Its reservoir pool is
atmosphere.
Its reservoir pool is
lithosphere.
Atmospheric input
of carbon through
rainfall is appreciable.
It is negligible.
There is exchange
of carbon between
organisms and environment.
There is no such
exchange.

 

Question. Name the two types of nutrient cycle existing in nature. Where are their reservoirs present?
State the functions of reservoirs.
Answer. Two types of nutrient cycle existing in nature are gaseous and sedimentary. Atmosphere and lithosphere are reservoirs for gaseous and sedimentary cycle respectively. The function of reservoir is to meet deficiency of nutrient which occurs due to difference in rate of influx and effiux.

Question. Carbon cycle in nature is a biogeochemical event. Explain.
Outline salient features of carbon cycling in an ecosystem. 
Answer. Carbon forms the backbone for complex organic molecules of protoplasm like carbohydrates, lipid, proteins etc. Main sources of carbon are air, water, fossil fuel and rocks. Carbon dioxide is added to the cycle pool of atmosphere by respiration, decomposition of organic waste, burning of fossil fuels, volanic eruption, etc. Carbon fixed by producers enters the food chain and hence passes to herbivores, carnivores, decomposers, etc. Natural exchange between lithosphere and hydrosphere or atmosphere is a very slow process. Major exchange in carbon cycle is between organisms (absorption by producers, released by all in respiration) and the atmosphere or hydrosphere. This cycling is a self- regulated feed back system but has recently been upset due to rapid deforestation and increasing combustion of fossil fuels.

Question. What does the term ‘standing state of soil’ signify? How are the nutrients recycled in the ecosystem? Write a cyclic account of carbon movement in nature.
Answer. Standing state signifies the amount of biogenetic nutrients present at any time in the growth medium (soil/water) of ecosystem. It tends to vary from season to season and ecosystem to ecosystem. Nutrients are exchanged through various components of ecosystem (living and non living) so that they can be used again and again.

Question. Healthy ecosystems are the base of wide range of (ecosystem) services. Justify. 
Answer. Healthy ecosystems are the base of a wide range of economic, environmental and aesthetic goods and services. Some of these are as follows:
(i) Purification of air and water
(ii) Nutrient cycling
(iii) Provide habitat to wildlife
(iv) Maintenance of biodiversity
(v) Mitigation of droughts and floods.
(vi) Forests provide storage site for carbon etc.

Question. Biodiversity must be conserved as it plays an important role in many ecosystem services that nature provides. Explain any two services of the ecosystem.
Answer. For best services the ecosystem must be healthy. There are several advantages for keeping ecosystem healthy. Ecosystem services provided by nature are as follows:
(i) Soil formation and soil protection are the major ecosystem services accounting for nearly 50% of their total worth. Plant cover protects the soil from drastic changes in temperature. There is little wind or water erosion as soil particles are not exposed to them. The soil remains spongy and fertile. There are no landslides and no floods.
(ii) Plant litter and humus prevent run off water, hold water like sponge and allow percolation of water. A lot of water is held in the soil which slowly passes towards perched water table. It comes out as springs. They are source of perennial freshwater which is quite pure.
(iii) Plant cover of natural ecosystems absorb polluting gases, cause settling of suspended particulate matter, removes CO2 and releases O2. Hence, purified air becomes available.
(iv) They protect the land from floods, remove sediments and other pollutants and recharge ground water.
(v) There is increase in atmospheric humidity, good rainfall and moderating effect on climate.
(vi) Producers of the ecosystem release lot of oxygen during photosynthesis. Release of oxygen by the producers helps in replenishing this gas being consumed in respiration and combustion.
(vii) There is nutrients cycling and the same are repeatedly circulated and recirculated. It keeps the fertility of soil intact.
(viii)Producers of the ecosystem pick up CO2 from the atmosphere and convert it into organic compounds in the process of photosynthesis. This sustains ecosystem and also provides food to the heterotrophs of the ecosystem.
(ix) Bees and other insects of natural ecosystems visit nearby farmlands and pollinate the crop plants. Elimination of pollinator will eliminate the plant species due to non-reproduction.
(x) A large number of tribals live in forests.
(xi) They provide grazing areas for numerous cattle.
(xii) Natural ecosystems are a source of spiritual, cultural and aesthetic values.

Question.  State the difference between the first trophic levels of detritus food chain and grazing food chain. 
Answer. In a grazing food chain, producers constitute the first trophic level. Producers are autotrophic living green plants which can manufacture their own food through photosynthesis utilising inorganic raw materials and sunlight. In a detritus food chain detritivores and decomposers constitute the first trophic level. Decomposers are the saprophytes, chiefly bacteria and fungi that breakdown the complex compounds of dead organisms into simple substances.

Question. Justify the importance of decomposers in an ecosystem. 
Answer. Decomposers are microorganisms that obtain nourishment from organic remains by secreting digestive enzymes over it. They help in converting complex organic substances into inorganic substances like carbon dioxide, water and nutrient. Sun is an endless source of enregy, but the chemical materials of the environment are not inexhaustible. The producers utilise inorganic substances (carbon dioxide, water etc.) and fix solar energy into the chemical energy of organic compounds. This stored energy is then passed to consumers by repeated eating and being eaten. Decomposers act on dead animals and plants (organic matter) and return the chemical nutrients to the environment. They also make space available for new producers. Without this, all life will ultimately cease to exist. Thus, the decomposers have a crucial role in the ecosystem. The decomposers are found in the soil and at the bottom of ponds, lakes and oceans.

Question. “It is possible that a species may occupy more than one trophic level in an ecosystem at the same time”. Explain with the help of one example. 
Answer. In an ecosystem, a species may occupy more than one trophic level simultaneously. As the trophic level represents a functional level, not a species as such. A species may occupy more than one trophic 
level in the same ecosystem at the same time, for example, a sparrow is a primary consumer when it eats seeds, fruits, peas etc. and a secondary consumer when it eats insects and worms.

Question. Discuss the relationship between detritus food chain and grazing food chain in a terrestrial ecosystem. 
Answer. In a terrestrial ecosystem, producers constitute the first trophic level or base of a food chain. Producers are autotrophic organisms which are able to manufacture organic food from inorganic raw materials in the process of photosynthesis. Part or whole of the plants enters the food chain as food for consumers. Herbivores, the primary consumers feed on plants or plant products.
Primary carnivores prey upon herbivorous animals and are eaten up by secondary carnivores. The top carnivores are the last order consumers or carnivores which are not preyed upon by other animals because of their size and ferociousness.
Decomposers may act on any trophic level. They feed on dead organic remains of plants and animal and return the biological nutrients stored in them to the soil for reuse by autotrophs i.e., they again enter the food chain. Hence, both detritus and grazing food chain are interrelated.

Question. How is detritus decomposed step-by-step by different agents and made available as nutrients to the plants? Explain.
Answer. Detritus (dead remains of plants and animals) gets decomposed and is made available as nutrients to plants by the process of decomposition. The steps in the process of decomposition are fragmentation, leaching, cataboslim, humification and mineralisation.
(i) Fragmentation of detritus : Detritivores (e.g., earthworm) breakdown detritus into small particles.
(ii) Catabolism : Bacterial and fungal enzymes degrade detritus into simpler inorganic substances.
(iii) Leaching : Water soluble inorganic nutrients go down into the deeper layers of soil and get precipitated as unavailable salts.
(iv) Humification : It is the formation of humus from detritus or organic remains. Humus is dark coloured amorphous substance rich in lignin and cellulose. It is highly resistant to microbial action and undergoes decomposition at an extremely slow rate. It is colloidal in nature, a reservoir of nutrients and helpful in maintenance of soil moisture as well as aeration.
(v) Mineralisation : It is the release of inorganic substances by the degradation of humus with the help of microbes.

Question. Describe the process of decomposition of detritus under the following heads: Fragmentation, leaching; catabolism; humification and mineralisation. 
Answer. Detritus (dead remains of plants and animals) gets decomposed and is made available as nutrients to plants by the process of decomposition. The steps in the process of decomposition are fragmentation, leaching, cataboslim, humification and mineralisation.
(i) Fragmentation of detritus : Detritivores (e.g., earthworm) breakdown detritus into small particles.
(ii) Catabolism : Bacterial and fungal enzymes degrade detritus into simpler inorganic substances. (iii) Leaching : Water soluble inorganic nutrients go down into the deeper layers of soil and get precipitated as unavailable salts.
(iv) Humification : It is the formation of humus from detritus or organic remains. Humus is dark coloured amorphous substance rich in lignin and cellulose. It is highly resistant to microbial action and undergoes decomposition at an extremely slow rate. It is colloidal in nature, a reservoir of nutrients and helpful in maintenance of soil moisture as well as aeration.
(v) Mineralisation : It is the release of inorganic substances by the degradation of humus with the help of microbes.

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