NCERT Textbook: Our Environment | Science Class 10 PDF Download

 Page 1


Science
208
Activity 13.1 Activity 13.1 Activity 13.1 Activity 13.1 Activity 13.1
Our Environment
13 CHAPTER
W
e have heard the word ‘environment’ often being used on the
television, in newspapers and by people around us. Our elders tell
us that the ‘environment’ is not what it used to be earlier; others say that
we should work in a healthy ‘environment’; and global summits involving
the developed and developing countries are regularly held to discuss
‘environmental’ issues. In this chapter, we shall be studying how various
components in the environment interact with each other and how we
impact the environment.
13.1 ECO 13.1 ECO 13.1 ECO 13.1 ECO 13.1 ECO-SYSTEM — WHA -SYSTEM — WHA -SYSTEM — WHA -SYSTEM — WHA -SYSTEM — WHAT ARE ITS COMPONENTS? T ARE ITS COMPONENTS? T ARE ITS COMPONENTS? T ARE ITS COMPONENTS? T ARE ITS COMPONENTS?
All organisms such as plants, animals, microorganisms and human
beings as well as the physical surroundings interact with each other
and maintain a balance in nature. All the interacting organisms in an
area together with the non-living constituents of the environment form
an ecosystem. Thus, an ecosystem consists of biotic components
comprising living organisms and abiotic components comprising
physical factors like temperature, rainfall, wind, soil and minerals.
For example, if you visit a garden you will find different plants, such
as grasses, trees; flower bearing plants like rose, jasmine, sunflower;
and animals like frogs, insects and birds. All these living organisms
interact with each other and their growth, reproduction and other
activities are affected by the abiotic components of ecosystem. So a garden
is an ecosystem. Other types of ecosystems are forests, ponds and lakes.
These are natural ecosystems while gardens and crop-fields are human-
made (artificial) ecosystems.
n You might have seen an aquarium. Let us try to design one.
n What are the things that we need to keep in mind when we create
an aquarium? The fish would need a free space for swimming (it
could be a large jar), water, oxygen and food.
n We can provide oxygen through an oxygen pump (aerator) and
fish food which is available in the market.
2024-25
Page 2


Science
208
Activity 13.1 Activity 13.1 Activity 13.1 Activity 13.1 Activity 13.1
Our Environment
13 CHAPTER
W
e have heard the word ‘environment’ often being used on the
television, in newspapers and by people around us. Our elders tell
us that the ‘environment’ is not what it used to be earlier; others say that
we should work in a healthy ‘environment’; and global summits involving
the developed and developing countries are regularly held to discuss
‘environmental’ issues. In this chapter, we shall be studying how various
components in the environment interact with each other and how we
impact the environment.
13.1 ECO 13.1 ECO 13.1 ECO 13.1 ECO 13.1 ECO-SYSTEM — WHA -SYSTEM — WHA -SYSTEM — WHA -SYSTEM — WHA -SYSTEM — WHAT ARE ITS COMPONENTS? T ARE ITS COMPONENTS? T ARE ITS COMPONENTS? T ARE ITS COMPONENTS? T ARE ITS COMPONENTS?
All organisms such as plants, animals, microorganisms and human
beings as well as the physical surroundings interact with each other
and maintain a balance in nature. All the interacting organisms in an
area together with the non-living constituents of the environment form
an ecosystem. Thus, an ecosystem consists of biotic components
comprising living organisms and abiotic components comprising
physical factors like temperature, rainfall, wind, soil and minerals.
For example, if you visit a garden you will find different plants, such
as grasses, trees; flower bearing plants like rose, jasmine, sunflower;
and animals like frogs, insects and birds. All these living organisms
interact with each other and their growth, reproduction and other
activities are affected by the abiotic components of ecosystem. So a garden
is an ecosystem. Other types of ecosystems are forests, ponds and lakes.
These are natural ecosystems while gardens and crop-fields are human-
made (artificial) ecosystems.
n You might have seen an aquarium. Let us try to design one.
n What are the things that we need to keep in mind when we create
an aquarium? The fish would need a free space for swimming (it
could be a large jar), water, oxygen and food.
n We can provide oxygen through an oxygen pump (aerator) and
fish food which is available in the market.
2024-25
Our Environment 209
n If we add a few aquatic plants and animals it can become a self-
sustaining system. Can you think how this happens? An aquarium
is an example of a human-made ecosystem.
n Can we leave the aquarium as such after we set it up? Why does
it have to be cleaned once in a while? Do we have to clean ponds
or lakes in the same manner? Why or why not?
We have seen in earlier classes that organisms can be grouped as
producers, consumers and decomposers according to the manner in
which they obtain their sustenance from the environment. Let us recall
what we have learnt through the self sustaining ecosystem created by
us above. Which organisms can make organic compounds like sugar
and starch from inorganic substances using the radiant energy of the
Sun in the presence of chlorophyll? All green plants and certain bacteria
which can produce food by photosynthesis come under this category
and are called the producers.
Organisms depend on the producers either directly or indirectly for
their sustenance? These organisms which consume the food produced,
either directly from producers or indirectly by feeding on other consumers
are the consumers. Consumers can be classed variously as herbivores,
carnivores, omnivores and parasites. Can you give examples for each of
these categories of consumers?
n Imagine the situation where you do not clean the  aquarium and
some fish and plants have died. Have you ever thought what
happens when an organism dies? The microorganisms, comprising
bacteria and fungi, break-down the dead remains and waste
products of organisms. These microorganisms are the decomposers
as they break-down the complex organic substances into simple
inorganic substances that go into the soil and are used up once
more by the plants. What will happen to the garbage, and dead
animals and plants in their absence? Will the natural replenishment
of the soil take place, even if decomposers are not there?
Activity 13.2 Activity 13.2 Activity 13.2 Activity 13.2 Activity 13.2
n While creating an aquarium did you take care not to put an aquatic
animal which would eat others? What would have happened
otherwise?
n Make groups and discuss how each of the above groups of
organisms are dependent on each other.
n Write the aquatic organisms in order of who eats whom and form
a chain of at least three steps.  ?  ? 
n Would you consider any one group of organisms to be of primary
importance? Why or why not?
2024-25
Page 3


Science
208
Activity 13.1 Activity 13.1 Activity 13.1 Activity 13.1 Activity 13.1
Our Environment
13 CHAPTER
W
e have heard the word ‘environment’ often being used on the
television, in newspapers and by people around us. Our elders tell
us that the ‘environment’ is not what it used to be earlier; others say that
we should work in a healthy ‘environment’; and global summits involving
the developed and developing countries are regularly held to discuss
‘environmental’ issues. In this chapter, we shall be studying how various
components in the environment interact with each other and how we
impact the environment.
13.1 ECO 13.1 ECO 13.1 ECO 13.1 ECO 13.1 ECO-SYSTEM — WHA -SYSTEM — WHA -SYSTEM — WHA -SYSTEM — WHA -SYSTEM — WHAT ARE ITS COMPONENTS? T ARE ITS COMPONENTS? T ARE ITS COMPONENTS? T ARE ITS COMPONENTS? T ARE ITS COMPONENTS?
All organisms such as plants, animals, microorganisms and human
beings as well as the physical surroundings interact with each other
and maintain a balance in nature. All the interacting organisms in an
area together with the non-living constituents of the environment form
an ecosystem. Thus, an ecosystem consists of biotic components
comprising living organisms and abiotic components comprising
physical factors like temperature, rainfall, wind, soil and minerals.
For example, if you visit a garden you will find different plants, such
as grasses, trees; flower bearing plants like rose, jasmine, sunflower;
and animals like frogs, insects and birds. All these living organisms
interact with each other and their growth, reproduction and other
activities are affected by the abiotic components of ecosystem. So a garden
is an ecosystem. Other types of ecosystems are forests, ponds and lakes.
These are natural ecosystems while gardens and crop-fields are human-
made (artificial) ecosystems.
n You might have seen an aquarium. Let us try to design one.
n What are the things that we need to keep in mind when we create
an aquarium? The fish would need a free space for swimming (it
could be a large jar), water, oxygen and food.
n We can provide oxygen through an oxygen pump (aerator) and
fish food which is available in the market.
2024-25
Our Environment 209
n If we add a few aquatic plants and animals it can become a self-
sustaining system. Can you think how this happens? An aquarium
is an example of a human-made ecosystem.
n Can we leave the aquarium as such after we set it up? Why does
it have to be cleaned once in a while? Do we have to clean ponds
or lakes in the same manner? Why or why not?
We have seen in earlier classes that organisms can be grouped as
producers, consumers and decomposers according to the manner in
which they obtain their sustenance from the environment. Let us recall
what we have learnt through the self sustaining ecosystem created by
us above. Which organisms can make organic compounds like sugar
and starch from inorganic substances using the radiant energy of the
Sun in the presence of chlorophyll? All green plants and certain bacteria
which can produce food by photosynthesis come under this category
and are called the producers.
Organisms depend on the producers either directly or indirectly for
their sustenance? These organisms which consume the food produced,
either directly from producers or indirectly by feeding on other consumers
are the consumers. Consumers can be classed variously as herbivores,
carnivores, omnivores and parasites. Can you give examples for each of
these categories of consumers?
n Imagine the situation where you do not clean the  aquarium and
some fish and plants have died. Have you ever thought what
happens when an organism dies? The microorganisms, comprising
bacteria and fungi, break-down the dead remains and waste
products of organisms. These microorganisms are the decomposers
as they break-down the complex organic substances into simple
inorganic substances that go into the soil and are used up once
more by the plants. What will happen to the garbage, and dead
animals and plants in their absence? Will the natural replenishment
of the soil take place, even if decomposers are not there?
Activity 13.2 Activity 13.2 Activity 13.2 Activity 13.2 Activity 13.2
n While creating an aquarium did you take care not to put an aquatic
animal which would eat others? What would have happened
otherwise?
n Make groups and discuss how each of the above groups of
organisms are dependent on each other.
n Write the aquatic organisms in order of who eats whom and form
a chain of at least three steps.  ?  ? 
n Would you consider any one group of organisms to be of primary
importance? Why or why not?
2024-25
Science
210
Figure 13.2 Figure 13.2 Figure 13.2 Figure 13.2 Figure 13.2
Trophic levels
13.1.1 Food Chains and Webs
In Activity 13.4 we have formed a series of organisms feeding
on one another. This series or organisms taking part at
various biotic levels form a food chain (Fig. 13.1).
Each step or level of the food chain forms a trophic level.
The autotrophs or the producers are at the first trophic level.
They fix up the solar energy and make it available for
heterotrophs or the consumers. The herbivores or the primary
consumers come at the second, small carnivores or the
secondary consumers at the third and larger carnivores or
the tertiary consumers form the fourth trophic level (Fig. 13.2).
We know that the food we eat acts as a fuel to provide us
energy to do work. Thus the interactions among various
components of the environment involves flow of energy from
one component of the system to another. As we have studied,
the autotrophs capture the energy present in sunlight and
convert it into chemical energy. This energy supports all the
activities of the living world. From autotrophs, the energy goes
to the heterotrophs and decomposers. However, as we saw in
the previous Chapter on ‘Sources of Energy’, when one form
of energy is changed to another, some energy is lost to the
environment in forms which cannot be used again. The flow
of energy between various components of the environment
has been extensively studied and it has been found that –
n The green plants in a terrestrial ecosystem capture about
1% of the energy of sunlight that falls on their leaves and convert it
into food energy.
n When green plants are eaten by primary consumers, a great deal
of energy is lost as heat to the environment, some amount goes
into digestion and in doing work and the rest goes towards growth
and reproduction. An average of 10% of the food eaten is turned
into its own body and made available for the next level of
consumers.
n Therefore, 10% can be taken as the average value for the amount
of organic matter that is present at each step and reaches the next
level of consumers.
n Since so little energy is available for the next level of consumers,
food chains generally consist of only three or four steps. The loss
of energy at each step is so great that very little usable energy
remains after four trophic levels.
n There are generally a greater number of individuals at the lower
trophic levels of an ecosystem, the greatest number is of the
producers.
n The length and complexity of food chains vary greatly. Each organism
is generally eaten by two or more other kinds of organisms which in
turn are eaten by several other organisms. So instead of a straight
line food chain, the relationship can be shown as a series of branching
lines called a food web (Fig. 13.3).
Figure 13.1 Figure 13.1 Figure 13.1 Figure 13.1 Figure 13.1
Food chain in nature
(a) in forest, (b) in
grassland and (c) in a
pond
2024-25
Page 4


Science
208
Activity 13.1 Activity 13.1 Activity 13.1 Activity 13.1 Activity 13.1
Our Environment
13 CHAPTER
W
e have heard the word ‘environment’ often being used on the
television, in newspapers and by people around us. Our elders tell
us that the ‘environment’ is not what it used to be earlier; others say that
we should work in a healthy ‘environment’; and global summits involving
the developed and developing countries are regularly held to discuss
‘environmental’ issues. In this chapter, we shall be studying how various
components in the environment interact with each other and how we
impact the environment.
13.1 ECO 13.1 ECO 13.1 ECO 13.1 ECO 13.1 ECO-SYSTEM — WHA -SYSTEM — WHA -SYSTEM — WHA -SYSTEM — WHA -SYSTEM — WHAT ARE ITS COMPONENTS? T ARE ITS COMPONENTS? T ARE ITS COMPONENTS? T ARE ITS COMPONENTS? T ARE ITS COMPONENTS?
All organisms such as plants, animals, microorganisms and human
beings as well as the physical surroundings interact with each other
and maintain a balance in nature. All the interacting organisms in an
area together with the non-living constituents of the environment form
an ecosystem. Thus, an ecosystem consists of biotic components
comprising living organisms and abiotic components comprising
physical factors like temperature, rainfall, wind, soil and minerals.
For example, if you visit a garden you will find different plants, such
as grasses, trees; flower bearing plants like rose, jasmine, sunflower;
and animals like frogs, insects and birds. All these living organisms
interact with each other and their growth, reproduction and other
activities are affected by the abiotic components of ecosystem. So a garden
is an ecosystem. Other types of ecosystems are forests, ponds and lakes.
These are natural ecosystems while gardens and crop-fields are human-
made (artificial) ecosystems.
n You might have seen an aquarium. Let us try to design one.
n What are the things that we need to keep in mind when we create
an aquarium? The fish would need a free space for swimming (it
could be a large jar), water, oxygen and food.
n We can provide oxygen through an oxygen pump (aerator) and
fish food which is available in the market.
2024-25
Our Environment 209
n If we add a few aquatic plants and animals it can become a self-
sustaining system. Can you think how this happens? An aquarium
is an example of a human-made ecosystem.
n Can we leave the aquarium as such after we set it up? Why does
it have to be cleaned once in a while? Do we have to clean ponds
or lakes in the same manner? Why or why not?
We have seen in earlier classes that organisms can be grouped as
producers, consumers and decomposers according to the manner in
which they obtain their sustenance from the environment. Let us recall
what we have learnt through the self sustaining ecosystem created by
us above. Which organisms can make organic compounds like sugar
and starch from inorganic substances using the radiant energy of the
Sun in the presence of chlorophyll? All green plants and certain bacteria
which can produce food by photosynthesis come under this category
and are called the producers.
Organisms depend on the producers either directly or indirectly for
their sustenance? These organisms which consume the food produced,
either directly from producers or indirectly by feeding on other consumers
are the consumers. Consumers can be classed variously as herbivores,
carnivores, omnivores and parasites. Can you give examples for each of
these categories of consumers?
n Imagine the situation where you do not clean the  aquarium and
some fish and plants have died. Have you ever thought what
happens when an organism dies? The microorganisms, comprising
bacteria and fungi, break-down the dead remains and waste
products of organisms. These microorganisms are the decomposers
as they break-down the complex organic substances into simple
inorganic substances that go into the soil and are used up once
more by the plants. What will happen to the garbage, and dead
animals and plants in their absence? Will the natural replenishment
of the soil take place, even if decomposers are not there?
Activity 13.2 Activity 13.2 Activity 13.2 Activity 13.2 Activity 13.2
n While creating an aquarium did you take care not to put an aquatic
animal which would eat others? What would have happened
otherwise?
n Make groups and discuss how each of the above groups of
organisms are dependent on each other.
n Write the aquatic organisms in order of who eats whom and form
a chain of at least three steps.  ?  ? 
n Would you consider any one group of organisms to be of primary
importance? Why or why not?
2024-25
Science
210
Figure 13.2 Figure 13.2 Figure 13.2 Figure 13.2 Figure 13.2
Trophic levels
13.1.1 Food Chains and Webs
In Activity 13.4 we have formed a series of organisms feeding
on one another. This series or organisms taking part at
various biotic levels form a food chain (Fig. 13.1).
Each step or level of the food chain forms a trophic level.
The autotrophs or the producers are at the first trophic level.
They fix up the solar energy and make it available for
heterotrophs or the consumers. The herbivores or the primary
consumers come at the second, small carnivores or the
secondary consumers at the third and larger carnivores or
the tertiary consumers form the fourth trophic level (Fig. 13.2).
We know that the food we eat acts as a fuel to provide us
energy to do work. Thus the interactions among various
components of the environment involves flow of energy from
one component of the system to another. As we have studied,
the autotrophs capture the energy present in sunlight and
convert it into chemical energy. This energy supports all the
activities of the living world. From autotrophs, the energy goes
to the heterotrophs and decomposers. However, as we saw in
the previous Chapter on ‘Sources of Energy’, when one form
of energy is changed to another, some energy is lost to the
environment in forms which cannot be used again. The flow
of energy between various components of the environment
has been extensively studied and it has been found that –
n The green plants in a terrestrial ecosystem capture about
1% of the energy of sunlight that falls on their leaves and convert it
into food energy.
n When green plants are eaten by primary consumers, a great deal
of energy is lost as heat to the environment, some amount goes
into digestion and in doing work and the rest goes towards growth
and reproduction. An average of 10% of the food eaten is turned
into its own body and made available for the next level of
consumers.
n Therefore, 10% can be taken as the average value for the amount
of organic matter that is present at each step and reaches the next
level of consumers.
n Since so little energy is available for the next level of consumers,
food chains generally consist of only three or four steps. The loss
of energy at each step is so great that very little usable energy
remains after four trophic levels.
n There are generally a greater number of individuals at the lower
trophic levels of an ecosystem, the greatest number is of the
producers.
n The length and complexity of food chains vary greatly. Each organism
is generally eaten by two or more other kinds of organisms which in
turn are eaten by several other organisms. So instead of a straight
line food chain, the relationship can be shown as a series of branching
lines called a food web (Fig. 13.3).
Figure 13.1 Figure 13.1 Figure 13.1 Figure 13.1 Figure 13.1
Food chain in nature
(a) in forest, (b) in
grassland and (c) in a
pond
2024-25
Our Environment 211
From the energy flow diagram (Fig. 13.4),
two things become clear. Firstly, the flow of
energy is unidirectional. The energy that is
captured by the autotrophs does not revert
back to the solar input and the energy which
passes to the herbivores does not come back
to autotrophs. As it moves progressively
through the various trophic levels it is no
longer available to the previous level.
Secondly, the energy available at each trophic
level gets diminished progressively due to loss
of energy at each level.
Another interesting aspect of food chain
is how unknowingly some harmful
chemicals enter our bodies through the food
chain. You have read in Class IX how water
gets polluted. One of the reasons is the use of
several pesticides and other chemicals to
protect our crops from diseases and pests.
These chemicals are either washed down into
the soil or into the water bodies. From the
soil, these are absorbed by the plants along
with water and minerals, and from the water
bodies these are taken up by aquatic plants
Figure 13.4  Figure 13.4  Figure 13.4  Figure 13.4  Figure 13.4  Diagram showing flow of energy in an ecosystem
Figure 13.3 Figure 13.3 Figure 13.3 Figure 13.3 Figure 13.3
Food web, consisting of
many food chains
2024-25
Page 5


Science
208
Activity 13.1 Activity 13.1 Activity 13.1 Activity 13.1 Activity 13.1
Our Environment
13 CHAPTER
W
e have heard the word ‘environment’ often being used on the
television, in newspapers and by people around us. Our elders tell
us that the ‘environment’ is not what it used to be earlier; others say that
we should work in a healthy ‘environment’; and global summits involving
the developed and developing countries are regularly held to discuss
‘environmental’ issues. In this chapter, we shall be studying how various
components in the environment interact with each other and how we
impact the environment.
13.1 ECO 13.1 ECO 13.1 ECO 13.1 ECO 13.1 ECO-SYSTEM — WHA -SYSTEM — WHA -SYSTEM — WHA -SYSTEM — WHA -SYSTEM — WHAT ARE ITS COMPONENTS? T ARE ITS COMPONENTS? T ARE ITS COMPONENTS? T ARE ITS COMPONENTS? T ARE ITS COMPONENTS?
All organisms such as plants, animals, microorganisms and human
beings as well as the physical surroundings interact with each other
and maintain a balance in nature. All the interacting organisms in an
area together with the non-living constituents of the environment form
an ecosystem. Thus, an ecosystem consists of biotic components
comprising living organisms and abiotic components comprising
physical factors like temperature, rainfall, wind, soil and minerals.
For example, if you visit a garden you will find different plants, such
as grasses, trees; flower bearing plants like rose, jasmine, sunflower;
and animals like frogs, insects and birds. All these living organisms
interact with each other and their growth, reproduction and other
activities are affected by the abiotic components of ecosystem. So a garden
is an ecosystem. Other types of ecosystems are forests, ponds and lakes.
These are natural ecosystems while gardens and crop-fields are human-
made (artificial) ecosystems.
n You might have seen an aquarium. Let us try to design one.
n What are the things that we need to keep in mind when we create
an aquarium? The fish would need a free space for swimming (it
could be a large jar), water, oxygen and food.
n We can provide oxygen through an oxygen pump (aerator) and
fish food which is available in the market.
2024-25
Our Environment 209
n If we add a few aquatic plants and animals it can become a self-
sustaining system. Can you think how this happens? An aquarium
is an example of a human-made ecosystem.
n Can we leave the aquarium as such after we set it up? Why does
it have to be cleaned once in a while? Do we have to clean ponds
or lakes in the same manner? Why or why not?
We have seen in earlier classes that organisms can be grouped as
producers, consumers and decomposers according to the manner in
which they obtain their sustenance from the environment. Let us recall
what we have learnt through the self sustaining ecosystem created by
us above. Which organisms can make organic compounds like sugar
and starch from inorganic substances using the radiant energy of the
Sun in the presence of chlorophyll? All green plants and certain bacteria
which can produce food by photosynthesis come under this category
and are called the producers.
Organisms depend on the producers either directly or indirectly for
their sustenance? These organisms which consume the food produced,
either directly from producers or indirectly by feeding on other consumers
are the consumers. Consumers can be classed variously as herbivores,
carnivores, omnivores and parasites. Can you give examples for each of
these categories of consumers?
n Imagine the situation where you do not clean the  aquarium and
some fish and plants have died. Have you ever thought what
happens when an organism dies? The microorganisms, comprising
bacteria and fungi, break-down the dead remains and waste
products of organisms. These microorganisms are the decomposers
as they break-down the complex organic substances into simple
inorganic substances that go into the soil and are used up once
more by the plants. What will happen to the garbage, and dead
animals and plants in their absence? Will the natural replenishment
of the soil take place, even if decomposers are not there?
Activity 13.2 Activity 13.2 Activity 13.2 Activity 13.2 Activity 13.2
n While creating an aquarium did you take care not to put an aquatic
animal which would eat others? What would have happened
otherwise?
n Make groups and discuss how each of the above groups of
organisms are dependent on each other.
n Write the aquatic organisms in order of who eats whom and form
a chain of at least three steps.  ?  ? 
n Would you consider any one group of organisms to be of primary
importance? Why or why not?
2024-25
Science
210
Figure 13.2 Figure 13.2 Figure 13.2 Figure 13.2 Figure 13.2
Trophic levels
13.1.1 Food Chains and Webs
In Activity 13.4 we have formed a series of organisms feeding
on one another. This series or organisms taking part at
various biotic levels form a food chain (Fig. 13.1).
Each step or level of the food chain forms a trophic level.
The autotrophs or the producers are at the first trophic level.
They fix up the solar energy and make it available for
heterotrophs or the consumers. The herbivores or the primary
consumers come at the second, small carnivores or the
secondary consumers at the third and larger carnivores or
the tertiary consumers form the fourth trophic level (Fig. 13.2).
We know that the food we eat acts as a fuel to provide us
energy to do work. Thus the interactions among various
components of the environment involves flow of energy from
one component of the system to another. As we have studied,
the autotrophs capture the energy present in sunlight and
convert it into chemical energy. This energy supports all the
activities of the living world. From autotrophs, the energy goes
to the heterotrophs and decomposers. However, as we saw in
the previous Chapter on ‘Sources of Energy’, when one form
of energy is changed to another, some energy is lost to the
environment in forms which cannot be used again. The flow
of energy between various components of the environment
has been extensively studied and it has been found that –
n The green plants in a terrestrial ecosystem capture about
1% of the energy of sunlight that falls on their leaves and convert it
into food energy.
n When green plants are eaten by primary consumers, a great deal
of energy is lost as heat to the environment, some amount goes
into digestion and in doing work and the rest goes towards growth
and reproduction. An average of 10% of the food eaten is turned
into its own body and made available for the next level of
consumers.
n Therefore, 10% can be taken as the average value for the amount
of organic matter that is present at each step and reaches the next
level of consumers.
n Since so little energy is available for the next level of consumers,
food chains generally consist of only three or four steps. The loss
of energy at each step is so great that very little usable energy
remains after four trophic levels.
n There are generally a greater number of individuals at the lower
trophic levels of an ecosystem, the greatest number is of the
producers.
n The length and complexity of food chains vary greatly. Each organism
is generally eaten by two or more other kinds of organisms which in
turn are eaten by several other organisms. So instead of a straight
line food chain, the relationship can be shown as a series of branching
lines called a food web (Fig. 13.3).
Figure 13.1 Figure 13.1 Figure 13.1 Figure 13.1 Figure 13.1
Food chain in nature
(a) in forest, (b) in
grassland and (c) in a
pond
2024-25
Our Environment 211
From the energy flow diagram (Fig. 13.4),
two things become clear. Firstly, the flow of
energy is unidirectional. The energy that is
captured by the autotrophs does not revert
back to the solar input and the energy which
passes to the herbivores does not come back
to autotrophs. As it moves progressively
through the various trophic levels it is no
longer available to the previous level.
Secondly, the energy available at each trophic
level gets diminished progressively due to loss
of energy at each level.
Another interesting aspect of food chain
is how unknowingly some harmful
chemicals enter our bodies through the food
chain. You have read in Class IX how water
gets polluted. One of the reasons is the use of
several pesticides and other chemicals to
protect our crops from diseases and pests.
These chemicals are either washed down into
the soil or into the water bodies. From the
soil, these are absorbed by the plants along
with water and minerals, and from the water
bodies these are taken up by aquatic plants
Figure 13.4  Figure 13.4  Figure 13.4  Figure 13.4  Figure 13.4  Diagram showing flow of energy in an ecosystem
Figure 13.3 Figure 13.3 Figure 13.3 Figure 13.3 Figure 13.3
Food web, consisting of
many food chains
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Activity 13.3 Activity 13.3 Activity 13.3 Activity 13.3 Activity 13.3
and animals. This is one of the ways in which they enter the food chain.
As these chemicals are not degradable, these get accumulated
progressively at each trophic level. As human beings occupy the top
level in any food chain, the maximum concentration of these chemicals
get accumulated in our bodies. This phenomenon is known as biological
magnification. This is the reason why our food grains such as wheat
and rice, vegetables and fruits, and even meat, contain varying amounts
of pesticide residues. They cannot always be removed by washing or
other means.
QUESTIONS
?
1. What are trophic levels? Give an example of a food chain and state the
different trophic levels in it.
2. What is the role of decomposers in the ecosystem?
13.2 13.2 13.2 13.2 13.2 HOW DO OUR ACTIVITIES AFFECT THE HOW DO OUR ACTIVITIES AFFECT THE HOW DO OUR ACTIVITIES AFFECT THE HOW DO OUR ACTIVITIES AFFECT THE HOW DO OUR ACTIVITIES AFFECT THE
ENVIRONMENT? ENVIRONMENT? ENVIRONMENT? ENVIRONMENT? ENVIRONMENT?
We are an integral part of the environment. Changes in the environment
affect us and our activities change the environment around us. We have
already seen in Class IX how our activities pollute the environment. In
this chapter, we shall be looking at two of the environmental problems
in detail, that is, depletion of the ozone layer and waste disposal.
13.2.1 Ozone Layer and How it is Getting Depleted
Ozone (O
3
) is a molecule formed by three atoms of oxygen. While O
2
,
which we normally refer to as oxygen, is essential for all aerobic forms of
life. Ozone, is a deadly poison. However, at the higher levels of the
atmosphere, ozone performs an essential function. It shields the surface
of the earth from ultraviolet (UV) radiation from the Sun. This radiation
n Newspaper reports about pesticide levels in ready-made food items
are often seen these days and some states have banned these
products. Debate in groups the need for such bans.
n What do you think would be the source of pesticides in these food
items? Could pesticides get into our bodies from this source through
other food products too?
n Discuss what methods could be applied to reduce our intake of
pesticides.
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