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Module 1: Ecological Interactions and Energetics

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Food Chains, Food Webs & Trophic Levels

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Today we begin a new module and that is Ecological Energetics.
(Refer Slide Time: 00:19)

In this module we will have three lectures; Food chains, Food webs and Trophic levels
followed by primary production and nutrient cycles.

(Refer Slide Time: 00:28)

So, let us begin with the first lecture food chains, food webs and trophic levels.
(Refer Slide Time: 00:30)

This is an image that we all must remember from our school days. It says that there is grass
and grass is eaten by some insects and then there is a frog that eats these insects, then there
is a snake that eats the frog and then there is the hawk or the eagle that eats the snake. And
the plants are getting their energy from the sun and this is what essentially a food chain is.
In the case of ecology, we are going to study this food chain in greater detail and also

understand what are the kinds of nuances that we observe in different food chains and food
webs.
(Refer Slide Time: 01:08)

Let us begin by defining what a food chain is. The transfer of food energy from its source
in plants through herbivores to carnivores, detritivores or decomposers is referred to as the
food chain. So, detritivores or, waste or debris feeders and the food chain has primarily
got to do with the transfer of food energy.
The sun is providing energy to the plants and that energy is getting stored in the form of
carbohydrates and in the form of different organic molecules so that energy from sunlight
is getting converted into chemical energy. This chemical energy is moving through
different organisms in this community in the form of the food chain.

(Refer Slide Time: 01:54)

We also have a number of other definitions that we will use later on. Autotrophs are
organisms responsible for primary production. They are known as primary producers and
they include trees, plants and algae. Auto means self, troph means nutrition.
(Refer Slide Time: 02:12)

Auto is self, troph is nutrition. These are plants that are making nutrition by themselves.
On the other hand, hetero is other, so heterotroph is an organism that is using some other
organism for its nutrition. Autotrophs include trees, plants and algae and also a number of
bacteria that get their energy from chemicals which are known as chemoautotrophs.

Heterotrophs are organisms that cannot produce their own food. They are organisms like
us, they rely instead on the intake of nutrition from other sources of organic carbon mainly
plant or animal matter; and example includes most animals such as us.
(Refer Slide Time: 03:04)

Now autotrophs are divided into photoautotrophs and chemoautotrophs. Photo is light; so
photoautotrophs is light self-nourishment or light self-nutrition. Photoautotrophs are
organisms that use light as a source of energy to manufacture organic molecules and food;
example most plants. And chemoautotrophs, chemo is chemical; so, chemoautotrophs are
organisms that use chemical reactions as a source of energy to manufacture organic
molecules and food.
An example is this bacterium, Hydrogenovibrio crunogenus, which is found in deep sea
hydrothermal vents. Hydro is water, thermal is heat. You have heated water vents that are
found inside deep seas, they are mostly of volcanic origin and you have these bacteria that
you that utilize this chemical energy to make their own food.

(Refer Slide Time: 04:00)

Then when we talk about food chains we can divide all the organisms into producers and
consumers. Producer is an organism that makes its own food which is the autotrophs;
including both the photoautotrophs and the chemoautotrophs. Consumer is an organism
that consumes some other organism for food; some other organism or some other part of
that organism.
The consumers which are using some other organisms or parts of some other organisms as
food are further divided into primary, secondary, tertiary and quaternary consumers.
(Refer Slide Time: 04:32)

Primary consumer is an organism that consumes the producers. This is an organism that
consumes the plant matter or the autotrophs. An example is a grasshopper or another
example could be the cow or all the herbivorous animals: chital, sambar, elephant and most
of these organisms are very important, prey species in the ecosystem.
These species are eaten up by their consumers, their predators which go by the name of
secondary consumers. Primary consumer is something that is a herbivore and secondary
consumer is a carnivore. It could also be an omnivore which is also feeding on the plants,
but primarily it is a carnivore. It is an organism that consumes the primary consumer. So,
that is the frog; so we have the producer.
(Refer Slide Time: 05:29)

You have the producer, that is eaten up by the primary consumer, which is then eaten up
by the secondary consumer, which is then eaten up by the tertiary consumer, which is then
eaten up by the quaternary consumer and so on. Tertiary consumer is somebody who eats
a secondary consumer.

(Refer Slide Time: 05:57)

And an organism that consumes the secondary consumer example is a snake. And
quaternary consumer is an organism that consumes the tertiary consumer such as the hawk.
In this particular example, we are seeing that you have producer in the form of grass, and
the primary consumer is a grasshopper, the secondary consumer is a frog, the tertiary
consumer is a snake and the quaternary consumer is a hawk; a hawk or maybe an eagle.
(Refer Slide Time: 06:34)

These consumers can also be classified as herbivores, carnivores, omnivores, detritivores
and so on.

Going through the terms - Herbivore is an organism that eats only plants, their primary
consumers; example is a cow. Carnivore is an organism that eats other animals and they
can be secondary, tertiary or quaternary consumers. So they can be anywhere, but they
cannot be the primary consumer and they cannot be the producer. An example is the tiger.
(Refer Slide Time: 07:00)

Omnivore is an organism that eats both plants and animals and they are generally
secondary or tertiary consumers; example is the bear. The bear can eat some insects, it can
also eat some amount of flesh, but it also feeds on fruits, it also feeds on roots of plants
and so on; so, it is an omnivorous.
Next is a decomposer; a decomposer is an organism that converts dead material into soil
and recycles the nutrients. When we are talking about any food chain so in this food chain
not only is energy getting passed, but also other nutrients such as minerals. For instance
when we talk about proteins, so proteins have nitrogen in them. The plant proteins have
nitrogen, from there the nitrogen has moved into the grasshopper, from there to the frog,
from there to the snake, from there to the hawk and eagle. But then, if all the nutrients, all
the inorganic materials and the nutrients if they get locked up in all the organisms, in that
case the plants will not be able to get the nutrients.
What is the role of the decomposers? The decomposers break up the bodies of all of these
and also of the plants when they are dead and then they release all of these nutrients back

into the soil. A decomposer is an organism that converts dead material into soil and
recycles nutrients. Decomposers include detritivores and microorganisms.
(Refer Slide Time: 08:47)

When we say herbivore. So herbivore: vore is eating and herbi is herbs. So, this is an
organism that eats herbs or that eats the plants. In the word carnivore; ‘carni’ means flesh.
(This is the same word root that is also seen in ‘carnival’ which is a festival of the flesh;
that is how the word root goes.) So, carnivore is an organism that eats up ‘carni’ or that
eats of the flesh; so, that is a carnivore.
In the word omnivore; Omni means both or Omni means all. Omnivores is something that
eats up everything. It eats up plants, it also eats up the flesh. When we say detritivores: so
here you have something that eats up the detritus. Now what is a detritus? The detritivores
consume detritus which is decomposing plants and animal parts as well as feces. So, any
dead part, any decomposing part, so the organisms that eat up those are known as in
detritivores. And once they eat these up they make it more and more exposed to the action
of microbial decomposers such as bacteria and fungi that further break it down. A good
example of detritivores is an earthworm.
If you have dead leaves that are there in the soil, the earthworm will eat up those dead
leaves. And then it will take out some amount of nutrients from there, but in this process
it will also convert those dead leaves into its own fecal matter. And by breaking off the
larger tissues into smaller chunks it makes it more and more exposed to the action of the

microorganisms, such as bacteria and fungi. Which further break it down to release all the
nutrients into the soil. So, that is the rule of the detritivores.
(Refer Slide Time: 10:43)

When you talk about food chains, there are two kinds of food chains; one is the grazing
food chain and the second one is the detritus food chain. Grazing food chain starts from
the plant base. So, it starts from grazing the plant base.
Grazing food chain starts from a plant base goes through herbivores to carnivores. And
grazing food chains are then further subdivided into predator food chains and parasitic
food chains. On the other hand a detritus food chain starts from the detritus or the dead
and decaying matter and then it goes through detritivores to carnivores.

(Refer Slide Time: 11:24)

Essentially in the case of the grazing food chain; so when you have the grazing food chain,
you have plants followed by herbivores followed by carnivores. And then these carnivores
can be; so they can be secondary, tertiary, quaternary and so on.
In the case of detritus food chain, you have detritus followed by detritivores and then these
are followed by the carnivores which can again be secondary, tertiary, quaternary and so
on. Now we will look at some examples.
When we see the grazing food chain you have two examples, two sub types; one is the
predator food chains. So, predator food chain, a good example is grass that is eaten up by
chital and that is eaten up by tiger or the one that we saw before; you have grass that is
eaten up by the grasshopper, that is eaten up by the frog, that is eaten up by the snake, and
which is eaten up by the hawk. Now when we observe a predator food chain the size of
the organisms generally increases as we move up the chain. So, grass is very small, chital
is a bit larger, its close to around 60-70-80 kg animal and then if you look at a tiger that is
a very large animal say around 300 or 400 kgs weight.
So, the size of the organisms increases as we move up the chain. On the other hand when
we look at the parasitic food chains, so a parasitic food chain, an example is rat flea
followed by a parasitic protozoa. So, again in this case the rat will be preceded by the grass
or say grains. In this case this portion which is the parasitic food chain, so from a grass to
rat will be a predator part of the food chain, but this part is a completely parasitic food

chain; rat to flea to parasitic protozoa. In this case the size of the organisms generally
decreases as we move up the chain. A flea is very much smaller than a rat and the parasitic
protozoa are microorganisms that are living on the flea. Here the size goes down as we
move up.
Now in the case of a detritus food chain, you have detritus followed by detritivores
followed by carnivores. An example is, you can have the fallen leaves of mangroves.
Mangroves are plants that grow in the vicinity of the sea shores or ocean shores, and when
the leaves of these plants, when they die and when they fall down, so they get into the
water, and there they are they are consumed by the detritivores.
Now these detritivores are then eaten up by the detritivore consumers. For example, small
fish or insect larvae; so, here we had some detritivores some very small organisms that are
eating these up and then these organisms are further eaten up by the insect larvae. And
from there they get eaten up by small fishes then large fishes, and then piscivorous; pisci
is fish vore is eating. There are fish eating birds. They are eaten up by the fish-eating birds,
so this is a detritus food chain.
(Refer Slide Time: 15:01)

If you look at the differences between both of these food chains we observe that, in the
case of a grazing food chain the primary source of energy is the sun, which is then used up
by the plants. In the case of the detritus food chain the primary source of energy is detritus.
It starts with the detritus or a dead tissue or a dead organism. Then the first trophic level

in the case of grazing food chain is herbivores, because they are eating up the plants. In
the case of detritus food chain they are detritivores.
Now the length of grazing food chain is generally longer, and the length of a detritus food
chain is generally shorter. What do we mean by that?
(Refer Slide Time: 15:45)

If you have say plants, then they are eaten up by herbivores, then they are eaten up by the
secondary consumer, then they are eaten up by the tertiary consumer, then the quaternary
consumer and so on. At each of these stages: so from plants to herbivores; plants, suppose
they have say 100 calories of energy that is stored in their body tissues, when the
herbivores have eaten those, they will spend quite a lot of energy to warm up their bodies
to ensure that the blood circulation goes on for the movement of their bodies. And so, only
about 10 percent of that energy will get stored in the bodies of the herbivores. Here you
only have 10 calories, from here to the next level again it will be 10 percent. This will have
only 1 calorie and then this will have 0.1 calorie and so on, 0.01 calorie.
The more up you go into the food chain then the lesser is the amount of energy that is
available. In the case of a grazing food chain, because it starts with the sun, so you have
ample amount of energy that is available in the grazing food chain and so it can support
longer chains. Whereas, in the case of detritus food chain because the starting material
itself is very small, it does not have a huge amount of energy. It cannot support a very long
chain of organisms. A detritus food chain is therefore, generally shorter in length.

(Refer Slide Time: 17:30)

That is about the food chains, but then if we look around in the nature, we will observe
that not a chain is present, but multiple chains that are present and all of these multiple
chains make up a food web.
(Refer Slide Time: 17:47)

For instance when we were talking about our simplest food chain in which we had grass
and then this grass was eaten up by the grasshopper, then this was eaten up by a frog, then
a snake and then a hawk. But then grass is also eaten up by say a locust and grass is also
eaten up by say some other caterpillars. Then a frog will also be eating up a locust and
maybe there would be some birds that would directly be eating up a caterpillar or they
would also be eating up a locust. And then, you could also have a bird that eats up a
grasshopper directly. And then, you can have a snake that maybe feeds on bird eggs or you
can have a situation in which there is a frog that is also eaten up by a bird.
When we look at these combinations, this becomes more and more complex. A
combination of different food chains that works together goes by the name of a food web.
(Refer Slide Time: 19:00)

We define a food web as a system of interlocking and interdependent food chains. All of
these different food chains they are linked together, they are interlinking and they are
interlocking and they are interdependent on each other.
Then we also define a trophic level. A trophic level is each of several hierarchical levels
in an ecosystem consisting of organisms sharing the same function in the food chain and
the same nutritional relationship to the primary source of energy. When we look at this
food web, so here you have grass, you may also have some trees, you may also have some
shrubs, you can also have some herbs and so on.

And all of these, if you look at this definition, all of these organisms share the same
function in the food chain that is all of them are producers, and the same nutritional
relationship to the primary sources of energy. All of these are using the energy of the sun.
All of these are using the energy of the sun and then they are passing it on to the primary
consumers or the herbivores.
All of these together will form a trophic level. Similarly, all of these together, the
grasshopper the locust, the caterpillars that are eating or that are feeding on these
organisms, the grass, trees, shrubs, herbs and so on, they will form another trophic level.
When we look at a food web we can define different trophic levels, we can define
organisms that are producers, that are primary consumers, secondary consumers, tertiary
consumers and so on. Although, it looks much more simpler in theory than it looks in
practice.
(Refer Slide Time: 20:53)

For instance this is a food web that is found in the Atlantic Ocean. As we can observe here,
there are so many interrelationships, so many who eats whom relationship here that it
becomes a bit difficult. But then, it is also possible that one organism may a be a part of
several different trophic levels.
For instance here we have a situation in which you have a bird that is eating up caterpillars.
When this bird is eating up caterpillars; so you have caterpillar as the primary consumer,
so the bird becomes a secondary consumer. But then if this bird eats up a frog, so, in this

case the frog is a secondary consumer; so, here the bird will also become a tertiary
consumer or if this bird eats up say some fruits from the trees, so it will also become a
primary consumer.
Essentially any organism in a food web may occupy more than one of those trophic levels.
We can understand the level of complexity that is involved here. You have so many
relationships and every organism or many organisms can occupy different trophic levels
at the same time. So, to reduce this complexity we make use of some tools.
(Refer Slide Time: 22:12)

And one such tool goes by the name of an ecological pyramid. Now, an ecological pyramid
is a graphical representation designed to show the biomass, numbers or energy at each
trophic level in a given ecosystem. It is a graphical representation that is designed to show
biomass. Biomass is the total amount of mass that is formed out of the biological entities
that are present at each trophic level or the number of organisms that are there at each
trophic level or the energy that is there in each trophic level.
And these ecological pyramids also go by the name of trophic pyramid, eltonian pyramid,
energy pyramid, food pyramid and so on. We will look at some different kinds of
pyramids.

(Refer Slide Time: 22:58)

The first pyramid is the pyramid of numbers. Pyramid of numbers shows you the number
of organisms that are present at each trophic level. What do we mean by that?
(Refer Slide Time: 23:10)

This is one pyramid of numbers. In the case of the first trophic level which is the plants or
the producers, we will go into the field and will count how many plants are there; how
many individual grasses are there. And when we total up all of them; all the trees, all the
herbs, all the shrubs, all the grasses that are there together will come up with a number.
And this number will be depicted by this particular portion of the pyramid.

The area of this pyramid or the area of this particular block is proportional to the number
of entities that we have at this trophic level. And all of these different slabs are showing
different trophic levels. This is the trophic level of the producers, this is the trophic level
of the primary consumer, this is the trophic level of secondary consumer, this is for the
tertiary consumer, this is for the quaternary consumer and so on. This pyramid will show
us the number of individuals that we have at each trophic level.
For instance, if there is an ecosystem in which you have tigers and you have leopards and
both of these are apex predators or both of these are quaternary consumers. This particular
block will tell you how many tigers are there plus how many leopards are there, you put
both the numbers together and you reach to this level.
Generally it is observed that the number of individuals will go down as we move up the
trophic level. Because, for instance in a tiger reserve like the Panna tiger reserve, you will
have say around 30-40 tigers. So, that will form this upper block. So, you will have say 40
tigers, but then if you look at the number of their prey species. So, if you look at the number
of chital or the number of sambars they might go up into several hundreds or maybe even
several thousands. So, as we move down, the size would increase.
If you look at say the number of grasses, that are there then that might even go up into
millions. As we move up a pyramid the number decreases; as you move down a pyramid
the number increases. You have a larger size base and you have an out narrower top. This
is something that is intuitively expected.

(Refer Slide Time: 25:41)

In this case you will have say, grass and you have say, 1 million individuals of grass. And
then you have these 1 million individuals of grass are supporting say chital and sambar,
and their number is say 4000. And then this number would be supporting say around tigers
plus leopards say, around 50 individuals.
We see that as we are moving up in the trophic levels, as we are moving up in the pyramid,
the number is going down. But then you can also have an inverted pyramid of numbers.
(Refer Slide Time: 26:32)

In an inverted pyramid of numbers, you will have a situation in which the number of
quaternary is much greater than that of the tertiary consumers, which is greater than the
number of secondary consumers, which is greater than the number of primary consumers,
which is greater than the number of producers.
(Refer Slide Time: 26:54)

One such example is given by the tree ecosystem. In the case of a tree, a tree might be
supporting some number of birds. Because a tree is such a large organism, so it might
support a number of birds. Let us say that there are 100 birds that are being supported by
one tree; so here you have 1, and here you have 100. Now if we consider any parasites that
live on the birds, each bird would be supporting say around 50-60 parasites. And each of
those parasites such as a flea would be supporting a number of hyper parasites say, such
as bacteria or protozoa.

(Refer Slide Time: 27:33)

In this case, what we are observing is that you have one tree that is supporting say, 50 birds
or let us say 100 birds. Now each of those birds is supporting say 10 parasites. So, if each
of those supports 10 parasites, so this particular block would become 10 into 100 is fleas
1000 fleas or 1000 parasites.
If each of these parasites is supporting say 200 bacteria. In that case, the number of bacteria
that will come on the top here would be very large, because each parasite is supporting
200 bacteria and you have 1000 parasites. You have 200000 or 2 lakh bacteria. This is a
pyramid of numbers that is inverted, because you have a smaller sized bottom and a larger
sized top.

(Refer Slide Time: 28:44)

You can also have other shapes such as a spindle. In the case of a spindle you have a
middle range that is larger and then that is followed by a smaller portion. For instance, in
place of having these parasites, so, we remove the parasites and the hyper parasites that
were there in our example and now you only have these birds that are supporting a
population of hawks.
And these are say frugivorous birds. Frugivorous means that these birds are living on fruits
- frugi is fruit; vore is eating, and they are supporting say 10 hawks. Here we have a

pyramid of numbers that looks like a spindle which is this shape. Another such example
could be from, one of the lakes or maybe from the seas.
(Refer Slide Time: 29:42)

So, now in the oceans what we have is we have the phytoplanktons. Phytoplanktons are
microscopic organisms that are able to sequester carbon. They perform photosynthesis and
they act as the producers in this case. These phytoplanktons support the zooplanktons.
Now zooplanktons are again microscopic organisms and these are animals. These are
microscopic animals that are feeding on the phytoplankton. These are microscopic plants
and these are microscopic animals.
It is possible that in a certain situation where the rate of reproduction of the phytoplankton
is very high. You have phytoplanktons that are able to multiply themselves very quickly.
They multiply themselves and then they also get eaten by the zooplanktons. These
zooplanktons are getting a sufficient source of food, because of the phytoplankton. You
have a large number of zooplanktons, but you can have a situation in which the
phytoplanktons are much lesser. Why much lesser? Because they are able to reproduce
very fast.

(Refer Slide Time: 31:05)

Even though you have a very small number, in this case, suppose you have 1000
phytoplanktons and suppose they are able to duplicate themselves every hour. So, in that
case in 1 hour from 1000 they will become 2000, in 2 hours from 2000 they become 4000,
in 3 hours from 4000 they become 8000, in 4 hours they become 16000 and so on.
If this is the rate of propagation, we will find a situation in which the population increases
exponentially. Here you have the number of phytoplanktons and this is the time. Because
nature is not able to support such a huge rate of growth; so, it is possible that at this level
their population is being consumed by those zooplanktons. Even though you have a lesser
number of phytoplanktons they are able to multiply themselves very fast and then they
multiply themselves fast they become much more. When they become much more they are
eaten up by the zooplanktons. In that case the number of phytoplanktons are again goes
down and the number of zooplanktons increases.
When you look at a snapshot of this ecosystem, we will find that you have less number of
phytoplanktons more number of zooplanktons. And then these zooplanktons have been
eaten up by the fish. So, the number of fishes is less and then suppose these fishes are eaten
up by a sea lion then that gives the number of sea lion is also less. Here again we are
observing a pyramid of numbers that is looking of this shape, so, this is the shape of a
spindle.

(Refer Slide Time: 33:01)

.

Or we can even have a pyramid of numbers that looks like a dumbbell. So, one such
example is grass, rabbit, flea. So, you have grasses, so there are quite a number of grasses
in your ecosystem and those grasses are supporting a small number of rabbits. But then,
because each rabbit can support a multiple or a large number of a these parasitic fleas. So,
the total number of fleas that will be found in this ecosystem will be greater than the
number of rabbits. And in that case the pyramid of numbers will look like a dumbbell.
Another pyramid is the pyramid of energy. We looked at the pyramid of numbers in which
each trophic level is showing how many individuals are present at that particular trophic
level. In the case of pyramid of energy, we look at the energy that is contained in organisms
at each trophic level. And in this case, the energy generally looks like this. You have a
pyramid that is a flatter or wider at the bottom and that goes on tapering to the top. The
amount of energy that is present at each trophic level goes on decreasing as we move up
the food chain or the food web.
The third kind of a pyramid is the pyramid of biomass which is the biomass of organisms
that is present at each trophic level. Here as well, in most situations we observe a pyramid
of biomass that looks like this. It is wider at the bottom and it goes on tapering as we move
to the top.

(Refer Slide Time: 34:35)

But another example is that of an inverted pyramid of biomass. An inverted pyramid of
biomass for example, can be the example of, again an oceanic ecosystem. Here again you
have planktons and these planktons are supporting small fishes. But then, because these
planktons have a faster rate of reproduction or a faster rate of multiplication, so a smaller
number of planktons can support a larger number of fishes. And because each of those
fishes are having the large amount of biomass, so if you look at the amount of biomass
that is being supported by the planktons, it will be less than the biomass that is there in the
fishes.
And then, from a small fish to a larger fish to an even larger fish. For instance, if we look
at a large sized shark. A shark will be having a huge amount of biomass that is stored in
its body and that might be much greater than the total biomass that is present in its prey
species. We can have a situation in which we have an inverted pyramid of biomass.

(Refer Slide Time: 35:50)

Now, in these pyramids we can also define the standing crop. A standing crop is the total
dried biomass of the living organisms that are present at each trophic level. In this case,
we are looking at the dried-out biomass. For instance, in the earlier situation we were
looking at a total amount of biomass that was present in the grasses for instance. Let us
say that we had 1 million tons of biomass and that was the green biomass. In the case of
standing crop, we will look at the dried out biomass after removing all the water.
(Refer Slide Time: 36:37)

The next concept is that of ecological efficiency. Ecological efficiency is the efficiency
with which energy is transferred from one trophic level to the next.