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Module 1: Introduction to Ecology and Evolution

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Species Abundance and Composition: Biodiversity

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In our Ecological Structure module, we now move to the second lecture, which Species
abundance and composition or Biodiversity. The ecological structure is comprised of a
number of species. And so, it becomes very important to understand how many species are
there?, how are they distributed? and what is the composition in a particular ecosystem?
So, how many individuals are there of each species and this is the study of biodiversity.
(Refer Slide Time: 00:45)

A number of ecological studies have been done in the forest, because forest provide an
avenue in which you can understand nature in its most raw form because they are
untouched by humans. So, we are able to see how nature operates without any
anthropogenic influence.
If you enter into a forest, what do you observe? The first thing you will observe is a lot of
serenity. You would not have any loud noises of vehicles, you would not have any smoke
and so on. And, you will also observe a number of trees if you are moving into forest that

is say a deciduous forest or it or a coniferous forest. You will also observe a number trees.
Now, these trees will also be divided into a number of canopies. So, you will have a top
canopy which is comprised of these tall trees, you will have a middle canopy, you will also
have some ground cover that is comprised of these herbs and shrubs and so on.
(Refer Slide Time: 01:53)

If you look around and if you are lucky, you will also observe a number of animals in the
forest. These are Chitals which are feeding on the grasses that are found in the forest. So,
you will observe some animals.
(Refer Slide Time: 02:05)

You will also observe some interactions between these animals. So, like we talked before
about the langur chital association. So, this is an example of a langur chital association.
We have this tree on which we have this langur and this langur is feeding and it is throwing
off some pieces of the leaves down into the forest floor. And so, there are a number of
chitals that have come to this area to feed on this leaves.
These chitals would not have had access to the leaves, because they cannot climb the trees
evidently. And, by coming in close contact with another species now they are able to
getting access to this food resource. And, they are also benefitted, because the langurs can
look very far because they are at a higher vantage position. If there is any predator nearby,
if there is a tiger nearby, they would give of alarm calls and the chitals would run away.
Similarly, the chitals are having a ground view of the situation and so, if a tiger is nearby
and if the langurs are not able to see it, because of say, tall grasses, the chitals might be
able to sense the presence of the tiger because of the smell of the tiger. If that happens, the
chitals would give off alarm calls and then they would start running off and that would
also alarm the langurs. Here we observe a number of interactions that are taking place
between different species when we move inside a forest area.
(Refer Slide Time: 03:35)

We will also observe a number of population level interactions amongst a number of
animals. In this case you are observing a troupe of monkeys that are doing allogrooming.

Allogrooming is grooming of someone else. So, we will observe monkeys and their
behaviours.
(Refer Slide Time: 03:55)

If, we look up, we will observe some birds; we will see parakeets most probably.
(Refer Slide Time: 04:03)

(Refer Slide Time: 04:04)

Or may be even birds like Mynas or Peacocks if you are lucky or we will also observe
some animals and birds that are migratory.
(Refer Slide Time: 04:06)

So, this is an example of Democil Cranes, which are migratory species that visit our
country for a while and then they move off.

(Refer Slide Time: 04:21)

We might observe some signs of some animals. So, for instance this is an image of a pug
mark of a tiger. A pug mark is a mark that is left on the ground when the animal is walking.
So, for instance, in this case we have this loose soil. When the tiger was walking here so,
it has left an imprint on the ground which is the pug mark. And, we have kept this pen to
give you a sense of scale of how large this pug mark is. Even though we are not able to
see some animals, we might be able to infer their presence in our forest by using these
indirect signs.
(Refer Slide Time: 05:00)

Another indirect sign is the scat of animals. So, scat is basically the fecal matter that is left
behind by the animal. And, every animal has a scat of different size and they also have
different behaviours when it comes to leaving their fecal matter. For instance, members of
the cat family would typically, after they are done disposing off their fecal matter, they
would just move like this. So, they will scratch the ground, we will see some scratch marks
near these scats and we can make a correlation about the size of the scat, the number of
scats or fecal samples that have been deposited by the animal. And, some other sign such
as scratching marks on the ground to infer which animal has give off this scat. So, we can
also get a sense of what animals are nearby, by looking at this ground.
(Refer Slide Time: 05:58)

If we look closely into the vegetation we might also observe some reptiles such as
chameleons or some other members of the lizard family.

(Refer Slide Time: 06:06)

This area might also have some flowers and may be some insects that are pollinating in
this area. For instance, here we have a bee that is pollinating and if you look closely, we
will also observe some other body parts of the bee. So, in this case these are the pollens
sacs in which it stores the pollen.
(Refer Slide Time: 06:28)

Then, we might observe some other inferences such as termite mounds. If there is a termite
mound. So, you also have termites in this area or things like fungi.

(Refer Slide Time: 06:38)

Fungi play a very important role in the forest ecosystem, because when there is any dead
wood or there are dead leaves that are lying on the ground. So, they are also storing a
number of nutrients inside. This dead log has a nitrogen inside, it has some phosphorous
inside, it has some potassium inside, may be some amounts of magnesium or iron as well
and by decomposing these logs, the fungi are able to release these nutrients back into the
ecosystem so that, they can be made use of by some other organism. We will also observe
some saprophytic organisms such as the fungi. There could also be a number of bacteria
in this area or may be some other fungi that we are not able to look directly from our eyes
but we can make an inference.

(Refer Slide Time: 07:28)

If we go near a water body, we might observe some reptiles such as muggers or some birds
that will be seen near this area. In these water bodies, there will be some fishes, may be
some frogs, may be some turtles.
(Refer Slide Time: 07:43)

We might be able to see some Turtles or maybe we might not be able to see some turtle
and there will also be a number of plant-like forms that are found in this area.

(Refer Slide Time: 07:53)

(Refer Slide Time: 07:56)

We will even see a Tiger somewhere or some Bears, or may be some Elephants.

(Refer Slide Time: 07:58)

Now, the point is, all of these are signs of biodiversity in this area, whether it is the Tiger,
whether it is the Elephant, the plants, the trees, the bacteria, the fungi, all of these are signs
of biodiversity.
(Refer Slide Time: 08:21)

So, how do we define biodiversity? Biodiversity is the variety of life in all its forms and
at all levels of organisation. Now, in the last lecture we looked at the different levels of
organisation that are found in the nature and biodiversity refers to the variety of life. So,

variety of life is different kinds of life forms that we are seeing; in all it is forms and at
different levels of organisation.
(Refer Slide Time: 08:49)

When we say, ‘in all it is forms’, it includes plants, vertebrates, invertebrates, fungi,
bacteria, and other microorganisms. And, when we say, ‘at all levels of organisation’, we
can say diversity at the levels of genes, diversity at the levels of species, diversity at the
level of ecosystems, or we might even look at diversity at some other level of organisation.
Now, these three are considered to be the most important.
1. The genetic level biodiversity,
2. The species level biodiversity and
3. The ecosystem level biodiversity
We will have a look at these three levels in greater detail.

(Refer Slide Time: 09:28)

Species as we know are groups of actually or potentially interbreeding natural populations,
which are reproductively isolated from other such groups. When we say actually
interbreeding, for instance, if we consider the chitals of kanha, they will be interbreeding
amongst each other. So, they are actually interbreeding a natural population. But, if we
consider the chitals of kanha and the chitals of Rajaji so, both of these are not interbreeding
with each other. Why? because one is in Madhya Pradesh and the other population is in
Uttarakhand, but they are potentially interbreeding natural populations. Why potentially
interbreeding? Because, say if you take a chital from Rajaji and take it to kanha and try to
mate it with the chitals of kanha, they will result in fertile offsprings. So, they are
potentially interbreeding and they are natural populations. So, species are groups of
actually or potentially interbreeding natural populations and these groups are
reproductively isolated from other such groups.
What do we mean by reproductive isolation? So, for instance you have a species that is
called chital, you have another species that is called, say tiger. If, you try to mate a chital
with a tiger they would not be able to mate, or even if in extreme situations if we are able
to coax them to mate, they might not result in offspring, or they might even result in an
offspring that is itself infertile. This is meant by reproductive isolation.They are
reproductively isolated from other such groups. So, species are groups of actually or
potentially interbreeding natural populations which are reproductively isolated from other

such groups. Now, species biodiversity asks how many species are there and how are they
distributed? In other words, the number and the distribution.
(Refer Slide Time: 11:29)

The genetic biodiversity on the other hand asks the diversity of genetic information that is
present at the level of phyla, families, species, populations, and individuals. Now, genes
as we know are units of heredity that are transmitted from parents to offsprings. So, we
might have a gene for say, eye colour, a gene for hair colour, a gene for skin colour, a gene
for tallness and so on.
When we consider all these different genes, genetic biodiversity asks, what is the diversity
of genetic information? For instance, if you consider the diversity of genetic information
at the level of a population. Consider a population of chitals in kanha. How are these chitals
different from each other genetically? Are all these chitals have the same gene for height?
or are they having different genes for heights?. Are they having different alleles for
height? or are they having the same coat colour? or are they having the same eye colour?
and so on.

(Refer Slide Time: 12:36)

Now, examples of genetic biodiversity include Polymorphism and Heterozygosity.
Polymorphism is the proportion or percentage of genes that are polymorphic. A gene is
considered polymorphic if the frequency of the most common allele is less than some
arbitrary threshold and this threshold is generally taken to be 95 percent. Now, what do we
mean by this?
(Refer Slide Time: 13:05)

Let us consider the coat colour; the coat colour of chital. Let us say that it is coded by a
gene called C. Now, this gene C might be available in a number of forms. So, we might

have a C 1 allele that codes for a very light coat colour. And, we might be having say C 2,
C 3, C 4, and C 5; and C 5 coats for a very dark coat colour. And, these 3 are coating for
some intermediate coat colours.
Now, any particular organism or any particular individual of chital in this population will
be having 2 alleles for the same gene C. So, basically it will be having 2 Cs , one is coming
from it is father and the second one is coming from it is mother. It is possible that you have
C 1 and C 1, C 1 coming from father and C 1 coming from mother or you could have C 1
and C 5 or say C 3 and C 5 and so on.
If you consider all the individuals in this population and you figure out what alleles are
there. And, then you do a counting of these alleles. It is possible that we might see that C
1 is present 200 times, C 2 is present 100 times, C 3 is present 150 times, C 4 is present
1000 times, and C 5 is present say 10,000 times. In the case of polymorphism, we will ask
the question; is this gene, the gene for coat colour that is represented by C, that is
polymorphic if the frequency of the most common allele is less than 95 percent.
In this particular example how many alleles do we have; number of alleles is 200 plus 100
is 300, 450, 1450 and 11450. So, these are the total number of alleles that we have.
What is the frequency of the most common allele? Here the most common allele is C 5,
which is present in 10000 copies. The frequency is given by 10000 divided by 11450 into
100. Now, this is the frequency of the most common allele. Now, the question is; Is it
more than 95 percent or is it less than 95 percent?
If we do this calculation 10 000 divided by 11450 multiplied by 100, it comes to 87.34
percent. Now is this less than 95 percent, the answer is yes. In this case we would say that
this particular gene is polymorphic for this particular population. Now, suppose we had
say in place of 10,000 genes we had it in 100000 genes. What would be the total in this
case?
So, we have 1450; 101450. And, in this second example what should be the proportion of
C 5. So, here the proportion of C 5 would be given by 100,000 divided by 101,450 into
100 percent. Now, if we do this calculation. So, we have 100,000 divided by 101450 into
100 we will get to a figure of 98.57 percent, which then would be greater than 95 percent.

So, in such a scenario we would say that this particular gene of coat colour represented by
C is monomorphic for this particular population. So, essentially what we are asking is that
if there is a gene that has only one allele. So, for instance if all these individuals are one
and the same with regard to coat colour everybody just had C 1 C 1.
In that case this particular gene would be called monomorphic; mono is one, morpho is
form. It has only one form which is C 1, but if we have more than one forms we then put
a threshold. When we have this threshold of 95 percent, we ask if the frequency of the
most common allele is greater than this or less than this. Because for instance, if you have
a population in which you have 1 lakh copies of say C 1 and say only 2 copies of C 2.
In that particular case we would say that even though we have 2 different alleles for this
gene, but it is still monomorphic because more or less we can say that all the individuals
are similar when it comes to the coat colour.
The second thing that we ask in the case of genetic biodiversity is the level of
heterozygocity. Now, heterozygocity is the proportion or percentage of genes at which the
average individual is heterozygous.
(Refer Slide Time: 19:11)

What we ask in this case is, suppose in the case of chital we have 100 genes; actually this
number of genes is very large it say between 20,000 to 100,000. But, for an example let’s

take 100 genes. Now, heterozygosity asks the question, what is the percentage of genes at
which the average individual is heterozygous?
Heterozygosity means every gene is present in 2 copies. Suppose this was present as 1 1
and 1 1. So, in this case we would call it homozygous for gene 1. Now, in the case of gene
2 suppose it was present as 2 1 and 2 4. So, here we will say that it is heterozygous for
gene 2. And, similarly we will move on till the last gene which is the 100 gene.
In this case suppose out of these 100 genes there were 40 genes for which this individual
was heterozygous. So, in this case 40 out of 100 genes it was heterozygous. We take
another individual for this individual it was say 35 out of 100, may be for another
individual it was 70 out of 100 for which it was heterozygous. Heterozygosity would ask
what is the proportion or percentage of genes for which the average individual is
heterozygous?
We can average all of these values to get an idea of the level of heterozygocity that is
present in this population. genetic biodiversity can be looked at, using these 2 examples
polymorphism and heterozygosity.
(Refer Slide Time: 21:11)

Next is ecosystem biodiversity. Ecosystem as we saw before is a group of interaction
organisms or population or community and the physical environment they inhabit at a
given point in time. If we have a group of different populations, that forms a community

and to this community you add the abiotic element, so the physical environment and you
get the ecosystem. Ecosystem biodiversity asks the question how many ecosystems are
there and how are they distributed?
(Refer Slide Time: 21:48)

What are the different kinds of ecosystems that we can have in a forest? So, suppose this
is our forest. It is possible that the whole of this forest is comprised of trees. So, in which
case we will say that it has only one kind of an ecosystem, but then it is also possible that
this forest has this area that is a wetland, then it has this area which is a grassland, then
this has this area which is a rocky outcrop; then probably you have this area which is very
close to a riverine. It gives you a different kind of an ecosystem and rest of the area is
comprised of trees.
In this case we would say that this area is comprising of a forest ecosystem, a grassland
ecosystem, a wetland ecosystem, a rocky outcrop ecosystem, and a riverine ecosystem. So,
in this case we have 5 different ecosystems. Ecosystem biodiversity is asking the question,
how many ecosystems are there. In the first example we had only one ecosystem.

(Refer Slide Time: 23:06)

In the first example we only had this forest and the whole of the forest was comprised of
trees. So, this is one ecosystem and this is another ecosystem. We can see very clearly that
this ecosystem with 5 different smaller ecosystems is much more diverse as compared to
the first case in which we have only one ecosystem in this area.
The second thing that it asks is how are they distributed? Why is distribution important?
Distribution is important, because say in this particular example in place of having a
scenario like this we had a rocky outcrop that moved like this. Now this is a rocky outcrop.
What is the difference that we get? In the first example we had this rocky outcrop and here
we had the water body.
In this case let us look at the number of edges that we have in this is small portion only.
So, we have this edge that is comprised of rocky plus forest and we have this edge that is
comprised of wetland plus forest, so let us call the situation 1 and let us call the situation
2. In this situation we have an edge which is rocky plus forest, we have this edge which is
wetland plus forest, but now we have another edge which is this one, which is rocky plus
wet land.
By having such a distribution by having more number of edges we also increase the amount
of biodiversity in this area why, because there might be some species that prefer living on
the edges. For instance, they could be a species that prefers living here so that it is able to
venture into the forest so that it can get its food, but when the predators come it should be

able to run into the rocky outcrop to save itself. So, edges have their own significance in
the ecosystem.
Now, by having a distribution which maximise the edges that are possible, we can
maximise the biodiversity, which is why the ecosystem biodiversity asks this question not
only how many ecosystems are there, but also how are they distributed?
(Refer Slide Time: 26:13)

Now, out of these different approaches of measure in biodiversity. The simplest one is
looking at the species richness and the species evenness which is the species biodiversity.
Why? Because in the case of genetic biodiversity you have to look at individual genes that
are present in the population, which is not only way technology intensive, but also very
much cost intensive, but if you just went into the forest and looked at the number of species
that are there. They can just be looked with your naked eyes or may be with using a
binocular or some microscope. It is the most easiest way of measuring biodiversity and so,
this is the most widely used measurement of biodiversity. Species biodiversity also asks 2
questions; 1, what is the number of species that are present in your area? and 2, what is the
distribution of individuals of different species? Now, what do we mean by that? The first
one is simple species richness, is the number of species present in an area.

(Refer Slide Time: 27:27)

Consider 2 forests. You have this forest 1 and you have forest 2. Say forest 1 has a total of
100 species and forest 2 has say 1000 species. Species richness asks the question how
many species are present. In this case you have 1000 species here you have 100 species.
We will say that forest 2 is much more biodiverse as compared to forest 1.
So, that is species richness the second thing is species evenness or the distribution of
individuals of different species. For instance in this case, suppose both the forest had equal
number of species. Let us have 100 species in forest 1 and 100 species in forest 2.
The distribution of individuals of different species asks the question, how many
individuals do we have of each species in both of these forests? In this case, suppose
species 1 has 10 individuals, species 2 has 12 individuals, species 3 has say 15 individuals,
and so on till species hundred that also has say 20 individuals.
This is the level of distribution of individuals of different species that we have in forest 1.
In the case of forest 2 suppose we have species 1 that has 2 individuals, species 2 has 3
individuals, species 3 has 2 individuals and so on, but there is one species that has say
10,000 individuals.
In this case we would say that even though both the forest have the same number of
species, both have 100 species, but if we look at forest 1. In this case all these species have
roughly equal number of individuals. When you move out into the forest if you take any

snapshot you might be able to see all 100 of these species. So, all these hundred species
are distributed everywhere.

Whereas, in the case of forest 2 we have 99 species with very few number of individuals
and one particular species that has 10,000 individuals. Now, if you move into this forest
you would observe only species number 100, because rest of the species are so few that
that your system is overwhelmed with species number 100. In this case we would say that
the species are not equally distributed or the individuals are not equally distributed
amongst different species.
When we ask this question, what is the level of species evenness, distribution of
individuals of different species, we would say that even though the species richness is
same in both these forest 100 species each, but this forest is much more even and this forest
is very much uneven when it comes to evenness. When we have the same level of species
richness we prefer an even forest. So, we will say that; the amount of biodiversity in forest
1 is much greater than the amount of biodiversity in forest 2.
Now, how do we know how many individuals are there in the forest?
(Refer Slide Time: 31:06)

That brings us to a concept of species accumulation curve. How do you get to know how
many species are there in a forest? Let us consider that we are considering the number of
mammals that are found in a forest.

(Refer Slide Time: 31:24)

We are seeing the number of mammals in the forest. So, you go into the forest and on the
very first day suppose you saw 10 species, this is 10 species. You have the number of
species and you have days. On everyday suppose you are venturing out into the forest and
you are spending say 6 hours in the forest, and you are looking at different mammals that
you can see in this forest.
On the very first day you will be able to see the maximum number of animals, because
these are the most common animals. You venture out into the forest and you saw say chital
or sambar or langur or maybe some macaques. These are the animals that are very easily
seen, Probably on day 2 you did not find any new animal. The number of species remains
the same. So, this is day 1, this is day 2, on day 3 you got a bit more lucky and you saw a
tiger.
On day 3 your number of species increased from 10 to 11, because you saw a tiger, on day
4 it is possible that you also saw 2 other animals. So, it increased from 11 to 13 may be
because you saw a leopard and you also saw an Elephant, but then with time as you are
observing more and more number of species, it will reach to a point of saturation.

(Refer Slide Time: 33:14)

So, when we draw the number of days and the number of hours that are spent goes by the
name of effort. We are drawing the number of species verses the effort. On the very first
day you will see the largest number of species, then this curve would increase with time,
it might also remain constant for some days, but then it will again increase as you are going
to see more and more number of individuals, but then after a while it will become flat.
Because for so, many days you are not seeing any new individual.
Here we see that on the first day we saw so many individual then every day we are seeing
some new individuals, but then after a while it is reaching to this level of flatness. And,
once we reach this stage we say that this is the number of species that are found in this
area. We draw a straight line and we see that this is the number of species that we have in
this area. Of course, the number of species may be a bit more or a bit less, but more or less
this is the number of species that we have.
This is a way in which we get to this species richness of an area. After richness the next
thing is to compute the species evenness. So, for all of these species we will keep a note
of how many individuals did you observe for this particular effort, and then we would
compute it in this way.

(Refer Slide Time: 34:54)

Say you have the i(th) species and you have the population of the i(th) species, so, 1 2 3 4
and so on. Suppose for the first species you saw 100 animals for 2 the second species you
only saw 2 animals, for species 3 you saw 30 animals and so on. You will make the stable
and once that is made, we measure biodiversity using different indices.
(Refer Slide Time: 35:25)

The first index goes by the name of Simpsons diversity index and is given by the symbol
D, which is equal to 1 over sum i is equal to 1 to S and P i square. Now, call it as small p
i and from this we will compute capital P i. Here we have total number of species. S is the

last species that we have, let us say we have 100 species that are found in this area. The
total number of individuals that we found in this area is 1000. This is the sum of all the p
i, that is 1000.
P i is computed as p i divided by the (sum of p i). So, in this case because this is 100 so,
you will have 100 divided by 1000 is equal to 0.1. In this case you will have 2 divided by
1000 is 0.002 here you will have thirty divided by 1000 is 0.03 and so on.
Here we are computing the p i. Now, in the case of the Simpson diversity index you have
one divided by the sum of P i square. In this case you will have P i square. Let us draw it
again.
(Refer Slide Time: 37:00)

So, here you had P i. So, P i was 0.1, 0.002, and 0.03 and so on. You do a P i square and
you get 0.01 and then this will be 0.000004, this will be 9 and so on. Here you are having
the P i squares. Now, you do a sum of all the P i squares and you will reach to a value of
x and then D is given by 1 divided by the sum of all the P i square or the sum of this value,
that is ‘x’.
This is the measure of biodiversity that is given by the Simpsons diversity index. D is the
Simpsons diversity index, S is the total number of species in the area and P i is the
proportion of the i(th) species in this area.

(Refer Slide Time: 38:11)

From this D we get to a value of evenness. Evenness or E is given by D divided by the
maximum value of D or D max.
Evenness is maximum if, for all of these species, you had the same number of individuals
or best if you had only one individual of all the species. For instance in this case species 1
had only one individuals, species 2 had only 1 individual, species 3 had only 1 individual
and so on till species number 100. If, that is the situation, so, mathematically the value of
D will become maximum and when you divide D by the maximum value of D you get the
equitability or evenness value of the Simpsons diversity index.

(Refer Slide Time: 39:05)

Another, index that is used is the Shannon diversity index. In the case of Shannon’s
diversity index you have is minus sum over I from one to the last number of species that
is i is equal to 1 to S. And, here you have P i log of P i. And this log is the natural logarithm
which is log to base e. Once you compute this value, this is the Shannon’s diversity index.
(Refer Slide Time: 39:32)

Here also you can get to an evenness value by H divided by H max. These formulae do
not have to be remembered for the purpose of this course, this is just to give you an
indication of how this biodiversity is actually measured in the field.