Module 1: Population and Community Ecology

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Community Nature and Parameters

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Today, we begin a new module which is Community Ecology. Community ecology is the
study of ecology at the level of the community or the application of the ecological
principles to understand how a community works, and how a community functions? In
this module we will be having 3 lectures.
(Refer Slide Time: 00:33)

The first one is community nature and parameters, what is the nature of a community?
What are the specific traits that we can discern from a community? How is one community
different from another community? What are the species that are found in a community?
Is there any specific relationship between species and communities? and so on.
In the second lecture we will have community changes and ecological succession.
Ecological succession is the process by which a community moves from being a certain
kind of community to another kind of community. For instance, if we have say a piece of

rock. This piece of rock might get invaded by certain species they will formal community
say a lichen community.
And then with time it will change, because these lichens will change the structure of the
rock; they will change the chemical composition; they will gave out certain chemicals,
certain enzymes, certain acids into the rock that might lead to the breakage of these rocks.
And at the same time they will also produce a lot of organic material. Once that happens
you will have some amount of soil formation, which will make way for other species such
as mosses, which will then out complete the lichens. And then from mosses will move to
grasses and then 2 shrubs to trees to a final community, which we call is climax
community, which remains stable and does not change.
Succession is the process in which these communities change with time. From a lichen
community, you have a moss community then you have another community that is
dominated by herbs, then another community that is dominated by shrubs and so on and
this process is known as ecological succession. So, it answers the question, does a
community change? And if so, how does it change? And what will be the final result of
such a change? What will be the climax community that will be formed?
And in the third lecture, we will have a look at community organization. Let us begin with
the first lecture which is community nature and parameters.
(Refer Slide Time: 02:45)

Let us begin with the first lecture which is community nature and parameters. We begin
with the definition of a community. “A community is an assemblage of populations of
living organisms in a prescribed area or a habitat”, that is, it is a collection of populations
of different species. Here you have a number of species that are living together they are
living in a certain area. So, the area concept here is important, if you have certain species
that are living in say Kanpur and there are certain species that are living in say Coimbatore.
These 2 will not form a single community, but then if you have different species that are
living together in say Kanha Tiger Reserve, we will say that it will form a community.
(Refer Slide Time: 03:27)

When you have 2 different communities the border between both of these communities is
known as an ecotone. It has a transition area where 2 communities meet and integrate and
it may sharp or it may be diffused.

(Refer Slide Time: 03:47)

Essentially what we are saying is, suppose here you have a water body and right next to
the water body you have a grassland. The grassland will be having certain species, that are
living together and the water body will be having certain species that are living together.
If we talk about 2 different communities what is the border? So, this line is telling us the
border and this is known as an ecotone. In this case the ecotone is a sharp ecotone, because
we can very clearly see that anything that is on the right side of this line is a water body
and anything on the left side of this water body is a grassland. In this case have sharp
(Refer Slide Time: 04:43)

Let us considered another situation where we are considering a forest area and in this forest
area we have certain trees and then after while you will see trees at a bit longer distances
apart. And then you also have the grasslands.
In such a scenario we can very clearly see that this is the forest region and this is a grassland
region. We can say that this is a forest community on the left side and this is a grassing
community on the right side. Then where is exactly the border of the grassland, or the
border between the forest and the grassland. Is it say here or is it here? That is something
that we do not know for surety. Because, we are seeing that the grassland has invaded into
the forest and the grassland has come to this end or we can say that the forest has invaded
into the grassland and we can see some trees species here in the grassland.
In this case the kind of ecotone that we are having is a diffused ecotone. It is a very wide
ecotone and this is known as a diffused ecotone. Coming back to the definition a transition
area where two communities meet and integrate and it maybe sharp or it may be diffuse,
that is an ecotone.
When we are talking about any community, when we are talking about say a forest
community or a grassland community, they will be certain attributes of that community
certain characteristics of that community.
(Refer Slide Time: 06:40)

Let us now have a look the community attributes. The attributes of a community are co
occurrence of certain species. For instance, if there is a forest and we say that there is a
tiger that lives in the forest. We are talking about the co-occurrence of certain

communities, where you have a forest you might have a tiger or we can talk about co-
occurrence of species of plants that come up together. For instance in the case of a Sal

forest, we have a number of associated species of Sal. There is a species that is known as
a Sinduri species. In the case of Sinduri, this tree will reside in the Sal forest.
(Refer Slide Time: 07:27)

Let us say that, here we have the tall trees of Sal and in the area in between you might have
these small trees, they are known as Sinduri or Mallotus and their associated with the Sals.
Essentially where you have a Sal forest you will also find the Mallotus trees. Here we are
talking about the co-occurrence of certain species. These species are occurring together,
that is one attribute. We will call this a community of the Sal forest because you are seeing
these tree that are always associated together. The second one is recurrence of groups of
the same species.

(Refer Slide Time: 08:20)

These groups tend to recur now; when we say recur we mean that if you have say in a
large landscape, if in this area you have a Sal forest and in this area you have another Sal
forest, and in this area you have a third Sal forest. In all three of these Sal forest you will
find Sal trees and you will find Mallotus trees. They tend to recur. Here also you have
Sal here you also have the Mallotus, here also you will have Sal and Mallotus together.
So, these groups of species they only occur together, but they also tend to recur
whenever you have another community of the same type.
It is not like in one particular patch of Sal forest, you have Sal and Mallotus. In another
patch you do not have a Mallotus maybe you have some other species. So, that will not
happen. It is a community because it has certain species that are occurring together and
wherever you have a similar community you will find, the similar species that are coming
together. The third attribute is that of homeostasis or self-regulation. In the case of a
community because you have so, many different kinds of species that are living together,
there is a certain amount of homeostasis or self-regulation that occurs in this community.

(Refer Slide Time: 09:47)

For instance, if you say talk about grass; we are talking about a grassland community. In
this you will have grasses of different species; let us say species 1, species 2, species 3,
then maybe there you will have some insects and these are insects that are feeding on grass.
Here you have species 4, species 5, species 6, then maybe you have some insectivorous
animals. Let us say you have species 7, species 8 and species 9.
What do we mean by homeostasis? Suppose this particular species of grass, it has
proliferated itself. Essentially you have this particular grassland in which the earlier
composition was let us say one-third was there for all the 3 species. So, you had, for any
patch you will have 33 % of species 1, 33 % of species 2 and 33 % of species 3 or let us
say 34 % of species 3.
Suppose from this state this community changes in a way, that now you have let us say 10
% of species 1, say 20 % of species 3 and let us say you have 70 % of species 2. So, in this
situation what we are saying is that, the species 2 is tending to proliferate at the expense
of species 1 and species 3. Once that happens the situation is that you have species 1, that
is eaten by all these 3 insects, you have species 2 that is eaten by all these 3 insects and
you have the species 3 that is eaten by all these 3 species of insects.
If a species 2 proliferates a lot. In that case, it is possible that the insect population will
now tend to preferentially feed upon species 2, because it is now more commonly
available. If you have certain species that are not having a very specific selectivity for the

kind of food that they want to eat, if any species tends to overshoot itself, so, they would
be a number of other organisms that will regulate it and they will bring the situation back
to the status-quo . It will tend to bring it back to a situation where all these 3 species are
If there is any small perturbation in the community. The community tends to move back
to the original state, which is known as homeostasis or self-regulation. It will self-regulate
itself so that the species composition remains the same.
(Refer Slide Time: 12:51)

These are the 3 community attributes. Now, we will look at some community
characteristics. Now, what are the different characteristics through which we can
differentiate one community from another community? In the case of attributes we were
asking the question, how do you define a community? What are the things that are common
between different communities? And in the case of community characteristics we are
asking, what are the things that are different in different communities?
If we say that there is a community of Sal forest and there is a community in teak forest,
how are both of these communities different from each other is something that will ask in
the case of community characteristics. The first characteristic is species diversity; which
species are living together. Here you can ask which species and also how many different
individuals are living in that particular area.

For instance in the case of a Sal forest, we will be having Sal, we will be having Mallotus.
In the case of a teak forest, you will be having teak plants, may be you will be having some
mango trees, maybe you will be having some Jamun trees and so on.
Species diversity is different in different communities, different species live in different
communities more or less. The second characteristic is the growth form and structure.
What is the vertical stratification in a community?
(Refer Slide Time: 14:24)

If we talk about an equatorial rainforest. In the case of an equatorial rainforest, you will
have a top community, a top canopy that is comprised of very tall trees. And these trees
are practically covering up the whole of the canopy and then you will be having some trees
that form a middle storey. So, these are those species that are more shade tolerant.
They do not require a very great amount of sunlight, because of which they are able to
survive below the first or the top canopy. Then you will be having some shrubs that are
growing, you will be having some grasses that are growing, then you might be having
some climbers on these trees, you might be having some other species that are living on
these trees. So, such as you can have some orchids that are living on top of these trees.
This will lead to a vertical structure or a vertical stratification, where we can talk about the
top canopy, we can talk about the middle canopy, we can talk about the ground cover, we
can talk about the climbers or we can talk about the epiphytes that are living on these trees.

Now, such kind of a structure or such kind of a growth form is different in different
communities, if for instance we go from this equatorial rainforest to say a shrub forest.
(Refer Slide Time: 15:59)

In the case of a shrub forest you would not be having this top middle canopies and all, but
what will see is that you have a shrub here probably a tree somewhere and maybe some
grasses somewhere. But, here we cannot talk about this vertical stratification. The vertical
stratification is seen in the case of an equatorial rainforest where you have different layers.
In this case you do not have different layers you just have species that are distributed here
and there.
The growth form and the structure, that is the trees shrubs herbs mosses and the vertical
stratification that is brought about by them is different in different communities. That is
also a community characteristic. The third it is characteristics is which is the dominant
Dominant species are those that are highly successful ecologically and which determine to
a considerable extent the conditions under which the associated species must grow. When
we talk about a dominant species, if you go to a Sal forest you will say that Sal is a
dominant species. Because, that is determining the characteristics of the Sal forest, because
you have so many tall Sal plants in that area.

The amount of moisture that is there available in the ground cover is very high, the amount
of sunlight is very low. Essentially the characteristics of that community are being
determined by this particular specious which is the Sal or Shorea robusta or in the case of
certain communities let us consider a community which is there on a ficus tree.
(Refer Slide Time: 17:44)

You have a large sized ficus tree and on that tree, now, this is a keystone species and it is
giving rise to a number of fruits and various parts of the tree are edible, the leaves are
edible and so on. And so, you have a community of a number of birds that are living in
this area.
You will have different species of birds that live on this particular tree, you will have
different species of insects that live on this tree, you will have certain mammals that live
on this tree, say things like squirrels, that are living on the tree, maybe you will be having
some reptiles that live on this particular tree, you might also have some epiphytes that are
living on this tree, epiphytes such as your orchids that live on this particular tree.
All of these species dependent on this particular tree which is the ficus tree. Now, if you
talk about this particular community of all these organisms that live together and whenever
you have a ficus tree in the surroundings you will find all these different species that live

There is this consortium and there is also a recurrence that is seen in different areas in
different Sal trees. And when we talk about such communities you have this ficus tree that
is responsible for this congregation to occur. We will say that ficus tree in this case is the
dominant species. Dominance can be there out of 2 different ways. The first thing is the
numerical abundance, if you have a tree that has there available in a very vast numbers
such as in the case of a Sal forest, when we talk about Sal trees there in a vast number;
plus they are also determining the conditions under which the associated species will have
to grow.
We will say that they are the dominant species or in the other case if there are any species
that are keystone species, in which case their impact on the whole community is much
greater than their numerical abundance we will say that they are also the dominant species.
In different communities we can have different species that are dominant. In a Sal forest,
we have Sal that is a dominant species. In a teak forests, we have teak that is a dominant
species. In the case of a mixed forest you can have a situation where there is no dominance.
Dominance also varies between different communities and which is why we study it as
part of community characteristics.
The fourth one is relative abundance. Relative abundance is the relative proportion of
different species in the community.
(Refer Slide Time: 20:42)

You can have, say a community where you are having species and their abundance.
You can have species 1 to 6. Let us say that we have this community of 6 species and the
abundance is say the species one has 1000 individuals; 2 has 5 and so on. Now, that is one
sort of a community. Let us talk about another community, where we have the same
species, but then their abundance is say 200, 205, 250, 180, 210 and 195.
Now, in this case, in the case of the first community; this is community 2. In the case of
the first community we will say that this species, species 1 dominates this community so
much that it looks very much like a monoculture. If you go into this community you will
say that you have only seen species 1, because species 2, 3, 4, 5 and 6 are so rare that you
will hardly find them. Whereas, in the case of the second community, you are seeing that
all of these species have roughly equal number of individuals so, wherever you go we will
find probably all of these different species together. Here we are talking about the relative
Now, relative abundance is asking, the relative proportions of different species in the
community. In this case in the case of community 1 the relative abundance of species 1 is
very high, in the case of community 2, the relative abundance of all different species is
roughly the same. This is a characteristic that will depend or that will differ between
different communities and so, we studied it as under community characteristics.
The 5th one is the trophic structure or who eats whom. This is something that we have
dealt with in the case of energetics and that is also a community characteristic, because in
different communities, you can have a different trophic structure, because you have
different species, they are having different roles and so on.

(Refer Slide Time: 23:06)

Next we move to the description of a community. Now, we have looked at the attributes
of communities, when do we call something a community. What are the common features
that you find in a community? We will look at community characteristics which is; what
are the things that are difference between one community to another community.
How do you describe a community? Let us say that we went to a Sal forest. How do we
describe this community of a Sal forest? In this case we will define it using these 3 terms,
the first point is species richness.
In any community we will define how many species are found in that particular
community? In this particular example, we are having 6 species. So, we will say that the
community richness in this case is 6.
Next, we talk about this species abundance in terms of number of individuals, biomass or
cover that is being used by these different species. Abundance is something that we already
saw in this particular example. In the case of community 1, the species 1 was over
abundant; in the case of community 2 everybody was roughly equal in abundance.
This is about the number of individuals that we found of different species, but in place of
numbers we could go by say the biomass of different species that are found. For example,
in the case of this particular community of the ficus tree, we can say consider that the ficus

tree has a mass of say 5000 kgs, then your bird species 1, has say 20 kgs, your bird species
2 has say 10 kgs, bird species 3 has say 15 kgs, insect species 1 has say 20 kgs and so on.
So, in place of going with the numbers we could go with the masses or the biomass or the
third way is to go by the cover.
(Refer Slide Time: 25:16)

In the case of a cover, if this is your area, or this is your community, you can say, so, for
instance if there is a grassland, we could take a view from the top, and suppose the species
1 is found here, then it is found here, and then it is found here. So, this is the first species.
The, second species is say found here, it is found here and maybe it is found here, in a big
Now, the third species is say found in this area only and then you can have a fourth species
that is found in this area; maybe a fifth species, that you find in this area and say this area.
And let us say a sixth species that you are finding only in this area. If you are able to see
all these species separately from a top view, in place of going with the numbers or going
with the biomass, you can just take the area that is being used by these different species.
In the case of species 1, we will add up this area, along with this area, and along with this
area. And let us say that species one is acquiring an area of say 10 hectares.
In case of species 2, let us say that it is 7 hectares, in the case of species 3, let us say 5
hectares and so on. So, we can come to our computation of abundance not just in terms of

the number of individuals or the biomass, but also in terms of the covered that is being, or
the area that has been covered by these different species. While describing the community
the first thing was how many species are there, second is how much is this abundance of
these different species and then on the basis of the abundance we can talk about the species
In the case of dominance we can rank as per the abundance, that is the number basal area
etc; and then we can say that this is the species that is the most dominant species in this
particular area. In this case, we will say, that this is species, the one is the dominant species.
In the case of this example we will say that this species that is having 250 individuals that
is the dominant species. Here, we will say that the ficus tree that has the highest amount
of biomass that isthe dominant species. Thisis the way in which we describe a community.
So, you talk about species richness, abundance and dominance.
Non dominance can also be in terms of the basal area.
(Refer Slide Time: 28:01)

What is basal area? Suppose, you have a forest and in this forest you have these different
trees. When we are talking about of basal area, we can take say, a standard height. We can
take the best height and at this best height we can say that this is the area. Let us use another
colour. This is the area that is used by these different trees.
So, in place of just the number or the cover we can also go with the basal area.

(Refer Slide Time: 28:34)

Let us now look at some ways in which we find these different descriptive factors. The
first one is relative density. Now, relative density is number of individuals of species x,
divided by total number of individuals of all the different species into 100. So, when we
are talking about the relative density what we are doing is, in this case we are totaling up
the total number. So, here you have 5, 6, 7, 8, 9, 10, 14 and 24. So, here you have 1024.
So, that is the total number of individuals that you have in this particular community.
(Refer Slide Time: 29:21)

Now, relative density of species 1 will be given by (1000 / 1024)*100 %.
Relative density of species 2 will be given by (5/1024)*100 %.
For species 3, it will be (3 / 1024)*100 %,
For species 4, (2 /1024) *100 %,
For species 5, (4 / 1024) * 100 % and
For species 6, (10 / 1024) * 100 %.
When we are talking about the relative density, we are asking this question what is the
number of individuals that are present in a particular species as a fraction of the total
number of individuals of all the species that are there in this particular community?
The second thing is relative frequency. Now, relative frequency is frequency of a species
x divided by sum of frequency values for all the species into 100, where frequency is
defined as the probability of finding the species in any one quadrate. Now, what does that
(Refer Slide Time: 30:39)

Suppose, we have this forest or this community and in this community you have
individuals of different species. So, let us say that this is species 1, 2, 3, 4 and let us say 5,
then this is 6.

Now, this is a particular community that is comprised of 6 different species. So, you have
the black species, you have the blue species, you have light green, you have dark green,
you have purple, I think that we left out dark blue. These are the 6 different species that
are found in this particular community. Now, let us take a quadrate. Now, when we are
talking about quadrate, we are taking a fixed size sample from this particular community.
Let us that we are taking a quadrate here. Now, in this quadrate you found one of light
green, one of black, one of purple and one dark blue. Let us take another quadrate of the
same size. In this case you found one of black, you found 2 of dark green, you found one
of light green. Let us take a third quadrate here. So, here you found 3 of black, you found
one of light blue, one of dark green and say one of dark blue.
In this case when we are talking about the relative frequency, the question that we are
asking is what is the frequency of a particular species?
The frequency of a particular species is, if you are taking this quadrates again and again,
what is the probability that will find this particular species in that particular quadrate. In
this particular example we took 3 quadrates. And in the case of species 1, we found that
this species was found in 3 quadrates, it was found here, it was found here, and it was
found here. So, the frequency of this particular species is 3/3.
In the case of the blue species we only found it once. So, the frequency is 1 / 3. In the case
of light green it is 1 / 3, and so on. Now, this was a very simple example, but then when
we do this quadrating sampling again and again we can come to a very precise value of
the relative frequency. So, relative frequency says that if you are going into the field, you
are drawing up a quadrate at any random location, what is the probability that you will
find species 1.
In the case of species 1, it is so common in this particular community, that wherever you
draw a quadrate, there is roughly a 100 percent chance that you will find the species 1,
whereas, in the case of the species that is light blue in colour, it is only found in one out of
every 3 quadrates.
Because, the numbers of these individuals is very less, it is only 3 individuals that are there
and this particular community. So, frequency also gives us in indication of how different
species are having different number of individuals. That is the relative frequency;

frequency of species x divided by some of the frequency values of all the species into 100
The third one is relative dominance. We can define dominance in terms of the basal area.
So, we can say that it is basal area of species x divided by total basal area of all the species
into 100 percent.
When we add all these 3 values; (relative density of species x) + (relative frequency of
species x) + (relative dominance of species x), we get to a value that is known as the
importance value.
(Refer Slide Time: 34:59)

Importance value of species x = (relative density) + (relative frequency) + (relative
In all these 3 we can see that the maximum values can be 100 percent. Suppose there is
this community in which you only have 1 species. In that case you only have species x.
Total individuals and the number of individuals here is equal. You have a value of 100
percent. Similarly, the maximum value of relative frequency is 100 percent and the
maximum value of relative dominance again is 100 percent. The importance value it varies
from 0 to 300 and the higher the value a particular species has we say that, that particular
species is very important for this particular community.

That is about the importance value of any particular species in any community. This is a
way in which we can describe a community.
(Refer Slide Time: 36:03)

After description let us have a look at the associations between species. Suppose, you look
at a number of different quadrates. For instance here in this example we had taken 3
different quadrates. We have this first quadrate, we have the second quadrate and the third
quadrate. In the first quadrate we say that the green, the black, the purple and the blue are
coming together. In the second one, we say that the green the black and the light blue are
coming together in the third one.
In the second, when we are seeing that the black, the dark blue, light blue and the green
are coming together. Similarly, if we take n number of quadrates, we can say that between
any 2 species we can have 4 different kinds of relations. The first relation is that if you
have species y. So, species x is present in most of the situations. In which case we will say
that there is a positive correlation, if y is present, x is present, if x is present, y is present
or you can have another positive correlation in terms of absence.
So, if you do not have x you do not have y. In which case we can say that either both of
these species are dependent on each other because of which they are found together in
most of the cases or they are so dependent on each other that if one is not there the second
will also not be there. They are interdependent on each other or maybe they are completely

associated with each other.