We move forward with our discussion on Community Ecology.
(Refer Slide Time: 00:20)
Today we will have a look at Community Organization and how communities change in
response to external forces.
(Refer Slide Time: 00:30)
We had defined these 3 terms earlier, relative density, relative frequency and relative
dominance. So, to recap relative density is the number of individuals of a particular species
divided by the total number of individuals of all the species multiplied by 100 percent.
This is telling us the number of individuals of any species, as a fraction of the total number
of individuals of all the species that are there in this particular community. We had also
defined relative frequency; as the frequency of finding the particular species in this case a
species x, in any quadrat divided by the sum of the frequencies or values of all the different
We had also defined relative dominance in terms of the basal area of the species or this
definition of relative dominance is primarily used in the case of trees, but in the case of
other species when we are talking about say herbs or shrubs, we can make use of other
definitions of relative dominance.
(Refer Slide Time: 01:34)
A more generalized definition of relative dominance is in terms of the importance of the
species for that particular community. So, we can define dominance in 2 terms. The first
is in terms of keystone species, which play a role that is much greater than their numerical
abundance. If we have a keystone species such as a ficus species in a particular forest
ecosystem will say that, the ficus species is a dominant species because it is a keystone
It is supporting a number of other species, which are found in that community plus a
number of parts of this particular tree, the ficus trees are edible, their fruits are edible, their
leaves are edible, even their flowers are edible. In that case even in the case of very pinch
periods like, extreme dry seasons they act as a food source. They are essentially they are
providing for the whole of the community. So, we say that the ficus trees are dominant
species for that particular community.
Secondly, we can define dominance in terms of the numerical abundance of the species.
So, if there is a species that is numerically very abundant or a species that is having a high
relative density, then we can say that that particular species is also dominant in that
community, because of it is numerical abundance it is able to regulate the characteristics
of the community. A good example is the sal trees in a sal forest so, because we have so
many number of sal trees, the environmental conditions or the habitat conditions for all
different species in the sal forest community are determined by the sal trees. Because, they
are very tall and they have a canopy. There they do not permit enough amount of sunlight
to reach to the ground. And, at the same time they also result in a very high amount of
moisture that is present in the whole of the community.
So, just because of their vast numbers the whole of the community is characterized by the
properties of the sal trees. So, in this case we can also say that the sal trees are dominant
species in the case of the sal community.
(Refer Slide Time: 04:04)
For any community we can define a term which is known as a community dominance
index. Community dominance index is the percentage of abundance that is contributed by
the two most abundant species. So, earlier we were talking about the relative abundance
of any one species. Now, we are talking about the relative abundance of two species.
CDI or the Community Dominance Index is defined as [(y1 + y2) / y] * 100 %
y1 is the abundance of the most abundant species, y2 is the abundance of the second most
abundant species and y is the total abundance of all the species. And, here abundance may
be measured in terms of density, biomass or productivity.
Why are we doing that? Now, if we are talking about the abundance of one species, we are
looking at the abundance of that particular species in the community, but it is often seen
that there are a number of other species that are associated with the most dominant species.
For instance in the case of a sal forest, you will normally find Mallotus where there is sal.
In that case we say that Mallotus is a dependent species or it is a sal associated species.
And, when we are talking about the dominance value in any particular community, we
should incorporate the abundances of both these species, because they are always
occurring together. So, sal is the most abundant species, Mallotus is, maybe the second
most abundant species.
We add both of these to determine the community dominance index, whether this
community is dominated by a few species or whether this community is not dominated by
just a couple of species, but nearly every species has an equal amount of abundance. That
is the question, that, we are asking. Is this community turning towards say a monoculture
or a biculture when you see only 2 species everywhere or is this community a more
diffused community, where you have a more number of different species that are found in
all the regions.
So, CDI = [(y1 + y2) / y] * 100 %
Now, the question is; is there a relationship between the community dominance index and
the biodiversity of an area?
(Refer Slide Time: 06:41)
In our lecture on biodiversity we had seen that biodiversity is dependent on 2 factors. Now,
the first factor is the number of species that is found in the area and the second one is the
distribution of individuals among these different species.
Essentially what we had said was if you have 2 communities and there is one community
that has say 7 species and there is another community that has 100 species. We will say
that this community is much more biodiverse as compared to the first community. So, here
we are talking about bio diversity. So, we will say that the second community is much
more diverse, but then if we have 2 communities.
The first community has 10 individuals, the second one also has 10 individuals, but then
in the first case you have the individuals such that, in the case of 1 2 3 4 5 6 7 8 9 10. The
first one has say 1000, the second one has say 10, then you have 2 3 1 2 5 6 7 8. Now, in
this case this community is having this particular species that is species 1, in so, much
abundance that if you go anywhere you will only find species 1.
Whereas, in the case of the second community, we have a situation where let us say we
have 200, 250, 300, 150, 225, 205, 210; let us say 175 190 and 200. If, this is the number
of individuals that we have of different species in, the second community then wherever
you go you will find a representation of all these different species because all of them have
a roughly equal numerical abundance.
If you have a situation where there is one community there is one species that is dominant.
The more is the amount of dominance or the more is the amount of the relative abundance
the lesser is the biodiversity. When we are talking about the community dominance index,
we take not just the most abundant species, but also the second most abundant species. In
this case what we are saying is that suppose the first species had 1000 individuals and the
second one; the second most abundant had say 800 individuals.
Here we have 1800 individuals that are there in just 2 species in all the other species have
very less number of individuals. Here again we will say that the amount of biodiversity in
this community is less. Why? Because if you go anywhere you will find either individuals
of species 1 or you will find individuals of species 2.
(Refer Slide Time: 10:05)
More is the amount of dominance, the lesser is the biodiversity. Also the more is the
number of species the lesser would be the dominance index, because when you have more
number of species, then it is also possible that the numerical abundance of the top 2 species
will be lesser because you have individuals from so many different species. So, this is
roughly the relationship that we expect.
(Refer Slide Time: 10:35)
But, this relationship is not that simple. If we actually look at the field values, so, here we
are looking at the relationship between dominance and species diversity in the invertebrate
community of decaying oak logs in a place called Wytham Woods in England. There is a
slight tendency for dominance to be lower in diversity is high, but the relationship is not
Here you have the community dominance index on this side you have the number of
species. We can say that you can try to put a curve like this, but then because here you
have so, much amount of variation, so, it is also possible that we might try to put a curve
not like this, but say like this; that is also possible. The relationship is not very tight in this
case or in terms of mathematics we will say that the r2
value is not very high or the amount
of correlation between both of these variables is not very high.
So, roughly we can say that if you have more number of species the community dominance
index reduces, but then this is not a very hard and fast relationship, the level of correlation
that you have between both of these variables, the number of species and the community
dominance index, the correlation is very less. But, then there is one correlation that is much
more prominent, which is the relationship between dominance and productivity.
(Refer Slide Time: 12:03)
Here we are looking at the relationship between dominance and productivity of grasslands
on sandstone and serpentine soils on jasper ridge. In this case, here you have the dominance
index, here you have the productivity. Productivity is in terms of grams per square meter
per day, that is the amount of production that is going on in this area.
And, we can see that if you have a lower dominance index, the productivity is roughly low
and if you have a higher dominance index the productivity is roughly high. Now why is
that so? Because here we can look at the organization of the community in terms of why a
certain species is dominant in a community. Any species will be dominant when it is able
to compete better than the other species.
Probably a species that is more dominant is able to say produce faster or maybe it is able
to take much more amount of sunlight and convert it into biomass. It is efficiency is much
greater. So, if you have a community where some species are dominant. They are typically
those species that have a very high level of productivity. And, in those communities where
you do not have a very high level of dominance, then most of the species that are there,
they are either all very low productivity species, or else they are also all these species are
spending quite a lot of energy in competing with each other.
Because, none of them is having a very high level of competence or competition as
compared to the other species. In this case those communities which have some dominant
species, they will be able to have a much higher level of productivity as compared to those
communities that do not have a species that is dominant.
(Refer Slide Time: 14:11)
This dominance also changes with the surroundings; it also changes with the biotic and
abiotic conditions that are brought about in the community. In this image, we are looking
at the changes in the phytoplankton community of a particular lake in British Columbia,
after artificial enrichment with nitrogen or phosphorus, 1 or 2 of the rare species increased
rapidly to form a “bloom” and then die back to their former status. Exactly which species
will “bloom” cannot be predicted.
(Refer Slide Time: 14:54)
What was done in this case was that we have already talked about eutrophication. In the
case of eutrophication, we have a situation in which you have a water body and in this
water body there is some discharge of nutrients into it. So, suppose you add N P and K
inside so, nitrogen phosphorus and potassium.
There will be some algal species that will form a very big bloom that will cover the whole
of the surface, because they are now getting very ample amount of nutrients and once there
is this algal bloom. So, after a while, this whole lake will be dead because all of these algae
after a while will start dying and when they die they will start sinking to the bottom. And,
when they are decomposed so, all the oxygen that is there in the water is taken away.
(Refer Slide Time: 15:40)
In this particular experiment the scientists tried to artificially do eutrophication in a
controlled manner. So, here on the x axis we have the days following the enrichment. So,
on this day 0 you added the nutrients, which nutrients were added nitrogen or phosphorus.
Here you have the nutrient that was added and this curve this dashed line is showing you
nutrient concentration that is there in the water. When you put the nutrients. This is the 0
level. So, before day 0 you have this line this dashed line, then you spread it with nutrients.
And, after a while, the concentration of the nutrients it starts to come down. Why does any
nutrient come down in concentration after a while? Because as we have seen in the case
of nutrient cycles, if you have nitrogen in the form of nitrates.
(Refer Slide Time: 16:37)
There will be some organisms that will be denitrifying organisms. And, in that case they
will convert these nitrates into nitrogen and oxygen and both of which will be later on
released from the water. Or in case you are adding the second nutrient; if you are adding
phosphorus into the lake what happens is, you have this lake in which you added the
phosphorus; let us just talk about phosphorus here.
(Refer Slide Time: 17:04)
The phosphorus was used up by all of these algae and then later on when these algae die
off, so their bodies have come to the bottom of the lake and with them, the phosphorus has
also come to the bottom of the lake. And, it typically takes a very long period of time for
this phosphorous to get released back into the water. So, if you look at the concentration
of phosphorus in this water after a while, it will start declining.
If we plot the concentration. If this was the normal concentration. In this case the
concentration has spiked and then it will start decaying and it will reach to the normal
levels after some days. So, this is what we are seeing. It increased and then it started
If we look at the number of species in this particular lake, the number of species all
throughout it remains constant, because in this particular short experiment there was no
extinction of species that occurred. So, none of the species was completely decimated from
this community. But if we look at those species diversity, so, if this was the species
diversity shown by these crosses, the species diversity reduced considerably and then it
started to increase.
Why do you have a situation of reduced species diversity? Because as we have seen earlier,
in this case you have these 2 communities, in the case of the first community you have 10
species, in the case of the second community you also have 10 species. But then, if the
number of individuals that are found in different species if that changes that can reduce
the level of diversity.
(Refer Slide Time: 19:09)
What happened in this particular case ? So, let us say that this is the species and say we
had these 6 species in the water. Now, the earlier numbers were say; we are looking at the
abundance say, per 100 ml of water. So, let us say that earlier we had 20, 25, 22, 24, 23
and 18 number of individuals. Now, once you have spiked it with the nutrients. So, let us
say after spike. So, once you added these nutrients there were some species that were able
to prolifically use these nutrients. So, suppose that was the second species. In this case
after spike here, you have 20, 22, 24, 23 and 18, but in place of being 25, let us say that it
increased to 10,000. Now, why did this happen? Because, when you have a lake that does
not have ample concentration of nutrients so, all these 6 species are competing against
each other, there is not a plentiful amount of nutrient that is available. So, none of the
species can become dominant. So, all of these are competing for the same scarce resources.
But then once you have given the resources so, again if you remember the Liebig’s law of
the minimum, so, those species that were constrained by the nutrient availability, they were
able to come out of that particular threshold and they were able to proliferate very rapidly.
Because, say in this particular example, suppose this was a species that was limited
because of the nutrients. Let us say this was a species that was limited by the pH of the
Probably it required a more basic pH, but the pH was more acidic or maybe this again was
another species that required an even more acidic pH, but then it was again limited because
the pH was not that much acidic. Or probably there was some other species that was limited
because of the light, that could be there in this area. Or some other species that was limited
by the temperature, or there was some other species that again required another range of
temperature, there was some species that wanted a more higher temperature there was
another species that wanted a lower temperature.
Suppose we played with these parameters; suppose we played with the pH. So, in that case
this species would have proliferated much better or say this species would have
proliferated much better, but then because of all of these species this one was the one that
was actually limited by the nutrients that were available. So, it was tolerant of the pH, it
was tolerant of the temperatures, it was tolerant of the light; the only thing that it needed
was the nutrients.
Once you put in the nutrients, the other species are not able to overcome their limitations,
but this species is able to overcome it is limitations and from 25, it becomes 10000. Now,
once that happens, what do we say about the diversity or the biodiversity that is there in
this community. In this particular community, that is in the earlier situation we had all the
individuals that are roughly the same.
In this case every species has roughly the equal or, more or less equal number of
individuals. In that case the level of biodiversity is high. In this particular case after the
spike this species became so large, that it now looks more or less like a monoculture. So,
there are so many individuals of this particular species that now if you take out any sample,
you will only find individuals of this species and others will be just overwhelmed because
of these numbers.
Because of these reasons the biodiversity so, you have the same number of species, but the
species diversity it reduces. Now, it reduces and then it starts to increase again. Why?
Because you had this peak and your species 2 was able to use this peak of nutrients, but
then later on when they speak again subsides, so, again you are getting to a situation where
the amount of nutrients in the water is less. So, in that case it is coming back to the status
quo. So, you started with this relative scale. So, it started here and it came back to here.
So, this value and this value are roughly the same, but then, what happened to the standing
crop in that time? Standing crop is the amount of biomass that is present in the lake.
In this case the standing crop it increased exponentially, then it reached a peak and then it
decreased. Now, why did that happen? This is because of the second species which was
using these nutrients and it led to an algal bloom in this area. Now, when that happened,
so, this peak coincides with the time where you had the algal bloom. So, this is how
dominance changes with the surroundings. Earlier when you did not have this spike of the
nutrients in this particular condition, in the earlier situation there was no species that was
very much dominant if you had to find out the community dominance index.
The two most abundant species are these two. So, this is roughly like; 49 divided by 132.
This is the community dominance index would be [( 49 / 132) * 100 %], which is roughly
we can say that this is around 50, this is say around 150. So, this will become close to
around 33 %.
This is the amount of community dominance index that we had in the lake before hand,
but then after the spike you have this value that is 10,000 and the second most diverse;
most abundant is 24. So, you have 24 divided by; and now this bottom value will also be
very close to 10000, but let us do the computation.. And, the final value is very much close
to 100 percent.
So, the community dominance index it shifted from 33 percent to 100 percent just because
of some changes that came up in the surroundings. The dominance changes with the
changes in the surroundings. But then in this particular case we can also see, that the
community, even though it suffered with these changes, even though it suffered with this
algal bloom, but then the later on values of everything are the same as that of the initial
If you look at the number of species it does not change, if we look at the species diversity
here suppose this was 100 percent. Here again it comes back to the same level. If, you look
at the standing crop, it also comes back to the same level, if you look at the amount of
nutrients, that also comes back to the same level.
This is what we mean when we said that a community shows some amount of homeostasis
or self-regulation. So, even though this community was given some changes it was able to
self-regulate and it was able to bring everything back to the normal state. At the very end
it had the same level of biodiversity, it had the same standing crop, it had the same number
of species as if nothing had happened. So, how is any community able to bring itself back
to the normal? Well it is able to bring it itself back to the normal, because we have this
concept of stability of different communities.
(Refer Slide Time: 27:24)
Community stability is defined as the ability of a community to defy change or to rebound
from change. There can be certain situations in which there is a community that is defying
the change. What do you mean by defying the change? That is, if there is a change, a
community is able to resist the change or in certain situation, it suffers from a change, then
it is able to rebound from the change.
How do you defy a change? A very good example of defying a change is the case of
(Refer Slide Time: 28:11)
Suppose you have a beaker and you have say water in it and the pH of the water is 7. And,
now if you add acid into this water. Now, as soon as you start adding acid, the pH starts
This is an example of a system that is not able to resist a change.
(Refer Slide Time: 28:37)
But, if we have another beaker. And, in this beaker suppose you have a buffer and the pH
of the buffer is again 7.0. Now, you add the acid. So, because you have a buffer solution
so, the pH will go down, but it will not go down at that faster rate. Suppose in this case
when you had the water inside and you added say 1 milliliter of an acid and the pH moved
from say 7 to 3.
In this case, here again you added 1 ml of the acid, but the pH change from 7 to 6.9. So,
this would be an example of a system that is the resisting the change.
A good example in the case of our communities would be that if you have a system in
which say you have again a lake ecosystem or a lake community.
(Refer Slide Time: 29:34)
Here you are adding your nutrients inside. Once these nutrients come in you have an algal
bloom. But then before this algal bloom can happen, suppose there are some fish species
inside this water and these fish species they eat up the algae that are being formed. So, in
that case this system is resisting the change, because you are putting in an external
disturbance which is trying to increase the algal population, but then your system is such
that you are maintaining our homeostasis because your fishes are eating up that algae.
In that case, there cannot be an algal bloom. But then, this amount of resistance will be
limited. Because, it will depend on the amount of changes that you are bringing in and the
rate at which you are bringing in the changes. For instance when we were talking about
this particular lake, in place of giving it this spike suppose we gave it a spike that was only
In that case probably the algae that started to proliferate they could have been eaten up by
the fishes and then probably we would not have seen an algal bloom. Or in other case, in
place of giving it a very sharp spike, suppose we gave it the same amount of nutrients, but
then probably we gave these nutrients in a span of say 30 days. What would happen in that
(Refer Slide Time: 31:07)
We are not talking about the fish that are eating up these algae, but let us say that we are
giving in the nutrients at a very slow rate. What happens when you have a very slow rate?
You have some algae that were able to proliferate, but then after a while they start dying
and so, these algae, they come down to the bottom of the lake. With that the phosphorous
that was there in the water that also came down. So, the phosphorous that was there on the
top it has come down.
And, when they have started to degrade, the nitrogen that was there it was released out by
the denitrifying bacteria that are present in this community. Again you have added very
small amount of these nutrients, again there was a small algal bloom and then again the
phosphorous went down; the nitrogen what lost. If you are giving these changes at a very
slow speed. If the quantum of changes is less or if these changes are coming at a very slow
speed, in that case the system is able to resist the change. Because, you are giving it, these
nutrients, but this system is able to push those nutrients down or it is able to push those
nutrients away so that there is no change in the community.
That is the resistance to change. And,so, that is defying the change or the ability to rebound
from the change. In our example of this lake, the community did suffer from a change, it
did suffer from a decline in species diversity, it did suffer from an extensive increase in
the standing crop, but then it was able to rebound back. So, it was able to bring all these
values back to the normal.
The ability of a community to defy a change, to resist a change or to rebound back from
the change is known as community stability. Community stability is a very integral part of
this study of ecology, because we are pushing different communities through different
changes, because of our anthropogenic influences, we are putting quite a lot of waste
materials into our water bodies into the environment and that is all changing different
It becomes important to know, how much is the amount of change that different
communities can tolerate. Whether the communities will be able to come back to normal
or not and, if we know the variables that govern the amount of resilience or the amount of
resistance in any particular community we can play with those variables, we can make our
communities much more stronger so that they are able to resist changes in a much better
way or maybe much more resilient so that even though these changes have occurred they
are able to come back very quickly. So, that is why a study of community stability becomes
very important these days.
(Refer Slide Time: 34:08)
Stability is divided into 3 different kinds. We can talk about global stability and we can
talk about local stability. We can understand it by the example of this ball that is there on
the surface. If you have a ball here and you take this ball up here and then you release it.
With gravity, this ball will come down and it will be here, you take it here to the other
direction and here again it will come down. Now, this is an example of a global stability.
In the context of a community this is a community that is in such a state that whenever you
take this community to some level of disturbance it is always able to come back. And, it
will always come back to the same initial state, because this is a global stability.
In this second example is an example of local stability. In this case you have a ball here, if
you push it to this direction it will again come down here. If you push it to this direction it
will again come down here, but then if you push it too far; if you; maybe bring this ball to
this particular point, now, it will not come to this point, but then it will start rolling in this
direction and it will come here. Now, in the context of a community this is an example of
a local stability.
In the case of a local stability, if there are small changes in the community the community
will be able to bring things back to the normal. But, if there are larger changes in the
community, then the community will change and maybe become a very different
community. And, a good example is when we are talking about the successions. Now, in
the case of an ecological succession, the community is shifting from one local stability to
another local stability. So, a grassland is a stable community, if you put some amount of
changes in the grassland, it will come back to its own state, but then if you bring about a
large change then maybe it would convert itself into a shrub-land or maybe it would
convert itself into a more primitive level, say a moss stage. So, that would be an example
of local stability.
If you have a situation where you have a global stability and you are pushing your species
to a very large extent.
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