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Module 1: Population and Community Ecology

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Community Changes and Ecological Succession

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We carry forward our discussion on community ecology and today, we will have a look at
succession. Now succession asks the question, how does a community form? And how
does a community change? So, ecological succession is one of the ways in which the
community changes.
(Refer Slide Time: 00:36)

It asks the question suppose you have a bare piece of rock so, you have this rock that has
come up because of say, some volcanic activity. This would be primarily a basalt rock or
for instance there is a granite rock that is deep inside the earth and because of some tectonic
activities because of some earthquake related activities, this rock comes up to the surface
of the earth. Now this rock does not have any soil associated with it. It is just a bare rock.
How is it that this rock is converted into soil? And then how do different kinds of
communities or different kinds of populations come into this place and then form a
community? And then, will that community remain constant with time? Or will that

community go on changing with time? And if this community changes, what are the factors
that are responsible for these changes? All of these things are considered in the case of
succession. In this case, we have this, this magma that is coming out and here we have the
rocks that have been formed. These rocks are very hard rocks. Essentially if you put any
plant here, the plant will not be able to grow. Even if you say make a hole somewhere here
and put a plant inside, the plant will not be able to grow because it does not have access to
water, it does not have access to the minerals.
This rock is all full of minerals. It has all the nutrients that a plant might need, but then all
of those nutrients are logged inside the various minerals that are formed that, that are there
in this rock. All these nutrients have to be released by some process and that process is the
ecological succession.
(Refer Slide Time: 02:25)

Once you have these rocks, in a short time, you will find that there are some lichens that
are coming up onto these rocks. Lichens are extremely hardy species. Here on the
background this grey color is the rock and these yellow colored things are the lichens.
These are very simple organisms these are very primitive organisms and they do not have
a huge nutrient requirement. Essentially they can make use of those small little minerals
that are present on the surface of this rock or maybe those nutrients that are brought along
with the wind when it blows some dust on this these rocks.

Any small amount of nutrients, any small amount of minerals that are available, these
lichens can make use of that. Plus these lichens are photosynthetic organisms. These are
photoautotrophs. They can make use of the sunlight to produce their own food. At the
same time their requirements for water is also very less. Any amount of rainfall that comes
into this area the lichens will absorb that water and maybe store it into their bodies or
maybe if this area is close to the oceans and the air has a lot of humidity, so, these lichens
can even make use of that humidity. What are these lichens doing here?
These lichens once they have come up on the surface of these rocks, they will use the
nutrients that are there on the surface or the nutrients that are brought along with the dust
particles; they will make use of the water, they will make use of sunlight and they will start
to proliferate in this area. And at this particular stage, there is no competition; there is
nothing other than lichens. The lichens can have all the space for themselves.
They are able to cover the whole of the surface. Now when you have any organic substance
that is growing up in any area and with this substance starts to degrade, you will have a
number of organic acids that are formed. When you have these lichens and when these
lichens die out, or probably even while they are living, they will be creating a number of
organic acids and some of them will be released into the rocks.
That is a very small amount of acids that is being released, but nonetheless it plays a very
important role because once you have these mineral acids once you have the action of
chemicals on these rocks, there is some more amount of fragmentation. Probably earlier
you only had the surface of the rock that was available for the growth of these lichens.

Refer Slide Time: 05:06)

Now because they have released some acids, then probably a very thin shell of these rocks
has now started to disintegrate. Once this shell starts to disintegrate what is happening is
that you have much more amount of minerals that are now available for the growth of any
plant material. Because earlier you only had the dust that was there, you only had access
to those minerals that were exactly on the surface, but now that you have this thin shell
that has started deteriorating, so, more and more minerals will start breaking apart more
and more nutrients will be released and so, now, some more plants can come into the place.
And what are those plants?
(Refer Slide Time: 05:54)

The next stage is the foliose lichen. Now earlier we had the Crustose lichen. In the case of
this crustose lichen, you can see that these are in the form of a crust that has come up on
the surface, they look like a crust, that is why they are called as crustose lichen.
In the case of foliose lichen, these look more like leaves. If you look at these portions
carefully, it looks like you have some leaf kind of arrangement in this place. Now crustose
lichen are followed by the foliose lichen. The requirements of foliose lichen are a bit more
than that of the crustose lichen. The foliose lichen cannot come up as the first organism or
as the pioneer species.
But once you have a these rocks that have been broken apart especially on their top surface
so, now because you have more amount of nutrients that are available. There is more
nutrients plus there is more amount of strata that is available for any autotrophic organism
to make a small indentation or make a small attachment to the rock.
Once that happens and when you have these foliose lichen. The properties of foliose lichen
are that they have a slightly larger nutrient requirement, they have a slightly more three
dimensional structure. They are coming out of the surface plus they are able to perform
photosynthesis in a much more efficient manner than the crustose lichen. In the case of the
crustose lichen their photosynthetic efficiency was less and in the case of the folios lichen
their photosynthetic efficiency is more. Once you have both these kinds of organisms in
this area, what do you think, who is going to out compete whom? Now of course, the
foliose lichen who have a better photosynthetic ability, they will be able to out compete
the crustose lichen.
The crustose lichen made way for the foliose lichen and the folios lichen in effect were
able to displace out the crustose lichen from this area. Now where are we getting all these
lichens from? These lichens are coming from the spores that are there in the air or are
being brought about from the winds. They were not present initially when we had these
rocks; you do not have any plant material, but then along with the air you will have some
spores that will come and land here, you will have some amount of dust that will come in
land here and then probably in the next rains we will have this process of succession that
has started from the crustose lichen into the foliose lichen.
When you have these foliose lichen, they are able to generate much more amount of
organic material as compared to the crustose lichen and in effect they are able to generate

much more amount of acids when they are living and also when their bodies are decaying.
Now in that case you will have that the layer of rocks that was exposed, it now depends a
bit. So, in place of having a very small shell, now you have a slightly larger shell and in
this case as well you do not actually have soil here, but then because these rocks have now
broken up on their shell. So, you have a scope for something larger to come up. Now from
the foliose lichen stage the next stage is the moss stage.
(Refer Slide Time: 09:43)

Now, the moss stage will come up when you have these rocks whose surface has been
broken up to a larger extent. They cannot come before the foliose lichen or before the
crustose lichen, they will come only after that and once they have come up. Now, you can
see that these are all more greenish in color. They have much more better photosynthetic
ability and then because of the inter specific competition, they will be able to displace off
the foliose lichen that are present there.
Now of course, it is not necessary that all of these areas should have a complete cover of
mosses. It is possible that in this particular rock, you have this area, the first area that has
say mosses, you have some other area that still has your crustose lichen, you can have
some other area that has your foliose lichen and so on. Because in this particular region
your succession started much before, than it was able to do in this area or probably in this
area in which you only have a bare rock.

In a large chunk of rock, it is possible that you have different stages in different places,
but then after a while this is the way in which your community we will move from one
organism to the next organism. In the case of these mosses; they are doing much more
photosynthesis, they have some roots that are now coming up. And once you have the
roots, they will able to break up the rocks even further. Why?
(Refer Slide Time: 11:36)

Because if this is your rock and then you have this crack which was there and the top
surface is having some amount of broken portions in which you can have the nutrients. If
you have a plant that is coming up in this area so, this plant will be having roots and
probably the roots will reach much deeper. In this case your roots are able to reach this
point and are probably able to secrete some chemicals inside this crack.
Probably, some acids because the roots also want to make space for themselves. In that
case, you will have some more amount of degradation that is happening very deep inside
and once that happens then probably this crack will enlarge. Similarly, if you say have a
very small crack here and you have a root that is coming up here so, this crack will also
start to enlarge. In effect what is happening is that, this rock surface is now getting more
and more cracked and in these cracks there would be other processes as well.

(Refer Slide Time: 12:52)

For instance, if you have a rock and you say have this crack here. Now in the night time
probably there would be some amount of dew that would accumulate. Dew consist of water
droplets. Probably some amount of water droplets will start accumulating inside and once
that happens and if the temperature is very less. In that case these water droplets might
start to freeze. They will form a piece of ice inside. Now we know that the density of water
is highest at 4 degrees Celsius. So, as your temperature is going down as it reaches 4
degrees.
So, you have the densest part of water, then once, the ice starts forming. So, you have
reduced its temperature from 4 to 3 to 2 to 1 and then 0 and maybe even to the sub zero
temperatures. What is happening now is that the density of water is now reducing which
means that the water is now becoming lighter which is why you have ice cubes that float
on the surface of water because they have a lesser density.
If something has a lesser density it means that it has a larger volume and once you have
something that has a larger volume, it requires more amount of space. When these ice
particles, when they start freezing in this area, they also exert some kind of outward force
on this particular crack and on this particular rock. What would that lead to? That would
lead to some secondary cracks that develop on the surface and once that happens, you will
have water that comes into these areas as well; then later on when it forms an ice here.

You will have larger quantities of ice that are being formed and then they again start
exerting the forces on the surface of these cracks. Ultimately this crack starts to expand in
size; you will have more and more number of cracks on the surface of this rock. This
process is also accentuated by the presence of the plant species because they are also living;
there they are also secreting out some acids which are further weakening the rocks and so
on.
(Refer Slide Time: 15:17)

Not just ice, but you can also have a situation in which you have a surface of the rock and
when you have the sun that is shining in the daytime.
Probably this area becomes very warm and then in the night time when the area cools
down. So, this area is now suffering. It is now becoming cold and so it isthe now shrinking.
For a number of materials when you heat up the material, the size increases; when you
cool it down, the size reduces. Now in this case because of the continuous action of sun
and the moon you will have some expansion in some contraction that is happening on the
surface at all times.
Once that happens that will also lead to the breaking up of the rock surface and in the case
of rocks such as granite, this is known by the term of onion weathering because just like if
you take a piece of onion, you will have a number of a leaves that are one. Similarly in
this case you will have a rock that will look something like this.

After a while you will have see a small section that has come out here, a small section that
has come out here, a small leaf that has come out here and so on. In this case your rock is
now getting separated layer by layer. Just like in the case of an onion, you have layers that
are forming on the surface which are also leading to cracks here.
(Refer Slide Time: 17:00)

Similarly, if you have us a rock with a small crack and probably, this area is close to the
seas. If this area is close to the seas then you might have some amount of sea water that is
coming into this area or probably some amount of salts that are being blown along with
the wind and are getting deposited here.
Now, what happens? If you have say a small amount of salty water, brackish water here
and when a you have the sun. So, this water starts to evaporate. Once that starts to
evaporate the, the salt particles they start accumulating here. Then later on maybe some
more amount of water came along with the salts. So, you again that evaporated.
After a while you will have a salt solution that is of a very high concentration and in this
high concentration when you again have the sun. You will start seeing crystals of salt that
are forming here. Now again, once you have this crystal formation the salt crystal that was
earlier say small in size that will start becoming larger and larger in size. During this
process of crystallization as your crystal is expanding, there also it will start exerting forces
on these surfaces and because of these forces the rock might crack further.

When we are talking about succession, it is not just the biotic organisms or the different
populations that are playing a role, but at the same time the climatic conditions or the
prevalent conditions are also playing a role in this case. Once you have these cracks you
started with the lichens, now you have reached the moss stage. The moss will further
accentuate the cracks and after a short while when these mosses start to die, their bodies
will start converting into the humus.
(Refer Slide Time: 19:12)

Now, you have an area where you have this rock. This rock is cracked in a number of
places, then this rock has also been converted into a powdery form at a number of places.
These rocks also have some lichens and maybe some mosses that have come up and they
are also dying. When they are dying you are also having some organic material that is
coming up into this area. What do you have now? You have rock particles which is
providing you minerals, you have the organic matter which is coming from the dead tissues
of these different species. Once you combine both of these together, what is this? This is
soil? Now, you have soil in this area.
So, because of the action of the climate, because of the action of different plants, now this
top layer of the rock has now been converted into soil. Once you have soil in an area, you
will start seeing some other species; maybe you will start seeing grasses in that area.

(Refer Slide Time: 20:20)

Grasses are much more prolific species, they are able to perform photosynthesis in a much
better way. Once you starts seeing grasses in this area so, the mosses and the lichens; they
have now been out competed. They get removed from the system and now you have lots
and lots of grasses.
Once you have these grasses, they are now further breaking up the soil or they are further
breaking up the rock surface that is down there because they have a very extensive root
system. In the case of your mosses you did not have an extensive root system, but now you
even have a more extensive root system. You have a much better photosynthetic ability.
You are able to generate a much larger amount of organic matter.
In this process the rocks are now breaking up even further and after a short while these
grasses will make way for shrubs.

(Refer Slide Time: 21:21)

Now, as we are moving from the grasses to the shrubs, we will observe that the requirement
of different nutrients will change; again because grasses can grow in the low nutrient
environments, but your shrubs will require typically a higher nutrient conditions. These
higher nutrient conditions are brought about because the grasses were able to break the
rocks, break the soil and so, much more amount of nutrients now becomes available for
the growth of these plants and after the shrubs you will start seeing a forest in a short while.
(Refer Slide Time: 21:59)

Now a forest is typically referred to as a climax community so, this is typically the end
where your succession is going to end. You start from a bare rock you move from lichens
to mosses to grass to shrubs to trees and ultimately you reach to a climax stage which is
the forest stage. This is what succession is all about. When you talk about succession, you
are asking, which species is making way for which species? Who comes after whom? That
is what you are asking when you are studying succession.
For instance even in the case of royalty when we say that there is this particular prince
who is going to succeed his father as the next king. That is succession. Here in the case of
ecological succession, we are asking which community is getting succeeded and which
community is succeeding the previous community.
(Refer Slide Time: 22:56)

We define ecological succession as the process of change in the species structure of an
ecological community over time. We have this ecological community which was having
your lichens in an area; maybe it was having say two or three different species of lichen.
But then, slowly and steadily the species structure is changing, you are getting other
species such as grasses you are getting shrubs.
The earlier species are getting out completed, they are dying off, they are making way for
these newer species and so, the species structure of the ecological community is changing
at all times. The process of this change is known as ecological succession. It takes place
over a long period of time.

(Refer Slide Time: 23:46)

We also have this other term which is called as sere or a seral community is an intermediate
stage found in ecological succession in ecosystem advancing towards its climax
community.

(Refer Slide Time: 24:05)

Essentially when we were referring to a community that ultimately became a forest. So,
this community started with your lichens which was both crustose and then followed by
your lichens that have foliose structure. Crustose lichen followed by foliose lichen,

followed by your mosses , followed by grass , followed by shrubs and followed by the
forest; so, all of these different communities.
If you are talking about our community that is all full of lichens that is called a seral stage
in this succession. If you talk about a community that has grass that is a seral stage in this
succession. A seral community is an intermediate stage; it is not the final stage, but it is
the intermediate stage that is found in ecological succession in an ecosystem advancing
towards its climax community.
In this case the forest is the climax community. When we talk about seres, they are of three
different kinds. The first one is known as a hydrosere. Now hydro is water, sere is your
seral community. This is a seral community that is found in water and we will have a look
at it in more detail in a short while. The second one is a Xerosere. Now ‘xero’ is dry. Dry
community; a community in a dry area; this includes a lithosere. A lithosere is a
community on a rock as we just saw and it could also be a psammosere. Now psammosere
here is you have sand and community. It is a community on sand.
If you look at ecological successions that are happening on sand dunes, that is an example
of psammosere. The third kind is a halosere; now halo is salt sere is a community. You
have a community in salt or a community in a saline body such as a marsh.
(Refer Slide Time: 26:18)

When we talk about the succession, this succession proceeds from a pioneer species
towards a climax species. In this case, we had started with rocks and the first community
that came up was the crustose lichen. This would be called as a pioneer species. Pioneer,
because this is the first one to come up. So, it is a pioneer. Pioneer is the first one, climax
is the last one and everything in between is a sere or a seral community.
Pioneer species are defined as, they are hardy species which established themselves in a
disrupted ecosystem and trigger the process of ecological succession. They may come up
in an area where you did not have any community beforehand or they may come up in a
disrupted ecosystem. What do we mean by a disrupted ecosystem?
(Refer Slide Time: 27:23)

Probably you have a forest and in this forest you have a number of trees. Probably you
have some shrubs, you have some herbs and so on and then you have a forest fire because
of which all of these die out. Now, you have a community in which you have disrupted
something. Probably a few trees remain in this area, but then everything else has been
disrupted. What would come up in these areas? The first species to come up in this area
we will again be called a pioneer species because it is the first one to come up in this
disrupted ecosystem.
Pioneer species are hardy species which establish themselves in a disrupted ecosystem and
triggered the process of ecological succession. If you do not have a pioneer species, you
will not have the ecological succession because there is no other species that can replace

these pioneer species. Why is that so? Because of their specific characteristics. Their
characteristics are their ability to grow on bare rocks, nutrient poor soil or water.
They are the first one to come up and they can come upon bare rocks where you do not
have any soil or they can come up in a soil that is nutrient poor or they can even come up
in water that is also nutrient poor. What are the examples of these soils that are nutrient?.
Consider a glacier. A glacier when it is moving from one place to another it is also grinding
the rocks that are below the glacier. Once this glacier melts the rocks that have been ground
up they come up in the form of a soil in that soil is extremely nutrient poor; it does not
have any organic materials, it does not have a number of minerals because it has never
been acted upon organically. The organisms that can come up in search of soil that will
also be the pioneer species. The other characteristics are the ability to tolerate extreme
conditions such as heat and cold.
(Refer Slide Time: 29:40)

When we are talking about a community that is coming up on a piece of rock. In the
daytime it is getting exposed to very hot conditions, in the night time it is getting exposed
to very cold conditions.
If we have a community that is coming up in a forest. In this forest you have much more
moderate climatic conditions because all the time you are having a high moisture level. If
you look at a location say here. So, you have a high moisture content, you are not exposed

to the direct rays of the sun because the plants are turning it out and then you are also not
exposed to a very cold condition because the air movement that would have happened in
this area is also being starved because of the trees. But in the case of a bare rock, you will
be exposed to all the conditions.
The pioneer species are able to tolerate these extreme conditions, they are able to tolerate
extreme heat. They are able to tolerate extreme cold, they have less nutritional
requirements and they are photoautotrophs because there is nothing else available for
them. Only those species can come up and have less nutritional requirements. If there is a
species that requires say nitrogen, phosphorus, potassium, in high concentration maybe in
a water soluble form; that is available in soil that cannot be a pioneer species.
They are typically small in size because again you have less amount of nutrients that are
available, you have very extreme conditions these species are not able to support a very
large body size. They are small in size. They often have a short life span with rapid growth
and they are mostly annual species. They are not perennial species, they cannot remain
there for a number of years. They probably come up in those conditions that are favorable
and then they die off.
Probably in the very extreme summers, they will die off; probably in very extreme winters,
you will not see these the species, probably these species will come up in the spring season
in that area. Because you have spring season only once in a year so, these the species will
come up only once in a year and then will die off. They often have a short life is span as
against forests species or the climax species that have a very long life span.
Then they have the ability to disperse through spores or seeds and they are also prolific
seed producers. They have to be those species that are able to disperse through spores or
seeds because otherwise when you talk about a bear piece of rock of say, a volcanic origin.
Where do you get these spores in the first place? Or where do you get this pioneer species
in the first place?
They have to have a dispersal mechanism through either air or through water. So, they
have to give out spores or seeds and typically these plants are also very prolific in their
seed production.

(Refer Slide Time: 32:47)

On the other hand, we have the climax species. In our example the forest was the climax
species. A climax species is defined as or a climax community is defined as a biological
community of plants, animals and fungi which through the process of ecological
succession in the development of vegetation in an area over time have reached a steady
state. Essentially when you talk about a climax community, it is a steady state community.
When we see a steady state it means that it is able to remain as this community for a very
long period of time.
Once you have these sal forests in this area so, probably they will continue for hundreds
of years because they are in a steady state. There is no other factor that is now pushing
them towards some other community or towards some other change. It is a biological
community of plants, animals and fungi. Why does it have all these together? Unlike a
pioneer species that is making way for itself, in the case of a climax community you have
so many species that are together that they are able to support each other.
When you have the plants that are growing. These plants will be giving out leaves. These
leaves upon dying they will fall onto the ground and if you do not have fungi, then you
will not be able to break these leaves down into the nutrients again. Or if you do not have
say these animals here so, in that case probably there would be some species that would
try to out compete everything else.

Now because you have these predatory organisms, because you have these animals and
because you have a rich biodiversity with a number of decomposers, so, this community
is able to sustain itself for a very long period of time so that it has reached a steady state.
Now there are four kinds of climaxes that have been recognized. The first one is a climatic
climax which is controlled by the climate of the region.
For instance when we talk about sal forest that is a climatic climax. It will come up in an
area that has sufficient amount of moisture that is probably a cooler area. So, that is the
species that will come up and that will come up and the community that gets formed
because of these sal trees the forest that would be a climatic climax because it has been
determined because of the climate and it continues to remain there for a very long period
of time.
The second one is in edaphic climax. That is controlled by the soil conditions of the region.
Different kinds of soils will be having different kinds of climax species or climax
communities. So, that is when you have a condition like that it is known as an edaphic
climax.
The third one is a catastrophic climax that is controlled by some catastrophic event such
as wildfire. A good example would be your teak forest. Now in a teak forest, you have
these deciduous a trees in the form of Techtona grandis and these trees shed their leaves;
when these trees shed their leaves, you have ample amount of dry fuel that is available
there and so they are very frequently prone to forest fires.