Module 1: Human Ecology and Ecology of Change

Study Reminders
Text Version

Set your study reminders

We will email you at these times to remind you to study.
  • Monday




























Today we begin a new module, which is the ecology of changes. We have seen how an
ecosystem functions, how different populations and communities interact and so on, but
then what happens if you bring in a major change to the ecosystem?
These changes are becoming important day by day because of the disasters that we are
observing in the world. For instance, if we talk about an oil spill. If there is a tanker that is
carrying oil and if it spills a large amount of oil on the surface of the oceans, then that is
going to have a negative consequence on different components of the ecosystem.
If such a thing happens, how does the ecosystem revert back? Is there some amount of
resilience in the system, so that it is able to bring itself back to normal or is it a situation
that will become doomed towards the extinction of a number of species? Now, that is an
important question these days, because we are observing more and more number of such
The disturbances can be of different kinds. There can be disturbances that are abrupt and
that are very large in their magnitude such as an oil spill or a forest fire. On the other hand,
there can be some other disturbances that take a very long period of time to manifest
themselves such as climate change. So, a climate change goes on for a very long period of
time, and that would also result in some amount of changes to different ecosystems.

(Refer Slide Time: 01:53)

In this module, we will be looking at three different case studies about oil spills. Plastics,
and the impacts of climate change. How they bring out a change in the ecology, and what
do we do to bring the system back to normal?
Let us begin with the first lecture which is Oil spills.
(Refer Slide Time: 02:09)

Whenever we are talking about any such disturbance, the impact would depend on what
was the status of the community before this impact came in and the nature and the
frequency and the magnitude of the impact. We can classify the situations into three

different categories. The first category is that you have a normal community that is
everything is normal, there is no heavy stress on that community, no high pressure of
diseases or say no high pressure of habitat degradation and so on. And you bring out a
single LID, now LID stands for a Large and Infrequent Disturbance.
Here we are saying that on the y-axis we have the community state, on the x-axis we have
the time. The community state is shown in two forms; one is this altered state, which is at
the bottom and the top portion is showing you the normal state. In the normal state there
would be some amount of fluctuations that we are observing in the community.
Where do we see these fluctuations? Because these are the normal variations that we
observe, because say, 1 year was slightly dry year or maybe it was a slightly more wet year
or probably there were some diseases that cropped up in this community, but then overall
it is more or less a normal community. These are the variations that we observe in a normal
At this time point there was a large and infrequent disturbance, now such a disturbance
could be things like forest fire. You have this community and there is a forest fire that
consume a major portion of this forest. What would happen then?
(Refer Slide Time: 04:02)

When we are seeing a large and infrequent disturbance, probably if this is your forest, you
had a forest fire that consumed all of these portions and only a small portion was left. If

that is the situation and if this community that was left out, if this is a normal community,
we would start observing the same scenarios we had observed in the case of population
(Refer Slide Time: 04:41)

In this case, this would be equivalent of a situation in which there is an island and in this
island, this much portion is having the organisms and the rest all portion is now clear, it
does not have any organisms. What would happen then? Then these organisms that are
already existing, because they are a part of the normal community, they would start
dispersing out, because these areas that are now vacant; so this is a vacant area. This vacant
area has the resources in the form of minerals. So, because it has minerals when it rains,
then would start observing some vegetative growth. There would be some trees that are
there in the community and these trees would be giving out seeds. Now, if there are some
birds in this area, those birds would be carrying those seeds from this area probably to this
area as well or maybe some amount of seeds would come out because of a wind flow or
because of a water flow.
There are a number of situations. At the same time it is also possible that when this area
was burned out, there were some seeds or maybe some rootstocks or some tubers that
remain below the ground.
If you have these root stocks that are below the ground. So, even if you have a fire on top
of this forest, these portions are still alive and so in the next rains, they would start giving

out a sprout, so we would start observing sprouts everywhere. And in a very short period
of time we would see that there are herbs, there are grasses, there are shrubs and maybe
even saplings of the big trees that are now coming up into this area.
Once that happens we will also have a situation where, you have ample amount of food
that is available to the animals. So the animals that are with a part of the community that
were left out or that was not consumed by the fire; now those animals would also now
come out and they would also start browsing or grazing on to these herbs and shrubs.
Once that happens, in a short period of time this whole area would start showing up the
characteristics of a healthy ecosystem once again, because you would have the plants, you
would have the trees, you would have the animals, you would have the birds, the micro
organisms the small little degrading organism the saprophyte and so on, they will start
coming up into this area.
In a very short period of time, we will have that this community is able to bounce itself
back to the normal situation. Here we have the normal community, you have a big forest
fire, but then with time it starts recovering and once it has recovered, it becomes a normal
community once again.
The most important points in this state would be that you have started with a normal
community and you only have one disturbance, but then what would happen if you have
say a number of disturbances;
In this particular example, let use talk about forest fire.

(Refer Slide Time: 07:49)

Let us consider another example where we have this big forest. In this big forest, there was
a forest fire and all these portions were consumed out and only this portion remained. As
before, all of these areas now behave as freely available areas and then you have the
organisms that are now starting to come out.
But then once that is happening once, you are observing some regeneration in these areas,
let us suppose there is another calamity. Probably, there was a river flowing in this area
and this river now floods the area. When it floods this area, probably all of these regions
are now inundated and once that happens, all these small seedlings and saplings that were
coming out, all of those small root portions that were giving out the sprouts. All of them
are inundated with water and when they are inundated with water they die out.
Now, we are observing a situation in which there is a community, a healthy community
that support one large infrequent disturbance and after that it was trying to come up again,
it was trying to show its resilient character, it was trying to come back to the normalcy,
but then we observe another disturbance in the form of another large infrequent
What would happen in that situation? In that situation, the saplings that were coming out
are now all dead. In this case, those seeds or seedlings that were saved from the forest fire
are now again dead, what will happen then?

(Refer Slide Time: 09:38)

So, this is how we can represent such a scenario, we have a normal community, you have
a large infrequent disturbance, now this community is trying to come back to the normalcy.
Then you have another large infrequent disturbance and now there are no more living
seedlings and saplings in this area, because anything that was saved from the forest fire is
now consumed by the floods.
Once that happens, if you have a series of disturbances that come up in a normal
community, we will have a situation in which the community after a while will not be able
to come back to the normalcy. It will behave as an altered community for a very long
period of time, because you now no more have any seeds or any rootstocks to bring this
community back to normal.
When such a situation happens, we could even say that it would reach a new normal. When
we say a new normal, we could say that, in place of having tall trees in this area, now you
only have those plants that grow near the rivers. So, that would be a new normalcy or a
new stage of succession. But then, when we talk about a community that was already in a
pre existing normal state, it will not be able to come back to that state for a very long period
of time.
Now, there can also be a third situation.

(Refer Slide Time: 11:06)

Now, this third situation is, when you have a community that is not normal from the
beginning. So, it is already suffering from some level of disturbances. Now, it is not a
completely altered community, it is not a normal community, it is somewhere in between.
A good example would be a forest that has a lot of insect infestation. In that case, most of
the trees are diseased trees and they are not normal trees, so to speak or an area that, let us
say there is a water body. And this water body is getting a number of contaminants in the
form of pollutants, because there is a municipal drainage that is draining into this water
In that case, all the organisms or most of the organisms in this water body are now already
in a state of stress. They are not a normal community anymore. If that is the situation and
if we see a large infrequent disturbance, so in that case the resilient power of the
community has already been lost because of the long period in which it was suffering this
stress. If that is the situation, if you have a single large infrequent disturbance, it is possible
that your community, will not be able to come back to the normalcy for a very long period
of time.
A good example in that case would be say, an animal community. So, you have a
community of animals and those animals are not the normal animals, because they have a
very high load of say parasites. If you have a very high load of parasites or maybe some
viral diseases in that community, if there is a large infrequent disturbance, then because

already they are very very stressed. So, their rates of reproduction or the rates of bringing
the system back to normalcy is already hampered.
In that case, if there is a large and frequent disturbance in the form of say a heavy amount
of poaching, so these animals that are left out, they are already so disease and so weak that
they will not be able to reproduce in a fast enough rate to bring the system to normal. So,
this is what we are seeing in this particular instance. Now, let us look at some examples of
these large infrequent disturbances.
(Refer Slide Time: 13:29)

There can be things such as fire. There is a forest fire; a forest fire does not happen very
frequently except in some specialised forest. We can say that it is an infrequent disturbance
and it is large because of its large magnitude and the large area that it covers. Another
example is a storm, so if you have a storm a very large area would be inundated, a number
of trees would get uprooted.
Another example would be tsunami. In a tsunami there is a large amount of inundation and
at the same time there is a heavy mechanical force that is applied by the large volume of
water that is coming in and for a very long period of time most of the areas are... will
suffer from a heavy amount of salinity. Or things like oil spills; oil spills again our large
infrequent disturbances because of their large impacts, they cover a very large area and
most of their situations and their impacts are very wide and very diverse.

Climatic extremes are yet another example. If you have a situation where your community
suffers from a very severe drought. So, a very severe drought or a high amount of flooding
would also count as a large infrequent disturbance, because in these situations are when
the productivity of the whole community would go down and the impacts would be very
severe. Or things like heavy pollution, so if there is an industry that is spewing out, say
mercury rich compounds into an ecosystem, so that would again be a large infrequent
(Refer Slide Time: 15:13)

And when we talk about a community that is a disturbed community, that is not a novel
community, it could be a community that is either diseased, it has a heavy load of
pathogens or parasites or it is weed infested. So, a good example would be a forest that has
a lot of cover of lantana, so because you have so many lantana around. In a normal forest,
you would have these trees and these trees would be giving out seeds.

(Refer Slide Time: 15:37)

Once these seeds come to the soil, they start giving out the new regeneration. If you have
a scenario in which the whole of the ground cover is now covered with these shrubs of
lantana, in that case, the seeds will not be able to reach to the ground, they will remain on
top of these lantana bushes, because they are not able to reach to the ground. So, they will
not germinate or even if they germinate, so if you have these lantana, your seedling will
not be able to get sufficient light from above.
In that case also the your seedling will not be able to germinate or grow properly or there
would be some amount of allelopathy chemicals that this plant would be putting into the
soil. Allelopathy as we have seen in a previous lecture is a situation in which one organism
is trying to hamper the growth or to kill another organism by giving out some chemicals.
Now, lantana also being a member of the verbenaceae family is known to give out certain
chemicals into the soil, which then hampered the growth or even try to kill other seedlings
and saplings so that the amount of competition that it faces reduces. If you have such a
forest that already has a heavy growth of lantana and if you now have a forest fire and in
that forest fire a number of trees get burnt out.
In that case, the remaining trees, even if you are able to remove all of these lantana and in
that situation even if the seeds of this tree are able to reach to the ground, because you
already have a very heavy dose of the allelopathy chemicals; so, these seeds will not be
able to germinate or probably in this case as well, you will have the lantana that also has

its root stock below the ground and when there is a chance of coming back, so, lantana
will be able to out compete your seedlings. In that case this forest will not be able to reach
back to the normalcy state. So, a heavy amount of weed infestation also counts as a
disturbance in the community.
(Refer Slide Time: 18:12)

Or facing competition from livestock. Let us say, you have a situation where you have a
forest and in this forest you say have a few trees and you have a very high amount of
competition because of the livestock. Now in this case, even if these trees give out seeds
that are able to reach the ground and they are able to give out these seedlings. The livestock
comes and it eats away the seedlings. So, there is already a very heavy amount of pressure
or disturbance in this community.
Now, if you have a forest fire and a few trees are now dead, but then you already have this
heavy pressure of livestock. In that case, whenever a new seedling comes up, the livestock
will eat it away. When that happens, then the impact of the large infrequent disturbance
such as the forest fire will never be completely removed from the system. So, the system
will not be able to come back to normalcy.

(Refer Slide Time: 19:19)

Or a situation in which you have a forest and in this forest you have a number of wild
animals. Let us say we are talking about black bucks. Now if you have these black bucks
and this area is also suffering from a heavy amount of competition from the livestock, so
the red ones are the livestock. Now, because of a heavy pressure of livestock we might
observe a situation of habitat displacement. Now, in the case of habitat displacement as
we had seen, the animals are forced to move into the sub-prime habitats. So, in this
situation, we will say that the black bucks are forced to move to the subprime habitat say
in the hills. Now, in the subprime habitat you do not have sufficient amount of food
available to these animals, you do not have sufficient amount of cover available to these
animals. Now, these animals are already facing a lot of stress, so in that condition of stress
the rate of reproduction would go down.
Now, if that is the situation and there you put in a large infrequent disturbance, say you
have a spell of a disease or say a heavy amount of poaching or any other such large
infrequent disturbance and you say that a few animals die out. Now, these animals that are
remaining, because they are already in a stress environment, because they already have a
heavy amount of competition because of which they had to suffer from habitat
displacement. And they are now in a place, where they are not having sufficient access to
food or water or shelter and so on. So, they will not be able to reproduce at a fast enough
rate that they would be able to come back to the normalcy.

In this situation as well, a heavy amount of competition from the livestock also counts as
a significant disturbance to the community which will prevent it from coming back to
Another disturbance is a rich amount of pollutants that are already there in the system that
will also have a very similar impact. So, with a heavy polluted dose a number of members
of the community will be facing a huge amount of stress and in that stress they will not be
having a high rate of reproduction. So, in that case if you give a single large infrequent
disturbance, the community will be doomed.
Or a community that is already facing climatic changes. So, there is a community that say
lives in a cold area. Now, because of the climate change there was say a 1 degree rise in
the main temperatures and in that case this community is already in a heavy amount of
stress. Now, you give it a single large infrequent disturbance a single LID and because this
community is already are stressed community, because it is already not in a situation where
it has an optimum rate of reproduction or an optimum rate of population growth. So, a
single LID will then wipe out this community for a very long period of time.
So, these are the large infrequent disturbances and the disturbances that communities face.
Now, we will look at oil spills as a case study of a single large infrequent disturbance, the
kinds of impacts that it brings about to the ecosystem and how ecosystems are able to go
back or at the same time, how are we as human beings, how do we act on these ecosystems
in an effort to help them to recuperate back to the normalcy.

(Refer Slide Time: 23:04)

We will begin with oil spills now. Now, oil spill is the release of liquid petroleum
hydrocarbon into the environment. That is the definition. You have liquid petroleum
hydrocarbons that gets released to the environment and that is an oil spill.
(Refer Slide Time: 23:17)

These oil spills can be anywhere. So, you can have a terrestrial oil spill, where the oil spill
is coming out on a portion of land. A good example is the Kuwaiti oil leaks that were
formed during the Iraq’s invasion of Kuwait in 1990 – 91.

(Refer Slide Time: 23:37)

When we say a terrestrial oil spill, this is how the oil spill looks like. So, you have this
area that is predominantly a desert area and then you have an oil spill here. All these areas
are now filled with oil. So, it becomes a lake of oil. So that is a terrestrial oil spill.
(Refer Slide Time: 23:55)

The second is a marine oil spill. In the case of a marine oil spill, the liquid petroleum
hydrocarbons get released into the marine environment. So, they are released into the seas
and the oceans.

(Refer Slide Time: 24:06)

This is an example of the deepwater horizon oil spill of 2010. In this case we observe that
here you have the ocean and here you have a heavy amount of oil that is there on the
surface of the ocean.
(Refer Slide Time: 24:20)

Based on their origins they can be classified into natural oil spills, accidental oil spills or
intentional oil spills. Now, natural oil spills are - example is the oil seeps in the Gulf of

(Refer Slide Time: 24:42)

In this case what happens is that you have the ocean and in the seabed, there is some
amount of natural leakage of these hydrocarbons. When that happens some hydrocarbons
are coming to the surface. So, this is how it looks like.
(Refer Slide Time: 24:59)

This is the gulf of Mexico oil seep and these streaks that we are observing, these are the
oil spills that are coming and these are coming out naturally, these are not manmade, these
are not intentional, these are not accidental.

(Refer Slide Time: 25:14)

The second kind is an accidental oil spill example the deepwater horizon incident. So,
nobody wanted to release this amount of oil, but just because of an accident it got released
into the environment. Another example could be a situation in which there is a tanker that
is full of oil and it collides with some other tanker or maybe with a rock. In that case the
tanker capsizes and when it capsizes, it releases all the oil that it had into the environment,
so that is an accidental oil spill.
And third one is an intentional oil spill, in which case human beings intentionally release
oil into the environment in order to cause harm to somebody. So, a good example is the
case of the Kuwaiti oil spills, in which there were people who spilled out oil into the

(Refer Slide Time: 26:09)

When oil gets released into the environment, we have all these hydrocarbons that come up
in the environment. Now, a hydrocarbon is an organic compound that consists entirely of
hydrogen and carbon, which is why we have this term ‘hydrocarbon’ - hydrogen plus
carbon. So, these are compounds that are made of hydrogen and carbon. And they form a
major chunk of the petroleum that is released into the environment.
(Refer Slide Time: 26:37)

Now, these can be straight chain molecules, these can be cyclical molecules or these can
be aromatic hydrocarbons. So, these are some common hydrocarbons that we see in oil
and all of these will have different impacts on the ecosystem.
(Refer Slide Time: 26:51)

We classify these hydrocarbons on the basis of their specific gravity. Now, group 1
hydrocarbons are those that have a very low specific gravity such as kerosene. Now, it
becomes important to classify them on the basis of a specific gravity, because the group 1
hydrocarbons, when they are released in a marine environment, they will remain on the
top surface of the water. So, they will come to the top surface and they will be exposed to
air, they will be exposed to the sun; whereas, group 5 hydrocarbons are those that have a
very high specific gravity that is greater than 1.
In these situations, because they have a high specific gravity, they will sink down to the
bottom. Now, why is that important? It is important, because this helps us understand the
impact of these hydrocarbons on different ecosystems. So, if there is an oil that comes to
the surface, now if it comes to the surface, so birds will get exposed to it or maybe in the
mammals that are coming to the surface to breathe. Examples include whales or dolphins
or even reptiles that come to the surface to breathe like turtles, they will all get exposed
with this hydrocarbon, because this is on the top surface of the water.
Any organism that is either reaching the waters from the top, for example, birds that are
looking out for fishes, they will get exposed. Or any organism that is moving from the

bottom to the top to have access to the air, it will get exposed. And in the case of group 5
hydrocarbons with high specific gravity, they are coming down to the bottom of the
oceans. Now, in the bottom of the oceans you will have a number of benthic communities
or bottom dwellers.
Those bottom dwellers will then get exposed to the oil. Now, good examples include things
like starfishes or a number of crab species that are living inside the oceans, so they will get
exposed. So, this classification is used when discussing the fate and persistence of the oil
spills. Fate and persistence, because if you have an oil that is coming to the top, so, on the
top, it will be oxidized because it is exposed to the air, it will also be acted upon by the
UV rays of the sun.
And so the fate will be very different and also the persistence will be very different,
because in the case of the hydrocarbons that are coming to the top, after a while they will
be removed from the system because of various processes whereas those hydrocarbons
that are coming to the bottom will probably persist for a very long period of time.
(Refer Slide Time: 29:30)

These hydrocarbons are also classified as petrogenic. Petrogenic is ‘petro’ is rock, ‘genic’
is formation; so petrogenic is something that is formed from the rocks or the petroleum.
So, these are derived directly from the mineral oils. Now, the second one is pyrogenic;
these are derived from incomplete burning of mineral oils. So, if you have an oil spill that
is coming to the top and it is a petrogenic oil spill, you try to burn it off, and when you are

burning it, any amount that remains, which is incompletely burnt will be called a pyrogenic
The third classification is that of a biogenic hydrocarbon. Now, biogenic hydrocarbons are
those that are derived from biological processes acting on the mineral oils. So, if you have
some amount of microbial degradation that happens, so anything that that is left out or that
is half acted upon will be termed as a biogenic hydrocarbon.
(Refer Slide Time: 30:32)

Now, talking about the fate; the fate will be very different depending on the categories of
these hydrocarbons. Now, suppose there is an oil spill; now if it is there on the top surface,
now here we are talking about the group 1 hydrocarbons. Now, the group 1 hydrocarbons
are there on the top and because they have a low specific gravity, they have come to the
top and because they also have a low density,so they will also be preferentially evaporated.
So, you have the heat from the sun and these are exposed to the air. So, there would be
some amount that would get evaporated and that would be removed from this particular
ecosystem. So, it will be carried away with air to some other location. The second thing
that happens is photo-oxidation. Now, in the case of photo-oxidation you have, photo is
light and oxidation is the chemical reaction that occurs because of the oxygen.
Now, these hydrocarbons are exposed to the air and they are also exposed to the sun. So,
in that case there would be some degenerative reactions that will happen and they will also

remove some part of this oil away from the ecosystem. Now, but because these are on the
top and you have this water surface, so some amount will also be spread out; so because
oil is lighter than water, so it will spread on the surface of the water and a large area will
be impacted. So, in the case of group 1hydrocarbons there is spreading and it impacts a
very large area.
On the other hand, if we talk about the group 5 hydrocarbons, they would settle to the
bottom, there would be some amount of sedimentation. And in this case, the impact will
be more and more localized to that particular area. But then because the group 1
hydrocarbon that had come to the top they are spread to a very large area, so the impact
that they will have on the ecosystem will probably be comparatively much lesser than the
impact of these sedimented hydrocarbons, because they are getting concentrated in the
Now, the other things that would happen are things like emulsification. Now,
emulsification is the process in which the oil droplets are able to come inside the water.
So, in this case the oil is broken down into drops and these drops are then further broken
down into very small droplets and these droplets are able to remain suspended in the water,
so that is known as emulsification.
On the other hand, we will also have some amount of dissolution. So, if there are some
other components of these hydrocarbons and that suppose have an OH group or maybe an
acidic group, so they will be able to dissolve into the waters. So, there will be this process
of dissolution. And also another process will be that of this coating and ingestion. So,
because you have these hydrocarbons on the surface, if there is a bird that comes to the
surface, if there is a dolphin that reaches the surface to get air, so these animals will be
coated with the oil and if this oil is able to reach their mouthparts, so, they will also be
eating up some part of oil. So, there will be these processes of coating and ingestion.
Then there will be the process of dispersion in which the oil droplets get dispersed out,
and if these oils are able to reach to the beaches, so, we will observe the process of beach
stranding in which case this oil will coat up portions of the beach and will also have an
impact on the terrestrial ecosystem. So, for instance, if it covers up the roots of the
mangrove, so the mangroves might start dying