Module 1: Introduction to Ecology and Evolution

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Biodiversity - II

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In the last lecture, we had started with our discussion of biodiversity and in today’s lecture
we will look at biodiversity in greater detail.
We had ended with the last lecture with the slide, biodiversity hotspots of the world and
we had defined a hotspot as a region where we have a large amount of species richness
that is a number of species are found per unit area there. They are also regions with a high
degree of endemism. A number of species that are found there are found only there, they
are not found anywhere else. If we lose them at those locations, we would not have any
backup of these species. Third, these are areas that are facing a large amount of threat
because of say diseases or forest fires or maybe anthropogenic influences like people
wanted to convert these forests into plantations or agricultural lands or residential lands or
for roads and so on.
If we look at this map these are the locations with a high degree of species richness. Now,
a question arises why are there some regions that have a high degree of species richness?
and why are there some locations on the earth that have a low amount of species richness?

Now, in particular we can see that the areas between the Tropic of Cancer and the Tropic
of Capricorn are the areas that predominantly have a larger amount of species richness. On
the other hand, if we look at the areas that are on the very north or south that is the Arctic
region or the Antarctic region there is very little amount of species richness.
What makes certain areas to have more amount of richness? and what makes certain areas
to have less amount of species richness or more biodiversity or less biodiversity?
(Refer Slide Time: 02:13)

There are a number of hypotheses that have been put up and essentially this is a
combination of the evolutionary processes and the ecological processes. It is difficult to
dissect for any particular region whether it is the evolutionary processes that are
dominating all the ecological processes that are dominating. So, which is why we are still
talking about the hypothesis and we have not formulated any theories about them. But,
then these hypothesis gave us a good understanding of the factors that can be responsible
for having more or less amount of biodiversity in any area.
The first hypothesis is known as the evolutionary speed hypothesis. Evolutionary speed
hypothesis suggests that all the biodiversity that has been created because of the process
of evolution. For any pocket of land or water on the earth the number of species that would
be there would depend on
1. the time for which evolution has happened there and

2. the speed at which the evolution has happened there.
So, essentially there is more biodiversity in areas with more time to evolve. Basically,
areas that are older would be having more number of species and more rapid evolution. If
there are areas that have shorter generation times or higher mutation rates or natural
selection that is acting more quickly than those areas would be having a greater amount of
biodiversity. An intuitive example for areas that differ regarding the time of labour for
evolution would be the case of old islands versus new islands.
(Refer Slide Time: 04:13)

Suppose, under the surface of some ocean we have seabed and suppose there is a volcanic
eruption here which gives out the magma and with time we have a new island that has just
appeared on the surface of the sea. Because this island was constructed using molten
materials so, essentially it would not be having any species whatsoever on the surface of
this island. If we consider an older island nearby which was created say many millions of
years back, in that case this particular surface would be teeming with a number of
organisms or teeming with a large amount of biodiversity.
Because, here the time available for evolution has been say several million years whereas,
here the time available for evolution is essentially 0 years. Now, even this particular island
would start getting biodiversity which is through a process that we call as succession and
we look at succession in greater detail in one of the later lectures, but then it is important
to note that the time that is available for evolution is an important factor to determine the

amount of biodiversity that we will have in an area. The second thing is more rapid
evolution now, more evolution could happen because of shorter generation times.
(Refer Slide Time: 06:01)

Shorter generation times means that for species; so, let us consider two species here: one
is a small species and the second one is a large species. Now, in the case of this small
species one pair of parents give rise to say 1000 off springs and these off springs will then
give rise to further number of off springs and the time between these two stages is, say of
the order of a few minutes or a few hours. Let us consider the case of bacteria. E.coli takes
around 20 minutes to perform a cell division.
In this particular case, let us consider some sexual reproducing organisms. So, for instance
we could consider a species of mouse. In the case of mouse the generation time would be
say a few months, say 4 to 6 months. Now, in 4 to 6 months you have moved from 2
individuals to say 20 individuals to say 400 individuals. On the other hand we have some
other species that are large in size. A good example could be the tiger. In the case of a
tiger, when you have a mating, it would generally result in say 3 to 4 cubs and the time to
sexual maturity for these cubs is of the order of a few years say around 6 to 7 years. So,
let us call it as 7 years. In this period of 7 years the smaller population of mouse would
have had a number of generations. Let us consider that this had 6 months of generational
time. So, in that case, when it has 1 generation it would be having as much as 14

generations and at the same time the number of off springs that have been produced is also
If you have a shorter generation time, if you have say just a hundred thousand years for
evolution and if the species are such that they have shorter generation times; So, they will
be having more number of generations and so, there would be a more rapid evolution. We
will start observing differences in the populations with the time. Also areas which have
high mutation rates; now you can have a high mutation rate in area say that has a uranium
mine. So, that has a larger amount of background radiation or areas that are say having
some other heavy metals or say some chemicals that are leading to higher mutation rate.
So, those areas will also be observing a more rapid evolution and third is natural selection
that is acting more quickly.
(Refer Slide Time: 09:13)

The second hypothesis is known as the geographical area hypothesis. Geographical area
hypothesis says that there is more biodiversity in areas which have larger areas and which
have physically or biological complex habitats because these regions support more niches
or roles for organisms. What do we mean by that?

(Refer Slide Time: 09:43)

Suppose you have two areas; one is this small area and then this is the second larger area;
let us consider that these are two islands. So, this is island 1 and this is island 2. Now,
because this island 1 is a small island, probably it would be having a fewer number of
habitats that are around. It is quite possible that in a very smaller island the whole of the
areas covered with sand. So now, you have sand and then you have water around it. Now,
in this case you only have a single kind of habitat.
In the case of the larger island you would have a strip of sand that would be there on the
periphery, then it is possible that in the centre you have some hills, you could even have a
small rivulet of sorts that flows through these hills. There could be some areas that are
plains, there could be some areas that are forested, there could be some areas that are
mostly full of grasses, there could be some areas that have a swampy vegetation. In the
case of island 2 we have a larger variety of habitats.
Island 1 would be able to support a very smaller biodiversity because every species has
specific requirements regarding its habitat. It would only support those species that can
live on sand or can live on the confluence between sand and water. However, in the case
of island 2, here we can observe that we will have one species that lives in the forest,
another species that lives on the conflict between sand and water, a third is species that
lives in the swampy lands, a fourth species that lives in the grasslands, a fifth the species
that lives on flat lands, a sixth species that lives in rivers, a seventh species that lives on

the hills and also other species that live at the other confluences. So, here we will be having
a species that lives on the confluence between forest and grasslands. So, here you have an
eighth species, then you might even have a species that lives at this conference, at the ninth
conference which is between the forest and the swampy lands.
So, more amount of complexity in the habitat, physically or biologically more complex
habitats will correlate with a larger amount of biodiversity and for areas that are larger in
size, it is more probable that you will be having a more varied habitat condition that are
available. Geographical area hypothesis says that you will have more biodiversity in areas
that are large and in areas that are physically or biologically more complex.
(Refer Slide Time: 12:47)

The third hypothesis is called interspecific interactions hypothesis. ‘Inter’ is between,
‘specific’ is a species; so, we are considering the interactions that are happening between
species. These are community level interactions not population level interactions; and this
hypothesis says that there is more biodiversity in areas with competition because it affects
niche partitioning. Now, what does that mean?

(Refer Slide Time: 13:23)

Suppose you have an area where you have a tree and in this tree suppose we have say two
species of birds. This is species 1 and then this is species 2 and because these species are
not having a very large number of individuals so, the amount of competition is less. If that
happens when the competition level is low, both of these species could use the same niche.
Niche the term refers to the position and role of an organism in the ecosystem.
When we see that both of these organisms are occupying the same niche we mean that they
are in the same position. So, both of these are canopy dwelling birds for instance and they
are performing the same role. For instance, both of these species are performing the role
of eating fruits and dispersing the seeds. When that happens we will have a situation where
both of these can live in the same niche.
Suppose with time you now have more number of birds; you have a third species and a
fourth species. When that happens that the number of fruits that are given out by this tree
are limited. Now you have a situation of competition; not everybody can have access to
the food. When that happens some of these birds will try to become more specialized.
Now, what do we mean by more specialized?
As we know in the case of a tree the fruit has the largest amount of nutrients. So, that is
the most edible part that is available in a tree, but then there could be other parts, say
leaves. In the amount of nutrition that you can have from leaves is typically less than the
amount of nutrition that you can have from the fruits, but when this bird tries to become

more specialised, it is because it is not able to compete with the other species, it now tries
to live on the leaves. It tries to eat leaves and get its nutrition from there.
When that happens, with time will observe that the body parts of the bird will start evolving
into structures that are more and more adapted to its new way of life, that is with
generations we might see that in place of beaks that were adapted to eating fruits now, the
structure of beaks is changing because of natural selection in a way that it is becoming
more and more adapted to eating leaves. If there is more amount of competition, then
probably some might even start eating the branches or maybe eating the bark or maybe
some would become insectivorous birds or maybe some would start getting down on the
ground and looking for their feed there.
With more and more competition we will observe that the niches would, the number of
niches that would occupied they would increase and when that happens, when you have a
particular group of birds that is feeding on fruits, then their evolution will drive them
towards having more and more adaptations to fruit eating. When you have a group of birds
that are eating leaves, we will have adaptations with because of natural selection that
become more and more adapted to eating leaves and when that happens the amount of
biodiversity increases. So, earlier when we did not have any competition, then you had a
certain number of birds, but with competition the number of bird species increases.
Similarly biodiversity also increases because of predation because predation retards the
competitive exclusion. Now, what do we mean by that?

(Refer Slide Time: 17:39)

Suppose you have a pond and in this pond you have say three species of fishes. You have
this purple fish, you have red fishes and you have green fishes. We have these three species
of fishes. Suppose these three species are such that the red one is the most suited or the
most competitive and it is so much competitive because it is so efficient in getting its food
that it would even eat the food of the purple fishes and the green fishes.
If you have this situation, in a very short period you will find that this fish would increase
in its numbers and the other fishes would get exterminated and then after a while you will
have this pond that only has a single variety of fish which is the red fish. Now, that is when
you do not have a predation. Let us put a predator in this picture. We had these three
varieties of fishes, but then you also have a crocodile in the system. You have this crocodile
and this crocodile does not have a preference regarding which fish to eat, whichever fish
it gets it will go and eat it.
What happens now, if the population of the red fishes increases then the crocodile starts
eating more and more of the red fish and so, this population is kept in check. On the other
hand if the number of green fishes increases then the crocodile will go and eat the green
fishes and keep their population in check. Now, just because you have this crocodile here
in the system, so that the red fish will not be able to increase in its numbers so much that
it would outcompete the other two fishes.

So, predation also helps in having more and more amount of biodiversity. In the absence
of crocodile, in the absence of the predator you would be having a situation in which you
have only one variety of fish, the red fish. When you put up a predatory pressure so, in that
case you now have three varieties of fishes. So, predation can also increase the amount of
biodiversity that we have in the system which is also a food for thought, why we require
tigers in our forest because tigers are predators and they keep the system in check. But, we
will look into it in greater detail in one of the other lectures.
(Refer Slide Time: 20:29)

Another hypothesis goes by the name of ambient energy hypothesis. There is more
biodiversity in areas with more energy for fewer species can tolerate climatically
unfavourable conditions. This hypothesis essentially states that those areas that have more
amount of energy in the form of say sunlight and are having more favourable conditions
would be having a greater amount of biodiversity.

(Refer Slide Time: 21:01)

We can look at it by considering the structure of earth. In the case of earth, here we have
the equator, then we have the two tropics – the Tropic of Cancer and the Tropic of
Capricorn, and then the Arctic circle and the Antarctic circle. If we consider this region,
the arctic or the Antarctic regions, both of these regions get a very little amount of sunlight.
So, because of that, not enough amount of energies available for plants to thrive and at the
same time the temperature is so low that everything is kept frozen. So, it makes sense that
we will not have a great amount of biodiversity in these areas.
But, consider the central region, the region between the Tropic of Cancer and Tropic of
Capricorn. In these regions we have abundant amount of sunshine. We have abundant
energy that can be used for by the plants. We also have more access to water nothing is
kept frozen in these areas and so more number of species would thrive in these areas, but
then we can also look at some other extremes.
Consider an area of desert that is found here. In the case of the desert, you have abundant
amount of energy, but you have a very great shortage of water and when that happens then
you will not have a large number of species that would be able to thrive in that area. So,
essentially what this hypothesis says is more energy would give you more species and
more favourable conditions; say more favourable conditions to life, will give you more
species. So, this is also another way in which we can explain why certain areas have greater
biodiversity and certain areas have lesser biodiversity.

(Refer Slide Time: 23:15)

Another hypothesis goes by the name of intermediate disturbance hypothesis. Till now,
we were looking at areas with more biodiversity. Now, we are concentrating on areas with
less biodiversity. Now, there is less biodiversity in areas with very high or frequent level
of disturbances because it leads to extinction of species.
What do we mean by frequent level of or high level of disturbance? Suppose you have a
forest and you have a human community nearby and these humans are going into the forest
every now and then they are poaching the wild animals and because the level of poaching
is very high; because they are going into the forest every day, so the level of disturbance
here is very large or probably they are getting into an equatorial rain forest and cutting all
the trees to convert them into plantations. Here we have a very fast rate of disturbance.
So, a forest that took millions of years to come up into its current shape is chopped down
and is converted into a barren land in a period of say a few months or say 1 or 2 years.
That is a very fast level of disturbance or a very high level of disturbance. If you have very
high level of disturbances or very frequent disturbances, so the species that are found in
that area do not have enough time to adapt themselves or to evolve themselves and to and
to avoid their own extinction. So, if you have very high level of disturbances or very
frequent disturbances that will lead to extinction of species.
However, if you look at the other extreme; if there are areas that do not have any
disturbances, very low disturbances or very infrequent disturbances. So, in those areas, we

would observe a competitive equilibrium and loss of species of low competitive ability,
that is, if we go back to our example of the pond, if we did not have this crocodile that was
putting a disturbance in the system, then very likely there would be some species that
would outcompete everybody else and they would reduce the level of biodiversity that is
present in this system.
(Refer Slide Time: 25:39)

Essentially if we tried to make a curve of the number of species or the species richness
versus the level of disturbance, we will find that at very low level of disturbances we have
low number of species, at very high level of disturbances we have low number of species,
but in the central region, with intermediate level of disturbances we have a larger number
of species. Essentially we will have a curve that goes like this. This is known as the
intermediate disturbance hypothesis. If you have an intermediate level of disturbance
which occurs at a frequency that is neither very high nor very low, so you will have more
amount of biodiversity.

(Refer Slide Time: 26:45)

Another good example for intermediate level disturbance is that if you have an area that is
only full of trees, so, in this area there would be say n number of species. If you create
some amount of disturbance by say chopping off few trees, so, these two trees are taken
off. What happens in this case is that the ground level which was earlier not receiving any
sunlight because this area was all full of trees, now you have created some pockets in
which you have sunlight that is now reaching the ground. When that happens, then these
areas that were earlier only having a tree will now be having some grasses or maybe some
other herbs and shrubs.
When that happens, then this also leads to the creation of a more complex habitat. So,
earlier you had only one kind of habitat that was available and which was the trees. Now,
you have multiple kinds of habitats because you have trees you have areas that are barren,
you have areas that are full of grasses, you have all these different ecotones. Ecotones are
areas that are joining two different areas. You have this particular line in which you have
a confluence of a grassland and a forest. You will have some other species that can thrive
on these confluences because you might have say a bird that wants to sit on top of a tree,
so that it can have a vantage look around and whenever it sees an insect or say a small
mammal like a mouse then it can go there and it can eat that species.
If you have an area that only has forests, so, in that case your bird of prey might not be
able to see down below what is happening on the ground. Now that you have created this

opening, what happens is that, now you can have a bird that sits here and gets the view of
the whole of this area and as soon as a mouse ventures out, it is able to grab that species.
So, some amount of disturbance will lead to more number of species that are formed. So,
that is all about why there are certain regions that have more level of biodiversity and why
there are certain regions that have less number or less amount of biodiversity.
But, then we have another question, why should we care about this biodiversity after all?
I mean what do we humans derive out of this biodiversity? Because, any subject even
ecology needs to meet certain aspirations of human beings. So, if we say that yes we need
to have more amount of biodiversity the first question that somebody would ask us why
do you need biodiversity. I mean say in the case of cheetah. So, cheetah is an animal that
was found in India and in the early 1950’s it was hunted down and now you do not have
any cheetah.
Somebody might ask - this cheetah is now extinct from India, but then it did not lead to
any collapse of the system. What is wrong if we take all the tigers out or say if we take all
the leopards out? It should not also not make very much of a difference. To understand
and to counter such questions we need to understand, why do we need biodiversity after
all? One way to understand is to look at the economic value of biodiversity.
(Refer Slide Time: 30:13)

That is, what are the benefits that we are getting out of biodiversity. The total economic
value is divided into use value and non use value. Use value is something that you get

because you are using that particular resource and non use value is some benefit that you
are deriving even when you are not using that particular resource and use value is divided
into direct value, indirect value and option value and non use values divided into existence
altruistic and bequest values. We will now have a look at all of these.
(Refer Slide Time: 30:47)

Now, use value is value that is arising out of use of resource.
(Refer Slide Time: 30:53)

And, non use value is value that is arising through even-though the resource is not being

(Refer Slide Time: 30:57)

(Refer Slide Time: 31:03)

So, we had use value and use value is divided into the direct value, the indirect value and
the option value. Now, these are the three use values. So, what is direct value? Direct value
comprises of consumptive values and non consumptive values. Consumptive value is a
benefit that we derive when we are consuming a resource. Now, in this case consuming a
resource means that if I use a particular resource then it is not available to be used by
somebody else. For example, if we have biodiversity in the form of forest, so, forest will
have a number of trees; with those trees we can derive timber, fuel wood, firewood,

medicines, we can have access to fodder, we can have non-timber forest produced, we can
have water and so on.
But, these are known as consumptive values because if I take timber out of a forest then
that timber is something that I can use, but any other person cannot make use of the same
timber. So, essentially if we are using a consumptive resource, the total amount of resource
that is available to be used by others, reduces. On the other hand, we also have non
consumptive resources. Non-consumptive resources or non-consumptive values include
recreation, ecotourism, education and research, human and wildlife habitat and so on.
For instance, if I go into a forest and I enjoy the scenic beauty; if I go into a forest and I
observe a tiger and I am very happy that yes, I was able to see a tiger. So, have I reduced
that value? The answer is no, because I can see this tiger if you go into this forest later on;
you might also be able to see the same tiger. By seeing this tiger, I was able to gain a
benefit, I was able to gain some amount of excitement or joy, but by using this value in a
non-consumptive manner, I am ensuring that this tiger is still there. On the other hand, if
I went for a consumptive use of a tiger, so, if I went into a forest, killed a tiger and took it
out and made a trophy out of it. So, if you go into the forest you would not see this tiger.
That is the difference between consumptive values and the non-consumptive values.
If we are conserving biodiversity, then we are ensuring that we have all of these direct
values that are available to us. If you have a forest and there are a number of medicinal
plants that are found in the forest. So, for instance, trees like cinchona. Cinchona, the bark
of this tree was the first one to be used against malaria. It is an anti malarial drug or for
instance things like Rauwolfia serpentina; which is another medicinal plant. The forests
are till date quite unexplored. If we get into the forest, it is quite possible that we might
find some other medicinal plant, but we can only make use of these if the forest is still
surviving there. So, that is the first value, that is the direct value.

(Refer Slide Time: 34:33)

Now, indirect value includes things like watershed benefits, that is agricultural
productivity, soil conservation, groundwater recharge, regulation of stream flows. These
are indirect values, but even in this case, I am not taking out soil to be used in my own
farmlands, but because we have a forest so, it is conserving the soil. That is not just being
useful for me, but it is useful to the society at large. It is an indirect value or things such
as ecosystem services like nitrogen fixation or waste assimilation.
(Refer Slide Time: 35:23)

For instance, we went to an area near Agra at the Kakraita plant where there are drains that
are carrying out the municipal waste. Now, and here you have a river. Now, if you directly
drain this municipal waste into the river, what will happen is that the river water will get
polluted. So, other people will not be able to use the river water. It might lead to spread of
diseases or it might even lead to a situation in which the fishes that are living in the river,
the turtles that are living in the river or the birds that are dependent on the river will all die
off. So, that would also affect the economy and the employment of a number of people
such as fishermen. So, it makes a lot of sense to treat this wastewater.
Now, to treat this wastewater you could go for a treatment plant; a wastewater treatment
plant, but then what people in Kakraita did was that, before the water was left out into the
river, this wastewater was divided into a number of channels and around those channels
they planted trees. Now, what happens is that, in the case of municipal wastewater you
have large amounts of nitrogen and large amounts of other minerals that can act as
fertilizers if they are used by plants, but that can act as toxins when you put them into the
Now, just by doing this, you ensured that all of those nutrients get absorbed by the trees
and then whatever water remains is so devoid of these nutrients that if you let that into the
river, it does not produce any amount of harm. Such types of services are known as
ecosystem services and this was an example of waste assimilation. A forest can act as a
very good waste assimilator. So, in place of constructing a wastewater treatment plant that
could cost you lakhs or even crores of rupees, you can just have a small forest and divert
all of your water there and that water will get used or things such as carbon sequestration.
Now, we all know that we are in a period of global warming. So, because a large amount
of carbon dioxide has been released into the atmosphere and is currently also being
released into the atmosphere, we are observing greenhouse effect. Because of it, the earth
is warming up and we are seeing climatic consequences of that. Now, if you have a forest
that holds biodiversity, that holds different kinds of trees so, all of these trees will take up
all this carbon dioxide from the atmosphere and fix it in the form of wood.