Today we begin a new module which is distribution and abundance. This module will have
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The first one is biogeography which is the analysis of geographic distributions.
Biogeography is the field of ecology that asks the questions, why are things where they
are? And we look at this topic in more detail in the second lecture which is why are things,
where they are?
Here we will ask if there is a species that is found in a certain area. So, why is that species
only found in that area? What is constraining the range of that species? why is it not found
anywhere else? And the answer to that is some push factors and some pull factors.
So, push factors are those factors that are pushing that species away from other areas and
pull factors are those factors that are attracting the species to that particular area. So, every
species tries to remain in an area or it is best able to survive in those areas that have the
most suitable climatic conditions most suitable biotic conditions for the survival of that
particular species. So, which is why we get a certain geographic distribution for every
species and this is what is asked in the topic of biogeography.
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Biogeography is the study of the geographical distribution of life on earth and the reasons
for the patterns one observes on different continents, islands and oceans. Essentially it is
asking why are certain species found in certain areas and it also asks; it tries to document
which species are found in which areas.
It is the study of the geographical distribution of life on earth. So, that is a cataloguing of
different species in different areas and the reasons. So, not only do you catalog, but you
also ask, what is the reason behind such a particular geographical distribution and the
reasons for the patterns one observes on different continents, islands, and oceans.
Now depending on which particular areas you are concentrating on there are further
subdivisions or biogeography such as island biogeography that asks the question, how do
species come into an island? How many species will there be in any particular island?
What would that depend on? Does it depend on the structural diversity of the habitats that
is therein on the island? Does it also depend on the size of the island? And so on.
Similarly, you can have oceanic biogeography which shall ask the question, what are the
species in the oceans that are found near the continents? what are the species that found
away from the continents? what are the species that are found in the upper layers of water?
What are the species that are found in the seabed? What are the species that are found in
the column of water and so on? You can have different sub disciplines of biogeography in
the term in the form of continental biogeography, island biogeography, oceanic
biogeography and so on.
Now, when we are asking which species is found in this area the other scientific term that
comes into picture is the range; the range is the distribution of the species. The range or
distribution of a species is the geographical area within which that particular species can
be found. Now when we are asking this question, which species is found in which areas?
And how are these different areas different from each other? It makes sense to have an
understanding of different kinds of habitats that exist on this planet.
Or at least the kinds of habitats that we have in India, because India itself is a very varied
country by geographically and so, we have different kinds of habitats, we have very high
mountains, we have deserts, we have the oceans, we have different kinds of forests, we
have grasslands and so on.
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Now we will have a look at some major Indian habitats and their residents.
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Let us begin with the Alpine meadows. Now, alpine is a term that refers to the mountains.
So, this is a meadow. A meadows is the grassland. This is a grassland that is found in the
mountains and a good example is the Dachigam National Park which is there in Srinagar.
Now in the case of Dachigam National park here you can observe that you have these hills
and these hills have these meadows or the grasslands. Now a place like Srinagar is having
an extremely cold climate and that is the climate that is therein and Dachigam hill.
Climatically we can seen that these areas that have the alpine meadows they have a cool
or a cold climate. Typically they are at a great height because you have mountains, so you
have hills here, typically if you talk about the wind speeds, the wind speeds will be very
high because this is a mountainous area. In a mountainous area you have winds in the
daytime you also have winds in the night time.
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Now if you look at a hill, so, let us consider a hill and if you have the sun here, now in the
daytime what happens is, this particular area it preferentially gets heated because of the
sun raysso, this area becomes warmer. Now when this area becomes warmer the air around
it also becomes warmer and it starts rising, now when it starts rising the cold air that is
there in the valley. So, here you have the valley and the cold air that is there in the valley
it will start rising upwards to fill up the gap. This is the kind of wind pattern that we will
observe in the daytime and this is known as a valley breeze.
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Now, at night time, when do not have the sun. So, now, what happens is the valley area
because it is sort of secluded from both the sides. So, here the air remains as such whereas,
the hill areas or the top areas they are able to lose out the radiation very fast. So, they are
able to lose out the heat. Now when that happens, the air here becomes cooler. So, here
you have a cooler air and the cooler air is more denser and so, this air now starts flowing
towards the valley, now this wind is known as a hill wind.
In the case of these alpine meadows not only do they have a cold climate, but at the same
time they also have a very high wind speed. At the same time we can talk about the soil
characteristics that we have in these areas. So, typically the soils here will not be very
fertile, why because they have been in this particular state for say thousands of years.
Whenever there is a rainfall so all the minerals that are there on the top layer of the soil,
they will start dissolving in the rainwater and they will start moving down slope. So, they
will reach into the streams and then they will reach into the rivers and ultimately they will
get drained into the seas.
These areas are typically not very fertile. At the same time especially in the case of this
particular area, you have the rocks that are making these mountains and there is a
continuous process of weathering that is going on and in the process of weathering the
rocks are getting broken into smaller fragments and they are ultimately making soil in
these areas, now that soil with the rains it moves down exposing more rocks which
ultimately makes more soil and so on.
So, this is a process that goes on. Now, if we are asking the question why are these
particular species of grasses found in these areas we will have to make a correlation
between the requirements of the species and the actual climatic conditions that we have in
this area. So, it is possible that the grasses that are growing here do not require a very high
level of fertility and they are tolerant to high wind speeds and they are also tolerant to low
temperatures. These are the kinds of correlations that will make in the case of
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Alpine meadows are found in Jammu and Kashmir. These are also found in Uttarakhand
and in a number of areas where you have hills and typically you will find that the species
are more or less common between these areas.
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Another kind of habitat that we have is known as an Alpine forest, now again Alpine is a
mountainous area. So, alpine forests are those forests that are found in the mountainous
areas. Typically you will find trees that are coniferous trees and you will also find some
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Now, in the case of these conifers trees you will have a structure that permits snow to fall
down and they will have a very specific shape of the tree. So, for instance, if you have a
tree like this and if there is a very heavy snowfall, so all of this snow comes on the canopy
of these trees which will make it extremely top heavy and that would facilitate the toppling
of this particular tree.
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In the case of these areas, typically you will find that the trees have a conical structure.
Now this particular shape facilitates that if you have snowfall so this is snowfall moves
down to the ground. It is not able to accumulate very much on top of these trees and
because of which these trees are able to withstand heavy snowfall as will.
In the case of Alpine forests we will find some of these conifers trees together with some
associated species which would even be broad leaved species. When we are talking about
these meadows and these forests, there would also be very specific animal species that are
found in these areas.
For instance in the case of Uttarakhand you will also find species like the pica, now pica
is a very small mouse species that is only found on this area. Now these are the alpine
forests and then if you move southwards with reach the moist deciduous forests.
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These are again the deciduous forests that are found in Uttarakhand; now when we say
deciduous, now deciduous forest is a forest type in which the trees have this adaptation
that they shed their leaves in a particular season. Now you could have trees that shed their
leaves in the summer season. In certain areas, when trees are shedding their leaves in the
summer season the main reason is that they want to conserve water, because in the summer
season it is typically the pinch period for water and water is lost from the leaves through
the process of transpiration. If you lose out all your leaves, so the amount of water that you
will be losing out every day becomes less.
On the other hand there could be some other species that shed their leaves in the spring
season. In those cases, these trees typically store their waste materials into their leaves and
then they shed their leaves so that the waste materials are gotten rid off.
So, these are the kinds of adaptations that we will find in these areas. Typically in a moist
deciduous forest you will find a very heavy ground cover. These are the moist deciduous
forests of Uttarakhand. If you talk about the climatic conditions here the climatic
conditions will not be that extreme it is not very cold, but then it is cooled plus in these
areas you have ample amount of moisture available throughout the year and the wind
speeds are not very high plus the amount of solar insulation that you get in this area is also
not very high, because typically these areas are on a higher latitudes.
These would be the characteristics of the terrain of this area or the characteristics of this
particular region. The soil typically again is not very fertile, but all of these trees like these
are the Sal trees that are found in this area and these trees are adapted to these conditions.
Now in the case of biogeography when we ask the question, why are Sal trees found in
these particular areas? That is because a Sal requires these conditions. So, it requires ample
amount of moisture, it cannot tolerate a very heavy cold and it does not require a very
fertile soil. This is why we can say that Sal is found in this particular area.
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In certain other areas we will find dry deciduous forests. Now again, this is a deciduous
forest because it is shedding it is leaves. In the case of a dry deciduous forest you will have
typically less amount of moisture that is available to the plants and a good example of a
dry deciduous forest is a teak forest. Teak forests are found extensively in the states of
Madhya Pradesh, Chhattisgarh, Maharashtra, Gujarat.
In the case of teak forest, these forests shed their leaves right before the summer season so
that they are able to conserve moisture and they are found in these areas because they are
able to tolerate quite a heavy amount of draught in these areas.
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Now moving westwards in our country, we have the scrub forests, now a scrub forest is,..
you can typically find a scrub forest in Rajasthan or Gujarat. In the case of a scrub forest
you will have such a situation that the land is exposed in a number of areas plus you have
these small shrubs in the area. Typically these shrubs are very thorny shrubs.
Here again you have a very low amount of moisture that is available to the plants and the
plants also show adaptations to conserve this moisture. There would be a number of plants
that would have reduced their leaves into the spines or the thorns. When you have a leaf
that is converted into a thorn so, it not only gives protection to the plant, but that is also an
adaptation through which it is able to reduce the amount of water losses through
In these areas because you do not have very tall trees, these areas have such low amounts
of moisture that they are not able to support tall trees. Typically the ground flora is exposed
to a very heavy amount of solar radiation. The plants would be adapted to that as well. So
that you would find a number of species where the leaves are covered in a waxy coating
not only to reduce the amount of moisture that is being lost, but also to reflect the solar
radiation that is incident on these plants.
When we talk about the animals of these areas, these areas do not support a very high
density of animals typically because the amount of moisture is less and also you do not
have ample amount of fodder that is available to the animals and green fodder is only
available in certain seasons in plenty. Otherwise the animals have to make use of this dry
fodder or maybe some amounts of leaves that some animals can have access too.
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If you move further westward in the case of Jodhpur, you will find these sand dunes. Now
we have moved from a scrub forest to a sand dune. A sand dune is typically a very dry
area and these trees they have been imported and planted to stabilize the sand dune
otherwise the sand dunes typically do not support a very large number of trees.
The sand is very fragile; It moves from place to place and again here the amount of
moisture that is available for so supporting life is very less and there would be very great
amount of adaptations to severely constrain the amount of water that is being lost or to
reduce the requirement of water per day.
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In this case you will find very specialized species such as this spiny tailed lizard that is
only found in this area. Now again if you ask this question, why do you have this spiny
tailed lizard in this area? The answer would be because this particular species is adapted
to this area plus the conditions in this area are so harsh that the predators of this particular
species are not able to live in this area.
Otherwise, if you have a very large number of predators in this area they would eat up the
all the spiny tailed lizards that are found in this area. Again you have this species because
there are certain pull factors for it for this particular region. It is well adapted plus also it
has a very less number of predators that are there in this area and also the other areas have
push factors. So, other areas have conditions that this lizard is not comfortable with or the
other areas has predators that would eat up this lizard. Typically we find the distribution
on this lizard more in this area.
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We also find Estuaries. Estuaries are very specialized habitats where you have a river that
is coming and meeting the seas. Here you have a confluence of fresh water and the saline
water. Now here again the species that would be found would be those that are tolerant to
both these levels of salinity. So, they can tolerate fresh water and also they can tolerate the
salty water plus there would be a number of species that will make use of all these 3 kinds
of habitats that will be found in an estuary. When you have estuary, you have this river
that is coming and it is meeting the ocean.
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In the case of the ocean, you have a very high salinity whereas, in the case of the river you
have a very low salinity. Typically you will find that these intermediate areas have an
intermediate level of salinity and here you will have a number of species that would spend
part of their time in the high salinity areas, part of their time in the intermediate salinity
areas and part of their time in the low salinity areas.
So, this provides a very specialized habitat and the species that are found in these areas are
found here because these are the only few areas where you can have all these 3 kinds of
salinities that are available in the same area. This is why you will have these species, these
specialized species that will be found in these areas.
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Near the estuaries of Gujarat you also have the Rann of Kutch. Rann of Kutch is home to
the wild asses that are found in our country. So, you have the Indian wild ass sanctuary.
Here again, if you talk about the habitat conditions, you will find that it is a very flat land.
There are hardly any hills around.
It is a very flat land and if you go, there you can see kilometers and kilometers of very flat
land. Now in the case of the rainy season, this area gets somewhat inundated and in the
case of the dry season, it will just act like a very flat dry piece of land. Here again, the
habitat is so specialized that you have only few months of the year when these areas are
inundated, during the other months of the year it is completely dry. So, the species that
will be found in this area will also be extremely specialized.
And especially, in the dry season you do not have access to water in a number of places
and so the species will be very constrained in their movement. Now the wild asses that I
found in this area are adapted to these dry conditions. They do not require a very large
amount of water plus they are able to feed on the native vegetation that is found in this
area. So, this area cannot support other predators and this area can support the wild asses
because of which you have the wild asses that are living in this area.
Typically the water sources in this area are extremely saline, because this area when it gets
inundated, it also receives some amount of salty water from the seas and also the
groundwater is extremely salty. So that also makes this habitat extremely specialized.
If you take these wild asses out and place them into some other forest. Say, if you keep
these in the forests of Madhya Pradesh. So, the tigers will come and hunt these wild asses.
But then, because tigers are not able to survive in this area, so, the wild asses are able to
sustain themselves in this particular area.
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In certain regions of the Rann of Kutch, we will also find these water bodies that will
support a very dense population of flamingos. Flamingos again are very specialized birds.
So they are migratory birds they spend some time in India and sometime they move out
and when they are here in India you can see that all of these are pinkish in color because
of their very specialized diets.
In their diets they have certain plants that are rich in carotenoids and also certain animals
that are taking these carotenoids and these carotenoids, these are compounds that are
getting accumulated into their bodies. If you talk about why this bird is found in this area,
you again have to make a correlation between the requirements of the bird. This bird
requires water. It is feeding on the organisms that are found in water.
It will only be found in areas that have water plus this has other requirements. Typically
these flamingos come here and they also breed in these areas. If this bird is coming here
to breed, it requires an area that does not have predators, it requires an area where it can
have ample source of food which it can feed to it is young ones. Because this the
surrounding area is extremely dry and you do not have a number of predators. So, you can
have a sustained population of these birds in this area.
(Refer Slide Time: 23:37)
Another very specialized habitat as that of the lagoons. Here we are seeing the Chilika
lagoon. Similar to an estuary, a lagoon is a large water body.
(Refer Slide Time: 23:51)
In the case of your Chilika lagoon, you have this water body. Here you have the oceans
and this particular case you have the Bay of Bengal. The lagoon is drained by a number of
rivers. Now these rivers are bringing in fresh water or a low salinity water. In the case of
the Bay of Bengal, you have high salinity; so you have saline water here. Now these
lagoons are separated from the sea with these sand bars.
When you have these sand bars here you have fresh water coming in from this area and
you have a saline water that is coming in from this area. If you went ahead and try to
measure the amount of salinity that you will have in the lagoon, you will typically find that
these areas that are near to the sandbars have a very high level of salinity. These areas that
are near the mouths of the rivers have low level of salinity and the other areas have an
intermediate level of salinity.
Here again, if you ask, what are the species that are found in these areas? So, there is a
dolphin by the name of the Irrawaddy dolphin that is found in this area and this dolphin
makes an extensive use of the organisms that are found in this area. Typically, the plants
that would grow here near the banks, which have a very low salinity, will be very different
from the plants that would be growing in these salinity areas. This makes a habitat that is
structurally very diverse plus you will have, especially in the case of this Chilika lake, the
depth of the water is very less so, the depth of the water is typically like 2 or 3 meters.
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