Today we move forward with our discussion on biogeography and look in detail at ‘why
things are where they are?’
(Refer Slide Time: 00:21)
In the last lecture we began by looking at the definition of biogeography. It is a discipline
of ecology that asks the question, what species are found where? In different areas of the
earth what different species are seen in those areas. Then it ask the second question why
are those species found only in those areas? So, if we say that lions are found in Gujarat,
why are lions found in Gujarat? What is there in Gujarat that lions are finding it a
hospitable place? Then it ask the third question, what are the other factors that are not
permitting lions to live somewhere else?
For instance, why do not we have lions in the Sundarbans? Or why do not we have lions
in Jammu and Kashmir? Is there something that is related to the characteristics of the lion
or the characteristics of the area that is different physical or chemical constituents that are
there in that area that is not suitable for a lion to live there. And, if you go for a very
simplistic analysis we can ask this question, why does a fish live in water? Why does not
a fish live in the land? Or why does not a fish fly? So, what is there in water that is making
it a suitable habitat for the fish? What is there on the land that makes it an unsuitable habitat
for the fish? These are the three questions that biogeography asks. 1: What species are
found there? 2: Why are they found there? and 3: Why are they not found somewhere else?
(Refer Slide Time: 01:59)
When you ask this question, ‘Why things where they are?’, we looked at this particular
diagram, now in this particular diagram we said that here we are putting up the altitude of
the location and here we are putting up the latitude of the location. Now we say that these
kinds of habitats a subarctic forest or a cool temperate forest these are typically found at
higher latitudes, but then if we have a mountain that is very tall and it is very near to the
equator then we also see the subarctic or the cool temperate forest near the equator.
So, essentially it is not related to where exactly these kinds of forests are found, it has got
more to do with what kinds of characteristics are available for these forests. So, if you can
make so like, typically you will not find a subarctic forest near the equator, but then if you
make the conditions such that you have a low temperature or maybe the kinds of wind
speeds that we find in the subarctic forest, if you make these sorts of conditions available
near the equator you will start finding a subarctic forest, near the equator as well. And this
is a fact that we regularly make use of when we are doing the conservation of different
species using ex-situ conservation. So, for instance we know that polar bears live in Arctic
and Subarctic areas. They live in very cold areas. But then we can also have a polar bear
in New Delhi. How? If we can create a condition, if we can create a chamber that is cold
enough for a polar bear, if we can give it a big sheet of ice to live on, then the polar bear
will not know whether it is living in the Arctic or whether it is living in New Delhi.
This is a fact that we regularly make use of. If you want to have a botanical garden
somewhere and you want to have say species from Africa. You can just bring those seeds
from Africa, you can give or you can create those sorts of conditions in your botanical
garden that those species can grow there. Typically in the case of many botanical gardens
we go in for areas that have an altitudinal variation.
(Refer Slide Time: 04:25)
If you have say a forest that is there near a hill, so now, the climate here will be cooler, the
climate here will be more moderate, in the climate here will be warmer. Now, if you select
such an area to construct say a botanical garden. In the cooler areas you can bring the seeds
of those species that are found in cooler areas, in the warmer areas you can bring the seeds
of species that have found in the warmer areas. And then in this small area that you have
designated as a botanical garden you can have multiple different kinds of species.
Here again we have to ask the same question, what are the conditions that are making those
species live in those areas? So, that we can copy paste those conditions into our areas, but
then this also brings us a second question. Let us say that we have a hill in India and we
have a hill in say South America and both of these hills have similar soil types and they
have say similar climates, but then again we do not find all the species of South America
in our hills in India.
What is causing certain species to be found there in South America, but not in India? When
we are talking about say the cat families, why is puma found in south America, but it is
not found in India even though we might be having several areas that have very similar
kinds of climates and vegetation? This is also another question that we are going to look
forward when we are studying biogeography.
(Refer Slide Time: 06:01)
To summarize biogeography, if we look at any distribution, if say the distribution of snow
leopard, the first question would be where is snow leopard found? The second question is
why is it found in these areas? The third question is why is it not found in these areas? And
then the fourth question is if suppose we look at another location at the same latitude and
and having very similar conditions so if snow leopard is found here why is it not found
here? Now this place might also be having a very similar climate. What are the factors that
are limiting the distribution of this species, only to the areas where it is currently found
and you can do this for n number of species.
(Refer Slide Time: 06:47)
We saw that we can look at island biogeography, we can look at continental biogeography,
we can even look at oceanic biogeography. So, this was another case that we looked at in
the last lecture, that if we have coral reefs that are found in these areas, why are they only
found in these areas? If you look at the distribution of coral reefs in greater detail you
would find that there are a number of conditions that make their survival possible in these
areas, they cannot live in very cold waters, they cannot live in very warm waters.
This is typically the area where there will be found where the water is having a good
enough temperature for them to be found. Then they have some specific nutritional
requirements and also some other requirements such as the condition that they cannot live
in areas that are extremely polluted or in areas that have a high sediment load. For instance,
if we look at this area which is near Sundarbans; here we have the Ganges river and the
Brahmaputra river. Both of these are falling into the Bay of Bengal in this area, but then
because they are carrying a huge amount of sediment load so we will not find a coral reef
in this area. So, we are looking at those factors that are causing coral reefs to be found here
and are causing the coral reefs not to be found in this area. This is all what it biogeography
is all about and the observations have come from a very long period of time.
(Refer Slide Time: 08:23)
Even if we look at Darwin’s journals; Charles Darwin as we had seen in one of the earlier
lectures. Charles Darwin happened to be a naturalist who went around the world in a ship
called the Beagle and when Charles Darwin was visiting the Falkland Islands so there he
made this journal entry. Two kinds of geese frequent the Falkland’s, the upland species
Anas Megellanica is common in pairs and in the small flock throughout the island, they
do not migrate, but build on the small outlying islets.
The rock goose, so called from living exclusively on the sea beach, Anas Antarctica, is
common both here and on the west coast of America as far north as Chile. In the deep and
retired channels of Tierra del Fuego, the sno-white gander invariably accompanied by his
darker consort and standing close by each other on the same distant rocky point is a
common feature in the landscape.
This naturalist, when he was going around South America and when he visited the Falkland
islands he made this observation that there are two species of geese. Geese, as we know
are birds and now even in the case of these birds we are think that there are two different
species and these two species are found in different areas. So, the upland species is found
throughout the island of Falklands, they do not migrate, but they build on small outlying
There is another goose that is the rock goose which lives exclusively on the sea beaches.
It does not go it does not venture inside and it is common both here and on the west coast
of America as far north as Chile. He is making the observation that there are two species
of geese, now we can say that geese when we refer to geese as a species of bird or as a
kind of bird.
Both geese would be having similar requirements; they would be feeding on similar kinds
of food, they would be having similar kinds of wings. They would be having similar size
and shape of the body, there are only some minor differences because of which we are
saying that these are two different species of geese.
They are not able to interbreed amongst each other, but then they requirements are more
or less the similar. But, even in those species that are very similar to each other we are
observing that they have very specialized requirements or very specialized area that they
are inhabiting. Then the question arises; why are they only in those areas and not in the
(Refer Slide Time: 11:11)
This brings us to the push and pull factors. When we are asking the question, ‘Why are
things where they are?’, we need to talk about the pull factors in the push factors. The pull
factors are conditions that attract the organisms to an area. For example, food availability,
amiable climate. If you have plentiful amount of food there is a good climate you want to
live in that area.
The other factors go by the name of push factors, can drive organisms away from an area.
Example is food scarcity or inhospitable climate. So, these are in general the push and the
(Refer Slide Time: 11:51)
If you look at this image; this is again an image from Uttarakhand and here we are
observing, that we have these hills and on these hills there is vegetation on these slopes
and there is no vegetation on this particular slope. The question is, why do we have
vegetation on the slope and why do not we have vegetation on the other slope? What can
be the push factor, what can be the pull factor? Here again we can say that because this
area is getting more amount of sunlight so probably this area is drier or probably the slope
here is greater than the slope here.
If you have of a mountain phase that has a greater slope, so in that case soil will not be
able to accumulate in that area. So, the depth of soil will be less, now if there is a less depth
of soil so the roots cannot penetrate deep, they cannot hold on to that area and at the same
time they will not be able to get sufficient amount of nutrients.
That could be some push factors that are keeping plants away from these slopes. Now what
are the pull factors that are bringing plants here? Probably because this area is a depression
area so it might be retaining much more amount of moisture, it is also having a slope in a
way that it is getting less amount of sunlight. Probably that is also another reason why the
losses of water is less.
And also because the slope here is less so probably more amount of soil is able to
accumulate here. Here we can see that even in the case of a single hill there can be a vast
variety of differences in the microhabitats of two different areas on the same hill that might
be responsible for a plant to live in certain area and to not live in certain other areas. But
then push and pull factors are not the only things that govern where a species is found and
why is it found there?
For instance, we were talking about the availability of puma in the hills of South America,
but not in India. Now it is also possible that if you bring puma from South America and
you allow it to settle in India, then probably it would be able to survive here, probably it
would out compete some other species and make a niche for itself.
It will occupy some of the niches that are available or say create a niche for itself. But then
you do not have puma here in India, not because India is providing some sorts of push
factors, but because puma has not been brought to this area because puma is very far away
from India, there are oceans that are separating the continents of Asia and the South
America. So, probably that is one reason why puma is not found in India.
(Refer Slide Time: 14:47)
Apart from the push and the pull factors the next factors that we need to look at are the
dispersal factors, what are the ways in which the organisms are able to move from one area
to another area. Dispersal is the movement of individuals away from their place of birth or
hatching or seed production into a new habitat or area to survive and to reproduce. So, this
is the movement of individuals from one place to another place, now why would there be
a movement of organisms?
(Refer Slide Time: 15:17)
Let us consider this particular tree. Here we have a tree and this tree bears fruits and inside
the fruit so you have the seeds. Let us suppose that this fruit fell down here and then later
on it gave rise to a seedling, so here we now have a seedling that is coming up. This is a
seedling of the same species as that of the tree, but then in the case of the tree we would
have roots down here and because this tree is very old so these root should also be very
Maybe they are able to tap the groundwater; so they are able to tap the groundwater and
they are also able to tap various minerals throughout the soil. So, because they are already
have a very extensive network of roots, so they are better able to get water, they are better
able to get the nutrients. On the other hand the roots of this plant are very small and so
they are neither able to reach to the groundwater table nor are they able to reach to the
nutrients so extensively as the larger plant.
In this case, what is happening is a case of intra specific competition. There is competition
for water, there is competition for nutrients, there is competition for sunlight and so many
things. Now in the case of this intra specific competition we can say that in a number of
situations, because this tree is much more older, it has got a much better network so it will
be able to out compete its own sapling and the sapling would die out.
Now, if the sapling dies out, then evolutionarily we would say that the plant is not having
that good a fitness because fitness not only means that you produce more number of
offsprings. But, also that the offsprings should be able to live and grow to their maturity
and produce their own offsprings.
In this case, if the plant adopted a strategy that all of its seeds just fell down below it so all
of or most of those seedlings would die off or else they would be competing with the
mother plant itself. In this case, this is not the best of the strategies. Now let us think about
another strategy; another strategy could be a strategy of dispersion.
(Refer Slide Time: 18:05)
Probably the fruits are such that a bird comes here; there is a bird that is coming here and
this bird eats up the fruit and then this bird goes to some other area and through its
intestines the seed of the fruit moves and then it gets dropped somewhere else; so probably
as seed fell down here. Now in this case this again would give rise to a plant a sapling, but
then in this case because this is far away from the mother tree so there is less amount of
competition that this seedling will have to suffer from.
Probably, now it is not always possible that your seedlings will always have less amount
of competition because it is also possible that your bird went and sat on some other tree
and then, put the seeds there with its droppings. But, then it is also possible by chance that
your seeds fall into an area where there is less amount of competition where there is a
canopy opening for instance.
So, if there is a an opening in the canopy so some amount of sunlight is able to reach down
and reach to this particular sapling or probably if there is this canopy opening, so there are
less number of trees around. So, its roots are also not suffering from that amount of
competition from the neighbors and in that case this plant would be able to survive.
And when it survives when it grows up then we would say that the fitness of the mother
tree was greater than that of another tree, that was only having this is strategy that all the
seeds come down; because when all the seeds fall down then there is a very big chance
that all of them are going to die. Whereas, if there are some seeds at least that fall to some
other area or are able to move from their area then there is a better chance that they are
going to survive, which makes dispersal a very important component of ecology.
You will observe that most of the species would want to disperse, so most of the offsprings
would want to disperse from that area consider tigers. Now if there is a tigress that has
given birth to say three cubs, now when these cubs are say around three or four years of
age, in that case the male cubs will start moving away, they will go away to some other
area and they will establish their own territory. Because in that process, 1: they will not be
posing a competition to their own mother, 2: the mother will not be posing a competition
to their own kids and 3: the chances of inbreeding become less.
Consider a plant such as the coconut. Why does a coconut have such a big shell? The
coconut has this big shell so that when it falls to water and there is ample amount of air
that is trapped inside so this whole fruit and the seeds are able to move away. So, every
plant or every organism wants its individuals or its offsprings to move away; to move at
least slightly away so that there is less amount of competition and less amount of
inbreeding and the process by which these offsprings are moving away is known as
So, it is the movement of individuals away from their place of birth or hatching or seed
production into a new habitat or area to survive and reproduce, and this is one strategy that
increases the fitness of the species. Now there could be some other factors apart from
dispersal and apart from the push and pull factors which might be responsible for why
certain species are found where they are found. And those are the anthropogenic factors or
the man made factors. Manmade factors include things like clearing of forests or pollution.
A good example is this clearing of forest.
(Refer Slide Time: 22:03)
Here, if we ask the question, why do we have trees here, but we do not see a tree here?
This area is no different from this area, so this area also has all the pull factors that this
area has; this area has none of the push factors that this area has, and because these areas
are so well connected, so it is very much likely that the seeds from here are able to disperse
to this area, but still we are seeing that this area is not supporting these trees. Now, here
comes the impact of humans, so this is a man made factor, this is a man made clearing of
the forest for their own purposes.
Today if we are talking about biogeography we not only have to look at the natural factors,
but also the man made factors. If we ask the question that if we are considering a river, say
if he considered the river Yamuna, so why are there certain patches in the Yamuna river
that do not have many species of fishes? The answer is because of the man made factor
because that particular stretch of Yamuna is so polluted that species cannot; that fishes
cannot live in that area. Or, if you ask the question why do we not see forest in near
industrial areas or especially those industrial areas that give out quite a lot amount of sulfur
For instance, if there is in ore processing industry that is releasing a plentiful amount of
sulfur dioxide, why do not we see plants nearby it? Now that again is a man made factor;
because of acid rain the plants are dying out. So, today when we are talking about
biogeography we not only need to take into consideration the natural factors, the push and
the pull factors, the dispersal factors, but also the man made factors that might be
responsible for a species being found in an area or not being found in an area.
For instance, if you ask the question why are lions not found in India or say why are lions
not found in Madhya Pradesh or why are cheetahs not found in India? The answer is
manmade factor because the lions of Madhya Pradesh or the cheetahs of India were hunted
down long back. But then again there was a human intervention that led to the wiping up
of these particular spices in those particular areas.
(Refer Slide Time: 24:35)
Now, another factor that might play a role in biogeography is known as habitat selection.
Habitat selection refers to a hierarchical process of behavioral responses that may result in
disproportionate use of habitats to influence survival and fitness of individuals. What we
are asking here is, suppose you have two areas and both of these areas have equal amount
of push and pull factors for a particular species.
(Refer Slide Time: 25:09)
For instance, you have this forest and in this forest you have two trees. This is the first
tree; and then there is a tree of another species. Both of these trees look similar. There is
only some slight difference in the chemical constituents that are found in the leaves. But
then, if you have a bird that makes its nest on this tree and there is a bird that makes a nest
on this tree.
And if you look at the survival of the offsprings, you figure out that probably both of these
nests have statistically equal probability that the chicks are going to survive because this
tree, the first tree is also providing a place to the bird that it is able to nest at a higher place.
So, that it is away from a number of predators and then this is also in an area where you
have plentiful food sources and this is also in an area where the bird has access to water
You have a nearby pool of water and the same factors are also available in the second tree.
In this tree as well, because these both of these trees are so close by, here also if the bird
makes a nest, it will be at an elevated location so predators will not be able to find it, the
bird can access all the food that is nearby and the water that is nearby.
In this situation we might see that there is hardly any push or pull factor that is different
between both of these trees. But then it is also possible that because of some behavioral
selection that the bird prefers this tree and does not prefer this tree. Those behavioral
selections may come because of some amount of learning or maybe because of the
exposure of the bird. Probably, if this bird was born in this nest, so it knows that these
particular leaves are good for making a nest and so it makes a nest here.
Whereas, if you had shifted this bird here, so it would have known these leaves and it
would have known that these leaves are also good for making the nests and in that case it
would have preferred this tree, but because that did not happen then we can say that the
bird is choosing this nest. So, these kinds of factors in which the bird is not going after a
push or a pull factor, but then there is a behavioral choice that the bird is making is known
as habitats selection.
It is a hierarchical process of behavioral responses, here we are talking about the
psychology of the bird or the behavioral responses of the bird to different stimuli, that may
result in the disproportionate use of habitats to influence survival and fitness of individuals.
So, there is a disproportionate use of a particular species of tree and not other species of
(Refer Slide Time: 28:13)
And this can be because of both innate responses that is the responses that come from birth
and they could also involved the learnt components, that the bird is learning after it is born.
There was this experiment that was done in which a species of bird which is known as
chipping sparrow it was studied and then the scientist wanted to see how much time it
spends on two different species of trees.
There are these two species of trees pine and oak. If you catch these birds out from the
wild, so they have lived all their life in the wild and you catch these birds and then you
reared them in an aviary.
(Refer Slide Time: 29:05)
In this aviary you have say a large size open air aviary and here so this areas completely
covered with say your nets and in an inside you have these two kinds of trees, one is a pine
tree and one is an oak tree and then you catch these birds from the wild and you have
released these birds into these aviaries. How much time do they spend on a pine tree, how
much time do they spend on the oak tree was the question. If you catch these birds from
the wild you find that these birds spent as much as 71 percent of their time on pine and
only 29 percent of their time on the oak trees. So essentially the birds preferred the pine
If you rear these birds in the laboratory, so essentially you catch hold of the eggs or
probably you catch hold off the chicks before their eyes are open. You bring them into the
laboratory and they are not exposed to anything else. They are just kept in the laboratory
and once they have been reared like this then you leave them out into the same aviary. The
first case was, so let us say that, you have wild birds and they prefer pine. That was the
first experiment and the inference was that the birds preferred pine.
Now there was the second experiment. In the second experiment the scientist just caught
hold of the birds before their eyes were open or they caught hold of some eggs before the
birds had been hatched. They allowed these eggs to hatch in the laboratory or they allowed
these chicks to open their eyes in the laboratory and then these chicks when they became
slightly older, then they were released in the same aviaries.
And here what we are observing is that if there is no foliage exposure to these laboratory
treated birds, then the percentage of time they spent in pine was 67 percent which is very
close to 71 percent and the percentage of time that they spend on the oak tree was only 33
percent which is very close to what we see in the wild behavior. In this case, what we can
conclude is that if the birds have not learnt anything new then it is probably in their genes,
it is probably coming from the birth that they should prefer living in the pine trees and not
prefer living on the oak trees or not prefer spending more time on the oak trees.
This was the second experiment and so you have lab reared birds and they also prefer pine,
but then we have a twist we have a third experiment. Now in this third experiment, the
eggs were caught or the birds were caught before their eyes were opened and then they
were reared in the laboratory, but then the scientist also put oak leaves nearby. When this
bird opens its eyes it sees that there are oak leaves nearby. You have lab reared birds with
exposure to oak. And what happens in that case? Here you have laboratory reared birds
and oak foliage expose your only they were not exposed to pine, they were only exposed
After these birds have become older, then you release them in the same aviary. What
happens to the behavior now? Now, they spend 46 percent of their time in pine and 54
percent of their time in oak. Now, if we say that a bird that is spending more than 50
percent of its time on a particular species, we say that it is preferring that species. So, in
that case we would say that in these two situations the birds are preferring pine, but now
they are preferring oak. So, here we say that in the 3rd experiment they prefer oak.
This preference or this habitat selection by this way we can say that it has got both in-born
components, the innate components. Because, if the bird has not been exposed to anything
new then it would prefer the pine tree; we have these innate components. But also there
are some learnt components; if the bird has been exposed to the oak leaves it learns that
oak is a preferred species. So, this is about habitat selection it has both innate components
and the learnt components. So, we looked at a number of variables that can govern where
by a species is found in a certain area or not found in a certain area.
There can be push factors, there can be pull factors, there can be anthropogenic factors,
there could be factors of dispersal, the species is not found somewhere probably because
it has not raised it so far and there can also be factors of habitat selection. How do we
decide which of these factors are at play at for any particular instance? For any particular
species which of these factors is playing a role? We can dissect out the roles of all these
different factors by doing what are known as transplant experiments.
(Refer Slide Time: 34:55)
What happens in a transplant experiment? Suppose this green area is showing you the area
where a particular species is found. Let us say that there is a particular plant species that
is found in the green area and is not found outside. What we do in the transplant
experiments is that we take certain plants from this area and we transplant them to the
There are three experiments here.
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