We carry on with our discussions on population in ecology, and today we look at some
case studies of how the population studies are actually done in the field.
(Refer Slide Time: 00:25)
So, if you have any population study it begins with defining the problem, because if you
have any population there are n number of things that you can learn about the population.
Your problem could be say why is this population growing, or not growing, or if you have
two different populations what are the factors that are leading to different growth rates in
different populations. Or for instance if you have a population, why does it grow in a
particular season, why does it not grow in other seasons? There can be a number of
problems. The first thing that we need to do is to define the problem. What are we trying
to study in this case?
And secondly, to define the population because if you have a very large population then
maybe you could take a small subset or maybe you could look at some sub populations or
meta populations. You have to be very clear about what is the areal extent, what are the
animals that you are, or organisms that you are considering as part of your population?
And what are those organisms that are not a part of your population. Now, once you have
done that there can be two kinds of studies: one is known as the problems of dynamics.
Now, problems of dynamics ask the question how is a population changing with time.
For instance if we have a population of whales in an ocean and these whales are
reproducing, but at the same time there are poachers who are killing these whales for meat
and probably the population is reducing with time. If you are looking at anything that is
changing with time such as the population size in the case of these whales which is
reducing with time, we will call it a population of dynamics. Because it is dynamically
changing, it is changing with time. The second kinds of studies are known as problems of
In the case of statics you are looking at a static population, there is no change happening
with time. So, it asks the question what determines the equilibrium conditions and the
average values. For instance if you see that in your forest you have say 40 tigers, and this
year you have 40 tigers, 5 years ago you also had 40 tigers, even 10 years back you had 40
tigers. So, there is nothing changing in this population. So, we are having some births, we
are having some deaths, but more or less everything is getting cancelled out and the
population is remaining static at 40 tigers.
Now the problem of statics in this case could be, why is this population stuck at 40, why
does it not become 41, why does it not become 39? Or, you could ask the question what is
determining the carrying capacity of this habitat, why is this habitat only able to support
40 tigers why not more? Or, you could ask the question, what are the interactions that these
organisms have, when they have this fixed size of 40 tigers, what are their home range
sizes, how much amount of territory are they defending?
These kinds of questions will be known as the problems of statics. We have problems of
dynamics in which things are changing with time. And we have problems of statics in
which things are constant with time and we are asking the question what is determining
the equilibrium conditions and the average values.
(Refer Slide Time: 03:47)
Now, if you have a problem of dynamics. So, there is something that is changing with
time. Now to study such a change you can follow these 3 steps; the first one is; you will
ask the question - Does the change occur in a particular time of the year? So, for instance
if we are talking about the whales, that are there in the ocean we can ask the question when
do these animals breed? When do they have the young ones, and when is the time, when
they are getting poached? So, are these things having some amount of temporal
distribution? So, you have births only in a fixed season probably you have poaching only
in a fixed season.
This is the first question that you will ask. The second question is does the change occur
in a particular stage in the lifecycle of the organism? For instance when we are talking
about poaching, are people poaching mostly the adult organisms or are they poaching the
calves, is there any change between; are they poaching the male organisms or are the
female are they poaching the female organisms. Is there any particular thing that is related
to the lifecycle that is causing the change and the third one is what are the agents that
operate at these times or at these stages?
Now let us take another example, let us talk about some insect population. So, we are going
to look at the locust problem in detail in this lecture. In the case of locust, these are
organisms, these are small insects and these insects are very harmful for the crops. So, they
are very prominent agricultural pests. So, these organisms they will come in huge sized
swarms. So, there will be a swam that can have, as many as say millions of creatures.
There are millions of insects that are coming together. If you combine their weights
together, it can be as high as 50000 tons of weight. Now, each of these insect it eats in a
day, leaves that are roughly equal to its own weight. So, if you have a swarm that has a
weight of 50000 tons.
So, it requires 50000 tons of vegetation every day just to sustain itself. Now if you have
major pests, such as a locust you would want to know; when are the times when you are
getting such a huge swarm? Because you do not get these swarms every year, if you are
having these swarm every year then probably most of the ecosystems would have been
decimated by these organisms. So, you will have a swarm say once in every 15 years or
once in every 20 years. If that happens, you will ask the question, what is suddenly leading
to an increase in the size of this population.
Because the population was roughly constant, because of which we were not seeing any
swarms for past 15 years and this year we are seeing a big swarm. So, there is something
that is changed with time. Here you will ask these questions; Does this change occur in a
particular time of the year? If you are looking at a swarm of these insects, are they coming
in a particular time of the year? So, that would give you an indication of what are the
reproductive periods for this particular insect. The second is the changes that we are seeing.
So, we did not see a change in the last 15 years, and this year we are seeing a change.
Probably that had to do something with the rainfall that happened, or probably a draught
that happened. So, it could be related to some climatic conditions, or it could be related to
say an external condition such as a predator. Probably it is possible, that the predators of
locust they have gone down. Now if the predators of locusts have gone down in numbers,
then it is also possible that the locusts will increase in their population.
So, the population will blast. Now in such a scenario we can ask this question that, we are
seeing this locust in this particular season, but what was the stage that the predator was
preferentially feeding upon? Was it feeding upon the adult insects, was it feeding upon the
nymph stages, or was it feeding upon the eggs of this insect. If you asked that question that
is the second question, the change that are occurring; Is that related to some particular
stage in the lifecycle of the organism, or for instance we are saying that these and these
organisms are coming with the onset of rains. So, in that case we can say that yes; then it
should be related to the adult organisms, because they are probably laying more number
of eggs in this particular season. So, one is the time of the year, second is the stage of the
animal, and the third one is what are the agents that are operating at these times or at these
stages, and there can be a number of such agents.
(Refer Slide Time: 08:37)
You can have extrinsic agents, or you can have intrinsic agents. Extrinsic agents are those
agents that are acting outside of the organism. So, things such as weather probably it was
very hot, probably it was very cold, very dry, very wet. So, weather can be an extrinsic
agent. Probably it rained much better this year so, that is why we are seeing more number
That could be an extrinsic reason, or you can talk about predators; probably the predator
numbers went down because of which more number of locusts are able to survive and
because of which we are seeing more number of locust. Another extrinsic agent could be
parasites or diseases. Probably for the past 15 years; there were some parasites, or some
diseases, that were infecting these insects and probably this year those parasites died off.
So, that is very similar to the effect of predation, or it could be related to the quantity and
quality of the food that is available, probably the locust had a very good amount of food
because of the rains.
So, they had ample amount of fresh grasses to feed upon, and because they were having a
plentiful amount of food and this food was also of a good quality. They were able to devote
much more amount of resources into reproduction. This could also be an extrinsic agent
or another one could be the shelter. For instance till now your locust eggs were being
predated upon by say dragonflies, and this year after the eggs were laid there was some
amount of leaf fall and all the eggs that were laid they were covered with leaves. So, they
got a shelter and the dragonflies were not able to find the eggs. So, this could also be an
There could be some intrinsic reasons. Intrinsic reasons are things that have physiological
or behavioral. When we talk about physiological reason, probably there was something in
the shape of hormones that changed in the bodies of these organisms because of which
they were able to lay much more number of eggs, or there can be a behavioral change in
which case we can say that something changed because of which these organisms, they
just came together. And they were together, they did not have to spend quite a lot of time
in finding out a mate. In that case, there was much more amount of mating and because of
which we had a much larger number of eggs. So, this could also be another reason.
We can say that there are a number of extrinsic agents and intrinsic agents that are acting
on every population at all times. Now, the reason for the change in the population, or the
reason for your dynamic problems, could be one of these, it could be more of these, or it
could be all of these, or probably something even otherwise.
So, you will have to look at each and every of these changes and maybe have an idea of
which all changes are applying on your particular population. And that is about the
(Refer Slide Time: 11:57)
What about the static problems? Now if you population size is say, static. You can ask the
question that the population size is not changing. So, dynamic factors will not have to be
considered. And what are the habitat variables that are responsible for the size of the
population? So, this is the question that you can ask. How do you solve this question? You
can experimentally manipulate the habitat variables to look for the responsible factors.
(Refer Slide Time: 12:35)
For instance; you have say a part in the seas that is having a very high growth of algae. In
this case; you have this whole area that is covered with the algae. So, you have a 100
percentage cover. And we have this 100 percent cover, say for the past 5 years. So, nothing
is changing. Now you can ask the question, why is nothing changing in this area? Probably
that is because you have ample amount of nutrients. So, if you try to reduce the amount of
nutrients that you have in this area. So, probably you will cordon off this area and the salty
water can be filtered, and the nutrients, taken away. And then you will see that the
population is declining. So, probably it was constant because it was having ample amount
of nutrients or probably it was constant because it was not having any predators here. So,
in that case you can try to bring in some predators. For instance you can bring in a sea
urchin into this area, and, why were there no predators in this area?
Because probably there was a very huge amount of wave action so, in that case you can
just create a containment in this area and then you can leave the sea urchins. So, in that
case, you will come to know that yes you are having 100 % cover in this area. Because
the wave action is not permitting you the sea urchins to come into this area so, anything
can be manipulated and we do such kinds of habitat manipulations in small scales if we
have to look at a problem of statics. So, if there is a population that is maintained at a very
(Refer Slide Time: 14:27)
So, let us say, you have a forest; in this forest you have say some elephants and the
population size of the elephants is not increasing. So, let us say you have these 15
elephants. And the population size has remained constant at 15 for the past number of
years. Now, you can ask this question, why are these elephants not giving birth? or
probably, the question, why are these animals when they are giving birth? Why are the
young ones not able to grow up?
So, whenever we are talking about a static population, we have seen earlier that if you talk
about the population in the n
th+1 generation, that will be equal to the population in the nth
generation plus, the number of organisms that were born, minus the number of animals
that died, plus the number of animals that immigrated into the area, minus the number of
animals that emigrated out of this area.
So, population at the n + 1 generation is the population at the n
th generation, plus number
of births minus number of deaths, plus immigration minus emigration. Now if we are
saying that P n + 1 is equal to P n, because nothing is changing with time. So, we can say
that P n + 1 is equal to P n. In that case P n and P n get canceled away and you get this
equation that the number of births, plus the number of immigrations is equal to the number
of deaths plus the number of emigrations.
Now in the case of a closed system, you can even have a situation where there is no
emigration, no immigration and so, you can have a very simple scenario that the births are
equal to the number of deaths. Now if the population is constant, you can ask this question,
why is the birth equal to the number of deaths? What is there in this population that is
keeping it at a low level. Probably there are some predators, probably there are some
diseases, probably there are some parasites, probably the habitat is not good enough. So,
the animals are not getting nutritious feed, and in that case you can tinker each and every
of these variables. If you think that the cause is a disease that is there in the animals, you
can go and check if they have these diseases. You can take say blood samples, you can
take faecal samples, and you can look for what all pathogens are present in them. You look
for parasites that are there, now every organism in the wild will have some diseases, it will
have some parasites. So, probably you saw that there were 3 parasites that were there in
most of the elephants.
So, you have your parasite 1 parasite 2, and parasite 3. Now the next question could be,
which of these is responsible for keeping the population at a low level? There could be one
parasite because of which the calves are dying at a very young age. So, that is keeping the
death rates in the population at a very high level. So, in that case you can start treating
these calves. So, you kill off all the parasites once and probably these calves are still dying
Then so, you can say that parasite 1 was not responsible for the high juvenile mortality
that we had that we were seeing here, probably you remove the second parasite again
nothing happens. Then you treat for the third parasite and then suddenly you see that the
calves are not dying. In that you can say that this parasite P 3 was responsible for keeping
this population static at this number of 15 elephants.
(Refer Slide Time: 18:01)
Again in the case of a static problem, the population size is not changing. So, dynamic
factors do not have to be considered. So, you are saying that P n + 1 is equal to P n or
essentially births minus deaths plus immigration minus emigration is equal to 0. So, that
simplifies our calculations to quite a high degree. And then you look for how many animals
are immigrating, how many animals are emigrating?
In a number of cases we will find that these numbers are also very small or probably 0. In
that case we will ask the question, what is keeping the birthrate at the current level? What
is keeping the death rate at the current level? Then we can manipulate different habitat
variables to look for the responsible factors that are keeping this population at a particular
(Refer Slide Time: 18:49)
Now we will look at one such population study which is the problem of the locust, now
locust are certain species of grasshoppers that have a swarming phase. So, there is still
very less amount of certainty, when do you call an organism a grasshopper? When do you
call it a locust? They are both very closely related organisms, but mostly grasshoppers will
be seen solitary, but then locusts will be seen in large sized swarms. So, we can say that
they are grasshoppers with a swarming phase.
(Refer Slide Time: 19:25)
Locusts are known to have these two phases. The first phase is a solitary phase and in the
case of a solitary phase they will behave more like grasshoppers, in which here is they are
innocuous. They have low numbers, and they do not pose a threat to either agriculture or
to the habitats. Then there is the second phase which is a gregarious phase, which is a
nomadic phase. These locusts they make large swarms. So, it is a swarming phase.
They make large sized swarms, then they move to other areas and once they start moving
to other areas they will have a huge requirement of leaves and because of which will
become very important pest for agriculture and also for conservation. This is something
that we know today that these locusts are having these two phases. How do you get to this
understanding that this is the same organism that is having these two phases?
(Refer Slide Time: 20:25)
This is what we are going to look at in this case study. Let us begin with looking at these
two phases. In the case of the solitary phase there are morphological differences between
the solitary and the gregarious phases. In the case of the solitary phase you have short size
elytras which are the wing sheaths, which are covering the wings, and in the case of
gregarious phases you have long sized elytras. In the solitary phase you have long hind
femora and you have a short hind femora in the case of gregarious phase. You have males
that are 20% smaller than females in solitary, you have males that are 4% smaller than
females in gregarious.
Essentially in this case, you can say that the males are very small. There is a huge amount
of sexual dimorphism that you will see. In the solitary phase the animals are pale colored
mostly light green in color and in the case of gregarious phase they are dark in color. And
solitary as the name implies they have a solitary nature, they do not form groups; in the
case of gregarious they form last sized groups.
Now this was known for quite some time that you are seeing these grasshoppers, you are
seeing this locust that come in sized swarms and when they come they decimate the whole
of the crops.
This has been known since antiquity even if you look at the old testament you will have
references of these locusts that are coming and that are acting as a plague for the society.
(Refer Slide Time: 22:01)
But then when people try to look at these locust, they found these two kinds of organisms.
You have this lighter colored version, and you have this dark colored version. And, both
of these look so different that for a very long period, people used to think that these are the
grasshoppers that live in our in the grassland, and these are the locusts that come into our
agricultural fields. And they were actually considered two different species.
Now the question was that this species, you see this is this particular species, once in every
15 years, or once in every 20 years, whenever it comes there is a huge amount of
decimation to the crops. There is a huge amount of decimation to the grasslands, which
also affects a number of other animals especially the dairy animals, and then once these
locust are vanishing, they completely vanish off. You have no trace of them for the next
15, 20 years and then suddenly they come up again and you do not see them in the
So, in the intervening period you only see these green organisms. So for a very long time
people used to think that these are two different species altogether.
(Refer Slide Time: 23:15)
Locusta migratoria is the migrating species or the dark colored species the one that we
have downwards. The second one was known as Locusta danica which is the light colored
species. So, we used to think that these are two different species. So, if you start with such
a foundation and you perform any number of population studies you are going to be wrong.
Why? We will come to that.
(Refer Slide Time: 23:51)
So, people started looking at these differences and for a very long period the scientists
were completely perplexed what is happening, we do not see these dark colored animals
for 15 to 20 years, and then they come up again in a jiffy. But, then when people started
doing the population studies; so, we have this person by the name of Plotnikov, and he
started looking at the different these two different forms. And he started looking at the
larvae of these. So, this is an extract from his paper and he writes,
“The most obvious difference between the two phases being in the coloration of the larvae,
this character and its variability have been studied in the experiments. The larvae may be
classified according to their coloration into 3 and not 2 categories. First is the migratoid
category, the second is green, and third is danicoid”.
Migratoid is something that is related to the migratory form, danicoid is something that is
related to the dynamic form and there is the third variety which is green. Intermediate
forms also occur between all three types of coloration and the various types can usually be
recognized definitely in the third larval stage. Now as in a number of insects this particular
insect also has a number of larval stages, and the differences can typically be seen in the
third level stage.
“Larvae of the coloration which I call migratoid have the upper side of the head black the
pronotum velvety black below.”
So, he is essentially describing all these three different varieties. You have this migratoid
variety, you have the green variety and you have the danicoid variety.
Now it is important to note here that when you are looking at the larvae you can see these
three different varieties; we are not going to get into details of all these three, but then
there are three different varieties that you can very easily see based on their appearances,
based on their morphology as well as based on the colors. Now just as we had seen that
you have these two varieties and you can very easily make them out using their colors.
Similarly you can look at the larvae also using their colors. So, that is the first level of
understanding that we get. This is how you proceed in doing any of the population study.
The first thing is to look at the population and to describe things.
(Refer Slide Time: 26:19)
He further writes,
“On the 29
th of May, I took from an ordinary cage”.
So, he is describing an experiment in this case, now, what was the experiment? The
experiment was that the scientist took out these larvae and he put these larvae into
containers in different numbers.
(Refer Slide Time: 26:41)
So, you can keep these larvae either solitary. So, in that case you have a container or a
cage in which you have only one larvae or you can keep them out in a very dense format.
So, you have a number of larvae that I kept together or you can keep them at some medium
densities. So, this is the experiment that he did. He took out these larvae and kept them at
different densities. So, he describes the cage that is not very important, but then he says
that after a while it was observed that the larvae in the ground cage began to turn green
and on the 11th of June they attained the 5th stage, they had become quite green, but 6 of
them became dark grey almost black. In the original cage all the larvae kept their migratory
So, essentially when you are keeping these larvae in different densities then you are seeing
that there are some larvae that are changing their colors. Then the same experiment was
repeated with larvae of danicoid coloration and with the same result. But, only green larvae
were obtained in the ground cage. As many as 15 experiments with larvae of the second
brood of danica reared under conditions of overcrowding yielded interesting results. When
the larvae were kept from the first stage in small cages in jars, a density of 30 to 50 larvae
to 450 to 675 cubic centimeter of space, a typical migratoid coloration was invariably
obtained. In experiments in which the density of the population was less, 20 larvae to 2000
cubic centimeter of space, besides the migratoid larvae some green and danicoid, but not
dark specimens were obtained.
So, essentially if you are keeping these larvae at a very high concentration. In this case
you are typically getting the migratoid variety and in these cases you are getting some that
are migratoid, some that are danicoid and some that are green colored larvae. So,
essentially through this experiment what he proved was that both of these varieties were
actually one species; they are not two different species. Even though they look very
different from each other, but they are one species. So, that was the first level of
So, remember when we started we said that whenever you are doing a population study
you have to define the problem and you have to define the population. Before this paper,
whenever there was a study on the locust they only focused on the green locust or the only
focused on the dark colored locust. Whenever you are talking about only the dark colored
locust that is leading to the harm to your crops, you are not defining your population
Because you are missing out all the lighter color versions that are actually also a part of
the same population. They are the same species, they are living together in the same area,
they are capable of interbreeding together and probably they are interpreting together. And
in that case it has to be defined as one population. If you are wrong with your fundamentals,
if you are wrong with defining of your population, then the rest of the results are not going
to proceed correctly.
This was the experiment and he showed that both of these are the same species; now when
the larvae were kept singly in glass jars they began turning green as early as in the second
stage and in the four to fifth stages they invariably became quite green. When larvae in the
first stage were placed in groups of four in small glasses about hundred cubic centimeters,
the resulting larvae of the fifth stage presented a mixture of migratoid and green larvae as
well as some transitional forms.
So, not only do you have these two varieties, here you get a third one. If you keep this
larvae singly, you get only get the green larvae. If you keep these organisms at a very low
density. So, there is just one organism that is having plenty of space to itself, it becomes a
green colored variety. If it is kept in a very high density, it invariably becomes a migratoid
variety and if it is kept in an between condition, then you get all these three different
So, just by looking at the morphological and the color characteristics of the organisms you
should not rush to say that these are different species, it is also possible that what you are
observing is a trait that is coming out of some extrinsic factors. Remember we talked about
the extrinsic factors right before and this is one extrinsic factor; how much space do you
have per animal?
(Refer Slide Time: 31:57)
Now, this particular work was made done by another Russian - British scientist by the
name of Uvarov and he used these results to say that actually these locusts are coming in
two different phases. He said that these dark varieties and these light varieties are two
different phases and they can change from one to another.
(Refer Slide Time: 32:23)
If you look at this seminal paper that was written by Uvarov, he said that there are these
two phases and he named it as phasis solitaria, that is the solitary phase and the phasis
transiens, which is the transient phase and then there is also a third phase. Phasis solitaria
is a term to be applied to the extreme form by which the species is represented in in a given
locality when only isolated individuals are present and no swarms exist or have existed
within the last one preceding generation.
Now, not only are these larvae different but also the adults that are coming out of them
they are also different and Uvarov looked at the behaviors of these two these two different
forms and he said that we call them as phasis solitaria and phasis gregarious and the
transitional phase. In the solitary phase, when you have these green colored organisms
they will live in that particular place they will have a large space for themselves.
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