Module 1: Population and Community Ecology

Study Reminders
Text Version

Population Parameters and Demographic Techniques

Set your study reminders

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




























Today, we begin a new module which is Population Ecology.
(Refer Slide Time: 00:18)

In this module we will have 3 lectures. The first one is population parameters and
demographic techniques, the second one is population growth and regulation and third one
is population studies and applications of population studies.
Let us begin with Population parameters and demographic techniques. Here we want to
understand what are the parameters through which we can describe a population. How do
we measure those parameters? What are the demographic techniques that we have?
Demography - demo means people, graphy means to write.

(Refer Slide Time: 00:54)

When we say demo graphic. So, demo is people as in the case of democracy which is the
rule of people, graphic is to write. Here we want to ask the question what are the methods
through which we can write about the population; so, that is a demographic technique.
(Refer Slide Time: 01:22)

We begin with the definition of population. As we had seen before, all the organisms of
the same group or species; which live in a particular geographical area and have the
capability of interbreeding. These are organisms of the same species, they are living
together and they are able to interbreed.

(Refer Slide Time: 01:40)

A good example is this picture which is showing us a population of impala. Here we have
this deer which go by the name of impala. These are found in Africa. Here we have a
population of impala and if we remember these are some cheetahs that we had seen before
and so, this is also showing us a population of cheetahs that are living together.
Why do we need to understand the population parameters? That is important because we
want to regulate certain species and also because we want to understand more and more
about certain phenomena that are happening in nature.
For instance if we need to increase the population of impalas in this area; now why would
we want to increase the population of impalas? Well impala are a very important prey
population and so if you have more number of prey you will also, with time increase the
population of the predators.
If for instance you are managing this area for cheetahs or you are managing this area for
say lions; you would want to increase the impala population. Similarly in our case in India
we want to increase the population of chitals and sambars so, that there is more food
available for tigers.

(Refer Slide Time: 02:58)

Now, if that is our aim we want to understand how this population will move with time.
So, this is the equation that regulates the growth or decline of a population.
So, we have Pn+1 which is the population at a time that is n+1 years is equal to Pn that is
the population at the nth year. So, here we are saying that this was the population in the
nth year; what is the population in the next year that is Pn+1. Now Pn+1 is given by Pn plus
natality. Now natality is the birthrate; how many number of animals increased because of
birth in the existing population.

How many animals were added; now addition can be because of natality or it can be
because of immigration. Immigration is when you have animals that are coming from some
other area into this area. If you look at the definition of population again; here we have,
which live in a particular geographical area.

(Refer Slide Time: 04:05)

Now if we have say two populations; so here we have a population that is P1; this
population called P2. And then there is some number of animals say Pi that move from P1
area to the P2 area.
So, they are moving from the first population to the second population. So, for the first
population we will see there that there is an emigration that is there are individuals that are
leaving this population and going somewhere else. But in the case of P2, we are observing
immigration that is there are animals that are moving from some other place into this
So, in this equation what we see that Pn +1 is given by Pn plus the addition; addition is
because of birth and because of immigration, minus the number of animals that are
removed from this population. Now why do we have removals or how do we have
removals? We have removals because some animals die off and then there are some
animals that move away from this area.

(Refer Slide Time: 05:11)

So, essentially what we are saying is that Pn+1 is Pn plus the number of additions minus the
number of subtractions. So, you have some animals that are getting added addition can be
because of births or they can be because of immigrations and subtractions similarly can be
because of deaths. So, some organisms are dying off or they can be because of emigrations.
So, this is the basic equation through which we can understand how a population is
moving. Now why do we need to make these assessments, why do we need to understand
how much is the population, what are their parameters and how is it going to change?
(Refer Slide Time: 06:02)

It is important because numbers are essential at every stage of management. So, basically
any management goes through these 4 stages which go by the name of the PDCA cycle.
(Refer Slide Time: 06:16)

Here we have P, D, C and A; so this is planning, so you plan something. So, essentially
for instance we are planning that we want to have more number of tigers in our area. So,
for that our plan would say for instance that you need more amount of protection, you need
more amount of preys, you need to control diseases that are there in this population and so
Once you have made this plan, what are the things that you are going to do? Next you do
those acts. From plan we move to the doing stage. So, in doing you actually implement
those plans. Doing is the stage of implementation. Once you have implemented something;
next you want to check whether those implementations are having those impacts or not.
For instance you increase the population of chitals, but then the population of tigers did
not increase; that could be a situation because tigers are getting poached up, they are
getting removed from this system because of hunters. After the doing stage you need to
move to the next stage which is the checking stage. Next you do checking. Are your
interventions having the desired impact or not and based on the results of your checking
stage the next stage is acting.

Acting is if suppose your tiger population is increasing do you further substantiate the
number of chitals so that you further increase the number of tigers. Or if the number of
tigers has increased to a level that you are satisfied with, now, should you stop increasing
the number of chitals or for instance the number of tigers is still decreasing what do you
do so that you are able to have a grasp on the situation. So, that is the acting stage and from
that act, we come up to the next stage of planning again. For instance in this case suppose
we figured out that you have tiger numbers that is reducing because of poaching. You will
make another plan you will say that now we need these steps to reduce the poaching that
is there in this population. It goes by the name of the deming cycle and at all of these stages
planning, doing, checking, acting we require numbers.
(Refer Slide Time: 08:31)

Why do we require numbers? Because if we do not have these numbers, if we are not able
to measure how much population do we have; how are we going to manage that
Because the first question that we are asking is we want to increase tiger numbers; if we
want to increase tiger numbers the first thing that we should know is what is the current
population of tigers and what is the population of tigers that we want? If we are doing
something to the system we want to ask is this doing having an impact or not? So, at every
stage you require numbers, which is by population assessment becomes extremely

(Refer Slide Time: 09:04)

Probably assessment is also important because numbers are crucial inputs for decision
support. For instance we need to make these decisions; do we want to increase the number
of tigers because the numbers are getting low or the numbers are already low or do we
want to reduce the numbers because you have you have too many tigers and that are
leading to conflicts or maybe your system is not able to support so many tigers.
Do you want to reduce the number of tigers or do you want to maintain a status quo? Do
you want to maintain the equilibrium state that this many number of tigers is good for the
system; so, we should have these same number of systems. However, we remain ignorant
of the actions required of us till we actually know the numbers.
Do we want to increase, reduce or maintain status quo is something that we will only know,
if we know the number of tigers that are existing and the number of tigers that we want.
So, this makes assessment of the animal populations extremely important.

(Refer Slide Time: 10:03)

And we also require to assess the population status to assess the risk of a population decline
or a crash. For instance if you have a very small population; you have say 4 tigers in any
area and these 4 tigers turn out to be all males. If you have 4 males in an area the population
is not going to increase any further. With the death of these 4 individuals the whole
population would collapse down to 0.
(Refer Slide Time: 10:45)

To avoid such a scenario; you need to assess what are the probabilities that if you have
alone female in your area or if you have very few number of individuals in your area what
is the possibility that all the offsprings turn out to be males ?
What is the probability that all the offsprings are males? So, you want to have an
assessment of the probability of having such a scenario. Or for instance you want to
understand if demographic stochasticity is going to have any impact. Demographics
stochasticity is one example what we have already seen; the second example is a chance
variation in births and deaths.
For instance every population is suffering from some births, some deaths. What will
happen now, is that you have say a death rate of say 20 percent and a birth rate of 30
percent; if this is the scenario your population is going to increase.
But then by chance it is possible that this year in place of having 30 percent a birth rate;
you only have a 10 percent birth rate and your population is going towards a collapse. So,
you want to make an assessment of what is the probability that you might have such a
situation or to understand if there is a possibility of an environmental stochasticity.
Now environmental stochasticity means a chance event in the environment that may lead
to a population crash such as you want to understand if any drought or any flood or a
famine or a disease is going to have an extremely negative impact on the population; now
why is that important?

(Refer Slide Time: 12:26)

Suppose you have a population of say 100 individuals. Now, out of these 100 individuals
say 99 die out and only 1 remains. Now you have these 99 that are dying out because of
say a forest fire.
Now if you only have 1 individual that remains then you will have a population crash
because you do not have any further births in this population. Now this happened because
you already started with a very low population of only 100 individuals. Now in place of
having 100 individuals say we have 10000 individuals; let us start with 10000 individuals
and because of this forest fire which was an environmental stochasticity; you have a
situation in which there is a massive number of deaths and as many as 9000 individuals
die out.
Here we only had 99 individuals that were dying out here you have 9000 individuals that
are dying out, but still you will have 1000 that will remain in the population and so the
population will persist. Now to understand if we have the same scenario in both the
situations; we have a forest fire that is clearing off a very large area of the forest. Now
what is the impact of any of these environmental stochasticities can only be understood if
we know the size of the original population.
With this we can make an assessment of the probability that you will have a population
crash or a survival of the population.

(Refer Slide Time: 14:16)

Or you can even try to understand what is the possibility that you will observe genetic
problems in your population. So, if your population size is very less that you have a very
high amount of homozygosity in this population; you might even see situations of
inbreeding depression or genetic drifts or loss of heterozygosity or if you have a very small
population you can also start seeing behavioral problems, behavioral problems and Allee
For instance, in the case of pack hunting animals where individuals are less efficient and
may not be able to hunt alone. A very good example is the case of the wild dogs; now in
the case of wild dogs when the group goes out for hunting some animals remain back with
the young ones, to protect the young ones.

(Refer Slide Time: 15:08)

Now, for instance if we are considering the wild dogs and suppose you have a pack size
of 50. Now, if you have 50 adults and say you have 10 young ones; now when these
animals have to go out for hunting, then out of these 50, you will have 40 that go out for
hunting; for instance then you will have 10 that remain for protection and you have these
10 young ones that also remain in the cave.
So, you have these 10 young ones that also remain in the cave. So because you are able to
spare these 10 individuals to take care of the young ones; so this population is able to
persist; so in this case the population is able to persist because the young ones are getting
a protection. On the other hand if you have a very small size; so, you have a pack size
which is say 10 individuals.
So, you have 10 adults and say you have 8 young ones. Now because these wild dogs hunt
using a pack strategy; so all of these individuals they have been evolved in such a manner
that when all of them go together and attack or they go together in a large number and
attack a prey; then they are able to kill that prey. But then if you have only a single dog
that might not be able to kill the prey. So, to have a possibility that you have some amount
of food that you can bring back to the young ones all these 10 adults will have to go out.
So, all 10 go for hunting and you have all the 10 young ones that remain in the cave. Now
in this case you do not have any amount of protection for these young ones. So, what
happens is some other predators; say, leopards might come into this area and hunt these

puppies. So, once that happens the whole population is going to collapse. So, this is an
impact that is known as the Allee effect; in which you have a very small number of
organisms that are not able to perform their biological functions properly.
So, they are either not able to hunt properly or they are not able to find their mates properly
and so on; so, we start seeing behavioral problems and Allee effects. Now these problems
only occur when you have a very small population, if you have these 50 individuals you
would not have an Allee effect, but if you have these 10 individuals you will have an Allee
effect; now to understand whether your; whether your population is at a scenario where it
can suffer from these behavioral problems or Allee effects or genetic problems or so on;
the first thing that you need to know is how many individuals are there in the population,
which makes population assessment extremely crucial.
(Refer Slide Time: 18:19)

Now, population assessment also helps us to plan scenarios and take steps.

(Refer Slide Time: 18:29)

When we say planning of a scenario; what do we mean? We ask this question that suppose
this is your number of animals of any species that you are interested in and this is time.
And suppose we have this many number of individuals and you have a probability of say
drought of say 25 percent.
Once out of every 4 years you will see a drought; probability of a disease is say 33 percent.
So, one out of every 3 years you will see a disease; probability of say a forest fire is say
around 40 percent which means 4 out of 10 years you will see a disease. Now you can
make scenarios you can ask; what is the possibility that this population is going to persist
or not? So, for instance this is the first year and suppose in the first year you suffered a
drought; so, because of the drought the population collapsed.
Then the next year you got a situation where you had a drought as well as a forest fire. So,
your population decreased even further maybe the next year was a good year. The
population started to increase, but then you had another year in which you had a drought
a disease and a forest fire and the population collapsed.
Such a scenario is highly likely if you have a very small population size. So, you can make
an assessment; you can ask this question what is the scenario that is possible because you
have these many probabilities of having a drought, a flood or forest fire or disease and so
on and in all of these scenarios, what is going to be your action point?

So, for instance if we see that our starting population is so less that all of these impacts are
going to crash this population; say 10 percent is our probability that this population is
going to crash within 10 years; so, that is a very high possibility. In that case to overcome
this possibility we might want to substantiate our population, we might want to put in more
number of individuals or maybe we would want to protect our individuals from the
common diseases; maybe we would want to vaccinate our animals or maybe we would
want to avoid any situation of forest fire.
So, maybe we will go for a very intensive protection in that area, but that is only possible
if we have the scenarios and scenarios is only possible, when we have the numbers.
Assessment of a population is extremely important and once we have these numbers, once
we have the scenarios we can go for adaptation or mitigation. In the case of adaptation you
try to make your population supple enough to respond to changes.
So for instance, if you have a disease you would want to give more amount of nutrition to
the animals so that they are able to fend off these diseases or you would want to give them
some amount of vaccinations. On the other hand, you could you even go for mitigation
where the causes of the change are analyzed and addressed. In the case of disease you
might want to kill off all the parasites or kill off all the vectors and so on. So, essentially
an assessment of a population size or the numbers is extremely crucial.
(Refer Slide Time: 21:49)

Now, when we are talking about a population assessment; the next question is do we want
to see the numbers or do we want to see the trends?
So, numbers and trends means last year we had say 2500 tigers; so that is a number. And
when we say trends we are saying that this year the number of tigers is greater than what
it was in the last year. So, do we need numbers? or do we need trends? We will actually
we need both of these. Trends are helpful when we need to analyze and address the gross
movement of population numbers; whether the population is increasing, decreasing or
remaining constant. And this is especially important for prey species such as chital or
sambar where exact numbers are hard to compute due to their large population sizes.
For instance in the case of prey species you do not want to know whether there are 10000
chitals in your area or whether there are 10010 chitals in your area. But as long as you
know that the number of chitals is same or is increasing you are happy. So, in that case
you do not have to go and count each and every chital.
(Refer Slide Time: 22:52)

Whereas for some other organisms such as your critical species like tigers or dugongs you
would want to know the exact numbers; now numbers give us finite data then trends. So,
if you know the number of animals that was there this year, last year and so on, you can
compute trends from these numbers, but you cannot go the other way.

So, if we just know that this year the number of tigers is greater than what we had last year
and last year was greater than what we had previous to last year and that was less than
what we had the year before. From that information you cannot figure out the number of
tigers that we have today, if we have the number of tigers we can very easily compute the
(Refer Slide Time: 23:40)

So, when we are talking about a population, what are the population parameters what are
the demographic information that we are trying to assess in this study.
So, we require a number of parameters. The first one is the population size. Population
size is the number of individuals in a population. When we say that Panna Tiger Reserve
has say 35 number of tigers what we are referring to is the population size that we have,
of tigers in the Panna tiger reserve. Now in place of population size, we could go for the
population density; population density is the number of individuals of the population per
unit area. So, if say Panna tiger reserve has an area of 350 square kilometers.

(Refer Slide Time: 24:28)

In that case we would say that we have 35 tigers in 350 square kilometers. That would
mean 1 tiger every 10 square kilometers or 10 tigers per 100 square kilometers.
When we put the numbers in terms of per unit area or per hectare or per square kilometer
or per 100 square kilometer; what we are referring to is the population density. Now
population density can vary a lot between different organisms.
(Refer Slide Time: 25:01)

For instance if we look at density in say; numbers per cubic meters or per square meters;
so things such as diatoms, now diatoms are small animals that are found in the oceans and
their numbers are represented in per cubic meter.
You have 5 million diatoms per cubic meter. When you talk about soil arthropods; so you
have this number of 500000 per square meter. But if we look at other organisms; so here
we have these numbers of 500000 for arthropods. In the case of trees this goes down to
something like 0.05; in the case of people it can go even down to something like
0.00000003 per square meter.
You have a very large diversity in these numbers; so for instance in the case of our tiger
reserves the density of chital or the density of sambar is going to be much higher than the
density of tigers. Now it is important to know this fact that you have a large variation so
that you can discern the best technique that you will use to measure the population density.
(Refer Slide Time: 26:15)

When you talk about population density we can look at absolute density or relative density.
Absolute density is the number of organisms per unit area and relative density is only
asking the question whether in area x has more number of organisms than area y or does
area y have more number of organisms than area x.

In that case we are not interested in the actual numbers, what we are interested in is only
which area has more number of individuals; so, that is the relative density. Now we look
at both of these population densities.
(Refer Slide Time: 26:52)

When we talk about the absolute densities; we can measure absolute densities in three
methods. The first one is total count. Total count is you go and measure each and every
organism. For instance, census of India is the total count.
The census officials will go to every household and ask how many males are there, how
many females are there, how many children are there and so on.

(Refer Slide Time: 27:20)

The second method is a sampling method. Now if you have a large area; so, you have this
large area that is a forest and you cannot measure the animals everywhere. So, what you
can do is you can take some samples out of this area, measure your animals, or count your
animals get a density and then extrapolate that density to the whole of the area; so, that is
the sampling method. In place of doing our total count you are doing a sampling in some
area and this sampling can be in the form of quadrats or in the form of capture-recapture
method and we will look at these in greater detail in a short while.
And the third method is removal method; in the case of removal method you have put up
traps, you kill the animals and you look at the rate at which these animals are getting
removed from the system. This method says that if you have more number of animals; then
more number of animals would die off and if you have less number of animals less number
of animals would die off. So, with that you can make an estimate of the number of animals
that are actually present in your area. Now this is not very useful in our Indian scenarios
because we do not kill animals, but then this is also one method. So, we should know that
this method exists.

(Refer Slide Time: 28:27)

Now, when we talk about quadrats; so in the case of sampling we were talking about
quadrats; in the case of quadrats you can have a square quadrat. These areas that we are
using they can either be a square or they can be a rectangular area or they can be a circular
area or we could even go for some irregular areas; so, all of these combinations are
possible. But, when you are using this quadrat method, the question is how many samples
do we need? where to set up these samples? and so on.
So, for that it is important to know the process of sampling.
(Refer Slide Time: 29:03)

Now, sampling goes by the objective to secure a sample which will represent the
population and reproduce the important characteristics such as the population under study
as closely as possible.
(Refer Slide Time: 29:21)

For instance if we have this area and in this area the right side has a much greater density
of animals than the left side; so in this case when you are taking a sample, you would want
to have a sample that is representative of the whole area.
For instance if you take with just one sample here and you take this density and you
extrapolate it to the whole of the forest; then your final estimate would be a very gross
underestimation because you will say that here you have a very less density of animals.
So, you are extrapolating and you are saying that the whole area has a very less density.
On the other hand if you are taking quadrat only here then you might overestimate;
overestimate the number or the density of animals.
So, the sample has to be chosen in such a way that it represents the population and
reproduce the important characteristics. In this case what you should do; is you should...
if you have this information ab initio, that this much area say 40 percent of the area is
having a high population density. In that case you will take samples in a way that 60
percent of your samples fall in this area and 40 percent of your samples fall in this area;
so, that it will becomes a representative of the whole population.

(Refer Slide Time: 30:36)

Next, we have sampling units. Sampling units may be administrative units or natural units
like topographical sections and sub compartments or it may be artificial units like strips of
a certain width or plots of a definite shape in size.
The unit must be a well defined element or group of elements identifiable in the forest area
on which observations on the characteristics under study could be made. The population
is then subdivided into suitable units for the purpose of sampling and these are known as
sampling units.
(Refer Slide Time: 31:12)

Essentially if you have an area; you can make use of natural sub units or administrative
sub units or artificial units. In the case of our forest we might go for say compartments;
compartments are management units or we could go for beats which are administrative
units. So, a beat is an area that is being managed by a single forest guard or we could go
for natural units.
So, natural units for instance, if this is the forest and then you have a river that is dividing
it into two parts; so we can say that these are natural units or we would even go for artificial
units in which case we can define certain in grids and we can say that these are what we
have defined as our sampling units. Next we defined a sampling frame.

(Refer Slide Time: 32:12)

A list of the sampling units is called a frame. In this case if we say that we have all these
grids 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13 and so on; when we make a list of all of these
then we call it as a sampling frame and from this sampling frame we pick up a sample.
(Refer Slide Time: 32:33)

Now, a sample is one or more sampling units that is selected from a population according
to some specific procedure to constitute a sample. So, for instance in this case we had 14,
15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38,

and 39; so, we have these 39 subunits and if we say that we are selecting this unit 9 and 19
and 29 and 39.
So we are just taking these 4 out of these 39; so these 4 will form a sample. So, these are
one or more sampling units that are selected from our population according to some
specified procedure and here our procedure was that any subunit that ends with a 9 is part
of our sampling.
(Refer Slide Time: 33:34)

Next, we have sampling intensity; sampling intensity or intensity of sampling is defined
as the ratio of the number of units in the sample to the number of units in the population.
In this particular case our number of units in sample is 4 and the number of units in
population is 39. So, in this case sampling intensity is given by 4 by 39. So, more the
number of subunits that we take into our sample, more is the sampling intensity and if we
take all the subunits that are there in our sampling frame as part of the sample, then you
have a sampling intensity of 100 percent in which case your sample turns into a census.

(Refer Slide Time: 34:30)

Now next we have the kinds of pl