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Module 1: Distribution, Abundance and Measurement of Threatened Species

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Threats to Species

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Today, we begin a new module which is management of threatened species. Conservation
is a field in which we utilize all our learnings from ecology for the benefit of organisms
and for the benefit of habitat. We will be using all different learnings that we have had so
far in ecology to understand why some species are facing this threat of extinction and what
can be done for those particular species.
(Refer Slide Time: 00:39)

This particular module will have three lectures. The first is ‘Threats to Species’.Why are
there some species that are the threat? The second would be In-situ conservation; which is
conservation on site and third will be Ex-situ conservation; which is conservation away
from the natural or the existing habitats of the organisms.
Let us begin with the first lecture which is ‘Threats to species’.

(Refer Slide Time: 01:05)

If you ask this question why are some organisms facing the threat? So, we can discern an
answer from ecology. The factors that are responsible for threats to organisms are the
opposite question to why things are found, where they are found.
In one of our earlier lectures we had looked at why are organisms found in certain locations
and not in other locations. Organisms are found in certain locations because they have
situations that are useful or that are helpful for the survival of those particular organisms
in those particular areas.
For instance, if there is an organism that requires a low temperature may be requires some
specific kind of foods, may be require some specific kinds of shelters and if we have those
conditions available in certain portions of the earth, so, that organism will be found in
those particular location
A threat to that organism would be if we remove all of these suitable conditions, so if we
remove that particular source of food; if we remove that particular source of shelter or may
be remove that particular life style of the organism; may be encroach upon those habitats
so that would result in a threat. So, this is the opposite question to why things are found
where they found.
If you have a push factor everywhere and a pull factor nowhere, so that would mean that
this organism is getting a stress from all different sides to vacate this particular portion of

habitat, but it does not have any other place to go. That would form a major threat to that
particular species. So push factors everywhere and pull factors nowhere.
(Refer Slide Time: 02:43)

What could be these push factors? This push factors could be no suitable habitat for the
organism either, it is too hot nor too cold or there are no trees, no food, no nutrients for
this organism to survive on or if it lives in the forest and this forest is completely burnt out
or if this area becomes polluted. If there is an organism that lives in a lake and you use that
particular lake as a dumping site. So, you are adding a number of pollutants into that area
so that it is now not able to live in that particular lake or that particular... or it’s habitats
are now no more suited behaviorally; in which case we will have a look at habitat selection.
Habitat selection is a behavioral process in which an organisms selects a particular habitat.
For instance you can have a mosquitoes in paddy fields, but then different kinds of
mosquitoes, different species of mosquitoes will utilize different kinds of paddy fields to
lay their eggs. So that is a behavioral selection.
If you remove any particular habitat that is being selected by an organism behaviorally,
so, which is in terms of the organism, that is, the best suited habitat; if you remove that
habitat, so behaviorally it will not find itself comfortable in the other habitats. The other
push factor could be a lot of competition. If you have a number of invasive species in a
forest. If you have a forest in which you have invasive species like lantana. So, lantana is
now growing very fast in a number of forest and it covers a major portion of the ground

that is there in those forest. If you have lantana that is covering up all the land so other
herbs and shrubs would be competed out, so that is the push factor. Too many predators
or diseases in any area is another push factor or if an organism is actively being hunted, it
is actively being killed out, it is actively being poached.
So, that is another push factors or small population dynamics, they also act factors push
factors or threat factors such as things like Allee effect. Now Allee effect is an effect in
which if you have an organism that lives in large sized packs. If you have a pack living
organism, if you reduce the size of the packs, so the efficiency of this organism to get food,
to get a mate, to protect itself gets reduced or in certain situations when the density of
organisms is very less so the organisms are not able to find their mates.
In those situations we are seeing Allee effect in picture or you could have a small
population dynamics such as stochastic death. So, just by chance you have a large number
of death. These are all different threat factors. So, we have push factors that are resulting
in to threats and there are some factors that are coming out because of chance events.
(Refer Slide Time: 05:47)

These factors can be divided into two parts; one are those factors that push a population
towards a smaller numbers through population dynamics. We had a look at population
ecology, we know how populations survive how populations increase in their sizes and if
you have a scenario in which there is something that is threatening the growth of the
population, there is something that is increasing the death rate, there is something that is

reducing the birth rate.
All these kinds of factors that will play a role when your population is large in size and
are trying to reduce the population size through population dynamics are called as
declining population paradigm. And the other factors; there are a number of other factors
that push a small population towards extinction and they are studied in the small population
paradigm.
(Refer Slide Time: 06:49)

Essentially what we are seeing here is, if you consider any species that is under a threat of
extinction; so you will have a large population, now this large population in time because
of certain threats becomes a small population and then, when it is already a very small
population they could be some other threats that would lead to its extinction.
When we are considering this portion, how does a large population become a small
population? In that case we are talking about the declining population paradigm. So, we
are asking the question, what are those threats that are causing this large population to turn
towards to turn into a smaller population? And when we consider this portion, how does a
small population become extinct? Here we are talking about the small population
paradigm.
We looked at a number of threat factors. Now, if we ask this question what are those factors
that are causing a population to decline to smaller population and what are those factors

that are pushing a small population towards extinction? So, will be talking about the
declining population paradigm and the small population paradigm.
Let us look at the declining population paradigm. What kinds of threat factors would result
in smallness of a population? If you are eating up the habitat of any particular organism;
if you are causing the habitat to shrink or if you are pushing this organism away from it’s
own habitat so that it does not have enough amount of habitats left for itself or if you are
putting it in some kind of competitive pressure. For instance, in a grassland that is being
used by chitals and sambar you are taking your cows and buffaloes for grazing. In that
case the chitals and sambar are now are not able to compete with the cows and buffaloes;
so in that case they have been pushed out.
Factors such as these; non availability of a suitable habitat or reduced availability of a
suitable habitat or being completed out or being killed out through poaching, all of these
would be studied under the declining population paradigm. And the other kinds of factors
that play a role in the small population dynamics, Allee effect, stochastic deaths and so on
will be studied under the small population paradigm.
(Refer Slide Time: 09:39)

We can say in other words that there are 2 kinds of factors that are playing a role at all
times, some of these are deterministic factors which are acting at large population sizes
and some of these are stochastic factors or chance factors. Deterministic factors are those
that play a determining role in the population dynamics and stochastic factors are those

factors that play a chance role in the population dynamics.
Chance factors become more important when you have smaller size populations. Why?
Suppose you have a population in which you have 10,000 young ones that are born. Now
on an average we can say that 50 percent of them will be males, 50 percent of them will
be females. So, roughly 5000 male offsprings and 5000 female offsprings.
It is possible by chance that in place of 5000 you get say 4990 male offsprings and 5010
female offsprings, but that would be roughly the amount of chance variation that you will
see in the large population whereas, if you consider a very small population so suppose,
you have only 4 offsprings that are born.
(Refer Slide Time: 10:59)

In those situations, roughly you have four off springs. So, in that case we expect 2 of them
to be males and 2 of them to be females, but then by chance it is possible that you only
have 1 male and 3 females or maybe even 0 males and 4 females or you could have another
situation; you could have 3 and 1 or you would have 4 and 0.
If you have a very small population so the probability that you can have one of these
situations increases and the probability that you do not have any organism of a particular
sex also increases very much. Stochastic factors or chance factors play a much more
important role when your population sizes are small as compared to when your population
sizes are larger.

(Refer Slide Time: 11:51)

What are the deterministic factors and what are the stochastic factors? Deterministic
factors could be things like birthrate, death rate and population structure. If you have a
population with a reducing birthrate, so that would be a deterministic factor. It will not
lead to an extinction in the near future, but then we can say that if the birthrate is going
down, then there is some issue and this population might be vulnerable to extinction or if
there is an increase in the death rate suppose. There are some diseases that have come into
the population and so they are increasing the rates of deaths in this population.
In this case as well, we will say that there is a chance that this population is being pushed
towards extinction. It is being pushed from becoming a large size population to becoming
a smaller size population or things such as population structure, if more and more
organisms in the population are becoming old. In that case you have a population structure
in which you have less number of young ones, less number of adults and many more
number of old organisms.
All these three factors the birthrates, the death rate and the population structure play the
role of deterministic factors and may push your population from being a large size
population to being a smaller size population.

(Refer Slide Time: 13:15)

Next, we have the stochastic factors or chance factors which are more important when
your population sizes are smaller. Here you have demographic stochasticity, so things like
all the off springs in your litter belong to the same sex all of them are males or all of them
are females. Now that is very much unlikely when you have a large size population, but in
a smaller size population suppose you only have two offsprings.
In the population so only two offsprings there is a very high chance that both of them are
male or both of them are females or say things like death in the litters. So, every population
would be having some amount of infant mortality and some amount of juvenile mortality.
There is a chance that you had only two offsprings in the current generation and both of
them tied.
It is much less likely if you have say 2000 offsprings in a population, it is very less likely
that all 2000 of them would die, but then if you have only two offsprings it is very much
much possible that both of them die out. These are demographic stochasticity. ‘Demo’ is
population; so ‘demographic’ is, we are talking about the about the characteristics of
population and these are chance factors that act on the population characteristics.
The second chance factors are environmental variations and fluctuations. Suppose you
have a drought, suppose you have a flood. So, that would also play a very crucial role if
you already have a very small population or catastrophes such as forest fires and diseases
or say genetic processes such as loss of heterogeneity and inbreeding depression.

(Refer Slide Time: 15:07)

In this case what we are saying is that if you have a large size population, you have a
number of males and a number of females and when they are breeding randomly. In that
case there is a very small chance that brothers and sisters would mate with each other, but
then if you have a very small population, suppose you only have three individuals left. So,
if you have any amount of mating then there is a very high chance that you will push this
population towards inbreeding depression in no time.
(Refer Slide Time: 15:33)

Other kinds of a stochastic factors are deterministic processes such as density dependent

mortality on exceeding the carrying capacity of the habitat. In this case, what we are saying
is that suppose, you have a habitat and this habitat can support 100 organisms.
(Refer Slide Time: 15:47)

In this case, suppose your population size is close to around 90. When you are reaching
close to 100,so, in that case the mortality increases because your populations now reaching
towards the carrying capacity. We had seen this and the case of the sigmoidal curve. So,
when we have the sigmoidal curve, this is the carrying capacity, this is the number of
organisms and this is your time.
When the population has reached to this level, so it is very close to the carrying capacity.
In this case ,the death rate would increase or probably the birthrate would also go down
because of some behavior reasons. If you can support only ninety of organisms and
suppose you have two different kind of organisms, so here you have chital and here you
have sambars in this population.
Suppose, you have both of these as 50:50. So we have 50 chitals and 50 sambars. In both
of these populations, we will find some amount of mortality that is going on, but then
suppose stochastically if you have a situation in which you have only say, 3 chitals left
and you have 97 sambars that are left.

(Refer Slide Time: 17:09)

In this case, it is possible that because of this density dependent mortality these 3 chitals
by chance die out and in that case also you will be pushing a very small population of
chitals towards a complete extinction.
(Refer Slide Time: 17:35)

Next, we have migration among population. In this case, you have two populations;
population A and population B. Suppose you have these two populations as say Kanha and
Pench, and both of these are connected. You have Kanha tiger reserve, you have the Pench
tiger reserve and they are connected through some amount of forest.

If there is some particular species that only has a 4 organisms left in Kanha. If these four
organisms move out to Pench then we will say that Kanha a suffered a local extinction of
this particular species. So, even things such as migration among population might be
responsible for a local extinction somewhere.
(Refer Slide Time: 18:19)

The factors that are leading species towards extinction can be remembered using this
acronym HIPPO. So, Hippo is short for Hippopotamus, but then you can used to
remember H is habitat loss. So, here you also have things like habitat degradation or habitat
fragmentation, but we classify all of this as habitat loss. So, we look at these in greater
detail in a short while. I stands for invasive species. So invasive species, if they come in
to your forest they will out compete all the other existing natural species and then they will
push them towards extinction.
P stands for pollution, so pollution also leads to habitat degradation. The second P stands
for human over population because the more number of humans that you have on this
planet, the more would be their requirements and to meet their requirements, they would
be using up the resources that are available in the habitats of different organisms. So,
human overpopulation also plays a very big role to in the extinction process and O refers
to over harvesting.

(Refer Slide Time: 19:31)

Over harvesting is the is a process in which suppose you have a particular lake and in this
particular lake you have certain fishes and because these fishes are also reproducing. So,
their population would increase with time and so we can remove certain amount of fishes
from this particular lake to be used as food for humans.
Suppose you have 1000 fishes here. Now out of these 1000 fishes if we remove say 100
fishes. So you will be left with 900 fishes in the lake and these 900 fishes would be able
to reproduce in a manner that they are able to restock the population. But then in place of
taking out 100 fishes, suppose we are taking out 990 fishes.
In that case what happens is that we are only left with 10 fishes in this particular pond or
in this particular lake. Now 10 fishes are not sufficient to restock the whole population of
the lake, so in place of exploiting 100 if you exploit 990 that is the case of over harvesting.
Over harvesting can also push a species towards extinction and very good examples of
over harvesting are not only fishing in the lakes, but also killing of whales from the oceans
or maybe even poaching of certain animals for their skin or for their fur and so on.

(Refer Slide Time: 21:01)

Different species will have different levels of impacts when we are having these factors of
hippo. The most important factor is the impact of human beings. If you have a larger sized
human population, if you have more amounts of impacts of humans on certain habitat, it
would lead to a differential impact on different species.
Sensitivity of a species to human impacts would be dependent upon these factors, the first
is adaptability and resilience of the species. Now if you have a species that is r-selected,
so here we are talking about r-selected and k-selected.
(Refer Slide Time: 21:41)

Now r-selected species are those that have a very high birthrate then they reach sexual
maturity faster and in a number of circumstances there is little parental care. So, examples
include things such as rats or mice or rabbits. Now in the case of mice, so you will have a
male and a female that would give rise to say 20 offsprings and then they would become
sexually mature in say around 6 to 8 months and then each of them would also give rise to
20 other offsprings.
Now, what happens in that case is that you have a very high amount of ro are the intrinsic
growth rate; so you have a growth rate that goes like this. So, this is the number of an
individual’s and this is time. Now in the case of r selected species because you have a very
high ro, so the population increases very fast and when that happens, even if you are you
are removing a number of organisms from that particular species the number of organisms
that are left out would be resilience enough to restock this species.
For instance, earlier suppose you had 1000 mice in a form and you are able to kill 900 out
of them. So, 900 got killed and your only left with 100, these 100 because they have a very
high birthrate because they reach sexual maturity very fast because they have little parental
care so they are mostly independent right from birth. So, there will be able to restock this
and increase their population to come back to 1000.
On the other hand, if we talk about the k-selected species, now k-selected species are
those that have the opposite characteristics of r-selected species. So, they have a very less
amount of birthrate, they reach sexual maturity in a very long period of time and there is a
lot of parental care that is required a good example is say humans or organisms like tigers.

(Refer Slide Time: 24:09)

Now, in the case of tigers one male and a female would produce a litter of say two three
or four cubs. Now those 2 3 or 4 cubs will take close to around 5 or 6 years to reach their
sexual maturity and for close to around three to four years these cubs will be under the
guidance and training of their mothers who will teach them how to hunt.
Now, in these species you have a very low amount of ro there, because there you have a
very less amount of intensive growth rate and. So, if you kill of these species; if you go
into a forest and if you hunt down tigers. So, the remaining tigers will take a very long
time to restock the population. So, in that we will see that the resilience of this particular
species, say tigers towards the impacts of humans are towards poaching is less, on the
other hand the resilience of organisms such as mice or rats or rabbits is much more. So, in
that case tigers will suffer a greater brunt from the human impacts as compared to the mice.
The second one is human attention; so we are talking about which species suffer a greater
amount of impact because of human activities. The second fact is human attention.
Charismatic species such as tigers are more sensitive because humans have high demand
for their skin bones and other parts because humans have are placing much more attention
on tigers. They will go out and want to hunt tigers whereas, they are not putting so much
attention on mice. They might not go out and hunt for the mice that are found in the forest
areas. That is another factor.
The third factor is ecological overlap between humans and the species. So, the greater the

overlap the greater is the impact of humans on that particular species; and fourth is the
home range requirements of the species. Species that require larger home ranges are more
sensitive to human impacts and good example is the case of elephants.
(Refer Slide Time: 26:21)

Elephants require a large size forest. Even if you take off this much portion of the forest.
It will place a very high amount of impact on the elephants. On the other hand, if you
consider another species that has a small home range requirement say rabbits. In the case
of rabbits you have these small populations that are in different areas. Even if you take this
part of the forest out so even in that case the other populations will be able to survive
because they have a smaller home range requirement.

(Refer Slide Time: 26:59)

The next question is when we talk about threats, when quantify this threat, how real is this
threat or what is the rate at which we are using the species, what are the rate of extinction;
can we put a quantifiable figure on to this? In this case, we can make use of another
ecological learning which is that of biogeography. Now there is this Island biogeography
model by Macarthur and Wilson that says that species richness of an island is given by the
following expression.

S = C x AZ

(Refer Slide Time: 27:35)

Here we are asking the question, if you have an island, you have, say this island that has
an area of A1 and suppose you have this another island that has an area of A2. What will
be the number of species that are found in A1 and what will be the number of species that
are found in A2? So, that has been worked out and we say that the number of species or
the species richness of an island is proportional to AZ, so that is it is proportional to some
power of A. So, now, that power could be say A2 or it could be A3 or just A or it could be
A1/2 and so on, but then it is proportional to some particular function of A.
Why do we say that? Because if you have a larger sized island, so this larger sized island
will probably have more diverse habitats because as we had seen in our biogeography
lectures, in the case of a smaller island probably you have sand everywhere, but then in
the case of a larger size habitat you probably have a sand on the beaches, but then on the
insides you might have say some hills or maybe you could have even a small stream that
is draining out into the ocean or you would have some areas that have some grasses or you
could have some areas that have certain herbs and shrubs. So, a larger sized island is able
to support more amount of habitats which would then support more number of species.
Secondly, it also supports more number of organisms because this has a larger size that
supports more home range species, that is if you have a very small island you will not be
able to have organism such as, elephantsthat have large home range requirements whereas,
if you have a larger size island you can even incorporate those species that have larger
home range requirements. So, the number of species that will be found in any area would
depend on the size of the island and this has been computed as S = C x AZ, where C and z
are both constants.

(Refer Slide Time: 30:03)

Now, we can use this equation to compute how many species will lose out, if the amount
of habitat that is available to these species reduces or if you are reducing the size of our
forest. Now it has been seen by looking at a number of ecosystems that z varies between
0.15 and 0.35.
Now, let us take a middle value of 0.3, so in that case if you have an area of A1, so you
will say that the number of species or the species richness is C x A1

Z which is 0.3. Now

we let this area decrease by as much as 90%.
(Refer Slide Time: 30:49)

So, what we are saying in that case is earlier we had this much of forest and then this much
amount of the forest has been encroached up for say human activities. So, only 10% of the
forest remains, so in that case how many species would remain in that area. So, we have
A2 that is the amount remaining is just 10% or 0.1 of A1.
So, in that case S2 which is the number of species that are found in this 10% area of the
forest now will be given by C x A1

Z ; here A = 0.1x A1 and z = 0.3, so it will be given as

C x 0.1 A1
0.3
. Now we can compute the ratio of these species. So in the first instance in
the whole of the forest we had the number of species that is given by S1 and in the second
instance we have only these many species remaining which is given by S2 and we are
finding out the ratio S2 / S1.
Now, the larger area is A1 and the smaller area is A2 which is 0.1 of A1. So, we already
know that A2 / A1 = 0.1, now if that be the situation what is S2 / S1?
(Refer Slide Time: 32:15)

Now, if we plug in the values will find that S2 / S1 = 0.10.3

,which roughly comes to 50%.
So, even if you have reduced your area by as much as 90%, the number of species that
remain is as high as 50%. So, even though you have removed 90% of the area you are not
loosing 90% of the species, you are only loosing 50% of this species, but then those would
be the species that preferentially required a larger sized of home range or probably those
are the species that have a very specific habitat requirement. So, they are more specialized
species.

(Refer Slide Time: 32:59)

We can use such a theory to compute how many species are we losing out by say losing
out our different habitats, such as the habitat in the tropical forests
We know from satellite studies that the rate at which tropical forest are actually decreasing
is around 2% per annum 1.8% per annum and if we even take the lowest value of z. So,
here we are not taking z is equal to 0.3, but we are taking z is equal to 0.15, we are taking
the most conservative estimate, it would translate to a loss of 0.27% per year and we are
estimating that the number of species that we have in the tropical forest is close to around
10 million species
If you are losing 0.27% of 10 million it means that we are losing as high as 27000 species
per year, which is a conservative estimate of the number of species that we are losing and
similarly we can estimate the losses from the other ecosystems.
Now, what are these species? If you ask somebody, he or she would say that yeah we lost
a few species, we have lost the Yangtze dolphin, we have lost the dodo, but then most of
the people will not be able to move more than say 10 species, that have been lost
throughout the whole process, but then we are losing around 27000 species per year and
most of these species are those that we do not even know that they exist. A number of
herpetofauna, a number of frogs species, a number of snake species, a number of lizards
species and all of them are also very important for the biodiversity of those area.

(Refer Slide Time: 34:45)

Which species get lost is not the same across all the species. So the amount of susceptibility
of a species to extinction is not the same across different species. The susceptibility
depends on the rarity of the species and if you have a species that is more and more rare,
so there is a greater chance that it gets extinct, because if you already have a rare species
it means that it has very less number of habitats that it can make use of or it already has a
very small population size. So, the number of stochastic phenomena that can play havoc
to this species are very high. Rarity is a function of the ecology and evolutionary
characteristics of the species.