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Push and Pull Factors in Greater Detail

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We will carry forward our discussion on the Distribution and Abundance of organisms.
Today, we will have a look at some push and pull factors in some greater detail. Now in
the last lectures, we had seen that push factors and pull factors govern the distribution and
abundance of different organisms and we had defined push factors as those factors that
make a certain area inhospitable or maybe less hospitable for a certain species. If an area
is very hot, if an area is very cold, very moist or maybe very dry for some particular
organism, that organism would not prefer to live in that area and different organisms have
different tolerances to all these factors.
A certain factor might be a push factor for a certain organism and might be a pull factor
for some other organism. Similarly we had defined pull factors as those factors that attract
organisms to any specific area. If an area has abundant amount of food, it has the amiable
climate; it is neither too hot for the organism nor too cold, it is neither too wet nor too dry,

if those are the conditions. The organisms would prefer to live in those areas. So, those
factors are known as pull factors. Today, we will look at some other push and pull factors
and some push and pull factors in more detail.
(Refer Slide Time: 01:44)

One push factor is the presence of predators; now this is an observation. This is a field
observation from an area in which we are seeing the number of sea urchins. In this case
we are seeing looking at sea urchins and this is at a particular line which is called as
‘Section at J’ and here, we are looking at the algae populations. And this is the depth of
the water and in this case we can see that in these two areas, where you do not have the
sea urchins; you have an abundant amount of algae.
Essentially if you have algae in an area, you do not have sea urchins in that area and if you
do not have algae in an area, this that area is having an abundant amount of sea urchins.
The question is, can predators act as push factors for an organism? In this case the sea
urchin is the predator and algae is the prey because the sea urchin feeds on the algae. How
do you discern if actually the distribution of algae is being governed by the presence or
absence or maybe the abundance of the sea urchins in that area?
Some experiments were conducted to establish the facts.

(Refer Slide Time: 03:11)

This is the first experiment. Now in this experiment, in early July 1959, a certain area in
the seas was cleared off of the sea urchins. Essentially all the sea urchins in that area were
removed. We are talking about an area that is having a population something like this. You
have abundant number of sea urchins and you do not have any algae in that area. Now you
experimentally remove all the sea urchins and then, you try to see what is happening to the
algae population.
In early July you removed that all the sea urchins. So, the algae population is 0. Now if we
look at late July, the algae have covered roughly 10 percent of the area because there are
no more predators to feed on these algae. Then, by August it has increased to 25 percent,
by September it has increased to 50 percent and by the next year 100 percent of that area
is now covered by the algae.
When we are looking at a push factor or a pull factor, when we said that we do not have
algae in these areas; then it is possible that you do not have algae in these areas for a
number of reasons. Probably, it was because of high temperature or say low temperature
or high salinity or low salinity or maybe some wave actions, they could be n number of
But, when you are removing this one factor; then everything else is remaining the same.
The only change that you have brought in this system is that you have removed the sea

urchins and just by removing the sea urchins, you see that the algae are able to come to
these areas.
(Refer Slide Time: 05:05)

As we had discussed in the case of the transplant experiments. In the case of a transplant
experiment, we take an organism from an area where it lives to an area where it does not
live. Essentially the green one is showing you the areas where an organism is living.
In this case this is showing you these areas; this one and this one. If you take an organism
and algae from one location to another location in this green area, it is able to survive. But
then, if you take it to another area and this algae now die is off, which is what will happen
if you take the algae from here and you bring it to this area.
Because you have sea urchins, the algae will be eaten up by the sea urchins or if you take
it to some other area and the algae is able to survive then, you will say that probably algae
has not reached into that area. In this case you were taking the algae into the red region.
So, the algae was brought into the red region, but then you made just one change which
was that you have removed the sea urchins and once you did that the algae was able to
populate this area as well.
From this experiment we can say that it was because of the presence of the sea urchins that
the algae was not able to colonize this area. So, sea urchin was the push factor. This is one
way in which we can distinguish between different push and pull factors and we can tell

which factor is doing what. The next question would be why don’t you have sea urchins
in this area. Because you do not have sea urchins, we are saying that you are getting algae
in this area. But then, why are you not having the sea urchins?
In these areas, because they were close to the coast; so some amount of wave action was
dislodging these sea urchins. Now, what will happen if you try to push a sea urchin into
this area? If you cover this area and if you put a sea urchin, in that case the algae population
will go down because in that case what you are doing is that you are adding your sea
urchins. If you add sea urchins, in that case this area will also now cease to be a good
locality for the algae. Now the third question is, if you are able to put the sea urchin here
and you put it in such a way that is just not able to eat the algae; what will happen then?
Probably you can make use of a trap and you can put your sea urchins and then you can
hang that trap in this area and probably feed your sea urchins with something else. In that
case, if there could be another reason, probably your sea urchin was giving out some
chemical compounds because of which the algae were dying off. To counter such
possibilities you can put your sea urchin in a trap, you can give it some amount of food
from outside and you can keep the sea urchin in these areas and in those situations, if the
algae do not die off, then we can say that it is because of a direct predation that the algae
population is being governed.
The researchers came up with these 4 criteria that will tell us if the predator is governing
the distribution and abundance of a prey. The first criteria is that the organism does not
survive when transplanted to a site where it does not normally occur, unless it is protected
from the predators by the cages. For instance, if you take the algae and you take this algae
into this area and you have these predators that will go and feed on to these algae. But
then, you can take this algae put it into a cage so that the predators are not able to reach
these algae.
This is also a second experiment that you can do. So, in that experiment what you are doing
is that you are not removing your sea urchins in this experiment. But, you are taking your
algae, you are putting it here into this area and normally it dies off.

(Refer Slide Time: 09:39)

What you can do is you can create a trap and in this trap because you have these wire
meshes. The sea urchins are not able to enter inside and you are putting your algae into
this trap. So, you will find that your algae is able to populate in this area and the algae is
able to populate because the sea urchin is not able to feed on it. Or in a very similar way
you can take some sea urchins and you can put those sea urchins here and probably in a
trap if you put it. If you take your sea urchins you add your sea urchins here and maybe
put them in a trap. In that case also they are not able to come out.
These two simultaneous experiments can be done.

(Refer Slide Time: 10:23)

And it is telling you that the organism does not survive in transplanted to a site, where it
does not normally occur, unless it is protected from the predators by the cages. If you
protect it, then it is able to survive in that area, which will tell you that there is nothing
other than the predator that is causing the absence of that particular species in that
particular area.
The second criterion is that there is an inverse correlation between the distribution of the
organisms and the suspected predator or alternatively in the places where it occurs, the
organism is inaccessible to the predator which is what we had seen in this case. So, there
is an inverse relationship; if you have the sea urchins in large numbers, you do not have
algae. If you do not have the sea urchins, you have the algae. Or you can tell it in the
reverse way as well. If you have the algae, you do not have the sea urchins and if you do
not have the algae, you have the sea urchins. You can put it in both of these ways, but there
has to be a negative or an inverse correlation between the prey and the predators.
The third criterion is that the suspected predator is able to inflict lethal damage on the prey
in experiments in cages or it can be observed to do so in the laboratory. So, if you are
taking your predator and in this area suppose, let us say that you take a cage in this area
and in this cage you add a number of algae. The algae have grown up in this cage. Inside
this cage you also put a sea urchin. Once it has entered into this cage or we can have a very
similar experiment here as well. If you have a sea urchin in a cage and you also have the

algae, then it should be seen that the predator is able to inflict a lethal damage that is the
predator is able to kill the prey or eat the prey either in experimental cases, in the case
when you are doing it in the wild or you can see it in the laboratory.
There has to be this condition that your predator is actually able to kill the prey, and the
fourth is that there is direct evidence that the suspected predator is responsible for
destruction of the prey in transplantation experiments which means that when you are
transplanting your algae into this area. You are taking your algae into this area and you
see that when you are taking this algae, the sea urchins are going to feed on these algae
and so all these algae die out.
It should be seen even in the case of the transplantation experiments that this is the reason
that your algae are not able to survive in this area. If all these 4 criteria are fulfilled, then
we will say that it is the predator that is governing the distribution and abundance of the
prey as a push factor.
(Refer Slide Time: 13:39)

This case we observed that there is a predator which is governing the prey. It is governing
the distribution and abundance of the prey. Now, one other question that we can ask is: Do
we have also situations in nature where the prey can govern the abundance and distribution
of the predator that is the prey can act as a pull factor for the predator?

What we are saying is; Do we have situations where you have the prey and it governs; so,
it is governing the distribution and abundance of the predator, say as a pull factor and the
answer is ‘yes’.
Let us have a look at this particular study.
(Refer Slide Time: 14:53)

In this study we have a species of drosophila by the name of Drosophila pachea.
Drosophila are very small fruit flies that we normally see in and around our fruit stalls.
Especially, if you go and take a bunch of bananas; you will find very small flies that are
hovering around or say if you go in to take some mangos, you will very easily find these
flies. Now, these flies are known as fruit flies and they normally feed on certain fruits and
they are also used as experimental animals, as model animals in the laboratories.

(Refer Slide Time: 15:41)

If you want to rear a fruit fly, what you do is that you take a vial. In this vial, you will put
some amount of food in the bottom and typically this food is starch in the form of corn
plus some sugar and maybe plus some fungicides so that it stays fresh for a long period of
time and then, you will put these fruit flies into this vial and then you will cover the top
with a piece of cotton. In this case the flies have access to air from outside. So, the air is
able to reach inside there good exchange of air.
So, oxygen can come inside; carbon dioxide can go outside. The fruit flies have access to
the food here and they also have ample space here and in that case they will be able to feed
on this food and they will multiply in their numbers. This is a very commonly used model
organism. Now, the model organism that we use in the laboratory is Drosophila
Now here again, if you look at the word roots it has a very interesting name Droso means
dew; Philly is affinity. So, when we say that something is hydrophilic it means that it
absorb water; it has a love of water. Similarly this drosophila has a love of droso and droso
in this case refers to the dew. So, this is called drosophila because it typically comes out
of its pupal stage when it is very early morning.
So, in the very early mornings you will see these flies coming out of their pupal stages,
when there is dew everywhere around. So, which is why we call it Drosophila. Melano is
black; Gaster is stomach. So, if you look at its stomach you will have a black color; so,

which is why it is called Drosophila melanogaster. But then, Drosophila comes in a
number of species and in this case there is this particular species of drosophila that is found
only in the deserts and even in the deserts it is found only near a particular species of
There is this cactus on which this drosophila is feeding and it is living there. Now, if you
take this drosophila. You take this Drosophila pachea and you try to grow it in this vial.
You have added corn, you have added sugar, you have added fungicide. So, you have
given it everything that you give to a normal drosophila melanogaster and this drosophila
is not able to breed. The young ones do not come out; so in one generation it is all gone,
but then if you take a piece of the cactus. If this is the cactus, you just take a piece of this
cactus and probably you put a piece of this cactus inside. Once you do that the your
drosophila will start to reproduce. You will be able to maintain a population.
You can also take this small piece of cactus you can autoclave it, in which case you have
heated it to as high as 120oC for as long as same 15 to 20 minutes so that all the living
organisms that are there on that piece of cactus, they are all dead and if you put an
autoclave piece of cactus here, still your drosophila will be able to breed. Another
interesting case is that if you take this cactus piece and if you put it into a vial and you are
adding some other drosophila.
(Refer Slide Time: 19:27)

Let us say that you are adding your normal drosophila; your Drosophila melanogaster and
you are raising it along with this particular cactus.
What will happen is, you will see that your drosophila start dying off. This is a very curious
case that you have this Drosophila pachea that is only able to grow when you have this
cactus around and if you have any other drosophila, that will die when you have this cactus.
What is the reason? One reason is that this particular cactus gives out certain sterol
molecules. Sterols are the molecules that are typically used in the making of several
hormones. In this case the sterol molecule is toxic to the drosophila species, but then this
drosophila pachea uses this particular sterol to make its own hormones.
What has happened in this case is that because of co evolution, this drosophila has been
living on these cactuses for a very long period of time and because of this co evolution it
is now able to make use of the chemicals that are given out by this particular cactus and
every other drosophila finds it toxic and so, every other drosophila dies. But then, that this
co evolution has is happened to such an extent that if you do not give this cactus, your
drosophila pachea is going to die. Why? Because it is now so much dependent on this
cactus that it now obligately ( or it 100 % ) needs this sterol from outside from this cactus
so that it can make its own hormones.
If you do not give this cactus, this drosophila is completely unable to make this hormone
by itself. If you have such a relationship, where your predator, in this case the predator is
the drosophila and the prey is the cactus. So, your predator and prey have co evolved to
such an extent that your predator can live only on the prey population. In that case you will
have a situation, where your prey will govern the distribution and abundance of the
predator. Why? Because in the desert if you have this cactus, you will have the drosophila;
if you do not have this cactus, you will not have this drosophila.

(Refer Slide Time: 21:58)

We are talking about the Drosophila pachea. If you have your cactus, you will have
Drosophila pachea on this cactus. If you do not have this cactus, Drosophila pachea will
not be there.
(Refer Slide Time: 22:23)

In this particular case, it is the prey that is the cactus which is governing the distribution
and abundance of the predator by acting as a pull factor and instances such as these are
very important because we can make use of these instances as biological controls. There

are huge implications for biological control of pests or invasive species. We look at one
such example now.
(Refer Slide Time: 22:41)

In the case of biological control what you are trying to do is that if you have a particular
plant and this plant is, may be acting as a weed for your area, you can bring in some
predator and this predator is one that feeds on this plant. You want to bring in a predator
that feeds only on this plant because this is the plant that you want to kill. If this predator
feeds on a number of other plants; so, in that case you will have a situation where all
different plants are dying off and that is not what we want.
We want to have those predators which are extremely specific for their preys or for their
plants. An example of this is given by the pine rust.

(Refer Slide Time: 23:26)

Now, pine rust is a fungus and this rust affects two species; so it affects the gooseberries
or ribes. So, Ribes is another word for gooseberries. This rust affects the gooseberries and
it affects the pine. It has evolved in such a manner that it will spend some part of its life in
the pine trees and it will spend some part of its life in the gooseberry trees and typically in
the forest, you will find both of these trees together. You have a pine tree and as well as a
gooseberry tree. Because of co-evolution it has so evolved that it requires both of these
species now.
You have a predator that is now specific to two different preys. If you want to control this
predator, you can eliminate the gooseberries because we want to maintain a plantation of
pine and we do not want our pines to be impacted by this particular rust. You can eliminate
the gooseberries in which case the rust will not be able to complete its life cycle.

(Refer Slide Time: 24:46)

Essentially what we are saying is that you have these two species. You have the pine and
you have the gooseberry and there is a particular fungus that moves from pine to
gooseberry and from gooseberry to pine. Here you have the fungus. And this predator is
so specific that it has to move from a pine to a gooseberry and it has to move from a
gooseberry to a pine to complete certain stages of its life cycle. Now, if you want to
maintain a pine plantation and if you remove all the gooseberries from this area, so, this
fungus will not be able to complete its life cycle in which case your pine trees will be saved
from the fungus.
This is a way in which we can make use of our theories of push and pull factors for our
own use; for generating a pine plantation or you can make use of these specific predators
to kill off certain plants that you consider as weeds. These are two instances that you can
make use of by looking at the push and the pull factors.

(Refer Slide Time: 26:06)

We will also look at one other push factor and which is your inter specific competition.
Inter means between. Here you have competition between two species. This is an
inharmonious interaction as we have seen before. This is an example in which you have
two different species of birds. Here you have this portion, this white colonies are the
redwing territories.
You have these birds that are known as red wing and you have this other bird that is known
as a tri-colored blackbird. If you have a red winged blackbird, and here you have this field
observation that on 15th of March 1959 in this area there were so many colonies of these
red wing blackbirds that had come up. After a short while you started seeing the tri-colored
blackbird that started appearing on the 20th of March. So, 5 days after these colonies were
established, you started seeing the tricolor blackbirds in these areas. And your tricolor
blackbirds are more aggressive and they are larger in numbers.
So, they are able to push your red wing blackbirds to the periphery. In this case the central
area which was earlier the area of the red wings is now all taken up by the tricolor
blackbirds. In this case what we are seeing is inter specific competition that is regulating
the distribution of a species. So, your red wings are now distributed because of the impact
of the tri-colors. This is also another push factor that we see in certain instances. If we look
at these areas then you will not find any more nesting that is being done by the red wings
in this area because they have been completely pushed away.

In a short while you will see that you only have these tri-colors that are there in this area.
So, this is another push factor that we are seeing. Now we had talked about allelopathy.
(Refer Slide Time: 28:20)

We look at allelopathy in more detail. Allelopathy is a phenomenon where you have a
plant that is giving out certain chemicals which is inhibiting the growth of other plants.
Allelo is someone else, and pathy is producing some sort of a disease. How do you prove
that there is this factor of allelopathy that is working for certain species? Here is a classic
experiment that you can do.
In this experiment people wanted to show that grass has allelopathic impacts on apple
saplings or apple seedlings. How do you prove that? In this case you do 3 different kinds
of experiments. In the first experiment, you keep both of these separate. You have these 2
pots. In the first pot you have grass; in the second pot you have the apple seedlings and
you give water to the first pot, you give water to the second pot and in this case you look
at the growth of the apple seedlings.
Essentially this is a control experiment in which you are keeping your apple seedlings
separate from the grass and you are giving it water. Another experiment is where you put
water into this pot which has the grass that is growing and then, you keep this pot in a way
that you are able to gather the water that comes out of this pot. Essentially this pot is a
permeable pot; so when you are putting water onto this grass bed, the water is going
through this grass it is reaching to the roots and then if there is any chemical that is given

out by this grass. It gets dissolved in the water and then, it comes out along with this water
and then this water is then given to the apple seedlings; so this is the second experiment.
If you see the growth of these seedlings as compared to these seedlings, you will find that
these seedlings are very stunted. They are not able to grow properly, that is they are having
some kind of a negative impact which is being given because you have the grass here. But
then somebody would ask that it is possible that you have a negative impact, but then
probably this negative impact is not because of the grass but because of the soil. Now to
counter that, you take this third experiment in which you take a permeated pot and here
you have only soil, you do not have any grass; you put water here and then you take out
this water that is coming out.
So the water has passed through the layers of soil and once it has come out you are putting
that water into the apple seedlings. And now, you compare the growth of these seedlings
with that of your control seedlings and you find that there is no change in the growth.
Essentially what you are doing is, you are doing three experiments.
(Refer Slide Time: 31:27)

Experiment 1, which is the control; here you have apple seedlings plus water which is your
normal water and then you are looking at the growth pattern. Let us call it an A kind of a
growth pattern.

The second experiment is where you are putting water, this water is moving through a
grass bed and then this water is being given to the apple seedlings and then you look at the
growth pattern and let us call it a B kind of a growth pattern.
In the third experiment, you have water that is moving through a bed of soil and this soil
does not have any grass. So, it is soil only. Then it is given to the apple seedlings and then,
you look at the growth pattern and let us call it a C kind of a growth pattern. When you
look at these three observations; so, here you have the observations. You find that A is
roughly equal to C, but B is very much less than A.
The growth, when you are giving water through the grass bed, the growth is very little
when you compare it with your control. But the growth when you put it only through the
soil is roughly equal to the control. So, both of these are equal and this one is very low. In
that case you can say that, yes, there is some inhibitory effect that is coming when the what
is passing through the grass bed and this inhibitory effect is not coming because of the soil.
It must be coming from the grass only.
This is one way in which you can demonstrate your inhibitory effects. In the case of
allelopathy, you have inhibitory effects that are coming from one species and they are
influencing the other species. So, they are coming from the grass and they are influencing
another species that is apple. But then can you also have inhibitory impacts that, in which
case you have one species that is putting an inhibitory impact on to members of its own
species. Is that also possible? Before we move on to that, let us think about the reason why
any particular species would want to inhibit the growth of its own species members.

(Refer Slide Time: 34:25)

Let us say that you have this tree in an area and this tree has long roots. Probably it is
covering a very large area and then, this tree is giving out certain fruits which have the
seeds and then through dispersal, these seeds can come into an area from here to here. Or
probably they can go off even further. Let us say that this is the region where you have the
roots. So, you can have seeds that come here or you can have seeds that come even after
In this case, this red region is showing you the influence zone of roots and the purple one
is ‘outside the influence zone’ and this is also ‘outside in the influence zone’. Let us say
that you have a plant that is say coming up outside the shade, but inside the influence zone.
Let us say that a seed fell into this region and now it is trying to grow in this region. So,
this seed; if you have a seed that is coming right under the plant and that is the first
situation, the second situation and then you have a third situation, where you have this
seedling that is coming up outside the influence zone of the roots.
In the first case, the plant is going to die. Why? Because it is not getting enough amount
of shade or enough amount of sunlight because it is there in the shade of the parent tree.
But then in the case of the second plant, it is outside the shade region, but then if it grows,
then it will also take up the nutrients that are currently being taken up by these roots. In
this case, if this plant is allowed to grow. This plant will be putting a negative influence or

a competition to the mother plant. If there is a competition, both of these plants will not be
able to get sufficient amount of nutrients or sufficient amount of water.
On the other hand, you have this third situation where you have these seedlings that are
coming up at a larger distance and there is no, or there is very little possibility that it would
so it would give a competition to the mother plant. Here we are considering these three
cases. In the first case, your seedling is already at a great disadvantage. Your seedling dies,
but then in the second and the third case your seedling is not at a disadvantage.
But in the second case, your seedling can give a negative influence to the parent. In the
third case it is so far away that it will not be able to give. Here you have no competition
because it is very far away. Now if you look at nature, it makes a lot of sense for the mother
tree to kill even this seedling because if it grows it will put up a negative influence or
competition to the parent.How can you kill off the seedling when it is so far away that it is not under your shade.
You have a situation in which your mother plant will also release some chemicals from its
roots which will inhibit the growth of any of its own daughters in the surroundings.surroundings.