Today we begin with a new module which is ecological interactions.
In this module, we will have three lectures. The first is Positive Interactions. We will begin
with, what are ecological interactions? and then move on to the positive interactions that
we observe in different ecosystems.
Then will have a look at negative interactions in the second lecture and the third lecture
will be a study of behaviors and behavioral ecology. So, let us begin with positive
(Refer Slide Time: 00:42)
What are interactions? Interactions are effects or impacts that the organisms in a
community have on each other. So essentially, if you see any organism that is there in the
ecosystem, it will be doing something or just by its presence; it is taking up some space.
And when it does so, it is putting and impact on the neighboring organisms.
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For instance if you have a forest and in this forest we have this tree; now just by its presence
here, it will be casting a shadow beneath itself and also to some degree on the sides. Now,
just by its presence, it is taking away all of these resources, all of this space, may be a lot
of water, a lot of sunshine and a lot of mineral nutrients. Because of which if there is any
plant that is trying to grow up here, they will not be able to get adequate amounts of water
or nutrients and then they will die out.
This is a process that we call as competition. This is a negative interaction that this tree is
causing just by its mere presence, because it is casting a shadow on the surface of the
ground. On the other hand, this tree by the same process might also be helping somebody
else. For instance if we have an animal and this animal cannot face the very high heat; that
is there in the summer seasons, so you will find that this animal will move in the shade of
this tree and so this animal will get some help out of this tree.
This is a positive impact that this tree is causing by its near presence here and also because
it is releasing water in the process of photosynthesis, it is taking water from the ground
and then it is releasing it through evapotranspiration. Now, through this process of
evapotranspiration; it is also cooling the surroundings. These surroundings are much more
cooler as compared to the warmer surroundings that are there in the vicinity. All of these
different impacts that any organism in a community is casting on the other organisms in
that community are known as ecological interactions.
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And these interactions can be divided into two parts; we have intraspecific interactions and
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So, essentially what we mean by that is if you look at the word roots, ‘intra’ is within and
‘inter’ is among. Now, ‘specific’ is related to species; now intraspecific would mean
interactions within the species. For instance, if we consider a population of say chitals; so,
any impacts that when chital is causing on another chital would come in the category of
intra specific interactions.
Whereas when we talk about in inter specific interactions; it would mean interactions
among different species. For instance, if we have chital and we have sambar and if there
is some interaction some impact that a chital is causing to a sambar; then we will call it as
inter specific interaction.
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The next thing is harmonious and inharmonious interactions; now harmonious means
something there in harmony or positive ecological interactions. How do we define a
positive or harmonious ecological interaction? It is an interaction where none of the
participating organisms is harmed. Whereas, inharmonious interactions or negative
interactions are those interactions, where at least one of the participating organisms gets
Even though there could be situations where none of the organisms is getting a benefit, but
if none of the organism is getting a harm; we will call it a positive ecological interaction
even if no none of the organisms is getting a benefit, the main principle here is the harm
(Refer Slide Time: 05:11)
If we look at different intraspecific ecological interactions; we see harmonious interactions
in the form of colonies and societies that we will look in greater detail in this lecture. And
there are some inharmonious interactions such as intraspecific competition and
Intraspecific is within the species; there is some competition. When we were looking in
the previous example; so in this example, if this tree, this plant and this tree belong to the
same species, then the kind of competition that is happening here will be called as in
intraspecific competition. So, that is an inharmonious interaction and we will look at it in
greater detail in the next lecture, and cannibalism where, an organism eats another
organism of the same species.
(Refer Slide Time: 06:10)
These are the intraspecific interactions; what about the inter specific ecological
interactions? Interspecific is between two different species. In this case we have
harmonious interactions that is positive interactions in the form of proto-cooperation,
mutualism and commensalism and we will look at these in greater detail in this lecture.
Inharmonious or the negative interactions which are interspecific competition; now what
do we mean by interspecific competition? In the case of our tree, if say this tree and this
plant belonged to different species; then such a kind of competition that this tree is not
allowing this plant of a different species to prosper to grow there; then we will call it an
interspecific competition. Other examples of inharmonious interactions are parasitism
which is related to the parasites that are found.
Predatism, which is related to predation or predatory behavior in which one organism eats
another organism of another species; so this is known as predation. And the third one is
ammensalism in which an organism is harming another organism of a different species
even though it is not getting a benefit out of it.
(Refer Slide Time: 07:31)
We can summarize these interactions in the form of this table. So, we have ecological
interactions, impact on the first organism and impact on the second organism.
In the case of competition, you have a harm to both the organisms. Here we are having
two organisms that are interacting in the ecosystem; if their interaction is in the form of
competition, so both the organisms are getting harmed. On the other hand, if it is
ammensalism, then one organism is harmed whereas, there is no impact on the second
And a very classical example of ammensalism is where you have cattle that are browsing
in a grassland. Now, on those grasses that they are eating they are showing a predatory
behavior; so they are removing the other organism in the form of exploitation. Whereas
just by walking they are trampling a number of plants and when they are trampling these
plants the cattle are not getting any benefit out of this behavior. So, the impact on the cattle
is 0; they are neither getting a benefit nor are they getting a harm; but just by walking on
the grass when they are trampling the grass the impact on the grass is negative, so such
processes are known as ammensalism.
Third one is exploitation; now exploitation is where one organism gets a benefit and the
second organism gets a harm. For example, we have predation, parasitism; so most of the
diseases are examples of exploitation. For instance, if you get tapeworms; so these
tapeworms will be getting food from you and they will be harming you. This is an example
Neutralism is a situation where there is no impact on any of the organisms that are
interacting. Now, examples of neutralism are very limited, because in a number of
situations we have not yet been able to dissect out clearly what are the impacts on the
organisms. For instance, you could say in the first instance that two trees that are separated
from each other.
(Refer Slide Time: 09:55)
Here we have a forest, here we have one tree and here we have another tree and this is the
canopy and this is the second canopy. In the first case, it might occur that both of these
trees are not having any impact on each other; neither a positive impact nor a negative
impact because they are very much separated from each other, but then it is quite possible
that the roots of the first tree are so long and so vigorously growing that they are tapping
into the water source or the water that is available to the second tree. And if that situation
occurs then we would say that there is one tree that is getting a benefit and another tree
that is getting a harm.
While in the first instance; it might appear that both of these trees are in a relationship of
neutralism, but it is also possible that they might be exploiting each other, but we call these
kinds of interactions as neutralism where the impact on the first organism and the impact
on the second organism are both 0.
Next we have commensalism; now commensalism is an interaction where one organism
gets a benefit and there is no impact on the second organism. Here also when we say no
impact; it might be a minimal impact, it might be an impact that we are yet to discover;
however, if more or less if we see that there is a very minimal impact or no impact on the
first organism; we will say that it is a relationship of commensalism. Here, one organism
is getting a benefit and the second organism is not getting any benefit or any harm.
A good example could be, if you are walking on the countryside and you watch a buffalo
and on top of that buffalo there is a crow that is getting a hitchhike. Now, this crow is
getting a benefit because it is not expending any energy; it is just using the buffalo as a
means of transport. And there is no impact on the buffalo because a buffalo is say around
400 kg organism. So, it does not make much difference if say 2, 3, 4 kg crow is sitting on
top of it.
However, there might be some negative, some negative impacts because even though it is
a very minimal weight, but still this buffalo has to explain a bit more of energy. But, we
can classify such interactions in the name of commensalism because the impact on buffalo
is minimal, it is very close to 0 and finally, we have the interaction called as mutualism.
Mutualism is a relation where both the organisms are getting benefitted.
To sum up here we have 6 different kinds of ecological interactions, competition,
amensalism, exploitation, neutralism, commensalism and mutualism.
(Refer Slide Time: 13:01)
When we are talking about intra specific that is belonging to the same species, harmonious
interactions we said that we have the examples of colonies and societies. Colonies are
defined as functional integrated aggregates formed by individuals of the same species.
They are functional integrated aggregates that are formed by individuals of the same
species. Good examples in this case are coral reefs, filamentous algae and microbial
colonies. What do we mean by functional integrated aggregates?
(Refer Slide Time: 13:41)
Let us have a look at a piece of coral reef. Now, this is how a coral reef would look like.
This is a small portion out of it and then, here we are observing that you have this huge
rock like structure and it appears that this is just one piece of rock, this is just one organism.
Whereas, actually we have a number of different organisms of the same species that have
colonized it together and they have formed an integrated aggregate; so this is an example
of a colony.
(Refer Slide Time: 14:15)
Another example of colony is microbial colonies; for instance in this petri dish and this is
an image from the Nobel lecture of Alexander Fleming, who discovered penicillin; the
Here we are seeing penicillium colony; so this is a colony that is made by a fungus and
here we are having normal staphylococcal colonies; so all of these are different colonies.
If you look at any one of these colonies, we will see that this is one structure; you will not
be able to differentiate different bacteria that are inside this structure. But then, all of these
bacteria have come from one single bacterium through the process of multiplication.
(Refer Slide Time: 15:03)
What is happening here is that on the surface of the petridish, and here you have a nutrient
agar medium. In this case, you had a single bacterium that came here and then it multiplied
and so it became from one to a million bacteria, but all of these bacteria are together.
When you look at this structure; you will not be able to see different bacteria from each
other; we will call this a colony of the bacteria. Here we are having colonies of
staphylococcus which are bacteria and here we have colonies of fungus, which is the
penicillium and because penicillium is releasing penicillin; so it is able to kill the bacteria.
So, this is how penicillin was discovered. So, intraspecific positive or harmonious
interactions are colonies.
(Refer Slide Time: 16:10)
And the second one is ‘societies’. What is a society? Interactions for labor division and
collaboration among individuals of the same species; so, these are interactions that are for
division of labor. So, for instance when we look at a colony; a society of bee in the form
of a bee hive; so we will be having a queen bee inside, we will be having a number of
drones and a number of worker bees. Now in that, case there is a strict division of labor;
so in the case of the queen bee, her role or her labor is to is to give rise to a number of
eggs, it is to lay eggs. The role of drones is to fertilize the queen and the role of the workers
is to go out for foraging to get food from outside, to defend the nest, to build up the nest,
to take eggs and when these eggs hatch, to feed the larvae and so on.
So, there is a strict division of labor; so, these are also interactions for labor division and
collaboration. Why is this collaboration essential? Because if we just consider a group of
worker bees, they will not be able to reproduce; so they do not have this ability. Whereas,
the queen bee that has the ability to reproduce that queen bee is not able to defend itself.
Only by coming together they are able to pass on their genes; so this is an example of
collaboration. These are interactions for labor division and collaboration among
individuals of the same species. Examples are bee hives, termite mounds and wolf packs.
(Refer Slide Time: 17:57)
Even in the case of a termite mound; you will also have termites that are serving different
(Refer Slide Time: 18:03)
When we look at interspecific harmonious interactions; so interspecific is between two
different species, two or more different species; harmonious interactions because these are
positive interactions, they are not harming anybody.
So, in this case the first example is that of protocooperation. Now, protocooperation is
defined as an ecological interaction in which both participants benefit, but which is not
obligatory for their survival. Now in the case of protocooperation, you have two entities
both of them are interacting and they are interacting in a way that they are mutually getting
benefited; the first party also gets a benefit, the second party also gets a benefit.
However, even though they are getting this benefit, they are not reciting together for a very
long time because this is not compulsory for their survival. When we say obligatory,
obligatory means compulsory; so, this is not compulsory for their survival; so they might
even go away from each other. A good example is, birds eating ectoparasites on the bodies
(Refer Slide Time: 19:18)
So, a good example is - these ox birds that are there on the surface of the giraffe. We went
to Kruger National Park and there we saw this giraffe and on the body of this giraffe, you
can observe that there are many birds. There are so many birds that are sitting on the body
of this giraffe.
What are these birds doing? These birds are eating the insects and the parasites that are on
this skin of this giraffe. Why are they eating these insects and parasites? Because these
insects are parasites are serving as food for these birds. With this interaction, by coming
close to the giraffe and by sitting on the skin of the giraffe and eating away all these insects
and parasites, these birds are getting a benefit, they are getting food out of it. This is a very
easy source of food; Why?
Because if these birds went out into the ground. It will be a very much difficult to see these
insects and to eat them; whereas, on the surface of this giraffe it is very easy to see these
insects. So there is benefit to the first party; now what is the benefit to the second party?
What is the benefit to the giraffe? Why is this giraffe allowing these birds to come to its
body, sit over it and do their job, get their food? Well, this is because the giraffe is also
getting a benefit out of it because the skin of the giraffe is getting cleaned in this manner.
These birds are removing the insects and the parasites that would remain on the body of
this giraffe. However, even though both of these parties are interacting together and both
of these are getting a benefit; even then, this is not obligatory for the survival of either the
birds or the giraffe. Even if there are no birds around; this giraffe will live and even if there
are no giraffes around; these birds would live because they would switch to some other
source of food; so which is why we call this interaction protocooperation. In this, both the
parties are getting a benefit; however, this benefit is not obligatory for the survival of any
(Refer Slide Time: 21:36)
Another example is cleaner fishes; so we will look at a small video to see what cleaner
fishes are. This is an example of what we call as a hippo spa; so this was featured in the
National Geographic some time back.
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Here we have a hippopotamus and that is lying in a river.
(Refer Slide Time: 22:02)
Similar to the case of the giraffe where we had so many birds that were eating its, eating
the insects and parasites on its surface, here we have a number of fishes that are known as
cleaner fishes; that are cleaning the body of this animal, the hippopotamus.
In this process, this hippopotamus is getting a benefit because its body is getting cleaned
up. It will have less number of diseases, whereas, at the same time the fishes are also
getting a benefit because the dead skin cells of the hippopotamus are serving as food for
these fishes. Not only do these fishes clean the outer skin, they also clean the mouth of the
animal. For instance we normally do a brushing of our teeth, but in the case of these
animals; they do not have a toothbrush. This is an alternative arrangement that nature has
provided to them. So, once they open up their mouth; these fishes will even get to the
inside of the mouth and clean it.
(Refer Slide Time: 23:10)
This animal is opening its mouth and the fishes will clean the insides as well. In such an
interaction, it is essential that these fishes should not be harmed by the animal, because if
these fishes think that this is a predatory animal; so they will not be coming there and
cleaning its body or its mouth. Similarly, the hippopotamus should not also be getting any
sort of harm through this process.
(Refer Slide Time: 23:42)
Another example is that of hermit crabs and sea anemones.
(Refer Slide Time: 23:51)
Now, hermit crab is a crab that has a soft shell on the top. Let us say that this is a crab and
if you have a crab in a water body, there are a number of predators that can come and eat
up this crab.
Now what the hermit crab does is that because it has a soft shell; it will make use of any
molluscan shell. So, any sea shell or any cowries shell that is available, so, it will just take
up any of these shells that are vacated and it will get inside. Essentially, when it goes
inside; it will have this larger shell on top of it and then it will use it as a means of
protection. On the other hand, we have these organisms; so this is something that we
observed in the Marine National Park in Gujarat. Here we have seawater and here we have
this animal and this animal is known as sea anemone.
This is how it will look; so it is a very beautiful looking animal and this is a predator
animal, but as soon as you touch it; it will get inside. What this animal does is that it also
has a number of stinging cells on its body and this animal can even prey upon some smaller
fishes. When this hermit crab has this shell on top, it uses the shell for protection. It can
get an even better protection if it could go to a sea anemone and maybe take off this small
portion and then stick it on top of its own shell. In that case, we will have a situation where
you have sea anemone on the top and you have all these tentacles that are around.
Now, how does this help; when you have these tentacles which have stinging cells, so a
number of fishes will be extremely vary of coming close to the hermit crab. This anemone
is providing an added protection to the hermit crab. At the same time, as we observed here
in the; so, as we observed here in this video, this animal is a sessile animal. So, it remains
attached only at one place. If it is found here and only when some fishes come near it, will
it be able to predate upon those fishes, but then if it has a connection to the top shell of this
hermit crab. So, this anemone will also get a benefit of transport which would mean new
sources of food. In this process, both of these organisms are getting benefit out of each
other and so this is also an example of protocooperation. Why is this protocooperation?
Because you can have hermit crabs that live without a sea anemone on their top and you
also have sea anemones that live without a hermit crab. Such type of relationship is not
essential for their survival, but in this relationship, both the organisms are getting a benefit.
(Refer Slide Time: 27:39)
Another kind of relationship is known as mutualism. Mutualism is an ecological
interaction in which both participants benefit and which is obligatory for their survival.
The only difference here is that both the participants are getting a benefit, but this is also
compulsory for their survival. Examples are microbes that digest cellulose in the stomachs
of ruminants and the bacterium rhizobium in the root nodules of legumes plants.
(Refer Slide Time: 28:20)
How does this relationship work? If we look at the root nodules of legumes plants such as
soya beans. This is the root of soya bean and here we will find that it has a number of
nodule like structures. Nodules are these small protrusions that have come up on the
surface. These nodules harbor bacteria of the rhizobium species. These rhizobium bacteria
get a benefit of shelter and food from the plant. When they are here in these nodules; they
are getting protected and at the same time they are getting a free, an easy source of food
through the plant and these bacteria are able to fix nitrogen that is found in the air.
They convert nitrogen into nitrites and nitrates so that it becomes amenable for absorption
by the plant. If you just have nitrogen, the plants are not able to absorb it through their
roots, but if you convert them into inorganic minerals like nitrates on nitrates, so, in that
situation the roots are able to absorb the nitrogen in the form of nitrates and nitrates. These
bacteria which are found in the legumes; they are benefiting the plants by providing them
with nitrogen which is essential for their growth.
If you have a plant and you do not give it nitrogen this plant would die. The presence of
these bacteria permits this plant to survive even on those soils that do not have a heavy
amount of nitrogen in them, and these plants are permitting these bacteria to survive in
those areas, where survival otherwise would have been impossible. In this case, both the
organisms, the plant as well as that the bacteria are getting benefit out of each other and
this benefit is so essential for their survival that it is obligatory. They cannot live without
each other; which is why we call these as mutualism.
(Refer Slide Time: 30:29)
Another type of interaction goes by the name of commensalism. Commensalism is an
ecological interaction in which one individual benefits, while the other is neither benefited
nor is harmed. Here also we have two different organisms; this is an inter specific
interaction and it is a harmonious interaction, it is a positive interaction; nobody gets a
harm. However, there is only one individual that gets a benefit, the other individual does
not get any benefit.
Examples are bacteria and other microorganisms that live on the skin without being
pathogenic or beneficial. For instance if you look at the surface of your hand; if you took
a sterile cotton swab and if you moved it across on your hands. If you put it into an agar
plate, you will find that there are a number of bacteria that grow on the surface. These
bacteria are not harming us and neither are they providing us any benefit, but then why are
these bacteria lying on the surface? Because they are getting food out of us, because, if
there are any dead cells; they will act as food for the bacteria. If we give out any oily
secretions, that would act as food for the bacteria; so the bacteria are getting a benefit
whereas, we are not getting any benefit out of it. At the same time, we are not getting any
harm because of these bacteria because these bacteria are non pathogenic bacteria, they
are not causing any diseases to us. It does not matter to us whether they remain there or
not. We are neither getting a benefit nor are we getting a harm.
Such types of interactions in which you have a situation where one organism gets a benefit;
while the other organism neither gets a benefit nor is harmed is known as commensalism.
Another good example of commensalism is egrets that feed with buffalos.
(Refer Slide Time: 32:33)
Here we have a very interesting example. When I was looking out of my building; I saw
this wall which was being constructed. This is an area in Bhopal that is getting constructed
and here we have this wall and there are a number of a egrets. Egrets are small birds and
these are insectivores birds; so they feed on insects. Where do they get these insects from?
Well they will find them in the soil.
You have these grasses here and these grasses will also be harboring a number of insects.
There might be some locusts, there might be some grasshoppers and a number of other
caterpillars and so on. You have a plentiful supply of food in the form of insects that are
found in these grasses and you have the predator in the form of these egrets.
These egrets came to this wall very early in the morning say around 7 o’clock or say 6.30.
And they would just come here and they would keep on waiting; they would not do
anything else. We used to wonder what they are doing there, because you have a bird that
is hungry, you have insects that are available, why is this bird not getting down and eating
the insects? We used to see this every day.
(Refer Slide Time: 34:09)
One day when it was a weekend and I was there at home. I observed that close to around
10.30 or 11.00; we used to see some buffaloes that came up to these grounds. And
immediately when these buffaloes came here, the egrets came down and started feeding
on the grass. Now what is happening here? If, you consider yourself to be an egret and you
see this grass around. This grass would be having a number of insects, but then how do
you get to that insect?
So, you come down and you find that there is an insect that is behind a leaf. Insects are
also very adapted camouflaging themselves. They will not show themselves off; most of
these insects will also be green in color and when you are around; when an egret is around,
these insects would hardly move. In that case it becomes very difficult for the egret to
catch hold of an insect or to put it in terms of energetics; the cost of a getting an insect is
much greater than the benefit that you will get out of eating that insect.
(Refer Slide Time: 35:17)
Essentially if you have an insect and if that insect say gives you let us say 3 calories of
energy upon being eaten. If you do not have a buffalo around, you go down and you do
the foraging and you spend say 5 calories of energy on average per insect that gets eaten.
What we are seeing here is; if you have the birds; if this bird gets down. So, basically this
bird would be living somewhere else. It came to this wall early in the morning; so it has
expended some amount of energy to come to this area that is rich in insects, that is rich in
food; so, that has expended some amount of energy. If this bird comes down here and
when it comes down here it would start searching for insects and these insects because
they are camouflaged, so, this bird will have to spend time; it will have to spend energy to
catch these insects. On an average, if you are looking for insects, because it is energy
intensive; suppose on an average you are spending 5 calories of energy per insect that you
are able to catch; so, that is the cost of the operation. Now what is the benefit that you are
getting out of it? You have those small caterpillars or you have these grasshoppers and you
are eating those and on an average 1 insect will give you 3 calories of energy.
On an average, we would find that this would become a lossy operation because the cost
of getting the insect is much greater than the benefit that you will get out of eating that
insect. The bird does not want to spend its energy, it does not want to get into a game that
is lossy for it. This bird will not get down; it will just keep on sitting there.
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