Namaste, In today’s lecture we will have a look at their Historical Overview of Ecology.
Any discipline of science goes through several stages in which its theories are formed;
those postulates that are not correct, they get overturned and we gather all new forms of
knowledge and we try to put them into a framework. In this lecture, we will have a look
at how ecology has come up through the years. In this particular lecture, we will not only
see who did what, but we will also pay a tribute to all the founders of this particular
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We begin with Theophrastus. Theophrastus was a Greek scholar who lived from 371 BC
to 287 BC. So, now, you can see that that our subject of ecology, it is not a new subject,
it is as long as 2300 to 2400 years old. Theophrastus was a student of Aristotle. He is
considered the father of Botany and he described interrelationships between animals and
their environments. Now a very basic tenet of ecology is that it tries to understand the
distribution and abundance of different organisms. So, in this case we ask this question
that if you have a certain organism in a certain area, why is it found in that area? So, first
try to ask where are different organisms found and then once you know that a certain
organism is found in a particular area, you ask the question why is it found in that area?
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So, essentially for instance if you have this forest and in this forest you have, say some
hills here, and you have a certain species of plant that is growing in this area on the top
of this particular hill.
So firstly, you will ask the question, where are different species found in this forest? So,
with that, once you had you have documented things, you find that this particular species
resides on the top of this mountain. Next you ask the question, why is it living in the top
of that mountain? what are the conditions that it gets there, that it is able to survive
there? Probably this particular plant requires a very cold temperature which is not found
in any other place and that is only found on the top of that particular mountain. So, we
will call such factors as pull factors. So, these are the factors that are pulling this plant to
this particular area.
And if we look at the other areas so, probably you will ask the question, why is this plant
is not found here? So, probably, you have a very high temperature in this area which
becomes a push factor. Now while discerning the pull factors and the push factors, you
also understand the requirements of different organisms; you also understand the
interactions that any organism has with members of its own species, with members of
other species; the kinds of interactions the organisms have with the environment. What
are the levels of fluctuations in the environment that any organism is able to tolerate or
not tolerate? All of these things when you study them together to understand the
abundance and distribution of different organisms, you are studying ecology. Now
Theophrastus was one of the very first people who described the interrelationships
between animals and their environment; which is why we say that, he was one of the
Now, in those days we did not have a number of theories, we did not have scientific
equipment, we did not have computers to take care of our data. So, how did people
understand or how did people study ecology in those days. Well, most of it was seeing
what is around you and documenting things.
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So, in the case of Theophrastus, he came up with a collection of 10 books and in these
books he classified plants according to their mode of generation, whether they are
coming out of seeds, whether they are coming out of birds (mode of transport of seeds),
whether they are coming out from some other vegetative parts and so on.
He also classified plants on the basis of their localities; what is found in which areas. He
also classified plants on the basis of their sizes and he also classified plants on the basis
of their practical uses such as food, juice, herbs and so on. As you can see here, he was
trying to gather information about as many plants as possible and he was then trying to
classify these plants on the basis of their modes of generation, location, size, uses and so
on. This is how ecology worked in the very early days. You tried to gather information;
you try to classify things. Then on the basis of these classifications, he was able to
discuss the importance of climate and soil to plants.
Now, here again when we are talking about this example of this plant that is living in this
area. We are talking about things like climate, because this area is colder, this area is
warmer. So, even today when we are taking these concepts for granted, Theophrastus
was the first person to point out these points; or differences in soil; probably the soil
cover in this area is very less. So, this has, say, a depth of soil, which is say around 5
centimeters; in this area you have a depth of soil which is safe around 1 meter.
Now, if you have a soil that is 1 meter deep. So, it can support a number of different
plants, but the soil that is just 5 centimeters deep it will probably not serve the needs of a
number of plants. So, because you have a very high density of plants that are found here
so, your particular species is getting out-competed. Whereas, in this area, because other
plants are not able to survive, this plant is able to survive because there is hardly any
competition in this area or things like the amount of nutrients that are there in the soil. If
you have a region that is on top of a mountain, whenever there is a rain in this area, the
nutrients that are there in the soil along with the rain, get washed to the bottom areas.
This is the reason why typically we see that the soils in the mountains are not very
nutrient rich, and these are the factors that are governing the distribution and abundance
of different organisms which is the science of ecology. Theophrastus was the first person
to discuss these factors; the importance of climate, the importance of soils to different
His book is known as ‘Enquiry into Plants and Minor Works on Odours and Weather
Signs’. He was the first person who we are talking about; but in the case of
Theophrastus, it was difficult for him to note all these different plants, it was difficult to
classify these plants because, say, even in the case of India, if I am talking in English, I
will say that this tree is a mango tree, but if I am talking in Hindi I will refer to it as “aam
Now, if you have different names for any particular species, it becomes difficult to
communicate with other people. So, this was a problem that remained for a very long
period of time.
(Refer Slide Time: 08:45)
For instance, in the case of Theophrastus, in his book, if he wrote the name of some
particular plant, and he was a Greek scholar, and if this text was taken to some other area
say, it was taken to Egypt and probably people in Egypt would be using some other
different terms. So, if you have different names for the same species in different areas,
you are not able to collate the information or the findings that are being taken up by
different scholars. So, this problem was sorted out by the next person who is Carolus
From around 300 BC, we are jumping to the 18th century. That does not mean that we
did not have any collate in this period, but it is simply that because of want of time, our
list cannot be an all - encompassing list. And so, we are just picking on different people
who in our opinion played a very major role in the science of ecology. Now Carolus
Linnaeus or Carl von Linne, was a Swedish botanist, physician and zoologist.
Here again you can see that there is one person who is a botanist, is a zoologist and he is
also a doctor because, he was prescribing medicines, he is a physician, because in those
early days all these sciences had not matured enough in a way that we are seeing them
now. If there is a person who is studying botany, a person who is a botanist, he has got so
many things to study because there is so vast amount of information that, probably, the
people were able to devote very less amount of time to other fields. But in these early
days, we had people who had a grip on a number of subjects and so, in most cases we
refer to them as naturalist.
So, a naturalist is a person who is studying nature, a person who is knowing about nature
and a person who is observing nature Now, that nature can be in the form of plants, it can
be in the form of animals, it can be in the form of rocks or soil or anything. So, in these
early people, you will see that they had a command over a number of topics and they
played a very important role in the development of a number of different fields of
science. He lived from 23 May 1707 to 10th of January 1778 and he is called the father
of modern taxonomy. Taxonomy is a process or it is a science in which you name
different taxa. If you are seeing a plant; if there is this mango tree somewhere, what is
the name that you give to this mango tree? and what is the family to which this mango
tree belongs? Essentially in the case of our biological world, we say that we have
different kinds of taxa.
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We start with kingdom. For instance, a mango tree belongs to the plant kingdom; it is a
plant it is not an animal. We have this hierarchy: kingdom, phylum, class, order, family,
genus and species.
In the case of mango, he said that mango will be written as Mangifera indica where
mangifera is the genus, indica is the species and it belongs to a family that is known as
the Anacardiaceae family. What he was doing here was that he used these Latin words
for different things. In place of calling it a mango, because different languages that are in
use today, they can change with time. So, for instance in the case of Hindi, it could have
changed from “aamru” to “aam” and probably in some other places in India people
would, in place of calling it “aam” they would call it “aama”. So, in different dialects, in
different languages, the names change and more so, if you are using a language that is
currently in existence.
He made use of all these Latin terminologies and he developed this science of taxonomy
which is a science of naming different species, in different taxa, and he is also called the
father of systematics because, after naming all these different species, you were able to
classify different species. Naming of different species, naming of different taxa and
classifying of all these taxa is the contribution of Carolus Linnaeus.
In the case of classification, this also has a number of other consequences because, once
you have classified a particular species and if you find some other organism or some
other species that is having very close connection, i.e.; it looks similar; probably it has
similar attributes; then probably you will put them together in the same genus. Similarly,
the organisms that are close together in the same genus, they will be put in the same
(Refer Slide Time: 13:53)
For example, in case of a dog, we call it a Canis familiaris. It belongs to the Canis genus
and the species is familiaris. In the case of a wolf, you call it Canis lupus. Both of these
belong to the same genera, which is Canis. In this case, it also tells you that, those
organisms that are placed together they probably evolved from a common ancestor
because of which they are having very common properties.
If we look at systematics, we can discern a number of ideas about evolution, and
similarly evolution has been able to refine systematics and taxonomy over the ages.
These are the contributions of Carolus Linnaeus. Once you were able to name each and
every species and each and every taxon, once you were able to classify them together,
then you could observe patterns amongst different organisms, amongst different taxa.
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Next, we will have a look at Thomas Robert Malthus and we will devote one complete
lecture to what he said. He was an English cleric and a scholar, who lived from 13th
February 1766 to 23rd of December 1834, and in 1798 he wrote a book, ‘An Essay on
the Principle of Population’, and this person, by writing this book has influenced a
number of studies in population ecology.
Now to put his ideas simply, he said that if you consider any population, it grows via
geometric progression. Basically, from two individuals, you become four individuals;
from four it becomes 8; 8 to 16; 16 to 32. Essentially you are doubling at every point of
time. Let us say that if you have a population that was 1000.
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From 1000 it becomes from 2000, from 2000 it becomes 4000, then it becomes 8000,
then it becomes 16000 and so on. Now that is about the population. And in this case, we
are talking about the population of human beings. But then, if you look at the resources
in terms of say agriculture, probably when you had 1000 population; you probably had
1000 bushels of wheat that were being grown. Now agriculture grows via arithmetic
progression; in which case, in the time it became from 1000 people to 2000 people, it
grew from 1000 to 2000 bushels of wheat.
When this population was able to grow from 2000 to 4000, this was a geometric
progression. Now in the case of a geometric progression, you have this situation that an+1
is given by an multiplied by a factor of ‘r’. So, which means that you can get to the next
term of the geometric progression by taking the previous term and multiplying it with a
common factor, in this case we are taking ‘r’ is equal to 2. So, 1000 multiplied by 2 is
2000 , 2000 multiplied by 2 is 4000, 4000 multiplied by 2 is 8000 and so on.
In the case of an arithmetic progression you say that an+1 is equal to an plus common
difference which is ‘d’. So, you add something that is common. So, with 1000, if you add
1000 you get 2000, but then this ‘d’ remains constant. So, then to your 2000 you add
1000 you get 3000, then you get 4000, then you get 5000 and then so on. Now it is very
easy to observe here that in a very short period of time the population would have grown
to such a large extent that the agriculture will not be able to compensate for that amount
of growth. So, essentially your population will grow much faster than agriculture will
So, you will have a dearth of resources and once you have a dearth of resources there
will be intense amount of competition and probably some individuals in the population
will have to die. So, he talked about preventive measures and he also talked about
positive measures through which you can keep your population in check. These were the
very early ideas that influenced a lot of thinking of population ecology.
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He was also the person who played a very important role in a sub-discipline of ecology
which is population ecology.
The next person we will consider is Alexander von Humboldt. He was a Prussian fellow.
Prussian is a person who belongs to the erstwhile empire of Prussia, which is now
Germany. So, he was essentially a German.
He was a polymath. He was a geographer, he was a naturalist, he was an explorer. Again,
we are seeing that he had a command over a number of languages and a number of
fields. And he lived from 14th of September 1769 to 6th of May 1859 and he performed
quantitative work on botanical geography and he is considered the father of
biogeography. He was an explorer, so, he took a contract from the Spanish empire and
then he went to South America. In South America he went to Peru, he went to
Venezuela, he went to Cuba and a number of other places.
And when he went to all these areas, he kept a lookout for what different organisms are
found in what different areas. So, in this case you see that there are certain organisms
that are found in the lower slopes of a mountain.
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So, there are certain organisms that are found here, certain others that are found here,
certain others that are found here and so on or probably in the South American continent,
you had certain organisms that were, say found in this area, there were certain organisms
that were found in this other area. So, essentially, he studied the distribution of different
organisms and he looked at it in a quantitative manner. So, he put everything down to
numbers and he is considered the father of this field of biogeography.
(Refer Slide Time: 20:50).
Next, we will have a look at Alfred Russel Wallace. He was a British naturalist, an
explorer, a geographer and an anthropologist and a biologist who lived from 1823 to
1913 and his contributions were that he independently conceived the theory of evolution
through natural selection and also worked on biogeography. So, Wallace’s line separates
animals of Asian origin from animals of Australasian origin. His main contribution was
about the theory of evolution. We generally attribute the theory of evolution to another
naturalist, Charles Darwin.
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Charles Darwin was an English naturalist, geologist and biologist. He lived from 1809 to
1882. He is famous for the theory of evolution and the theory of natural selection.
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So, essentially this was a contribution of both of these naturalists; Charles Darwin and
Alfred Russel Wallace.
(Refer Slide Time: 21:50)
Now, what do you mean by ‘Evolution’?
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Evolution refers to changes that happen with time in different species; they may lead to
differences that prop up in a species and they may also lead to the formation of a new
species. So, you can understand evolution very easily if you look at the sub-steps of
(Refer Slide Time: 22:16)
So, if you consider any population. Let us consider the population of human beings. In
the population of human beings, you will have different individuals who have different
So, there will be somebody who is tall, there will be somebody who is short, there is
somebody who is dark, somebody who is fair, there will be somebody with a black hair,
somebody with a brown hair, somebody with black colored eyes, somebody with blue
colored eyes. So, you see ‘n’ number of variations and these variations are not only in
the external traits, but these variations are also found in a number of internal traits such
as metabolic processes. There are some people who can drink milk very easily; there are
some other people who have a lactose intolerance.
We have variations in a number of these different traits, and similar to human beings we
have variations in all different species. If you take any species and if you consider the
individuals of that species, every individual will have some differences from the other
individuals, maybe less so in the case of homozygotic twins, but more so, in the case of
people who are distant relatives or maybe not related to each other. If you have these
variations, what can be these variations?
Let us say in the case of mosquitoes, you spray insecticides on a population of
mosquitoes. There will be some mosquitoes that will die off preferentially because they
are completely not able to tolerate at the insecticide, but then there will be some
individuals that will be able to tolerate the insecticide and they live. There will be some
mosquitoes that fly very fast or there will be some mosquitoes that are slow in flying.
Now those mosquitoes that are flying very fast are probably using immense amount of
energy and those mosquitoes that slow are probably using less amount of energy. But
then you will find variations everywhere. So, the first tenet in evolution is variation. So,
every species has its individuals and every individual is different from every other
There are some variations that are there in different populations, but then the second
tenet is about overpopulation. Now in this case, what we are saying is that every
species, the organisms of every species have this innate ability to produce ‘n’ number of
off-springs, may be much greater than what the nature can sustain. So, for instance if you
look at a single female mosquito, it may lay as many as 500 to 1000 eggs.
So, from 2 mosquitoes that are there in the parental generation; a male mosquito and a
female mosquito, you have 1000 individuals in the next population, in the next
generation. So, from 2 to 1000, you have a 500 times excess. If this thing continues for a
while in the next generation you again have 500 times excess and you have 500,000
individuals. And then in the next generation you again multiply it by 500 and you have
250 million individuals. What is happening in this case is that if you have two
individuals and suppose nature can support for say 1000 individuals, but then you can
see that in just first generation, second generation and third generation, you have
exceeded the capacity of nature by a very huge amount. So, every species has got this
trait that it can over-populate. Even in the case of human beings, people can produce as
many as 6 or 10 off-springs. So, in just one generation you can increase your population
by as much as 5 folds, 6 folds, or 7 folds. And then in a few generations you will have so
many human beings that you do not have ample resources for them. This brings us to the
3rd point which is the struggle for existence.
Now in the case of these mosquitoes you have these 250 million mosquitoes and the
nature can provide resources for say 1000 mosquitoes, so, what will happen? At every
stage there will be some number of individuals that will die off because those individuals
that are better able to get food, that are better able to find a mate, that are better able to
say fend off from the predators, they will survive and the others will either die of
starvation or maybe they would not find a mate. So, they are not able to pass on their
traits to the next generation or maybe they are killed off by the predators and so on. So,
there is a constant struggle for existence if you see the nature.
And because of this struggle for existence the 4th step is that of the survival of the
fittest or the process of natural selection. So, just because there is a huge struggle for
existence, you have less amount of resources, more number of individuals. So, there will
be some individuals that will be lost and when you have these processes that are going
on for a very long period of time, you will have certain traits that will preferentially start
showing up in this particular population and probably this would even lead to something
that we call as speciation, in which the population becomes so different from the
previous generations that it now becomes a new species in itself.
That is the contribution of Charles Darwin and Alfred Russel Wallace. Once you have
this idea, in the case of Carolus Linnaeus, he talked about different individuals, different
organisms; he was able to classify these organisms, but then Charles Darwin and
Wallace were able to show how these new organisms get formed; how these different
classes, how these different genera get formed. This was a very big contribution because
it was able to collate all the information that had been given by the previous generations.
(Refer Slide Time: 29:04)
Next, we will have a look at Herbert Spencer. He was an English philosopher, biologist,
anthropologist and a sociologist who lived from 1820 to 1903 and he actually gave this
term ‘survival of the fittest’. He also worked on the ideas of evolution.
Next, we will have a look at Ernst Haeckel. Ernst Haeckel was a German biologist,
naturalist, philosopher, physician, professor, marine biologists and artist who lived from
1834 to 1919. He made detailed multicolored illustrations of animals and sea creatures.
Here we can see that our quest with naming different things, classifying different things
has continued even till the 20th century because, Ernst Haeckel was also involved in
making a collection of what different organisms are, how do they look and so on and he
also coined the term ecology.
(Refer Slide Time: 30:13)
In the term ‘Ecology’, ‘eco’ is home and ‘logos’ is study. It is the study of the home.
And in the case of Ernst Haeckel he actually called it as Oekology. He was the person
who played an important role not only in making all these different illustrations which
guided the succeeding generations, but he also coined the term ecology.
(Refer Slide Time: 30:41)
Next, we look at Vladimir Vernadsky. Vladimir was a Russian scholar, He belonged to
Ukraine and he was a mineralogist and a geochemist. Here was not very much related to
biology, but then because of his command over minerals and over geochemistry he was
able to give vital inputs to the science of ecology. He lived from 1863 to 1945 and in
1926 he wrote the book “the Biosphere”. Now a biosphere refers to that portion of the
earth that is able to support life.
So, on the earth we talk about lithosphere. Lithosphere is the rocky portion; we talk
about atmosphere which is the air that is surrounding us; we talk about the hydrosphere
which is all the water bodies that we have. And there is life on land, there is life in the
air, there is life in the water bodies. So, a combination of all of these that are supporting
life is called the biosphere. Vladimir also played an important role in being one of the
first scientists to recognize that oxygen, nitrogen and carbon dioxide in the earth’s
atmosphere result from biological processes. And he also gave rise to the biogeochemical
cycles. What do we mean by biogeochemical cycles? If you consider any organism; if
you consider yourself, you are taking in a number of nutrients. So, you are taking
carbohydrates, proteins, fats, different mineral salts and so on. Now how are you getting
all of these? You are getting it from your food, whether you are a vegetarian or a non-
vegetarian, we are consumers, we are not able to produce our own food, we are not able
to perform photosynthesis.
We get it from something else. Now that “something else” can be a plant if you are a
vegetarian or that can be an animal if you are a non-vegetarian. Where does that animal
get its food from? Ultimately it will get its food from a plant source. So, plants are
ultimately supporting the whole of the biosphere.
(Refer Slide Time: 33:10)
Now if you consider any plant, while it is growing, it requires water. So, it draws up
water from the soil through its roots; it requires air, especially it requires carbon dioxide
which it takes from the air; it requires energy from the sunlight and it also requires a
number of minerals that it gets from the soil when they are dissolved in water. So,
through the process of transpiration this water is taken up from the soil and then it is
released into the atmosphere; with the water it takes up the mineral salts, it takes carbon
dioxide from the air, it takes energy from the sunlight and then it produces the food.
This food is in the form of carbohydrates, proteins or fats which are then stored in its
body and then from this plant, when a herbivore eats it, it gets into the animal kingdom,
then from one animal it moves to another animal and so on till it reaches the top predator
or the apex predator. When a plant dies, when an animal dies or any of these animals die
or when they are giving out some waste products, say, the animal defecates or the plant
is shedding its leaves. all of these will be then acted upon by the decomposers.
The minerals are also taken up by the plant. So, let us consider any one metal; let us say
the plant took up iron. Now this iron, when the animal ate this plant, the animal got the
iron; from this animal it moved to another animal and so on. And when the plant died or
the animal died or with their waste products the decomposers got this iron and then they
were able to break down these organic molecules so that the iron got released into the
environment and it was again able to reach the mineral pool that is there in the soil.
So, in this way we can see that there is a continuous cycle that is going on from the
minerals that are there in the soil, they are taken up through all of the biosphere and then
they are released back into the soil. Similarly, if we look at another element such as
carbon. Now carbon is a component of carbohydrates, it is there in proteins, it is there in
fat, so, it is there in most of the organic molecules that we have.
Where do the plants get carbon from? They take it from the air in the form of carbon
dioxide, then they make food, from there when the animals eat it, the animals are getting
the carbon, from one animal it is moving to another animal which are again getting
carbon. All of these organisms are releasing carbon dioxide through the process of
respiration. So, if there is any animal that is respiring, it is releasing a carbon dioxide and
when these organisms die or when they shed their parts or will they defecate, all of these
come to the decomposers. So these decomposes are also getting carbon and then these
decomposes breakdown this carbon into carbon dioxide and release it into the
Here we are seeing another cycle which is moving from the atmosphere through the
plants, through animals and then back into the atmosphere. So, these processes are
known as biogeochemical cycles because, they are involving biological beings, the living
beings in the form of plants and animals, they are also involving in the earth. So, which
is why you have the ‘geo’ component and these are chemical cycles because you are
seeing a movement of chemicals through all of these.
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