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LESSION-3

ECOSYSTEM: FUNCTION OF THE ECOSYSTEM

Dr. Nibedita Khuntia
Assistant Professor
Maharaja Agrasen College
University of Delhi

INTRODUCTION
Function means the working of a particular ecosystem of a particular area in. It includes the
interlinking of organisms, their nutritional requirements, the circulation of nutrients, energy
flow, and again decomposition of all the chemicals, both organic as well as inorganic and
their release to the atmosphere. Mainly all these activities have been explained under three
major categories as follows
1. Biogeochemical Cycle
2. Food chain and Food Web
3. Energy flow in the Ecosystem
4. Productivity

LEARNING OBJECTIVES
After going through this lesson, you will be able to
1. Know the categories and their respective sub-categories of biogeochemical cycle.
2. Understand working of food chain.
3. Distinguish between grazing and detritus food chain.
4. Give schematic representation of food web.
5. Explain energy utilization in an ecosystem.
6. Know the various concepts of productivity.

3.1 BIOGEOCHEMICAL CYCLE

As the name indicates, in an ecosystem, both biological as well as physical components,
passing/rolling through the underground and above the ground to complete a cycle. Through
Nutrient Cycling various, inorganic and organic compounds are formed and decomposed
(normally forty elements are required by the living organisms in the nature). This is also
called as nutrient cycles of the ecosystem. "Nature is self-sufficient by nature".
Biogeochemical Cycle has been divided into two major categories like atmospheric and
edaphic cycling based on the types of the substance they are dealing with.

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I) Atmospheric Nutrient Cycle

a) Water Cycle
b) Carbon Cycle
c) Oxygen Cycle
II) Edaphic Nutrients Cycle
a) Nitrogen Cycle
b) Sulphur Cycle
c) Phosphorous Cycle

3.1.1. Atmospheric Nutrient cycle

a) Water Cycle: Water is an essential compound for the photosynthesis in plants. Plants
absorb water from the underground through the roots, to its upper or aerial parts. This
water is used for the process of photosynthesis. Some water gets back to the atmosphere
from the plant body through the process of transpiration and some are utilized by the
plants for its metabolic activity(Fig.1).
The driving force for a water cycle is the solar radiation which is 15% of the total radiation
reaching on the earth. Though the radiation water gets evaporated from the surface of the
waterbodies, it comes back to the surface of earth as rain. The above figure clearly teaches us
how the cycling of this vital resource occurs on the earth.

Fig.1: The Water Cycle

(Ref: https://upload.wikimedia.org/wikipedia/commons /9/94/Water_cycle.png)

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b) Carbon Cycle: Carbon Dioxide is an important gas present in the nature Plants needs it
for the process of photosynthesis to prepare the food material that is carbohydrates for its
own as well as for heterotrophs. That means it gets locked into the organic matter
through the process of photosynthesis. Carbon Dioxide released to the atmosphere by
respiration of all the living organisms on the earth. Earth’s Crusts also releases carbon
which is present in forms of Calcium Carbonates (CaCO3) and Magnesium Carbonates
(MgCO3) which comes from the skeletons of marine organisms in the process of
mineralization. By subsequent weathering these Calcium Carbonates and Magnesium
Carbonates get mixed with the soil and add to the soil nutrition. In this way ultimately
these carbon rich organic matters accumulate in the ecosystems.

Fig.2: Carbon Cycle

(Ref: https://d32ogoqmya1dw8.cloudfront.net/images/clean/literacy/tlp
/carbocycle/carbon_cycle_diagram_1466775306722780178.jpg

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By burning of fossil fuel, coal & wood large amount of carbon dioxide get released to the
atmosphere. These carbon dioxides get assimilated by the plants. The animals consume these
carbohydrates, that is the photosynthetic products. The carbohydrates consumed, in this way
reach to the heterotrophs. When these heterotrophs die, carbon again goes back to earth’s
crust. Through the process of respiration also carbon dioxide released from the biotic
components to the atmosphere (Fig.2).
c) Oxygen Cycle: In the atmosphere the dry air contains 20.94% of oxygen. Oxygen is
required by all the living organisms for respiration. It is also associated with the moisture
content. It is the simplest but inevitable nutrient cycle in the ecosystems (Fig.3) .

Fig.3: Oxygen Cycle

(Ref:https://www.universetoday.com/61080/oxygen-cycle)
3.1.2. Edaphic Nutrient Cycle:
a) Nitrogen Cycle: It is a complex cycle occurs in nature through various steps. About
79% of the atmospheric air is Nitrogen. It enters to the biotic world and got assimilate
then again goes back to the atmosphere. Following steps are involved for the completion
of Nitrogen cycle (Fig.4).
A. Nitrogen enters to the living organisms: Pure nitrogen gas cannot be used by the
green plants. Only Nitrate & Ammonium forms of Nitrogen can be utilized by them.

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Thus, nitrogen gas first fixed into Nitrous Oxides, Nitric Oxides and Ammonium in
the nature. The production of nitrates from nitrogen is called nitrification.
Production of ammonia is called ammonification.
1. Nitrification: It can be done both by non-biological (physical) and biological
way.
i) Non- Biological fixation of Nitrogen or Nitrification: During lightening,
nitrogen gets combined with the oxygen in nature as this reaction needs a
high amount of energy.
ii) Artificially nitrogenous compounds also get produced in industries.
They are the chemical fertilizers. Farmers use these fertilizers to
enhance the yield of the crops and soil fertility.
iii) Biological Nitrogen Fixation: As name indicates biological nitrogen
fixation is carried out by the living organisms known as nitrogen fixing
organisms.
Ex. Blue-green algae, Bacteria, Fungi etc.
Biological Nitrogen Fixation are of two types.
i) Non-Symbiotic Nitrogen Fixation: It is carried out by organisms, who
live freely in the soil or water.
Ex. Azobacter, Anabaena, Nostoc etc.
ii) Symbiotic Nitrogen Fixation: Some microorganisms live inside the
root nodules of different plants in a symbiotic association. They have
the capacity to fix atmospheric nitrogen. This process is called
symbiotic nitrogen fixation. In the roots of the higher plants, primarily
in legumes (beans, peas, soybeans), the nitrogen fixing organisms form
nodules, multiply inside these nodules and carry out the process of
nitrogen fixation.
Ex. Bacteria, Rhizobium etc.
The nitrogen gas from the atmosphere gets converted into Ammonia (NH3)
which in then converted into amino acids. Amino acids are the building blocks
of nucleic acids (DNA and RNA). Due to the symbiotic association legumes
are regarded as a good biofertilizer for other crops which in turn reduces the
use of chemical fertilizer in the crop fields.
Without root nodules also some symbiotic association are there for nitrogen
fixation in the nature.
Example:(i) Anabaena – Azolla association
(ii) Cycas Coralloid roots etc.

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B) Ammonification: Ammonification is an important step in the nitrogen cycle. It is

the process of production of ammonia (NH3) or ammonium (NH4) compounds
from the decomposition action of bacteria on organic matter. Thus, on the death
and decay of the plants as well as animals the complex organic compounds are
released into the soil where they are again decomposed into simpler compounds
by the microorganisms and release energy.
Examples of bacteria – Nitrosomonas bacteria, Nitrosococcus bacteria

Fig.4.: Schematic Representation of Nitrogen Cycle

(Ref:https://en.wikipedia.org/wiki/ Nitrogen cycle #/media/File:Nitrogen_Cycle_2.svg)
C) Nitrification: When ammonium gets converted into nitrates, it is called nitrification.
Nitrates can be directly absorbed by the plants & incorporated into proteins, nucleic
acids & other nitrogenous organic compounds. Some nitrates may be stored in the
humus of the soil, immobilized by the bacteria & some may reach into the waterbodies
with the runoffs.
2) Nitrogen Gas back to the Atmosphere: Through the process of denitrification nitrogen
gas goes back to the atmosphere. Some bacteria are there, who can convert Nitrates
(NO3) to Nitrites (NO2).

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They are called denitrifying bacteria. Ex. Pseudomonas

Ultimately Nitrates, Nitrites & gaseous Nitrogen then released to the atmosphere.
b) Sulphur Cycle

Fig.5: Schematic Representation of Sulphur Cycle

Like other Nutrient Cycles, the movement of sulphur in the biosphere and the underground is
called “Sulphur Cycle” (Fig.5). Sulphur Cycle is a sedimentary type of nutrient cycle as the
reserve pool is buried underground in rocks, minerals as well as sulphates (SO4) in sea
sediments.
Sulphur found in nature in following form.
i) Hydrogen sulphide (H2S)
ii) Sulphur dioxide (SO2)
iii) Sulphates (SO4)
It enters to the living system as
i) Soluble form presents in the soil and pass on to the plants through the plants roots.

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ii) It is assimilated by the plant to synthesize protein, vitamins & same other important
products.

Fig.6:Schematic representation of Sulphur Cycle in detail

(Ref:https://en.wikipedia.org/wiki/Sulfur_cycle#/media/File:SulfurCycle_copy.jpg)
iii) Then the above compounds passed to the animal bodies through food chain.
Within plants & animal bodies the organic sulphur decomposed by aerobic bacteria to
sulphate (SO4) or hydrogen sulphides (H2S). Hydrogen Sulphides then get converted
into elemental sulphur by anaerobic bacteria (these bacteria do not require oxygen).
In this way sulphur gets back to the soil & sulphur cycle gets completed. Sulphur
Dioxide (SO2) is also released to the atmosphere by vehicular exhaustion i.e. the
burning of fossil fuels (Fig.6).
Example: Petroleum, Coal etc.
j) Phosphorus Cycle: In the ecosystem, more phosphorous is available in plants and
animal bodies in comparison to the abiotic system. In abiotic system phosphorous is
abundant in rocks and other natural deposits, formed during geological processes.
Phosphorous is desired for the structure of DNA, as coenzymes and for the conversion of
foods to release usable energy.
It is a simpler cycle.
Roots of the plants absorb Phosphorous present in a soluble form in the soil and assimilate it.
Through food chain it then gets transferred to the animal bodies and by the death and decay
of the plants and animals, or animal excreta it goes back to the atmosphere (Fig.7).

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The runoff from the soil and the loss of phosphorous to the sea is greater than the availability
of it to the land. Only 60,000 tons of phosphorous is returned to the soil through the birds,
fishes of the sea and the algae. A major amount of phosphorus gets lost to the sea.

Fig.7: Schematic representation of Phosphorous Cycle

Ref:https://en.m.wikipedia.org/wiki/File:Phosphorus_Cycle_copy.jpg

3.2.1. FOOD CHAIN

Definition: The transfer of food energy from the source in plants through a series of
organisms with repeated eating & being eaten is referred to or called as food chain (Odum,
1971).
How Food chain works:
The green plants are the autotrophs or the producer of the ecosystem. With the help of
chlorophyll, and by using CO2 and H2O from nature in presence of sunlight the plants can
prepare their own food known as carbohydrates. The process is known as photosynthesis
Sunlight
Plants CO2 + H2O + Chlorophyll Carbohydrates
(Photosynthesis Process)
That’s why plants are called autotrophs (auto-self; troph-to nourish)
Thus, plants are the first step in a food chain.

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The heterotrophs (hetero= other; troph = to nourish) are the organisms who depends on the
autotrophs for food and energy.
Heterotrophs are two types – Herbivores and Carnivores
Herbivores are the herb/plant eaters. They can only eat the green plants/autotrophs.
Ex. Grasshoppers, Rabbits, Goats etc.
Carnivores are the meat eaters in an ecosystem. They cannot have the ability to eat herbs.
For example, Frogs, Snakes, Hawks, Lions, Tigers. Thus, in an ecosystem a chain like
structure is found based on “-to eat and being eaten “scheme, which is also called “Pray-
Predator relationship” in an ecosystem (Fig.8).
Plants (Autotrophs) Herbivores (Heterotrophs ) Carnivores
(Heterotrophs)
or
Plants Primary consumers Secondary consumer Tertiary consumers

Fig.8: Schematic representation of a Food Chain

(Ref:https://www.ck12.org/biology/food-chain/lesson/Food-Chains-and-Food-Webs-BIO/)

Types of Food Chain:

Two types of food chains are observed in different ecosystems, known as
1. Grazing Food Chain
2. Detritus Food chains

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Grazing Food Chain – It is recognized by the starting level organisms of a food chain. They
are the living green plants. Then if we see the examples, in a grassland ecosystem the chain
will be starting with Greengrass, then Grasshoppers, then Frogs then Snakes then Hawks as
shown in the above figure.
Detritus Food Chain – Here the starting point is dead plants or animals. This type of Food
chain does not depend on the sunlight.
Ex- Dead plants and animals, Scavengers, Micro organism
This should be noted that the Grazing and Detritus Food Chain perform independent to each
other but different parts of a single ecosystem. More energy is transferred in a grazing food
chain than in a detritus one. Combinedly the grazing and the detritus Food chain complete the
nutrient cycle in an ecosystem.

3.2.2. FOOD WEBS

In a natural system the linear structured food chain does not happen always. Sometimes, if a
particular species is absent in a particular area, that species is replaced by another species so
that
the “to eat and to be eaten” system will be maintained. For example, in a grassland
ecosystem, if
frogs will remain constant and for the eagle, food will also be available.

Fig.9: Schematic representation of a Food Web

(Ref:https://www.tutorialspoint.com/environmental_studies /environmental_ studies
_functions_of_ecosystem.htm

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Additioon to this, inn Nature, a more compplicated nettwork of foood chains iss existed which
w are
interconnnected. Thhat is knownn as Food Webs
W (Fig.9)).
Thoughh Food Webbs are very complex inn nature. It playsp a signnificant rolee in the balaance and
stabilityy of an Ecosystem. If deer
d will bee eliminated
d from the nature,
n thenn number off grasses
i will invadde to the crooplands. In turn area off the croplannds will be reduced
will inccrease, and it
resultinng reduced products.
p Siimilarly, the upper trop phic level animals
a likee tigers willl not get
their foood and will die ultimattely.
So, the Food Web networkingg is an impoortant creatiion of the naature to maaintain and sustain
s a
balancee in the ecossystems.

3.3. EN
NERGY FL
LOW IN AN
N ECOSYS
STEM
According to first law of therrmodynamics, energy cannot be created
c or ddestroyed. It
I can be
transferrred from onne form to another.
a
In an eccosystem thhe energy geets fixed or in other woords in preseence of sunlight the au
utotrophs
or the producers
p prrepare their food materrial. The hetterotrophs get
g their foood / energy from
f the
autotropphs that is the
t plants. Energy is required
r by all the livinng organism
ms to perform their
work orr metabolism m (Fig.10).

Fig.10: Relationsh
hip between
n producerr, consumerr and decom
mposers

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Energy utilization in an ecosystem occurs in two ways.

i) Quantity of solar energy the plants receive from the sun for photosynthesis
ii) Quantity of energy flow occurs from the plants to the consumers.
This behaviour of energy transaction in an ecosystem is known as energy flow.
Energy flow occurs in two models within the ecosystem
a) Single Channel Energy Flow Model
b) Y- Shaped Energy Flow Models
a) Single Chain Energy Flow Model: This type of energy flow works as per the food
chain of the ecosystem. For example, in a grassland ecosystem, grasses are the
producers. They fix carbon dioxide2 from the atmosphere and produce carbohydrates as
the gross productivity. It is a one-way direction of energy flow. This clearly indicates
that, if the food chain is longer in length then the energy reaches to the top carnivores are
less and if the food chain is a shorter one, more energy will be available to the topmost
trophic level organisms.
b) Y – Shaped Energy Flow Model: In nature besides the single chain model, another way
of energy transfer is found. Food web shows the realistic picture of flow of energy which
is more complex with more combinations.
The Y-model explains the connection between a grazing & a detritus food chain (Fig.11).

Fig. 11: Y – Shaped Energy Flow Model

Ref:https://eco-intelligent.com/2016/11/17/y-shaped-model-of-energy-flow-who-eats-whom-
in-nature/
In this above figure at every stage the two food chains are linked. The herbivores can be eaten
up by the decomposer or the top predator in the detritus food chain. The other links are, when

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the herbivore or the top predator died. They get decomposed by the decomposer and again
used up as nutrients by the plants. In a nutshell, in balanced ecosystem hardly anything goes
to the waste. Sometimes, the decomposers are also eaten up by the top predators of grazing
food chain.
Ex. Earthworms in a grassland ecosystem can be eaten up by the hawks or eagle.
It is called so, because it looks like the English alphabet ‘Y’.

3.4. PRODUCTIVITY
Concept of Productivity:
The amount of organic matter i.e. the food prepared by a plant is known as productivity.
When it is measured at any unit time, it is known as rate of productivity of that ecosystem.
Productivity are of following types
A. Primary Productivity
B. Secondary Productivity
C. Net Productivity
A. Primary Productivity: The productivity is the production capability of a plant. Thus,
primary productivity is always associated with the autotrophs or the photosynthetic
organisms i.e. the green plants. Some of the microorganisms also can carry out this
process and are known as photosynthetic microorganisms.
Thus, primary productivity is the rate of fixation of solar energy from the sun by the
photosynthesis activity of the organisms.
Primary Productivity are 2 types
1. Gross Primary Productivity (GPP)
2. Net Primary Productivity (NPP)
1. Gross Primary Productivity: It is the total rate of photosynthesis or food
production by a photosynthetic organism. It depends on the chlorophyll content of a
plant. Thus, it is calculated as amount of CO2 fixed per gram of chlorophyll per one
hour. This can be represented as
GPP = amount of CO2 /gm Chl / hr.
2. Net Primary Productivity (NPP): We know, energy is required for each activity
they do, by the living organisms. Thus, for respiration work, plants also need energy.
So, the energy what was fixed during photosynthesis is being used here. As a result,
the remaining energy after loss in the respiration utilization is known as the Net
Primary Productivity. In simple words
NPP = GPP – Respiration energy
B) Secondary Productivity: This is related to the heterotrophs in contrast to the Primary
Productivity. Secondary Productivity is the energy stored at consumer level. Ecologist

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Odum (1971) prefer to use the term assimilation rather than production at this level.
Secondary production is not a fixed level of energy utilization of production rather
moves from one consumer level to the other through the food chain.
C) Net Productivity: Net Productivity is the storage energy of the consumers. That means
it is the energy which remains in the body of the consumer after utilization in respiration
or any other work done by the consumer. Thus, it can be measured as the biomass. Net
productivity can be expressed as production of carbon mg/meter2/day.
It can then be calculated how much energy per/biomass of the consumer gets in a year.

HOMEOSTASIS
Definition of Homeostasis is “The ability to maintain a constant internal environment in
response to the environment changes”. This is an unique principle of biology.
In the same way natural ecosystems are also capable of maintaining their internal regulations
i.e. self-regulations or self-maintenance at any point of time. This is called as stable steady
state of an ecosystem (Homeo means same, stasis means standing) Odum (1971) defined
Homeostasis as the tendency of an natural ecosystem to resist change and to remain in state
of equilibrium, which implies that within an ecosystem there is always stays a balance
between the production, consumption as well as decomposition and in all the living
organisms within a particular ecosystem in a particular time follow this kind of equilibrium.

LEARNING OUTCOME
The abiotic factors in an ecosystem maintains a dynamic role. Different components of the
atmosphere, like oxygen, hydrogen, carbon, nitrogen, Sulphur etc. cannot be used by the
living organisms in their elemental form. Thus, these elements form different compounds as
nutrients for the living world. In that way a cycle of “elements to compounds and again back
to elemental form” happens in the nature. That cycle is known as Biogeochemical Cycle.
Each organism has a place in an ecosystem pyramid. That is based on their food habit. This
characteristic can be easily understood by the food chain of an ecosystem. But if one of the
organisms if absent in an ecosystem. Then some other organism with same kind of food
choice will replace that place. Thus, the chain of” to eat and to be eaten” habit remain
maintained. So, in instead of only one chain, a web like structure found for the self-
sustenance of an ecosystem. That is known as Food Web.
Energy Flow is the theme of the ecosystem. Through different channels energy flow occurs
between the producers, consumers and decomposers.
We know that the external environment or climate has an impact on the living organisms,
growing in that area. But side by side the Nature also provided a system to them that, the
internal environment or the system within the body of the living organisms do not change
with the disturbances of the outside environment. This is a gift of the Nature to the biotic
factors of an ecosystem and called as Homeostasis.

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