UNIT 5 ECOSYSTEM F U N ~ T I O N ~ N G

Structure
5.1' Introduction
21
i
- 5.2
Objectives
Ecosystem as a Unit of Nature
1 5.3 Components of Ecosystem
Abiotic Components
, . Biotic Components
5.4 Tolerance Range and Limiting Factor
I
Tolerance Range
Limiting Factor
5.5 TrophicLevel
5.6 o Ecological Pyramids
Pyramid of Numbers
Pyramid of Biomass
Pyramid of Energy
.
Limitations of Ecological Pyramids
5.7 Energy Input in Ecosystem
5.8 Concept of Production
Primary Production
Secondar). Production
5.9 Energy Flow
5.10 Food Chain and Food Web
Food Chain
Food Web
5.11 Ecosystem Control
5.12 Summary
5:13 Terminal Questions
5.14 Answers

As you know from your study of Block-4, FST-1; and Block-1, LSE-02, that an
ecosystem consists of the community of organisms in a given area together with the
abiotic (non-living) components of the environment. The term ecosystem is applied
to both small and large ecological systems. Thus we might consider a small pond or
even a tree as an ecosystem. On a much larger scale; we can examine a.crop field, a
grassland, forest, ocean, or even our planet on the whole as an ecosystem.
Ecosystems have both structure and function. The structure of an ecosystem is
determified by the components that make up the system, while ecosystem function i;
determind by the manner In which these components interact in a complementary
way. Let us examine these interactions in more detail.
In this unit you would find certain terms and concepts that you have studied before
(in the above units). Here, these concepts have been elaborated further, or they are
used as a background material for explaining otber concepts. Before, you start going
through this unit, we'advise you to give a quick glance to the units mentioned below.
i) Unit-14, Block-4, FST-1; ii) Unit-1. Block-1, LSE-02.

Objectives
After studying this unit you would be able to :
explain why ecosystem is taken up as a unit of study;
identify the various components of an ecosystem and state the f u n c t i ~ ~ role
a l of
producers, primary consumers. secondary consumers and decomposer4 in an
ecosystem;
summarise the concept of limiting factors and tolerance range;
recognise different types of ecological pyramids, and realise the usefulnes4 and
limitations of ecological pyramids in describing ecosystem dynamics;
describe gross primary production, net primary production and secondary
production;
explain food-chain, food web, and flow of energy through the cco<y\tcm;
 Q ~
:~ u n ~~ g TYW
h
t i o n i nand d e t ~ n and
e use in proper context the terms 'ecological efficiency', 'energy budget',
and 'ecological feedback'.

~
I
5.2 ECOSYSTEM AS A UNIT OF NATURE
~
An ecosystem can be visualised as a functional unit of nature representing complex
interactions between living and non-living components. The study of any ecosystem
involves systematic description of the components and understanding of the close
relationship between the biotic and the abiotic components. Why consider ecosystem
as a unit of study? This, perhaps, is the question arising in your mind presently. If
-
-4
I
1
one wishes to study the various aspects of relationships of living and non-Jiving I
components of the environment, it would be easy to understand and interpret these
relationships in a smaller component of the biosphere,, that is the ecosystem. We shall i
elaborate this further with the help of an example. Let us consider a village ecosystem
(see Fig. 5.1). It is depicted here by the area enclosed within the dotted lines. The
boxes within the village ecosystem represent three sub-systems namely: producers or
'crop plants, cattle and humans. The solid lines connecting the boxes represent the
interactions. Solar energy, fertilisers and pesticides are the major inputs brought from
outside the village ecosystem. These inputs determine the quantum of output, that
is, foodgrain, fodder and other animal products which are exported from the village.
So, you see that, the village ecosystem could be considered as a model to study the
organisms and their environment as an integrated unit.

L-,-------- J
Fig. 5.1 : A model of village ecosystem

Ecosystems are mnceptual models and these models can be applied at any scale, from
a bowl o i water to the whole earth. Ecosystems represent enormous contrast in size
and complexity. For the purpose of study, an ecosystem can be delineated in almost
any way convenient to the interest of the investigator. In the case of some ecosystems
such as lake, river or pond, distinct boundaries can be recognised but in the case of
other ecosystems, such as a grassland, forest, village or town, boundaries are not so
sharp however, they can bk delineated according to the object ~f study or any other
practical consideration.

5.3 COMPONENTS OF ECOSYSTEM

All ecosystems possess both biotic and abiotic components. Let us now examine these
two components.

-
a -without, bios life 5.3.1 Abiotic Components
Three broad categories of abiotic components can be visualised,

i) Inorganic Substances : There are about forty elements that are required in
various processes of living organisms. Some of these are macronutrients which the
plants need in relatively large amounts, and others are micronutrients, that are
required in trace amounts. There are nine macronutrients : carbon, hydrogen and
 oxygen (the three elements found in all organic compounds), and nitrogen, EFosystenl Functioning
potassibm, calcium, phosphorus, magnesium, and sulphur. Some examples of
micronutrients are : iron, chlorine, copper, manganese, zinc, molybdenum and borcIn.
ii) Organic Substances : These include carbohydrates, proteins, lipids and their Detritus is derived from latirr
word 'detere' meaning near nnuy.
derivatives which are derived from the waste products of plants and animals or
are the remains of dead plants and animals. Organic fragments of different sizes
and composition formed as a result of decomposition of organic residues are
collectively called organic detritus. Decomposing organic matter releases
nutrients along with the formation of a dark, amorphous, colloidal substance
called humus which is important for the fertility of soil (also see Unit-4, LSE-02).
New humus is added as old humus gets converted into mineral elements.
:i l iii) .Climatic Factors : This includes temperature, rainfall, humidity, and light, and
their daily and seasonal fluctuations. These abiotic constituents are very
important for the survival and continuation of living beings and the ecosystem.

5.3.2 Biotic Components

We categorise the biotic components of an ecosystem into three categories on the
basis of how they obtain energy and nutrients.
i) Producers : Producers. also called autotrophs, are largely green plants that can Producer = Primary Producer
make food from simple inorganic materials. Food refers to complex organic aut, auto-self; trophus-feeding
compounds such as carbohydrates, fats and proteins. Green plants accomplish
food making through the process of photosynthesis. In this process, green plants
use carbon dioxide, water and some minerals, to produce carbohydrates first. and
later various other-organic compounds such as fats and proteins. Oxygen is given
off by plants as a byproduct of photosynthesis. During photosynthesis, radiant
energy of sunlight is conve'rted into chemical eriergy and is stored in the chemical
bonds of the compounds made by the plants (see the equation given below).

Chlorophyll
(in green plants)
6C0,+6H20 + Light Energy C,,H,,O, + 60,
(''~rbonWater Sun Gluco\e Oxygen
)\~de (sugar)

The major primary producers of aquatic ecosystems (freshwater and marine) are
various species of algae (see Fig. 5.2). In terrestrial ecosystems. the major
primary producers are predominantly herbaceous and woody plants. Somc
photosynthetic prokaryotic organisms such as blue green algae and a few bacteria
are also called primary producers. Besides the green plants there are certain
chemosynthetic bacteria that are also autotrophic. But they obtain the energy for
the'synthesis of organic compounds (amino acids, proteins) from sources other
than solar energy. Some of these sources are : ammonia (NH,), methane (CH,), Fig. 5.2 : Marine algae
a) Laminaria agardhii
and hydrogen sulphide (H2S). You can surely make a long'list of animals that
b) Nereocystis luetkeana
obtain their food from green plants. Can you also name a few organisms that
depend on the chemosynthetic bacteria for nutrition? We give you a few
examples of the same. Organisms like crabs, molluscs and giant worms, that are
present at o r near the oceanic floor where sunlight cannot penetrate, get their
food from chemosynthetic bacteria.
Consumers : These are also called as phagotrophs o r heterotrophs. The organisms Phago - eat
grouped under this category cannot manufahure h e i r own food but obtain their heteros - other different:
energy and nutrients by feeding on other organisms. Some eat primary producers trophos - feeding
(green plants) to get their food supply and are called herbivores. In terrestrial Herbivores = Prirnarv Consumers
ecosystem typical herbivores are insects, birds and mammals. Two important herba - grass. grreat crops;
groups of herbivores mammals are rodents and ungulates. Primary consumers vora - devour, cat
also include parasites (fungi, plants or.animals) of plants (see Fig. 5.3). In aquatic
ecosystems (freshwater and marine) the typical examples of herbivoresare : small
crustaceans and molluscs. Most of these organisms such as water fleas. copepods.
crab larvae, mussels and clams are filter feeders and extract the minute, primary
producers from water.
EXmysystern : Functioning and Types

Ungulates are hoofed, grazing

animals, such as horses, cattle and
sheep that are adapted for running
on the tips of their digits

a
Fig. 5.3 : Some Parasites of Plants a) Dodder (Cuscuta sp.) b) Mistletoe (Viscum sp.)

Carnis-flesh
Besides. there are animals which depend on herbivores for food and are cal!ed
secondary consumer=carnivore
secondary consumers . Since secondary consumers feed on herbivores, they & ~ - c
therefore. carnivores. There are also animals that feed on secondary consumers.
They too are carnivores, and are known as tertiary consumers. Secondarv and
tcrtiary consumers may be : a) predators which hunt, capture and kill their prey;
b) carrion feeders which feed on corpses; or c) parasites in which they are much
smaller than the host. and they live on it while the.host is alive. They depend on
the metabolism of their host for their energy supply.
There are some animals that have quite flexible food habits as they eat plants,
(therefore are herbivores) and animals (therefore are carnivores). They are
Omnib -all known as omnivores o f which man himself is a good example.
Sapros - decomposed, rotkn; iii) ~ e c o m ~ o s :e Also
~ s known as saprotrophs. Mostly, these are microscopic and are
trophos - reeding heterotrophic in nature. Decomposer organisms obtain their eneigy and nutrients
7:
by degrading dcacl organic matter. When plants and animals die, their bodies are
still a source of energy and nutrients, as are their waste productssuch as urine
and facces which they discard throughout their life times. These organic remains
are decomposed by micro-organisms, namely fungi and bacteria which grow
saprophytically on these remains. They secrete digestive enzymes from their
Decompser ,organisms secrete bodics on the dead and wasted materials. subsequently absorbing the products
dgrrJiveazlm~lhmtbclr of cligcstion. l'lie ri~teof digestion is variable. The organic matter of animal
bodies into the dead organic wastes such as urine, faeces and corpses is cpnsumed within a matter of weeks
material and absorb the digested
food. This is in contra& to whcrcas fallen leaves and brqnches may take years to decomposes. During the
c ~ ~ = U m e which
rs eat and digest decomposition of wood. fungi'act and produce an enzyme cellulase, that softens
it internally. S ,
the wood. This enables the small animals to penetrate and ingest the material.
Fri~gmentsof decomposing tnaterial are called detritus, and many small animals
Iced on these, contributing to the process of breakdown. They are called
detritivores. Because of the combined activities of the true decomposers (fungi
.and bacteria) and detritivores (animals), in the breakdown (decomposition) of
matel-ials, thtiy are sometimes collectively referred to as decomposers. Although,
stri~tlythe term decomposer relates to saprophytic organisms, Some typical
terrestrial detritivores are : earthworm (see Fig. 5.4a), woodlice, millipedes (see
1 Fig. 5.43) and other smallei' (< 0.5 mm) animals such as mites, springtail and
nematodes.
The important end result of the decomposer activity is that inorganic nutrients,
originally bound up in the tissue of organisms are converted into simple forms
that are usable once again by the producer organisms. Apart from processing and
clearing the organic wastes, decomposers are vitally important for regenerating
ecosystem fertility by releasing nutrients for utilisation by plants, that were
locked up in the organic matter.

5.4 TOLERANCE RANGE AND LIMITING FACTOR

Fig. 5.4 : Detritivores. Jn the above section you have studied the biotic and abiotic components of the
a) Earthworm, b) Millipede ecosystem and their functional roles and relationships. You have also seen how
Important these components are for the survival and well-being of the organisms and
ulti~natelythe whole ecosystem. These components are required in certain minimum
and maximum limits for the optimal functioning of the organisms.
'11
- -

Ekosystcm Funrtioninp
' 5.4.1 Tolerance Range
Organisms are able to survive only within certain maximum and minimum limits with
respect to each environmental factor such as water, light and temperature. These are
called the tolerance limits and the range in between these limit\ is the tolerance
ranges (see Fig. 5.5). Different organisms have different tolerance ranges (see Fig.
5.6). Beyond the maximum and minimum limits ot this range. no member of a
particular species can survive. For example, fish generally tolerate a narrow riunge of
water temperature. If the water cools below the range of tolerance. they die or rnovc
to warmer waters. Mlnlmum M B X I ~ U ~
Llmlt 01 Tolerance Limtt of Tolerance

I I
I I Optimum
I I
I
I Zone of I

C:
Low Temperature High

-
Fig. 5.5 : . b n g e of tolerance for a population of organisms of the same species, to an environmentalfactor
- in this case temperature. The organisms shown here are scarlet prawns.

, Human
, (naked)

Tc--
Call

Cow
: w n i n g and l y ~ 5.4.2 Limiting Factor i
In all ecosystems one factor, usually abiotic, limits the growth of organisms and is
therefore called a limiting factor. The limiting factor is one that outweighs all the
other factors that are necessary for the growth of organisms. It is the primary
t
determinant for growth because it lies beyond the minimum and maximum limits of
the range of tolerance. For example, phosphorus is a limiting factor in certain aquatic
ecosystem. It is the first to be used up. When phosphorus is reduced, the growth of
algae is impaired. So, this is an example, where pho3phorus is in short supply and is
thus a limiting factor.
As mentioned above. just as the shortage of any abiotic factor impairs the survival
of organisms In an ecosystem, so can an excess. Any factor that is in excess may be
detrimental for the living organisms, directly or iricl~~ectly.
You may be wondering,
how' Let us consider an example of a power plan1 from where the hot water pours
into a nearby stream. As a result, the temperature or water in the area nearby shoots
up from 10" C to 300 C. This sudden therryal vhock IS fatal for many fish and other
aquatic organisms. The above example, illustrates the direct effect of excess of a
factor.
How the factors indirectly affect living beings is illustrated by the following example.
If we over-water or tlood a patch of land having trees. on a prolonged basis. then the
excess water saturates, the soil by displacing air needed by the trees from the soil
pores, thus creating anaerobic conditions. As a result, the roots get deprived of
oxygen leading to the death of the trees. The excess of the water thus indirectly affects
the suwival of trees adversely.
 Ecosystem Functioning
5.5 TROPHIC LEVEL
In an ecosystem, the various biotic components are related to each other and form
food chains (see FST-1, Unit-14, Section 14.3). If we group all the organisms in a
food chain according to their general source of nutrition, we can assign them different
trophic (feeding) levels (Fig. 5.7). The producer organisms belong to first trophic
--

Fourth Trophlc
Consumers Level
(Top Carn~vores)

B Consumers
Thlrd Trophlc
Level
(Garn~vores)

Level

.
First Trophic
Producers Level
dPlants)
Fig. 5.7 : Diagrammatic representation of trophic levels in
1
an ecosystem.

level, primary consumers (herbivores) to the second trophic leve', secondary

consumers (carnivores) to the third trophic level and tertiary ct\ lsumers (top
carnivores) t o the fourth trophic level. Man, who is an omnivore may belong to more
than one trophic level (see Fig. 5.8). There are usually four or five trophic levels,
and seldom more than six - its reasons you would study in Section 5.9 of this unit.

.......................-
.... I I 1
.rig. 5.8 :Three food chains drawn separately to show that an organism can occupy different trophic levels.
-

In this diagram, the position of man in different food chains illustrates this point. The arrows indicate the
direction of food chain.

The study of trophic level gives us an idea about the energy transformation in an
ecosystem. It provides a useful conceptual basis to include all organisms that share
the same general mode of feeding into one group and they together are said to belong
to the same trophic level. This feeding level concept, implies that organisms obtain
food through the same number of steps from the producer. One thing should be clear
to you, that is, the trophic levels are numbered according to the steps an organisms
is away frorn the source of food or energy, that is the producei.

.
5.6 ECOLOGICAL PYRAMIDS
, The ancient Egyptians constructed elaborate tombs called pyramids. The base of the
pyramid is broad and it supports the upper levels of the structure. and it narrows to
a point at the top. A sim~larsituation is seen when we study and depict the trophic
relationships in an ecosystem. The different trophic levels of an ecosystem are related
to one another and can be summarised in the form of ecological pyramids. The base
of each pyramid represents the producers or the first trophic level while the apex
represents tertiary or high-level consu.mers; other consumer trophic levels are in
k w y s e m : Functioning and TYW between. There are three kinds of ecological pyramids possible which are discussed
below.

5.6.1 Pyramid of Numbers

A graphic representation of the total number of individuals of different species
belonging to each trophic level in an ecosystem is known as pyramid of numbers. It
consists of a number of horizontal bars depicting specific trophic levels which are
arranged sequentially from primary producer level through herbivore, carnivore
onwards (Fig. 5.9). The length of each bar represents the'total number of individuals
Number of
4 individuals/
\. square meter
.I \

Secondary Consumer
(Camhare)
/ \
/ \

Primary consumer
(Herbivore)

/ \
/ \
Primary Prducer / \
(Pmducer) \ 100
/ \
/
\
/
\
L--- ---A
, -
Fig. 5.9 : An upright pyramid of numbers. The number of individuals indicated h~the
Qgure are hypothetical, and the organisms are not drawn to the same scale.

at each trophic level in an ecosystem. The number of individuals drastically decreases

with each step towards higher trophic levels and the diagrammatic representation
assumes a pyramidal shape and is called pyramid of numbers. In such pyramids, you
would find that generally the higher trophic levels are occupied by relatively
large-sized animals which are also less abundant. This is due to the fact that animals
at higher trophic levels are larger than the animals which they capture from the lower
trophic level. In case of tiger or lion, the size relationship does not hold good as cattle
and other preys may be larger in size. This could be explained on the basis that these
wild cats are powerful and eat their prey by cutting into manageable pieces.
For most ecosystems, pyramids of numbers are right side up because numbering of
organisms decrease at successively higher trophic levels. However, there are some
ecological systems for which pyramids of number may be inverted. For example, if
we depict the situation of single tree along with its dependent insect population we
would get an inverted pyramid as shown in Fig. 5.10. Since the tree is a primary
producer, it would represent the base of the pyramid and the dependent
phytophagous insect population will represent the second trophic level.
Number of
indivdualsl
- -/square meter
r-
Primary \ /
Consumer \ / 500
(HerbiMre) \ /
/

Primary
Pmducer
(Producer)

Fig. 5.10 : An inverted pyiamid of numbers. The number of individuals indicated in

the figure are hypothetical, and the organisms are not drawn to the same scale.
 Describing the structure of ecosystem through a pyramid of numbers may be quite
instructive in some sense but it suffers from certain limitations that we shall discuss
now. i) You know that the producers vary greatly in size. In such pyramids, for
example, a single grass plant o r alga is given the same status as a single tree. This
also explains why a true pyramid \hape is often not obtained, and we get an inverted
pyramid. Also parasitic food c h a ~ n (you
s would study in Subsection 5.10.1, iii) of this
unit) may give inverted pyramid. ii) The range of numbers is so great that it is often
difficult to draw the pyramids to scale, although logarithmic scales may be used.

5.6.2 Pyramid of Biomass

' You have seen that pyramid of numbers is not a very good method to use if the
organisms at different trophic levels are of greatly differing sizes. T o overcome the
shortcomings of the pyramid of numbers, the pyramid'uf biomass is used. Biomass
represents the total dry weight of living beings of different species at each trophic
level at a particular time. And it is usually determined by collecting all the organisms
occupying each trophic level separately and measuring their dry weight. This
eliminates the size difference problem because all kinds of organisms at a trophic
level are weighed. Biomass is measured in g/m2. A t the time of s'ampling, the amount
of biomass is known as standing crop,or standing biomass. Generally, the biomass of
producers is much greater than biomass of herbivores and the biomass of herbivores
is greater than the biomass of carnivores and so on and so forth. In other words, we
find that biomass decreases at each trophic level if we move from producer to.top
carnivore. Therefore, diagrammatic representation of biomass of individuals
belonging to the different trophic levels invariably assumes the shape of an upright
pyramid (Fig. 5.11). This, however, is not always the case. In some aquatic

Biomass
(grams dry weight1
square meter)

Elomass if = Total Combined

Third Trophic Welght of AII
Secondary
Consumer Level Carnivores
(Carn~vore)

Blornass of = Total Combined

Primary Second Troph~c We~ghtof All
Consumer Level Herbivores
(Herbivore)

\
\
\ B~ornassof First = Total Combined
Primary \ Trophlc Level We~ghtot AII
Producer Producers
, (Producer) \
\
\
\
----A
- P

Fig. 5.11 : Pyramid of Biomass. Ihe numerical ligures ot biomass as indicated abote are hypothetical, and
the organisms are not drawn to the same scale.

ecosystems, like large lakes and oceans, the pyramid of biomass: sometimes assumes
an inverted form (see Fig. 5.12a). Since microscopic phytoplanktonic algae are
primary producers in the aquatic system, they have short life cycle, thus they
reproduce rapidly. Being single-celled organisms, they d o not accumulate much
biomass and they are eaten up faster by organisms like zooplankton, fish etc.
Consequently, at a given time, the total weight or the standing crop of phytoplankton
is less as compared to herbivores or othkr consumers. This is the reason for the base
of the pyramid in aquatic ecosystem being smaller than the super structure.
Fcosystem : Fanctioning and Types
Biomass
(grams dry weight1
square meter)

\
Secondarv Consumers / \
~Cernlvwes) / \

/
/
/
e w \
\
\

--- +- -
/-

\
\
- -/L
/
Primary Consumers \
(Herbivores) \
/

Pr~malyProducers
(Producars)

\ 1
v

Fig. 5.12 : Pyramid of biomass in an open ocean ecosystem at two different times of the year a) during
winters, b) during spring season. The numerical figures as indicated above are hypothetical and the
organisms drawn are not to the same scale.

For constructing the pyramid of biomass, the time of sampling is very important. You
could ask why? Let us discuss this point with a specific example, that is of ocean. 13
open ocean, the producers are microscopic phytoplankton and consumers range from
microscopic zooplankton to massive organisms like whales. Here, the biomass of
consumers may temporarily exceed that of primary producers, if sampling is done
when the number of phytoplankton is low such as in winters. During such sampling
periods, the pyramid of biomass would look as shown in Fig. 5.12a. However, if the
samples are taken during spring when phytoplankton populations are immensely
large, or if several generations of phytoplankton are included, the pyramid shape
would look like as shown in Fig. 5.12b. From this example, it should be clear to you
that the time of sampling is very crucial. In the same ecosystem, we can get an
inverted pyramid at one time of the year and an upright pyramid in a different season.

5.6.3 Pyramid of Energy

When we wis'n to compkre the funct'ional roles of the trophic levels in an ecosystem,
an energy pyramid is probably the most informative. It overcomes some of the
objections pertaining to the pyramid of numbers and biomass. An energy pyramid
The Erst law of thermodynamics more accurately reflects the laws of thermodynamics, with loss of energy being
states that energy may be depicted at each transfer to another trophic level, hence the pyramid is always upright
transformed from one form to (see Fig. 5.13). Energy pyramids in the case of aquatic ecosystem are also upright,
another but can be neither even where the pyramid of biomass is inverted. In energy pyramids, a given trophic
created nor destroyed.
level, always has a smaller energy content than the trophic level immediately below
it. This is due to the fact that some energy is always lost as heat in going from one
trophic level to the next. Each bar in the pyramid indicates the amount of energy
utilised at each trophic level in a given time, annually per unit area. The unit of
The second law of meakurement is kcal/m21yr.
thermodynamics states that the
conversion of energy from one A pyramid of energy must be based on determination of the actual amounts of energy
form to another is never 100 per that individuals take in, how much they burn up during metabolism, how much
cent efficient, that is, some energy remains in their waste products, and how much they store in their body tissues. The
is always wasted in energy
conversions.
energy inputs and outputs are calculated so that energy flow can be expressed per
unit area of land (or volume of water) per unit time. Though, these calculations are a
bit difficult than for the other pyramids, the advantages of energy pyramid are many:
i) It takes into account the rate of production, in contrast to the pyramids of numbers
 Fxosystern Functioning
Energy Kilocalories
Square meter/ Year
(KcaVsqmIyr)?

Ter;iary
Consumer
' (Top Carnivore)

Secondary
Consumer
(Carnivore)

(Herbivore)

10.000

Fig. 5.13 : Pyramid of energy. The cumulative energy content of primary producers is always higher as
compared to the energy in the next trophic level and so on and so forth, over a period of time. The numerical
figures as indicated above are hypothetical, the organisms are not drawn to the same scale.

and biomass which indicate the standing states of organisms at a particular moment
in time. Each bar of a.pyramid of energy represents the amount of energy per unit
area or volume that flows through that trophic level in a given time period. ii) Weight
for weight, two species do not necessarily have the same energy content.
Comparisons based on biomass may, therefore, be misleading. iii) Apart from
allowing different ecosystems to b e compared, the relative importance of populations
within one ecosystem can be compared and inverted pyramids are not obtained. iv)
Input of solar energy can be added as an extra rectangle at the base of a pyramid of
energy

,IN 5.6.4 Limitations of Ecological Pyramids

The pyramid of energy is a significant improvement over the previous two types of
ecological pyramids, yet all of them overlook one or another important aspect. Some
of these limitations are discussed below.

i) Some species practise more than one mode of nutrition or belong to two or more
trophic levels. This is particularly true in the case of consumers of higher trophic
levels. Man is an example. H e gets his food from primary producers as well as
from higher trophic levels. Such organisms which feed at more than one trophic
level are extremely difficult to depict in ecological pyramids.
ii) Saprophytes play a vital role in ecosystem but they are not represented in
ecological pyramids.
iii) Detritus such as litter and humus is an important source of energy and exerts
considerable influence on ecosystem function, yet it is not depicted in ecological
pyramids.
iv) Ecological pyramids do not provide any clue to seasonal and diurnal variations.
v) The rate of transfer from one trophic level to another is not reflected in the
ecological pyramids.
 5.7 ENERGY INPUT IN ECOSYSTEM -
C

Survival and functioning of ecosystem is dependent on the input of energy.

Continuous availability af energy is essential for supporting diverse ecosystem
processes. For any ecosystem, the ultimate source of energy is sunlight. And as you
know, it enters the kosystem through the producers. When a primary consumer
(herbivore) eats a producer, and is itself eaten by secondary consumers, we can say
that energy is flowing through the ecosystem. You have already studied some aspects
of solar energy input in Subsection 2.2.2, Unit 2, Block-1 of this course. The
Table 5.1 information provided therein, would serve you as a base material for understanding

~~
I
Mean total radiation of sun and
this and the subsequent sections of this unit.
sky, on a horizontal surface (in As you already know that the amount of solar energy received at the outer boundary

~
I
callcm2/day)(After Ramdad and
Yegnanarayanan, 1954)
TRIVANDRUM
BANGALORE
MADRAS
487
467
530
of earth's atmosphere is at the rate of 2cal/cm2/min.This quantity is fixed and known
as solar constant. You have also learnt that about 30 per cent ofthe sunlight reaching
the earth's atmosphere is reflected back into space, about 51 pet cent is absorbed as
heat by ground, vegetation or water, and about 19 per cent is pbsorbed by the
DHARWAR 480 atmosphere. Only a small fraction of sunlight, that is, about 0.b2 per tent reaching
BOMBAY 499 the atmosphere is used in photosynthesis. Nevertheless; it is this small fraction on
'POONA 506 which all the organisms of the ecosystem depend. The actual amount of solar flux'
AHMEDABAD 543
JODHPUR 534 received at the surface of the earth is dependent on various climatic, geographic and
JAIPUR 495 other environmental factors. On an average the total amount of solar energy that
I ALLAHAB'AD 511 reaches th'e earth's surface is about 3,400 kcal/m2/day.It varies significantly from one
~ CALCUTTA
DELHI
486
489
place to another, for example, it decreases with latitude and its input also varies
during different seasons at any given location. The solar flux values for fourteen
JULLUNDUR 496
different stations in India are given in Table 5.1, and it varies from 361 to 543

I~ ~ SRINAGAR 361
cal/cm/day .

5.8 CONCEPT OF PRODUCTION

I ;

i You have just studied that ecosystems are unable to function, unless there is a I

i 16 constant input of energy from an external source that is sun. Solar energy enters the

I
biotic components of the ecosystem through primary producers. And you know that Ecosystem Functioning
the plants store solar energy in the form of chemical bond energy through the process
of photosynthesis. In the following subsections you would study about this stored
solar energy in the plants and its availability to the next trophic levels.

5.8.1 Primary Production

Energy accumulated by plants during photosynthesis is called production or more
specifically primary production. It is the first and the basic form of energy stored in
an ecosystem. Production is defined technically as the amount of biomass or organic
matter produced per unit &ea in a given period. It can be expressed in terms of
weight (g/m2) or energy {kcallm2 ) . The .rate at which energy accumulates is
know$ as primary productivity.It is expressed in terms of kcallm 2/yr or g/m2/yr.
In case of plants, primary production is generally differentiated into two distinct
categories, namely gross primary production (GPP) and net primary production
(NPP). Gross primary production refers to the total amount of solar energy fixed into
organic matter by primary producers through photosynthesis. A considerable portion
of the solar energy fixed by plants (GPPI. is utilised by plants themselves in respiration
(R) to get the energy needed f ~their
r t?-,-tabolism and for other vital functions. The
amount of energy left after respiratory consumption (R) is incorporated into new
body tissue (growth) or is used for producing nzw individuals (reproduction). The
amount of biomass or organic matter accumulated by plants per unit area in a given
period is called net pr-mary prodi~ction.The overall relationship between GPP and
NPP can be written as :
GPP - R = NPP or GPP = NPP +
R
F r o q this equation,.you might have noticed that whatever energy is fixed by plants
(GPP) some of it is used for their own maintenance (R) and only remaining (NPP)
is available for the next trophic level. So net primary production is the only energy
available for the next trophic level.
The annual net primary productivity of the whole biosphere is approximately 170
billion tons (dry weight) of organic matter. Of this total, about 115 billion tons are
produced on land and about 55 billion tons in the oceans, despite the fact that the
oceans occupy about 70% of the earth's surface. We the human beings harvest about
1.2 billion tons per year as plant food.
Production efficiency : The maximum amount of solar energy harvested by plants is
about 5 per cent but the average for green plants, on the whole is only a small fraction
of sunlight, i.e., 0.02 per cent reaching the atmosphere. The production efficiency,
that is the ratio of net primary production to gross primary production (of green
plants) is on the average rather high. It varies between 40 to 85 per cent. The most
efficient are those plants which have low maintenance requirement due to minimum
non-photosynthetic (non green) tissues, such as in grasses, algae and phytoplankton.
Algae and crops like corn have an efficiency of about 80 to 85 per cent, submerged
aquatic plants 60 to 75 per cent, deciduous forests about 42 per cent.
Different ecosystems have different productivities (see Fig. 5.14). Productivity of
ecosystems depends on a variety of factors such as sunlight, temperature, rainfall and
the availability of nutrients. Those situations that provide the best circumstances for
plant growth are the most productive. Warm, moist, sunny areas with high levels of
nutrients in the soil are ideal. 'some areas have low productivity because one of the
essential factors is missing. Deserts have low productivity because water is scarce,
arctic areas too have low productivity because temperature is low, and open oceans
also have low productivity because nutrients are in short supply. Coral reefs and
tropical rain forests have high productivity. Marshes and estuaries are-highly
productive since waters running into them are rich in the nutrients and they also get
enough light.
You have just seen that some ecosystems have consistently high production. Such
high production usually results from an additional input of eriergy subsidy to the
system. The energy subsidy, as you have learnt may be in the form of high ambient
temperature, rainfall, or inflow of nutrients. Some agricultural systems also have high
productivity, e.g., sugarcane has a productivity of 1.700 to 1,800 glyr; corn 10.000
glyr; and some tropical crops up to 3,000 glyr. Can you now think of the energy
subsidies that are linked to high production? In agricultural system energy subsidy
Eco~ystern: Functionin and Types includes the use of fossil fuels for land preparation and the use of fertilisers and
t
pesticides. - -

. -
Fig. 5.14 : Comparative productivities of different ero~ystemsof the world. The numerical fiwres written
in bold are the average values and the ones written in parentheses represent the range of productivity
(Data from R.H. Whittakar, Communities and Ecosystems, 1975).

5.8.2 Secondary Production

You have seen that net primary prodyction is the only energy available to consumers
or heterotrophs including man. Herbivores such as cow or deel g a z e upon grass
and utilise primary produ'ction. T h e food is processed in the sfomach of animals.
Digested material is assimilated in the body and the unutilised material is excreted.
Some of the assimilated energy is used up in respiration to provide energy for the
metabolic needs of body such as maintenance and repair of tissues. The remaining
part is utilised for producing new tissues for reproduction. Production of animal
biomass on account of growth as well as addition of new individuals of animals is
referred to as secondary production. And secondary productivity is the rate of
formation of new organic matter by heterotrophs.
Very little of the plant matter that is consumed is actually converted to animal tissue.
In terms of energy content, the conversion is only about 10 per cent. This energy loss
is shown in Fig. 5.15, where a rabbit has 0.1 kcal of secondary production for every
1 kcal of food eaten. What happens to the other 90 per cent? Fig. 5.15 shows that Ecosystem Functioning
most of this difference is used in respiration to power the animal's movements and
maintain its body functions. A certain amount is not assimilated at all and is therefore
excreted in the faeces. Thus relatively little energy is left for the production of new
body tissues.

12 Kccll 10 K d
Gross Pnmary Prdduclbn Ntll Prlmary Production

€iten.
and not. Lost
Ass~milated . vla.

Uneaten

0 89 Iccal Eaten.
fl.1 Kcal
~rod'uhionby Carnivlves

Fig. 5.15 : Energy relationships in an ecosystem. The average values for energy transfer w e illustrated,
the actual values vary from system to system.

T o put this concept on a familiar level, let us consider an adult human. A

person eats daily, yet a healthy adult does not gain weight at all. T o summarise,
although there ere large variations from ecosystem to ecosystem, as a ,;p,neralisation,
for every 10 kqal of plant tissue available to herbivores, about 1 kral will be eaten,
and only aboui 0.1 kcal will bdstored in the form of body weigh:,
It must be clear to you that in contrast to primary production, secondary production
is usually not differentiated into 'gross and net' categories because heterotrophs
consume only already manufactured food.
,
Just as net prlmary production is limited by a number of variables, so is secondary
production. The quantity, quality (including the nutrient status and digestibility), and
availability of net production are the 'three limitations.

S.9 ENERGY FLOW

-As you know our world is a solar-powered system, and green plants are the entry
gates ol energy into ecosystem. In Unit 2, Subsection 2.2.5, Block-1, LSE-02 you
have already learnt that out of the total incoming solar energy, only a very small
fraction is absorbed by plants. And on this small fraction of sunlight trapped by plants
is built the entire living world. In this section we shall discuss with you as to how the
different biotic components of an ecosystem are related in terms of energy.
From your study of the First block of this course plus the Fourth block of FST-1, you
have sufficient background information on this topic. One thing you should
remember is that any organism derives its energy from the 'food' it consumes. And
you know that all organisms cannot make their own food and only the producers have
the capacity to do so. Therefore, various organisms in an ecosystem musifulfil their
energy needs by relying on producers directly or indirectly. In other words we can
say that energy flows from the first trophic level, that is, from producers to the
subsequent trophic levels. In an ecosystem energy is transferred in an orderly
sequence. See ~ g 5.16 . carefully before you proceed further.
Have you noticed the following two points in the figure? i) The flow of energy is in
one direction only, and ii) some energy is lost as heat at every successive step.
Now let us consider the first point, that is. the direction of flow of energv. E n e ~ g v
flows from lower (producer) to higher (herbivore, carnivore, etc.) trophic level. It
never flows in the reverse direction, that is, from carnivores to herbivores t o green
plants. Organisms at each trophic level depend on those at lower trophic levels for
&mystem : m o e md tne energy to sustain themselves and reproduce. For example, we cannot convert
energy directly from the sun into food. We depend on green plants to make such
transformation for us. This is in accordance with the first law of thermodynamics,
that energy cannot be created nor destroyed but may be transforxhed from one form
'into another. For example, the energy of visible light is transformed into chemical
energy of the glucose molecule synthesised by green plants through photosynthesis.
The living organisms including plants utilise glucose in respiration, which releases
chemical energy, and a part of which is ultimately dissipated as heat, that is, the third
form of energy.

Th~rdTroph~cLevel Fourth Trophic Level

Pr~maryConsumers Secondary Consumers Telt~aryConsumers

Decomposers
Fig. 5.16 : Energy flow through an ecosystem. Producers capture a small amount of solar energy and make
i t available for the subsequent biotic components of the ecosystem, whether they are herbivores,
carnivores, top carnivores or for that matter even the decomposers.

4.
Let us now take the second point that i$ the loss of some energy at each trophicievel.
You might recall that the second law of thermodynamics states that when energy is
transformed from one form into another, some fit is co~vertedinto u n u s h l t
energy, such as heat. Let us understand this with another example. When you slide
a box along the floor, some of the energy you are putting into pushing the box is
being converted into heat energy, due to friction. And this heat energy escapes into *
the surrounding environment. In the same way when the energy stored in muscle cells
is used to contract arm muscles, some of the useful energy is lost as body heat from
athe body. Since heat energy cannot be used to do useful work, more energy must be
supplied to a biological system from outside to compensate the inevitable energy loss.
In order to continue to function, organisms and ecosystems must receive energy
supply on a continuing basis.
Related to the various aspects of energy flow in an ecosystem is the question -why
only a few links in the food chain? The unavoidable loss of usable energy between
feeding levels explains why food chains are relatively short -at the most four or five
links. From your study of pyramids of energy you have seen that how the amount of
energy decreases from the first trophic level onwards. At the fourth or the fifth level
very little energy is left to support another trophic level. In general, there is about
90 per cent loss of energy mainly as heat as we proceed from one trophic level to the
next higher level. In other words, only 10 per cent energy of a particular trophic level
is incorporated into the tissues of the next trophic level. Thus, if 1,000 kcal of plant
energy were consumed by herbivores, about 100 kcal would be converted into
herbivore tissue, 10 kcal to the carnivores and 1 kilocalories to the top carnivore
tissues. Considering these aspects it is clear that in human communities, consu'mption
of food defived from animals such as meat, eggs and dairy products have high energy
cost as compared to foods obtained directly from plants.
In energy terms it is more economical to eat bread made from wheat than to feed
the wheat to hens and then eat the eggs and chicken meat (also see Fig. 5.17). T h i ~
is because eating wheat as bread avoids using any part of its energy 'to keep the
 Ecosystem Functioning
chickens ahvc and active. The crux of the whole discussion is the shorter the food
chain, the greater is the availability of usable energy.

1 Person

29 People

Fig. 5.17 : The relative energy erticiency of ditferent types of foods tor human consumption

This principle has also been practised by many animals in nature to fulfil their energy
needs. The example of baleen whale (Fig. 5.18) we shall discuss here. These whales

Fig. 5.18 : The largest mammal - the baleen whale

are typically found in the open oceans, often in areas where obv~ousfood sources are
insufficient to supply the energy needs of so large an animal. This animal has a special
adaptation that allows it to feed on micnoscopic zooplankton and tiny tlsh. A large
sheet of horny material called baleen, composed of a substance similar to our
fingernails hangs down from the roof of the mouth. These toothless animals can scoop
up a huge mouthful of water and then strain the water out through the fringed edge
of the baleen. trapping in its mouth enormous numbers of tiny plants and animals
that are then swallowed. In this manner a large carnivore is able to feed on primary
consumers of very small size in an ecosystem that is very poor in sizeable prey
organisms, and thus fulfil its energy requirements.
Energy Budget
We have seen that all living things must take in and use energy to maintain their
bodies, to grow, to obtain more energy and to reproduce. Each individual hzs an
'energy income' of all the energy that it acquires during a specified period. Jt also has
an 'energy budget', its allot~nentof different amounts of energy for various activities.
Similarly energy budget for ecosystem as a whole can be prepared. One such example
is given below (see Fig. 5.19). From such studies one can know as to how much energy
input there is in an ecosystem and its subsequent transformation from one trophic
level to another. The energy values are generally expressed in terms of calorie. Let
us now discuss the Fig. 5.19 that you have just seen. It shows that most of the energy
input is in the form of solar radiation while the output of energy is represented by
the waste heat dissipated from the system. ~t may be observed that the total energy
input amounts to 410486 kcallm2/yr (410,000 kcal/m2 solar energy and 486 kcallm21yr
in the form of organic matter imported into the system) match'es with the output of
I
1 Ecusystem : Functioning and Types

Fig. 5.19 : Energy flow diagram for Silver springs, Florida. All the energy figures are expressed ar
kcal1mVyr. (After Odum, 1957).

energy (407986 kcal1m2lyr) lost as waste heat and (2500 kcallmLlyr exported from the
system in the form of organic mat_ter).
5.10 FOOD CHAIN AND FOOD WEB
You are familiar with the concept of food chain and food web that you have studied
earlier in FST-1, Unit 14. Based on that we would discuss these in m o h detail in the
following sections.

5.10.1 Food Chain

In a food chain, the food energy is transformed from a given source through a series
of species, each of which eats the one before itself in the chain. This repeated series
of eating and being eaten is always initiated with green plants, which convert radiant

- - -
energy into chemical energy which is stored in food. A very simple food chain is :
Sun grass goat man
In the previous sections, you have also studied that at each transfer a proportion of
the food, energy is lost as heat. This limits the number of links or steps in a food
chain, usually to four or five. In aquatic ecosystems, microscopic green plants called
phytoplankton and algae play the same role as grasses in a pasture or trees in a forest.
Based on the kinds of organismsthat constitute the first trophic level, three types of
food chains can be distinguished. These are : i) grazing food chain, ii) detritus food
chain, and iii) auxifiary food chain.
i) Grazing Good Chain : Grazing food chains are quite familiar to most of us. Cow
or deer grazing in a field represen6 a grazing food chain. Similarly, eating of
phytoplanktonic algae by zooplankton and fish is another example of grazing
food chain. In most ecosystems, only a small proportion of the total community
energy flows through grazing food chains. Also ?t each step, significant amount
of organic matter is shunted to detritus food chain-through death, decay and
excretion by living organisms.
The grazing food chains in forest and ocean represent two extreme types. Ocean
food chains are among the longest, up to five links, in contrast to forest types
which mostly consist of three or rarely four links. One of the. reasons for the
longer length of grazing food chains in aquatic ecosystems is'the small size of the
phytoplankton and zooplankton that chiefly comprise the first two trophic levels.
If there are many small herbivores at level two, this means that the carnivores at
level three also can be relatively small and numerous, and an additional carnivore
level can be accommodated before the last level, represented by a relatively small
number of large carnivores.
 &mystem : Functioning and Types ii) Detritus Food Chains : Detritus food chains begin with dead organic matter which I:
is an important source of energy. A large amountpf organic matter is contributed \
by the death of plants, plant parts, animals and their excretion products. These :II
types of food chains are present in all ecosystems but they are over dorni;-~~+lng ?
in forest ecosystems and shallow water communities. ii
I
Various species of microscopic fungi, bacteria and other saprophytes play a
prominent role in decomposing organic matter to obtain energy needed for their
survivafand growth. In this process they release various nutrients, locked in dead
organic matter, which are used readily by the green plants.-Detritus food chains
are interconnected with grazing food chains and other auxiliary f o ~ dchains
through certain specific common organisms to permit crossing over of energy and
material flow from one ciicuit to another. For example, cattle do not assimilate
all of the energy stored in plants, undigested residues in faeces become available
I for the decomposers and the detritivores.
/
Detritus food chains are located mainly in the soil or in the sed;ments of aqbatic
ecosystems. They form an essential component of natural ecosystems and are
necessary for self-sustenance and for maintaining ecological balance. Detritus
food chains can be of great practical value for modern man for sewage treatment
and control of water pollution.
Most of the natural ecosystems possess both grazing and detritus types of food
chains. Their relative importance however, varies from onesecosystem to
another. In terrestrial and shallow water ecosystems, detritus food chains
dominate because a major proportion of the annual energy flow passes through
this circuit. In case of tidal marshes, almost 90 pqr cent of the primary production
is routed through the detritus food chains. In deep water aquatic systems rapid
turnover of organisms and high rate of harvest are responsible for the dominance
of grazing food chains. : -
iii) ~ u x % a Food
r ~ Chains : In addition to grazing and detritus food chains there are
other auxiliary food chains operated through parasites and scavengers. Some
parasitic food chains may be quite complex and may involve unrelated organisms.
A deer fed upon by internal roundworms and external ticks or a man with
malarial parasites in his. blood are examples of parasitic food chains. Oh,
parasitic relations are quite involved as parasites are transmitted through a
variety of vectors .or through unrelated intermediary host organisms. Like the
other food chains, the ultimate source of energy for all auxiliaiy food chains is,.
solar energy orginally harvested by plants..
5.10.2 Food Web
In nature no food chain is ~solatedor is simple asodescribed in the above examples.
A plant may serve as a food source for many herbivores simultaneously, erg.. grass
i:
I

plants can support d e e ~ cow,

, grasshopper or rabbit. Similarly, a herbivore may bc
food source formany different carnivorous species (see Fig. 5.20). Also food
4

Fig. 5.20 : A simplified version of a food web.

availability and preferences of herbivores as well as carnivores may shift seasonally Ecorvstem Functioning
(e.g., we eat mangoes in the summer and oranges in the winter).
In an ecosystem, when all interconnections between food chains are mmped out, they
form a food web (see Fig. 5.20). A food web illustrates, all possible transfers of .
energy and nutrients among thc organisms in an ecosystem, whereas a food chain
traces only one pathway in the food web.
The food web for most communities is very complex, involving innumerable k i d s af
living organisms. With many interlocking food chains the community rcmains stable
even if one or more of these relations are altered. For example, in a stream-side
ecosystem if the grasshoppers become scarce or their population is wiped out because
of some calamity, the frogs preying on grasshoppers are not forced to die o r move
out of that place. They can instead feed on other organisms such as flies or butterflies
(see Fig. 5.20). Obviously, then a food web introduces a strong element of stability
into an ecosystem. Larger the number of components involved, the more stable the
ecosystem is.

5.11 ECOSYSTEM CONTROL

In this section, let us discuss about yet anothel' important aspect of ecosystem
functioning, that is, how it maintains its ecological balance. By now, it must be
obvious to you that an ecosystem is a dynamic system, wherein a lot of events take
place. For example, animals eat and in turn are eaten, moisture and nutrients flow
in and out of the system, and weathers change. In spite of all these happenings the
ecosystems persist and recover from the slight disturbances. This cnqacity of an
ecosystem to self-regdate or self-maintain is called homeostasis. I s .l't this gbility of Homeo = same;
ecosystems to recover from certain perturbations remarkable? LCt us take a simple Stasis = standing
example to see that how is this balance maintained in spite of the f.!ght disturbances
in the ecosystem. Consider a grassland, when there is a drought, do not grow
well. The mice that eat the grass become malnourished. When this happens, their
birth rate decreases. And also the hungry mice retreat to their burrows and sleep. By
doing so, they need less food and are less exposed to predators, so their de'ath rates
decrease. Their behaviour protects their own population balance as well as that of
the grasses which are not being consumed while the mice hibcrnate. Such a
mechanism is known as feedback regulation and is very important to maintain the
ecological balance. It is the prime regulatory mechanism for the ecosystem as a
whole. You may know that there are several kinds of organisms comprising an
ecosystem. So all the organisms in an ecosystem are part of several different feedback
'loops. A feedback loop may be defined as relationship in which a change in some
original rate, altcrs the rate of direction of further change. In the above example, we
had deliberately taken a very small group of living beings, that has primarily the mice
and the plants.
Now we take up, another parameter of ecosystem balance. One factor that aftects
the stability or persistence of some ecosystems under small o r moderate
environmental stress is species diversity - the number of species and their relative
abundance in a given ecosystem. High species diversity tends to increase long-term
persistence of the ecosystem. It is because with so many different species and the
linkages between them, risk is spread more widely. An ecosystem having a good
variety of species has more ways available to respond to most environmental stresses.
For example, the loss or drastic reduction of one species ip an ecosystem, with
complex food web usually does not threaten the existence of others, because most
consumers have several alternative food supplies. In contrast, the highly specialised
agricultural ecosystem, planted with only one type of crop such as wheat or rice is
highly vulnerable to destruction from a single plant disease or insects. Therefore, the
essence of the whole discussion is that most balanced ecosystems contain many
different species.
The discussion so far, might have led you to conclude that the ecosystems have the
ability to cope up with any disruption. You should realise that this ability is limited.
Extremities like fires (destroy the landscape), over-exploitation (e.g., rampant
,ieforestation, mining) or excessive simplification !monoculture, plantation,
cropfields) or too severe and prolonged stre\\ch (like drought, pollution) seriously
trcaystern : Functioning and Types hamper the control mechanism, resulting in ecosystem degradation. The lesson is
obvious. We should check and control our actions, so that, we do not overload the
ecosystem.
 Ecosystem Functioning

5.12 SUMMARY
In this unit we have examined various aspects of ccosystem functioning. So far you
have learnt that :
Ecosystems are considered functional units of nature having no specified size or
limits.
Ecosystems comprise di'fferent biotic and abiotic components which are
functionally coordinated and operate in an integrated, holistic manner.
Every organism has a capacity to tolerate a certain range of a particular
environmental factor. This range is known as the tolerance range. At the
extremities of the tolerance range the factor becomes limiting.
The concept of trophic level tells us as to which organisms share the same general
source of nutrition.
Trophic relationships of an ecosystem can be represented graphically in the form
of ecological pyramid. The base of the pyramid represents the producers and the
successive tiers represent the subsequent higher trophic levels..
Ecological pyramids are of three types : one - pyramid of number depicts the
number of individual organisms at each trophic level; second - pyramid of
biomass represents total weight of the living organisms at each trophic level, third
pyramid of energy shows the amount of energy utilised at successive trophic
levels.
Ecological pyramids. give useful information about the functiofial structure of an
ecosystem, but they also have some limitations. Important among them are :
a) decomposers are not' represented; b) organisms which take food from different
trophic levels are not accounted for; c) one gets no idea about the seasonal and
daily variations and also about the detritus litter as an energy source; d) the rate
of transfer from one trophic level to another is not known.
Energy is transferred in an srderly sequence, i.e., from sun to producers, to
consumers,.to decomposers. Energy flow is always downhill and unidirectional.
Heat is constantly lost during the process of energy transfer as expressed in the
first and the second laws of thermodynamics. In an ecosystem,energy flow can be
quantified. Energy budget refers to the energy enterihg and leaving an ecosystem
in a given span of time.
Ecosystems are solar-powered systems. Green plants capture solar energy and
store in the form of organic substances. 'Gross primary productivity of an
ecosystem is the rate at which organic matter is produced during photosynthesis.
Net' primary productivity represents the rate at which some of this matter is
incorporated into plant bodies. Net primary productivity is less than gross primary
productivity because of the losses resulting from plant metabolism. Increase in the
weight of consumers which depend on organic food is termed as secondary
production.
Productivity varies from one kind of ecosystem to another and from one time to
another. The availability of water, the amount ofminerals and many other factors
in addition to incident radiation limit productivity in different ecosystems.
Ecosystem : Func- and Energy passes from one trophic level to the next. Approximately 90 per cent of
the energy is lost at each transfer. O n the average, about 10% of the energy
entering a particular trophic level is available to the next level in an ecosystem.
Therefore, the biomass that an ecosystem can support at each trophic level declines
rapidly. The loss of energy at each trophic level, limits the number of trophic levels
in a food chain to four or five.
Organisms of various trophic levels are related to each other through feeding
relationships, that can be represented in terms of food chain. Three main types of
food chains can be distinguished namely grazing, detritus and auxiliary food
chains. The relative importance of these chains may vary in different ecosystems.
0 Ecosystems are highly dynamic entities. They have evolved effective homeostatic
mechanism for self-regulation through feedback control.

I ) a) Fill in the blank spaces with appropriate words.

All ecosystems have the same three categories of organisms; ..................,
which use abiotic sources of energy and nutrients to synthesise organic
molecules; ................., which acquire energy and nutrients by digesting the
organic molecules of living organisms; and .................., which obtain
energy and nutrients by digesting the organic molecules of dead organisms,
their excretions and other organic (but no longer living) materials. Of the
three categories, an ecosystem could persist without ...................

2) a) Discuss the concept - range of tolerance. Can you think of any examples in
which the range of tolerance was exceeded in ecosystems you are fadiliqr
with? What happened during these incidents?

b) What js a limiting factor? What is the limiting factor in most terrestrial

ecosystems?

3) a) Which of the following pyramid can never assume an inverted shape?

pyramid of biomass, pyramid of number, pyramid of energy.
....................................................................................................
b) Which trophic level remains unrepresented in ecological pyramids?
.....................................................................................................
 c) Why pyramid of biomass in some.aquatic ecosystems like lakes and oceans i Ecosystem Funellon6ng
acquires an inverted shape?

4) a) The net primary productivity of an ecosystem is the total amount of producer

tissue formed per unit area per unit time, or the ..................I productivity,
minus the chemical energy used in ................... The amount of chemical
energy decreases with each step in a food chain, since each organism uses
some of the energy to ................ ..., a process that converts chemical energy
to .................., and this form of energy is lost from the food chain.
(Fill in the blank spaces with appropriate words)
b) Productivity can be expressed in terms of k ~ a l / m ~or/ ~g/m2/yr.
r Discuss the
differences and the similarities between these two expressiens.

c) What influences productivity?

d) Which.ecosystems have high net productivity?

e) What is secondary production? How does it differ from primary production?

...................................................................................................

5 ) a) Fill m the blank spaces with appropriate words :

Energy enters an ecosystem primarily as .................. and leaves an
ecosystem primarily as ................... Within the ecosystem, it is transferred
from organism to organism in the form of .................. energy.
b) Can energy be recycled through an ecosystem? Yes or No?

c) Assume that a plant convert's 1 per cent of the light energy it received from
the sun into plant material and that an animal,stores 10 per cent of the food
energy that it eats. Starting with 10,000 calories of light energy, how much
energy is available to a person who eats :
i) Wheat ....................... calories
ii) hen.. .....................calories
...
111) frog.. ....................;calories
~cosyscem:lhc6mtwmdT~prs d) A group of explorers is stranded on a barren island. All they have in their
stores are some hens and some wheat. To make these resources last as long
as possible, should they :
i) eat the wheat and when it is finished, kill and eat the hens, or
ii) feed the wheat to the hen, collect and eat the eggs laid and when the
wheat is gone, kill and eat the hens, or
iii) kill and eat the hens first and when they are finished eat tbe wheat?
Choose the correct choice and write its number in the box provided below.

Also justify you1 answer, in not more than 4 to 5 lines.

...................................................................................................

e) Is there one o i more than one food web in any ecosystem?

f) During the 1970s, shark-killing expeditions became a fad. Why are sharks
important in the food chain of the ocean, and what do you suppose,might
happen to other fishes if a large number of sharks disappeared?

6 ) In what way food web relationships promote ecosystem stability? Support

your answer with a suitable example.

--
5.14 ANSWERS
Self-assessment Questions
i$ a) i) v'
ii) x
iii) V
iv) x
v) x
primary producers
biotic components
E- consumer
decomposers

L abiotic components -4 energy

inorganic elements
substrate or medium
 Ecosystem Functioning
c) Write from your own experience
Hint : i) water in a desert
ii) salinity in an aquatic ecosystem

2) a) wheat, corn (first trophic level)

. goat, rat (second trophic level)
1ion;cat (third trophic level)
b) Hint : e:g., bear,
Second trophic level (herbivore) as it eats tubers and various other plant
products; third trophic level (carnivore) as it eats animals like deer which is
a herbivore; fourth trophic kvel (top carnivore) as it eats animals like frog
which are carnivores.
c) In-situations where the number of producers is less than the subsequent
trophic levels, we get an inverted pyramid, e.g., a large number of insects
feeding on a single tree.

e) In energy pyram~ds,a particular trophic level always has a smaller energy

content than the trophic level immediately below it. This is because some
energy is lost while transfer from one trophic level to the next. Since the
amount of energy decreases from the first trophic level onwards, therefore the
energy pyramids are always upright and never inverted.

3) a) In an ecosystem the producers utilise solar energy and store it in the food they
prepare which are mainly carbohydrates. The plant tissues that have the
stored solar energy in them serve as a source of energy for the herbivores.
And the herbivores pass on the energy to the carnivores and so on and so
forth. Thus the ultimate source of energy for our planet which on the whole
can b e . nnsidered as a large ecosystem, is sun.

Consumers

//

pDecomposers

c) i)
d) ii)
e) iij
f ) desert and semi-desert areas (7)

I
hi

:1
I/
savanna (5)
open ocean (6)
i1 estuary (3)
temperate deciduous forest (4)
tropical rain forest (2)
I1 coral reefs (1)

I extreme desert (8)

-:-~TJP 4) a) Your choice.
b) Hint : If one population of a food chain suffers a decline; then this particular
food chain could disappear.
c) v)
d) iv)
e) iii)
f) ii)
g) iii)
h) ii)
Terminal Questions
1) producers, consumers, decomposers, consumers.
2) a) Every living organism can tolerate certain range of a particular environmental
factor. Beyond this range the organism is unable t o survive, e.g., one of the
factors that\influence the life in lakes is pH. Due to acid rain, the p H of th-
lakes becomes low, consequently most of the living organisms perish. The
water of such lakes appear transparent as the lake becomes devoid of life.
(Recall from FST-1, Unit 16, Sub-section 16.2.1)
.( Another e x a i p l e is the accum"1ation of hazardous wastes in the bodies of

organisms like birds, man, etc. who are at the top of the food chain. (See
FST-1, Unit 16, Sub-section 16.2.1)
b) Living organisms are dependent on certain environmental factors for .their
survival and well being. If any of these factors is in short supply, o r even in
an excess, it becomes a limiting factor. In most terrestrial ecosystems, water
is a limiting factor.
3) a) pyramid of energy
b) decomposers
c) In lakes and sea, most primary producers are single-celled algae which are
very small and short-lived. These producers have rapid turnover as compared
to the animals of secondary and tertiary trophic levels, e.g., various kinds of
fish. These organisms of secondary and tertiary trophic levels are large in size
and outweigh the producers. So if we calculate the biomass of various trophic
levels in conditions when the biomass of producers is less than the consumers.
the pyramid assumes an inverted shape.
4) a) gross, respiration, d o work, heat
b) Similarities - both are the units t o measure primary productivity.
Differences - primary productivity in terms of weight is expressed as g/m'/yr.
and in terms of energy is expressed as k ~ a l / m ' / ~ r .
c) Productivity is influenced by a variety of factors such as sunlight, temperature.
rainfall and availability of nutrients.
d) Ecosystems like coral reefs, tropical rain forests and estuaries have high net
productivities.
e) Secondary production refers to the production by consumer organisms. In
primary production, the solar energy is trapped by producers resulting in the
increase of their biomass; whereas in secondary production, the consumers
utilise the stored energy of plants, for building their bodies.
5) a) Light; heat, chemical
b) No
c) i) 10 calories
ii) 1 calorie
iii) 0.1 calorie
d) If we sustain.the population of men as well as hen or the stored wheat, the
stock would exhaust faster. So first, hens be eaten and thentwheat. This will

wheat
wheat
- -
enable the wheat stock to last longer.

- hen
man
man .......... (1
.......... (2)
 In food chain (2) since man is neare.r the therefore, t h e energy loss
would be minimum and they can be sustained on the available wheat stock
for a longer-period.
t e ) O n e food web
f) Disappearance of shark would lead to a massive increase in the number of
small fish. This would exert tremendous pressure on the phytoplankton
population. In times, when the phytoplankton number is very low, there won't
be enough food-for the fish, and result would be increased mortality of small
fish. Thus the entire food chain would be disrupted.
6 ) A food web shows the feeding interrelationships which exist between various food
chains found within an ecosystem. A food web. has a number of alternative
routes for energy flow, which help in promoting ecosystem stability. Give an
cxample of your choice.