Prepared by Senjuti Saha, Associate Professor, Women's Christian College, Kolkata

1. Topic 8
Concepts of Trophic Structure, Food Chain and Food Web.
Energy Flow in Ecosystems
Senjuti Saha
Associate Professor
Women’s Christian College
Kolkata
2.19 GEO-A-CC-4-10-TH – Soil and Biogeography
Unit II: Biogeography

2. What is Trophic Structure
A characteristic feature of any ecosystem, measured and described either in
terms of the standing crop per unit area, or energy fixed per unit area per unit
time, at successive trophic levels
It can be shown graphically by
the various ecological pyramids
It is the pattern of movement of
energy & matter through an
ecosystem
It is the result of compressing a community
food web into a series of trophic levels

3. Example:

4. What is Trophic Level
• Every ecosystem has a trophic
structure: a hierarchy of
feeding relationships which
determines the pathways for
energy flow and nutrient
cycling
• The first trophic level
comprises the green plants or
the PRODUCERS
• The second is the herbivores
or the PRIMARY CONSUMERS
• The third is the Carnivores
which eat the herbivores or
the SECONDARY CONSUMERS
•The fourth is the DECOMPOSERS who
lives on the herbivores and carnivores
•Species are assigned to trophic
levels on the basis of their nutrition

5. Trophic Level Vs. Trophic Structure
Trophic Level Trophic Structure
All organisms in an ecosystem can be
placed in Trophic Levels depending on
what energy source they rely upon
and how they provides energy for
other organism in the food web
It refers to the way in which organisms
use food resources to get their energy
for growth and reproduction and is
often referred as ‘Food Web’ or ‘Food
Chain’
Any healthy ecosystem consists
of trophic levels that have
complex linkages to form a Food
Web

6. Special Cases
1. Hydrothermal Vent
Communities are an
exception: the producers are
Chemosynthetic bacteria that
derive energy by oxidising
hydrogen sulphide
1. A special class of consumers,
the detrivores, derive their
energy from the detritus
representing all trophic
levels

7. Energy Flow in an Ecosystem
The existence of living world depends upon the flow of energy and circulation
of materials through the ecosystem. The energy is required for the
performance of all the life activities. The living organisms utilize solar energy.
In photosynthesis, this radiant energy is transformed into chemical energy and
in cellular metabolism the chemical energy is transformed into mechanical or
heat energy
Energy flow is the movement of energy through an ecosystem: from the
external environment through a series of organisms and back to the external
environment, It is one of the fundamental processes common to all
ecosystems

8. Energy Flow & Trophic Level
Forms of Energy
four types
• Chemical,
• Electrical
• Mechanical and
• Radiant energy
Chemical energy is stored in the bonds of
chemical substances and later released
from potential to kinetic e.g. fuel
molecules
Electric energy results from movement of
charged particles e.g. electric current
flow in our homes
Mechanical energy is involved
in the physical movement of
any matter e.g. pushing a
table or paddling a bicycle
Radiant energy is energy of
electromagnetic spectrum, it travels in
waves e.g. X rays, infrared, radio wave
etc. The radiant energy is in the form of
electromagnetic waves which are
released from the sun during the
transformation of hydrogen to helium
constitutes the fundamental source of
energy

9. The basic supply of energy requisite by all life forms is the chemical energy of
their food. The chemical energy is acquired by the conversion of the radiant
energy of sun which is stored in the food of living organisms is converted into
potential energy by the constituent atoms of food in a particular manner
The first law of thermodynamics says energy
is neither created nor destroyed; it may be
changed from one form to another form in
numerous paths.
The second law of thermodynamics says
each time energy is converted from one form
to another, a part of it is lost in the form of
heat (entropy). No energy conversion is 100%
efficient.

10. Progressive decrease in energy
utilization at each trophic level
It is estimated that there is a loss of
about 90 percent energy as heat
during metabolism as it moves from
one level to another, therefore the
efficiency of energy transfer is only
10 percent. Thus, energy budget of
an ecosystem declines with each
energy conversion and the available
energy at higher trophic level
becomes too small
Energy Pyramid

11. Energy flow and Efficiency
The ecosystems integrate both
biotic (living) that comprises
producers, consumers and
decomposers; abiotic (non-living)
elements such as solar radiation,
soil, water air and heat
Ecosystems are energy systems that
include numerous food chains and food
webs connecting the enormous number
of organisms with their environment. The
energy passes through the categorised
various feeding (trophic) levels and in
this transformation of energy most of it
goes as heat in the respiration

12. This entire process of energy transformation explains that energy flow is uni-directional
(one-way flow) through the ecosystem and it is not recycled rather it is replenished by
the solar radiation
Energy flow models are basically of two types
1. Single Channel Energy Flow Model characterised by unidirectional flow of
energy through different Trophic Levels, involving a single food-chain and indicating
dissipation of energy at each transfer
Single Channel Energy Flow Model

13. 2. Y – Shaped Energy Flow Model, also known as Two Channel Energy Flow
Model that includes both the Grazing and Detritus food chain of an ecosystem,
This model is more realistic because
 They conform to the basic stratified structure of ecosystems
 They separately indicate the direct consumption of living plants and utilization
of dead organic matter both in space and time
 The detrivores differ greatly in size-metabolizm relations and in study
techniques
The proportion of the net primary production energy that flows down the two
pathways varies with different ecosystems, as well as with seasonal or annual
cycles in the same ecosystem.
However in all ecosystems the Grazing and Detritus food-chain are
interconnected, so shifts in energy flows can occur quickly in response to forcing
function inputs from outside the system

14. Y – Shaped Energy Flow in an Ecosystem

15. The flow of energy through the food chain depends
on the efficiency with which organisms consume
their food resources and convert them into
biomass. This is called trophic level efficiency of
ecological efficiency which depends on two types of
factors; the limitations on the availability of basic
resources for photosynthesis and disturbance due
to the external relationship to the environment

16. Universal Energy Flow Model
• It is applicable to any living component of any ecosystem
• For any given trophic level, such figure consist of a ‘box’ representing the
‘biomass’ at any given time and pathways through it representing the flow of
energy
• The size of the box is proportional to the total biomass at the trophic level
• Energy enters the trophic level as the food is ingested by the organism. Some
of the ingested energy is not used and egested out, while the rest is
assimilated
• Some of the assimilated energy is used in respiration or is stored in the form
of fat. The rest is used for growth or egested
• The rest amount of energy or biomass or production becomes available for
the trophic level
• Such model may well represent the dynamics of an entire trophic level or of
a single individual within the trophic level
• Thus, universal model the energy inputs and outputs of a particular
individual or of all the individuals at a trophic level

18. The ecological efficiency can also be seen with respect to climatic
limitations. For example the environments with no limitation which
includes the tropical areas with an annual average output of 2200 grams of
organic output per square meter. This is followed by environments of
seasonal limitation like Mid-latitude zones with the annual average organic
output of 600 to 1200 grams per square meter. The others are
environments with permanent limitation which include arid/arctic and
Polar Regions with 5 to 90 grams per square meter annual average.
Similarly aquatic ecosystems also show variation in energy efficiency
depending upon the depth of oceans

19. Food Chain and Food Web

20. Food Chain
Describe the energy flow between
species within an ecosystem
Producers use energy from the sun to
make food and therefore start the chain
The arrows represent the direction of
energy flow, pointing from the organism
being consumed to the organism
receiving the energy

21. Food Chain Types
Several types of Food Chains are there. Two of them are
discussed here: Grazing & Detritus

23. Significance of Food Chain
• The knowledge of food chain helps in understanding
the feeding relationship as well as the interaction
between organism & ecosystem
• It also helps in understanding the mechanism of
energy flow & circulation of matter in ecosystem
• It also helps to understand the movement of toxic
substance and the problem associated with biological
magnification in the ecosystem

25. Food Web
Group of interconnected food chains
Organisms within a food web can belong to more
than one trophic level, or feeding level

26. Food Web Types
• Soil food web
• Aquatic food web
• Forest food web
• Grassland food web
• Food web of Terrestrial & Aquatic ecosystem

32. Significance of Food Web
• Food webs distinguish levels of producers and consumers by
identifying and defining the importance of animal
relationship and food sources, beginning with primary
producers such as plants, insects and herbivores
• Food webs are important tools in understanding that plants
are the foundation of all ecosystems and food chains,
sustaining life by providing nourishment and oxygen needed
for survival and reproduction
• The food web provide stability to the ecosystem

34. Ecological Pyramid
• Ecological pyramid is a graphical representation in the form of
a pyramid showing the feeding relationship of groups of
organisms. It is often represented in a way that the producers
are at the bottom level and then proceeds through the
various trophic levels in which the highest is on top
• Ecological pyramid also shows the flow of energy or biomass
at each trophic level in a particular ecosystem. Biomass
pertains to the amount of living or organic matter in an
organism. Biomass pyramids are shaped that way to show
that biomass is largest at the base, and decreasing in amount
as it goes through the apex

35. Types of
Ecological
Pyramid
Pyramid of
Numbers
Pyramid of
Biomass
Pyramid of
Productivity

36. Pyramid of Numbers
• It depicts the number of individual organisms at different
trophic levels of food chain.
• This pyramid was advanced by Charles Elton (1927), who
pointed out the great difference in the number of the
organisms involved in each step of the food chain
• Successive links of the trophic structure decrease rapidly in
number until there are very few carnivores at the top

38. Two types of biomass pyramids are there
• the upright and the inverted
• The upright pyramid is found in
most ecosystems. It results when the
combined weight of producers is larger than
the combined weight of consumers
• An inverted type results when the combined
weight of producers is smaller than the
combined weight of consumers
Pyramid of Biomass

40. Pyramid of Productivity / Energy
 When production is considered in terms of energy, the pyramid
indicates not only the amount of energy flow at each level, but
more important, the actual role the various organisms play in the
transfer of energy. An energy pyramid illustrates how much
energy is needed as it flows upwards to support the next trophic
level
 The pyramid is constructed according to the rate at which food
material(in the form of energy) passes through the food chain.
Some organisms may have a small biomass, but the total energy
they assimilate and pass on, may be considerably greater than
that of organisms with a much larger biomass
 Energy pyramids are always slopping because less energy is
transferred from each level than was paid into it. In cases such as
in open water communities the producers have less bulk than
consumers but the energy they store and pass on must be greater
than that of the next level

42. Disturbances in Ecosystem
• Bioaccumulation: When plants/animals take up a chemical
from the environment do not excrete it, the chemical builds
up in the organisms over time to a potentially lethal level
• Biomagnifications: Refers to the sequence of processes that
results in higher concentrations of the chemical in organisms
at higher levels in the food chain. The concentration of
chemical may not affect lower levels of the food chain but the
top levels take in so much it can cause disease or death
• Extinctions of Species : Due to decrease in population of
various species the balance of various trophic levels have
more accumulation of species while others have very less
population

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