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7.
What is the percent % efficiency in a food chain?
How do you
calculate
the % percent efficiency of energy and biomass transfers along a food
chain - examples of calculations explained
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(7)
Calculations of the efficiency of
biomass or energy transfer up food chains
The fact of the matter is, that
up a food chain/biomass pyramid, only a small percentage of the mass is
passed on.
Reminder: Only ~1% of sunlight
energy ends up as plant biomass.
e.g. To make the numbers easy, lets
assume only 10% of biomass is passed on at each stage.
For the food chain 1 of 4 trophic levels, we would get in terms of
percentages ...
plant producers (100%) ==>
primary consumers (caterpillars, 10%) ==> secondary consumers (small birds
1%) ==> tertiary consumer birds of prey (0.1%).
Instead of percentages the
biomass might be expressed as mass in kg or g.
plant producers (100 g) == stage 1 ==>
primary consumers (caterpillars, 10 g) == stage 2 => secondary consumers
(small birds 1 g) == stage 3 ==> bird of prey (0.1 g)
The typical mass of a common
kestrel is 180g, so lets repeat the 10% exercise above ..
180000g of cabbage ==> 18000 g of
cabbage white butterflies ==> 1800g of blue tits==> 180g kestrel
So it takes 1000
kg of cabbage to make one 1 kg kestrel !!!
You can work out how much
biomass is lost by subtracting the mass left at one stage from
the previous stage. Suppose we start with 1000 g of
producer (cabbage) and on pass on 10% of biomass at each
stage from one trophic level to the next up the food chain -
using the example above.
at stage 1 the mass loss is
1000 - 900 = 100 g
at stage 2 the mass loss is
100 - 90 = 10 g
at stage 3 the mass loss is
10 - 9 = 1 g
At the end of stage 3 the
total mass loss is 900 + 90 + 9 = 999 g, only 1 g
becomes part of the bird of prey (kestrel) at the top of the food chain!
100 x 999/1000 =
99.9% of
the mass has been lost in the process!
The reasons for inefficient transfer of biomass and its loss are
discussed in part 6
You can also work out the
efficiency of biomass transfer at each stage in the food chain
Using the following formula
to calculate the efficiency of biomass transfer:
| |
biomass available
and transferred to next level |
|
|
Efficiency =
|
-------------------------------------------------------------- |
x
100 |
| |
total biomass available from the previous level |
|
For stage 1: Efficiency = 100
x 100 / 1000 = 10%
For stage 2: Efficiency = 100
x 10 / 100 = 10 %
For stage 3: Efficiency = 100
x 1 / 10 = 10%
You can also express the
efficiency of energy transfer for each stage in the food chain
Using a similar formula
to calculate the efficiency of energy transfer:
| |
energy available
and transferred to next level |
|
|
Efficiency =
|
-------------------------------------------------------------- |
x
100 |
| |
total energy available from the previous level |
|
Incidentally, you can draw pyramids of
energy, in exactly the same way as for biomass.
In the food chain: plants ==>
rabbits ==> foxes, all these fields of plants of large areas of grass
support a relatively smaller population of rabbits, which in turn support a
very small number of foxes - you only get a relatively small numbers of a
top predator!
This is the reason why you rarely get
food chains of more than five stages (feeding/trophic levels) because
there is so little mass/energy resource left at the end of the food
chain.
Once the energy is lost, it
can't be used by the animal in the next stage of the food chain i.e. the
next trophic level.
It can be difficult sometimes to
construct an accurate pyramid of biomass because some organisms may feed
at more than one trophic level - examples have already been
mentioned in the context of food webs.
Examples of biomass and
efficiency calculations
Ex. 1. Food
chain:
Grass ==> rabbits ==> foxes
Suppose 500 kg of grass
supports 25 kg of rabbit, which in turn supports 2 kg of fox.
(a) Calculate the biomass
efficiency at each stage.
| |
biomass available
and transferred to
the next level |
|
|
Efficiency =
|
--------------------------------------------------------------------- |
x
100 |
| |
total biomass available from the previous level |
|
For grass ==> rabbit:
Efficiency of biomass transfer = (25/500) x 100 =
5.0%
For rabbit ==> fox: Efficiency
of biomass transfer = (2/25) x 100 =
8.0%
(b) Calculate the overall
biomass efficiency from grass to fox.
For grass ==> fox: Efficiency
of biomass transfer = (2/500) x 100 =
0.4%
(c) Calculate the biomass
lost between the 2nd and 3rd trophic levels.
mass of rabbits - mass of
foxes = 25 - 2 =
23
kg
What is this as a
percentage of biomass lost?
(23/25) x 100 =
92%
Ex. 2.An
energy transfer efficiency calculation
A typical food chain (summarised) in a more complex ecosystem of
food webs,
but starting with a given
initial input quantity of sunlight energy of 1 MJ:
sunlight (1 000 000 J) ==>
plants 8 000 J ==> small animal herbivores (600 J) ==> top
predator carnivores (50 J)
After the 'sun' input, the
figures in () represent the chemical potential energy
store of the organisms.
| |
energy available
and transferred to
the next level |
|
|
Efficiency =
|
------------------------------------------------------------ |
x
100 |
| |
total energy available from the previous level |
|
The formula for calculating the
efficiency of energy transfer up a food chain.
(a) Calculate the efficiency of energy
transfers involved.
sunlight ==> plants: efficiency = (8
000/1 000 000) x 100 =
0.8%
plants ==> herbivores: efficiency =
(600/8000) x 100 =
7.5%
herbivores ==> carnivores: efficiency
= (50/600) x 100 =
8.3%
(b) What is the overall efficiency from
sunlight to top predator carnivores?
Efficiency = (final energy store of
carnivores/initial sunlight energy) x 100
Overall efficiency of energy transfer
= (50/1 000 000) x 100 =
0.005%
(c) What % of energy is lost from the 1st
to the 3rd trophic level?
1st trophic level are the plants (8000
J)
3rd trophic level are the top
carnivores (50 J)
Energy lost = 8000 - 50 = 7950 J
% energy wasted = (7950/8000) x 100 =
99.4%
Note: The efficiency of energy
transfer = (50/8000) x 100 =
0.63%
The
reasons for inefficient transfer of biomass and its loss are discussed
in part 6
Key points -
Summary of ideas
Based on
the syllabus-specifications for students taking the AQA, Edexcel and OCR
GCSE level biology examinations (~US grades 9-10).
In a food chain, the
percentage efficiency
of biomass or energy transfer refers to the proportion of
energy passed from one trophic level to the next.
It measures how much of the energy consumed by an
organism is successfully transferred to the next level when it is eaten.
Why is efficiency important?
Not all energy is transferred efficiently through a food
chain—energy is lost due to:
-
Respiration (used for movement, heat production,
etc.)
-
Excretion (waste materials like faeces)
-
Indigestible parts (bones, fur, etc.)
How is efficiency calculated?
The formula for energy transfer efficiency
is:
| |
biomass available and transferred to next level |
|
| Efficiency =
|
------------------------------------------------------------ |
x
100 |
| |
biomass available from the previous level |
|
Typical efficiency values:
-
Plants converting sunlight into biomass:
~1% (most energy is lost as heat)
-
Primary consumers (herbivores) converting
plant energy: ~10%
-
Secondary & tertiary consumers (carnivores)
converting animal energy: ~10–20%
Because of this energy loss at each stage, food chains
rarely have more than four or five trophic levels!
Summary of
learning objectives and key words or phrases
Know how to calculate the percent
efficiency of biomass transfer in food chains.
Be able to relate efficiency of biomass transfer to
energy transfer up a food chain.
Be able to explain examples of
calculations and problem solving exercises in biomass transfer.
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