5,4,3,2,1 go… can you talk about food
chains and food webs for 60 seconds
mentioning as many as the key words as
possible?
Food web
Producer
Consumer
Decomposer
Food chain
Primary
consumer
Detritivore
Quarternary
Secondary
consumer
Omnivore
Trophic level
Habitat
Carnivore
Energy loss
Tertiary
consumer
Heat
Respiration
Herbivore
Ecological pyramids
We Are
Learning:
About how energy moves and is lost
between trophic levels.
• What are the different types of ecological pyramid?
• What are the relative merits and disadvantages of
each?
•What percentage of energy is transferred from one
level to the next?
Today we are covering from the
specification:
Review – Energy in Food
Chains
Exam question
Ecological Pyramids
• Food chains and food webs are a useful
means of showing what different
organisms eat and therefore energy
flow.
• PROBLEM: They do not provide
quantitative information.
• SOLUTION: ecological pyramids., e.g.
Pyramids of Numbers
What information do they give us?
Number of organisms in
an ecosystem at each
trophic level.
BUT…
They can look odd
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Pyramids of Numbers - Issues
1) No account is taken of size – one tree is given the same
value as 1 aphid therefore they can look inverted.
2) The number of individuals is so great that it is impossible
to represent them accurately on the same scale as other
species in the food chain – e.g. one tree and 1 million aphids.
They can look odd
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Pyramids of Numbers – Another Example
Pyramids of biomass
• More reliable, quantitative description
of a food chain is provided when their
biomass is measured.
• Biomass is the total dry mass of the
plants and/or animals in a particular
place.
Pyramids of Biomass
Biomass = dry
mass of
organisms
Given as kgm-2 for
an area
Or
kgm-3 for a volume
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Measuring biomass
Problem:
finding the biomass of worms
Why is this so hard?
How would you do it?
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Problem 1: variability due to moisture content.
Solution: dry your earthworms at 60˚C for 24-48
hours to get “dry earthworm biomass”.
Problem 2: variability due to gut contents.
Solution:
• Keep the live earthworms in containers until they
empty their guts (24-48 hours, if they don’t die in
the process);
• Dissect the preserved earthworm and flush their
opened gut;
• “Ash-dry” worms leaving only mineral gut contents.
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What do we find?
A statistical relationship between length & dry biomass
Dry worm & weigh
“ash-dry” at 500 ˚C
Remove ash
Weigh gut contents
Subtract gut contents
weight from dry weight
Q. Why is the graph so useful?
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Pyramids of biomass
• Fresh mass is quite easy to access but
varying amounts of water makes it
unreliable.
• Use dry mass but the organisms must be
killed;
– Usually a small sample, which may not be
representative.
So, why do we usually see this
shape for pyramids of biomass?
• Energy is ‘lost’ at each stage in the food
chain (see previous lessons!), so there is
less energy available at each subsequent
trophic level to create new biomass.
Pyramids of Biomass – an exception to the rule
Anomaly from
pyramid of biomass
with aquatic food
chain
Q. Why is there less
biomass in producers
than primary
consumers?
A. Sample is made at a single point in time and plant biomass
may vary with season. Phytoplankton may also be reproducing
quickly and so large turnover of biomass.
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Pyramids of energy
• Most accurate representation of energy flow
through an ecosystem.
• Collecting data can be difficult and complex.
• Data are collected in a given area (e.g. one
square metre) and in a set time (e.g. a year).
• Results are more representative than those
for biomass as 2 organisms of the same
biomass may store different amounts of
energy.
(1g of fat stores 2x quantity of energy of 1g of
carbohydrate.)
Pyramid of Energy
Always a true pyramid-shape…why?
Energy stored in
each trophic level of
a Florida ecosystem
Energy KJ m-2 y-1
Q. Why might
deriving energy
content be difficult?
A. Two organisms of the same dry mass may have
different energy contents e.g. more or less fat which is
high in energy.
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Task
Past paper questions.