energy transfers in food chains -...
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Energy Transfers in Food Chains
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Feeding relationships
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Energy transfer in a food chain
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The length of food chains
This tiger is a top
predator – the final level
in a food chain.
Most food chains only
contain four or five
species, or less.
Why is this?
Firstly, plants cannot actually absorb all of the energy from
the Sun. Furthermore, energy is lost at all levels in
food chains. If a food chain was very long, animals at the top
of it would not get enough energy to survive.
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What can pyramids show about energy transfer?
Food chains and pyramids
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Pyramids of numbers only give an accurate impression of
the flow of energy in a food chain if the organisms are of
similar size. Measuring the biomass at each trophic level
can give a more accurate picture.
Pyramids of numbers are a quantitative way of
representing food chains.
What are the problems of
representing food chains in
pyramids of numbers?
They record the number of
organisms at each trophic level
in a food chain.
What are pyramids of numbers?
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In a pyramid of numbers, the length of each bar represents
the number of organisms at each trophic level in a
food chain.
As a single tree can support many organisms, this food chain
produces an unbalanced pyramid.
Understanding pyramids of numbers
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Understanding pyramids of biomass
In a pyramid of biomass, the length of each bar represents
the amount of organic matter – biomass – at each trophic
level in a specified area.
At each trophic level, the amount of biomass and energy
available is reduced, giving a pyramid shape.
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Constructing pyramids
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The problem with pyramids
Pyramids are a very simplified way of representing
food chains.
Some animals may belong to more than
one trophic level. For example, humans are
often primary, secondary and even tertiary
consumers. There would be no way to show
this properly with either form of pyramid.
It is also very difficult to accurately measure the biomass of
an ecosystem. This is done by sampling the biomass, and
then drying it out. While it may be possible to dry and weigh
some plants, there are ethical issues with doing the same
thing to animals – especially endangered species.
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Food chains and pyramids
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What are the benefits of eating plants?
Your friend is at university and has phoned you to
complain that all she can afford to eat is beans on toast.
How could you make her
feel better about her diet?
What are the energy
arguments for eating a
plant-based diet?
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A luxury item?
How can the costs of these food items be compared fairly?
Why is meat more expensive than bread?
A 400g steak costs £4.00.
This equals £1.00 per 100g.
An 800g loaf of bread
costs £0.80.
This equals £0.10 per 100g.
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How energy efficient is it to eat meat?
Food chains can be used to understand why some foods cost
more than others. Which of these chains is the most efficient?
The first food chain is the most efficient because it
contains fewest trophic levels, so less energy will be lost.
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Is eating meat less energy efficient?
Cattle and other livestock are fed grain and cereals.
Would it be more energy efficient if humans ate the plant
crops instead?
If a one-acre field of corn is
used to feed cows, it can
support one person.
If the same area is used to
feed humans directly, it can
support 10 people.
Why is so much energy lost?
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Energy loss in food chains
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Some animals need more energy
Mammals and birds are able to
regulate their body temperature.
This has many advantages but it
uses lots of energy.
Other animals, such as
snakes, lizards and fish,
are unable to regulate their
body temperature, so need
less energy.
Would it be more energy
efficient to farm snakes for
food than chickens?
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What happens to the waste?
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How do you calculate efficiency?
Energy efficiency can be calculated using the following
equation:
=efficiencyenergy used for growth (output)
energy supplied (input)
For example, if grass receives 1,000,000kJ of energy from
the Sun and uses 20,000kJ of energy for growth, then:
efficiency =1,000,000 kJ
20,000 kJ
= 0.02 = 2%
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Energy efficiency calculations
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Glossary
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Anagrams
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