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Chapter 5 – Energy and Ecosystems
How does energy enter an ecosystem?

The ultimate source of energy in an ecosystem is the sun, which provides light to be
converted into chemical energy by autotrophs
...
Energy can be transferred between organisms in a
food chain
...

Decomposer: organisms which break down complex materials into simple components after
an organism dies e
...
fungi/bacteria
What percentage of energy is transferred from one trophic level to the next? How is energy lost along a food
chain?

Approximately 10% of the energy falling on a plant will be converted into organic molecules
(gross production), even some of this is used in respiration so the overall energy stored is
the net production:

Net production = gross production - respiratory losses
Primary producers are so inefficient for several reasons:




Not all wavelengths of light are suitable for photosynthesis
Some light is reflected off the plant/falls on non-photosynthetic parts
Limiting factors reduce the rate of photosynthesis

In consumers, there are alternative reasons as to why energy transfer is so inefficient:


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Not all parts of the organism are consumed
Not all consumed parts of the organism are digestible and are egested
Heat losses in respiration (particularly endotherms, who must maintain a high body
temperature)
Why do food chains have no more than 5 trophic levels?




Energy transfer is too inefficient to provide sufficient energy to support a large
enough population at higher levels
The total energy stored at each level is lower each time
How is the percentage efficiency of energy transfers calculated?

Energy available can be measured in kJ/m-2/y-1 however calculating the efficiency of transfer
between levels of the food chain can be done using this equation:
𝐸𝑛𝑒𝑟𝑔𝑦 𝑡𝑟𝑎𝑛𝑠𝑓𝑒𝑟 =

𝑒𝑛𝑒𝑟𝑔𝑦 𝑎𝑣𝑎𝑖𝑙𝑎𝑏𝑙𝑒 𝑎𝑓𝑡𝑒𝑟 𝑡𝑟𝑎𝑛𝑠𝑓𝑒𝑟 (𝑡𝑜 ℎ𝑖𝑔ℎ𝑒𝑟 𝑙𝑒𝑣𝑒𝑙)
× 100
𝑒𝑛𝑒𝑟𝑔𝑦 𝑎𝑣𝑎𝑖𝑙𝑎𝑏𝑙𝑒 𝑏𝑒𝑓𝑜𝑟𝑒 𝑡𝑟𝑎𝑛𝑠𝑓𝑒𝑟 (𝑡𝑜 𝑙𝑜𝑤𝑒𝑟 𝑙𝑒𝑣𝑒𝑙)

What are the different types of ecological pyramid? What are the relative merits and disadvantages of each
one?

Pyramids of number – there are often more organisms at lower trophic levels than higher,
therefore the proportions of the bars on the pyramid are representative of that of the
typical number of organisms in a feeding relationship
 No account of size of the organisms at each level
 Number of organisms at one level may be so much greater than another that it is
impossible to accurately produce a to-scale pyramid
Pyramid of biomass – biomass is the total mass of the plants/animals in a particular place,
thus a pyramid of biomass compares the dry mass of organisms in a food chain at a
particular time (as the fresh mass contains water, which is difficult to measure and
therefore unreliable)
 To test the organisms dry biomass they must be killed
 Only measures biomass at a particular time – doesn’t take into account seasonal
differences
Pyramids of energy – an accurate representation of energy flow through a food chain by
measuring the stored mass of energy in organisms at each level over a year
...

How to natural and agricultural ecosystems differ?

A natural ecosystem has no additional energy inputs other than solar energy
...

However some of this stored energy is utilised in respiration, and therefore the energy
remaining after this is the net productivity
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A pesticide is a poisonous chemical designed to kill pests
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Cost effective – organisms can easily develop resistance to pesticides due to natural
selection, meaning it will have a limited use and development costs are high
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Completely eradicating the pest population often kills the natural predators of the pest,
allowing numbers to massively grow once again
...

The goal with biological agents is to keep the pest population levels low, so the control
agent and pest can live alongside one another
...

The control, in increasing order, goes:




Choosing plants/animals which are the most naturally pest resistant
Providing natural habitat for the predators of the pest
Monitoring the crop for signs of damage – allows earlier action



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Mechanically removing pests if they begin to exceed acceptable levels
Using biological agents if necessary
Using chemical agents if necessary
How does intensively rearing animals increase the efficiency of energy conversion?

Intensive rearing is designed to increase the efficiency of energy conversion from food to
body mass, by minimising energy losses in other areas

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