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Energy and Ecosystems
5.1 Food Chains and Food Webs
Ultimate source of energy is sunlight converted to chemical energy by
photosynthesising organisms then passed as food to other organisms


Photosynthetic organisms that manufacture organic substances using light
energy, water and CO2

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5.2 Energy Transfer Between Trophic Levels
As little as 1% of the Sun's energy is captured by green plants and so made
available to organisms in the food chain
These organisms then pass only a small fraction of the energy on that they
receive to each successive trophic level


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o The total mass of organisms in a particular place (biomass) is less at
higher trophic levels
o The total amount of energy stored is less at each level

Calculating the Efficiency of Energy Transfers

Energy available is usually measured in kilojoules per square metre per year

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5.3 Ecological Pyramids
Pyramids of Number

Usually, the numbers of organisms at lower trophic levels are greater than
those at higher levels ­shown by bars proportional to the number at each
trophic level
Significant drawbacks to using a number pyramid
o No account is taken of size ­ one tree…

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In both PoN and PoB only the organisms present at a particular time are
shown, seasonal differences are not apparent
This is particularly apparent when the biomass of some marine ecosystems
arre measured
Over the course of a whole year the mass of phytoplankton (plants) must
exceed that of zooplankton…

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5.4 Agricultural Ecosystems
Agricultural ecosystems are largely made up of animals and plants used to
produce food for mankind
There are considerable energy losses at each trophic level and as we are third
or even fourth in the chain we receive only a tiny proportion of the Sun's

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o The efficiency of the crop at carrying out photosynthesis. This is
improved if all necessary conditions for photosynthesis are supplied (no
limiting factors)
o The area of the ground covered by leaves of the crop

Comparisons of Natural and Artificial Ecosystems

Artificial ecosystems are based on the same ecological…

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Solar energy only ­ no additional energy Solar energy plus energy from food
input (labour) and fossil fuels (machinery and
Lower productivity Higher productivity
More species diversity Less species diversity
More genetic diversity within a species Less genetic diversity within a species
Nutrients are recycled naturally within the Natural…

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o Be costeffective ­ because development costs are high and new
pesticides are only useful for a limited time due to pests developing
resistance, making the pesticide useless
o Not accumulate ­ so it doesn't build up, either in specific parts of an
organism or as it passes along the…

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o Removing the pests mechanically (handpicking, vacuuming, erecting
barriers) if the pest exceeds an acceptable population level
o Using biological agents if necessary and available
o Using pesticides as a last resort if pest populations start to get out of

How Controlling Pests Affects Productivity

Pests reduce productivity in…




A well written set of notes with some useful, coloured diagrams to help illustrate the key points. They cover food chains, energy flow, pyramids, biological control, and intensive farming so would be useful to any student studying these topics. 



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