A2 AQA Unit 4 Energy and Ecosystems

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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
Green plants are producers
All organisms that obtain energy by consuming other organisms
Animals are consumers
Those that directly eat producers are PRIMARY consumers ­ first in the chain
Those that eat the primary consumers are secondary consumers
Those that eat the secondary consumers are tertiary consumers
Secondary and tertiary are usually predators but can also be scavengers or
Producers and consumers die and the energy they contain can be used when
the complex materials are broken down into single components again
In breaking these materials down they can be absorbed by plants and be
Majority of this is done by decomposers (fungi and bacteria) and some by
detritivores (earthworms)
Food Chains
Food chain ­ relationship in which producers are eaten by primary, who are
eaten by secondary, who are eaten by tertiary.
Longer chains may contain quaternary consumers
Each stage in the chain is a trophic level
Food Webs
Within a habitat many food chains will be linked together to form a food web
Problem is their complexity
A producer can be eaten by several different organisms etc.
Charting all feeding interrelationships is not feasible
Also, these relationships are not fixed, they depend on times of the year

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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
Energy Losses in Food Chains
Plants normally convert 13% of the Sun's energy into organic matter.…read more

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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
(KJm2year1)…read more

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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 is treated the same as an aphid
and each parasite has the same numerical value as its larger host.…read more

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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 (animals), but this is not seen at certain times of
the year. E.g.…read more

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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
Agriculture tries to ensure that as much of the available energy from the Sun
as possible is transferred to humans
It is channelling the energy flowing through a food web into the human food
and chain and…read more

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The efficiency of the crop at carrying out photosynthesis.…read more

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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 recycling is more limited and
ecosystem with little addition from supplemented by the addition of artificial
outside fertilisers
Populations are controlled by natural Populations are controlled by both natural
means such as competition and climate…read more

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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 food chain
Biological Control
It's possible to control pest by using organism that are predators or parasites
to the pest organism
The aim is to control not eradicate the pest as it could be counterproductive
If…read more

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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 agricultural ecosystems
Weeds compete with plants for light, space, water, mineral ions and CO2
As these are often in limited supply, any amount taken in by the pest means
less is available for the crop plant…read more



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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