AQA A2 BIOLOGY UNIT 4: Energy and Ecosystems

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  • Created on: 18-04-14 14:17
Preview of AQA A2 BIOLOGY UNIT 4: Energy and Ecosystems

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The many relationships between the members of a community in an ecosystem can be described by food
chains and webs.
Each stage in a food chain is called a trophic level, and the arrows represent the flow of
energy and matter through the food chain.
Producer Photosynthetic organisms that manufacture organic substances using
light energy
Consumer Organisms that obtain their energy by feeding on other organisms
rather than using the energy of sunlight directly
Primary consumer Animals that directly eat producers
Secondary consumer Animals that eat primary consumers
Tertiary consumer Animals that eat secondary consumers
Herbivore Animals that only eat producers
Carnivore Animals that only eat consumers
Decomposers Fungi and bacteria that break down complex materials into simple
components whilst releasing valuable minerals and elements in a form
that can be absorbed by plants
Detritivores Organisms such as earthworms that carry out decomposition
Food Chains
Trophic levels can also be described as food chains or energy chains.
When a food chain is taken to its natural conclusion, humans are found at the end of it, meaning that we
are dependant on an efficient transfer of energy along a food chain.
Every link in the chain acquires food and feeds on the link prior to it.
Each link is also consumed by the link that follows it.
Food chains are the process whereby energy that is trapped in carbon compounds is transferred through
an ecosystem.
Food Webs
Most animals do not rely upon a single food source and within a single habitat many food
chains will be linked together to form a food web.

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The Sun is the source of energy for ecosystems.
However, as little as 1% of this light energy may be captured by green plants and so made available to
organisms in the food chain.
These organisms in turn pass only a small fraction of the energy that they receive to each successive
stage in the chain.…read more

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Ecological pyramids show the mass or amount of energy stored by organisms at each trophic level.
Pyramids of Number
These show the numbers of organisms at each trophic level.
The number of organisms at lower trophic levels is greater than the number of organisms at higher
trophic levels.…read more

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Agricultural ecosystems are made up largely of domesticated animals and plants used to produce food.
There are considerable energy losses at each trophic level.
This means that the energy we receive from the food we eat is often only a tiny proportion of that
available from the sun at the start of the food chain.
Agriculture tries to ensure that as much of the available energy from the Sun as possible is transferred
to humans.
Productivity is the rate at which something is produced.…read more

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In natural ecosystems productivity is relatively low.
The additional input to agricultural ecosystems is used to increase the productivity of a crop by reducing
the effect of limiting factors on its growth.
The energy used to exclude other species means that the crop has little competition for light, carbon
dioxide, water and the minerals needed for photosynthesis.
The ground is therefore covered almost exclusively by the crop.
Fertilisers are added to provide essential ions, and pesticides are used to destroy pests and diseases.…read more

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Pest An organism that competes with humans for food or space and can be a danger to health
Pesticide Poisonous chemicals that kill pests
Chemical Control
An effective pesticide should be:
Specific So that it is only toxic to the organisms at which it is
It should be harmless to humans and other
organisms, especially the natural predators of the
Biodegradable So that once applied, it will break down into
harmless substances in the soil.…read more

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Integrated Pest-Control Systems
Integrated pest-control systems aim to integrate all forms of pest control rather than being reliant on one
The emphasis is on deciding an acceptable level of the pest rather than trying to eradicate it altogether.…read more

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Intensive rearing of livestock is designed to produce the maximum yield of meat, eggs and milk at the
lowest possible cost.
As energy passes along a food chain only a small percentage passes from one organism in the chain to
the next.
This is because much of the energy is lost as heat during respiration.
Intensive rearing of domestic livestock is about converting the smallest possible amount of food
energy into the greatest quantity of animal mass.…read more


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