Energy and Ecosystems

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Energy and Ecosystems

Food Chains and Food Webs

  • Organisms found in any ecosystem rely on a sour of energy to carry out all their activies.
  • The ultimate source of energy is the sunlight, which is converted to chemical energy by photosynthesising organisms.
  • This chemical energy is then passed as food between other organisms.
  • Organisms can be divided into three groups according to how they obtain their energy and nutrients. These three groups are: producers, consumers and decomposers.


  • Producers are photosyntheitc organisms that manufacture organic substances using light energy, water and carbon dioxide.
  • Green plants are producers.


  • Consumers are oganisms that obtain their energy by consuming other organisms rather than using the energy of sunlight.
  • Animals are consumers.
  • Those that directly eat producers are called primary consumers.
  • Those that eat primary consumers are called secondary consumers.
  • Those that eat secondary consumers are called tertiary consumers.
  • Secondary and tertiary consumers are predators, but may also be scavengers or parasites.


  • When producers and consumers die, the energy they contain can be used by a group of organisms that break down these complex materials into simple components again.
  • They release valuable minerals and elements in a form that can be absorbed again by plants and so contribute to recycling.
  • This is normally carried out by fungi and bacteria called decomposers and to a lesser extent by earthworms, known as detritivores.

Food Chains

  • Food chains describes a feeding relationship in which producers are eaten by primary consumers, who in turn, are each by secondary consumers and so on.
  • Each stage is known as a trophic level.
  • Arrows on a food chain indicate the direction of energy flow.

Food Webs

  • Most animals do not rely upon a single food source and withing a single habitat many food chains will be linked together to form a food web.
  • Food webs are very complex.
  • Feeding relationships presented by food chains are not fix, they change depending on the time of year, age and population size of the organism.

Energy Transfer between Trophic Levels

The sun is the source of energy in ecosystems, however as little as 1% may be captured by green plants and so made available to organisms in the food chain.

Energy Losses in Food Chains:

  • Plants normally convert between 1 - 3% of the sun's energy available to them into organic matter, this is because:
  • Over 90% of the suns energy is reflected back into space by clouds and dust or absorbed by the atmosphere.
  • Not all wavelengths of light can be absorbed and used for photosynthesis.
  • Light may not fall on a chlorophyll molecule.
  • A factor, such as low carbon dioxide concentration, may limit the rate of photosynthesis.
  • Gross production = the total quantity of energy that the plants convert to organic matter.
  • However, plants use 20 - 50% of this energy in respiration.
  • Net production = the rate at which they store the energy.
  • Net production = gross production - respiritory losses.
  • Only 10% of the plants energy is passed


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