What does interdependence mean?

Interdependence is the way living organisms interact and rely on each other to survive.

  • They eat each other.
  • Reproduction (pollonation)
  • Dead plants fertilise soil
  • We take milk from cows, sheep, etc.
  • Camoflage
  • Shells (protection)
  • Parasites
  • Nesting
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Food Chains/Webs

A community is a group of organisms of different species that live in the same place (habitat). Sometimes one animal will eat another. The transfer of energy from one organism to another in the community is modelled using food chains and food webs.

Food Webs

In a community, an organism usually feeds on several different types of food. Instead of one simple food chain there are many food chains that share the same organisms. If all the food chains are put together a food web is made. The arrows show the direction in which the energy flows.

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Population changes

In a food web the population of one organism may be affected in some way by the population of another.

REMEMBER - One organism may be the food supply of another.

For example, if the population of catapillers goes down so will the population of thrushes (its predator).

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Energy used by organisms

Use of energy entering a mammal

  • 70% Heat
  • 5% Cellular activities
  • 10% Faeces
  • 3% Urine
  • 10% Movement
  • 2% Growth (biomass)
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Not all animals are eaten by carnivores. Animals and plants also die from disease or old age. Organisms that eat dead material are called decomposers. Decomposers get their energy from organisms that have died. The main decomposers are bacteria and fungi. Decomposers break down large molecules in dead material and return the molecules to the soil. They respire like normal organisms so some energy is lost in heat. They are an important part of the eco-system because they recycle carbon and nitrogen compounds. They are often left out of food webs because they are small or microscopic.

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  • A food chain shows the passage of energy between organisms.
  • A producer traps energy from its surroundings.
  • Energy passes from producer to primary consumer, to secondary consumer and on to tertiary consumer.
  • Energy is lost from food chains, mainly as heat.
  • Pyramids of numbers show the decrease in number of individuals along a food chain.
  • Pyramids of number can be inverted if the organisms are of different sizes. 
  • A pyramid of biomass shows the decreasing amount of matter available along a food chain.
  • A food web shows all the feeding relationships in a community.
  • Dead matter from organisms is decomposed, mainly by bacteria or fungi.
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