Food chains, Food webs & Pyramids (Edexcel IGCSE Biology)

• Food chains are diagrams that show the energy flow between organisms in an ecosystem
  • The arrows in a food chain represent the transfer of energy between trophic levels
• All food chains start with producers; this is because they convert energy into a form that can be used by other organisms, e.g. plants convert light energy into chemical energy

Food chains show the transfer of energy that occurs when organisms feed

• Food webs are networks of interconnected food chains
• Food webs can aid in the understanding of the complexity of feeding relationships in ecosystems

Food web shows the complex feeding relationships within ecosystems

• A change in one population within a food web can affect other populations, e.g. for the food web above a decrease in the rabbit population might lead to:
  • an increase in grass growth, due to decreased feeding
  • a decrease in the fox population, due to reduced food availability
  • an increase in caterpillars and earthworms, due to increased growth of grass
  • a decrease in the frog and mouse populations, as foxes would need to compensate for the reduction in rabbits

Pyramids of numbers

• Pyramids of numbers show how many organisms are present at each trophic level of a food chain
  • The size of each bar indicates the number of organisms present
• A food chain containing three organisms can be represented in a pyramid of numbers with three bars:

grass → vole → owl

The bars in a pyramid of numbers indicate the number of organisms present at each trophic level

• In some food chains many small organisms may feed on few large organisms; in this case a pyramid of numbers will not be pyramid shaped
• E.g. for the food chain:

oak tree → ladybird → woodpecker

Pyramids of numbers are not always pyramid-shaped

Pyramids of biomass

• Pyramids of biomass show the mass of living organisms present at each level of a food chain
  • Biomass = mass of living matter
• Pyramids of biomass are always pyramid-shaped; this is because the available biomass always decreases at higher trophic levels of a food chain
• E.g. for the same food chain shown above:

oak tree → ladybird → woodpecker

The bars in a pyramid of biomass represent the mass of living matter at each trophic level of a food chain, and are always pyramid shaped

Pyramids of energy

• Pyramids of energy illustrate the stored energy within the biomass at each trophic level
• As with pyramids of biomass, pyramids of energy are always pyramid-shaped
  • This is because energy is lost from food chains at each trophic level
• E.g. for the food chain:

grass → snail → frog → wolf

Pyramids of energy show the stored energy at each trophic level of a food chain; they are always pyramid shaped

Drawing food pyramids

• When drawing any type of food pyramid, there are several conventions that should be followed:
  • the trophic levels of the organisms must stay in the same order as in the food chain, with producers at the bottom, followed by primary consumers, then secondary consumers, then tertiary consumers etc.
  • each bar must be labelled to indicate the trophic level
  • the length/width of a bar reflects the unit represented by the pyramid; a longer/wider bar indicates:
    • more organisms
    • greater biomass
    • more stored energy
  • bars should be the same height
  • if graph paper is provided then bars should be drawn to scale