Food Chains & Food Webs (WJEC GCSE Biology): Revision Note

Food Chains & Food Webs

What do food chains and food webs show?

• Food chains and food webs are diagrams that show the flow of energy in ecosystems

• They show the transfer of energy between organisms present at different trophic levels, e.g.

  • First trophic level

    • Contains producers; organisms that produce their own carbon compounds, e.g. plants use carbon dioxide and water to produce glucose during photosynthesis

  • Second trophic level

    • Contains first stage, or primary, consumers; these animals are herbivores that feed on producers

  • Third trophic level

    • Contains second stage, or secondary, consumers; these animals are carnivores that feed on primary consumers

  • Fourth trophic level

    • Contain third stage, or tertiary, consumers; carnivores that feed on secondary consumers

Energy transfer in food chains and food webs

• The sun's radiation is the energy source for almost all ecosystems on Earth

  • There are a few unusual ecosystems, such as in the deep sea, that rely on energy stored in chemicals

• Energy enters food chains when producers convert light energy from the sun into chemical energy that is stored in plant biomass

  • Biomass is living matter, e.g. the tissues of plant stems and leaves

  • Note that only a small percentage of the light energy that falls on plants is converted into chemical energy in biomass; most passes through leaves, falls on non-photosynthetic structures, or is lost as heat

• This chemical energy is transferred to the biomass of primary consumers when they digest the tissues of producers and use the products to build up their own tissues

• The chemical energy is then transferred to the biomass of secondary consumers when they eat primary consumers, and so on up the food chain

• At each stage of a food chain or food web there may be uneaten parts of organisms, dead organisms and waste products; energy stored in these tissues and substances passes to decomposers such as bacteria and fungi

Food chain diagram

Food chains show the transfer of energy between trophic levels in an ecosystem

Food web diagram

Food webs show the interactions between multiple food chains in an ecosystem; note that it is possible for organisms to belong to more than one trophic level of a food web