Food Chains & Webs

• The sun is the initial source of energy for nearly all living things
• Energy is lost between each step in the food chain and so the energy available for the next organism decreases
• Energy can be lost as heat during respiration or in waste material such as feces
• Food web


  A diagram of the interconnections between lots of organisms populations
• Organisms have more than one predator or don't have any predators
• When a food web is altered it has different effects on different populations
• If one species goes extinct it can affect the entire system with unpredictable results
• Altering one link in the food web has big consequences for the whole system
• How energy passes through an ecosystem

  1. Using food chains

  2. Predator prey cycles
• Food chain


  Shows what gets eaten by what in an ecosystem
• Food chain


  • Starts with a producer (photosynthetic organism like a green plant or alga)
  • Has primary consumers (eat the producers)
  • Has secondary consumers (feed on the primary consumers)
  • May have tertiary consumers (feed on the secondary consumers)
• As energy gets passed up the different levels of the food chain, most of it gets lost
• Arrows between levels in a food chain


  Represent the flow of energy up the chain
• Predator prey cycle


  Graph showing how the populations of a predator and its prey vary together over time
• In a predator prey cycle, the change in the predator population always lags slightly behind that of the prey
• Predator prey cycle


  1. At low predator population, prey population increases

  2. With more prey, predator population increases

  3. High predator population causes decline in prey

  4. Fewer prey causes predator population to decline

  5. Cycle repeats
• The main reason for the predator prey cycle is that it takes a long time for entire populations to increase or decrease