These notes will be updated this summer!


Energy is transferred through food chains and food webs in a community

Ecological pyramids
Pyramids of numbers
  • Total number of organisms in a food chain at each trophic level
  • Highest number at the bottom (usually producers, then consumers)
  • Pyramid will be inverted if lots of small animals are feeding off one large plant

Pyramids of biomass
  • Total biomass of organisms in a food chain at each trophic level
    • Always pyramid shaped
  • Organisms multiplying rapidly may have biomass less than primary consumers
  • Dry mass is measured / H2O stores no energy and varies in different organisms

Pyramid of energy
  • Amount of energy transferred to each level of a food chain in an ecosystem
  • Always pyramid-shaped / no energy loss

Transfer of energy between trophic levels
Food chains and food webs
  • Energy is used to produce new cells / remains fixed in that organism
  • Energy is passed on to the next trophic level through feeding
  • Producers are photoautotrophs (plants)
    • Transduce light energy into chemical P.E. by forming new tissues and storing organic compounds
      (starch, glucose, lipids, proteins)
  • Consumers are herbivores, carnivores and omnivores
  • Decomposers are detritivores and saprophytes
    • Break down dead complex organic molecules into simple inorganic molecules
  • Food chains are feeding relationships and linked with each other to form complex food webs
    • Some organisms feed on different trophic levels / leaves and insects
    • Some organisms feed on different foods when they are larvae (leaves) and when they are adult (nectar produced by different flowers)

Energy transfer and efficiency
  • 2% of light energy is converted to chemical P.E. by photosynthesis
    • Rest is lost reflection from leaves / heat loss / not all wavelengths are utilised / light strikes non-photosynthetic structures
  • 10% of that are passed on along trophic levels
    • Rest is lost in respiration / as heat/faeces/urine
  • Chemical P.E. / generates heat / stores energy by forming organic matter (new cells)
    • Mammals are homeothermic / must maintain constant body temp
    • Warm environment / less energy maintains body temp / more organic matter stored / more transferred to consumer
    • Small organisms / large surface area:volume ratio / lose high amounts of energy
  • Carnivores fix organic matter more efficiently than herbivores
    • Herbivores feed on plants
    • \ Take up cellulose and lignin / difficult to digest
    • \ More food passes through gut and is lost as faeces
  • Trout fix organic matter most efficiently, they are
    • Poikilotherms → must NOT maintain constant body temp
    • More energy is used to fix organic matter
    • Carnivores are harvested while they are still young and grow rapidly
    • Trout transfer most energy to consumer (human) in terms of food
  • [EXAM] Number of food chains is limited
    • Due to energy losses (at each trophic level)
    • In respiration/egestion/excretion/movement/as heat
    • (Too) little energy is left to sustain higher trophic levels/to be passed on

References and Further Reading
AQA (2006) GCE Biology/Biology (Human) 2006 specification, [PDF]

BYA5 SECTION: 14.1 14.2 14.3 14.4 14.5 14.6 14.7 14.8 14.9 14.10