Energy Conversion, Transfer and Loss in Food Chains and Webs

• Primary Productivity

  Gross Primary Productivity (GPP) is the measure of all photosynthesis, or creation of chemical energy, that occurs in an ecosystem. The rate at which autotrophs produce biomass in a given area and time period. Approximately half the GPP is used by plants for their own respiration. The remainder is either stored or utilised by the plants for new biomass production, and is available for consumption by heterotrophs.
• Net Primary Productivity

  The energy which is left after losses as a result of respiration, growth, heat and movement.
• Net Primary Productivity = Gross Primary Productivity - Respiration
• Secondary Productivity

  The assimilation of food into new biomass through the transfer of organic material between trophic levels.
• Ecological Efficiency

  The percentage of biomass produced by one trophic level that is transferred and incorporated into biomass at the next trophic level. On average, only 10% of the energy that enters a trophic level is passed onto the next trophic level. The remainder is used for respiration and movement, and is lost from the system as heat and indigestible waste. An ecological efficiency of 10% limits most food chains to 4 or 5 links, which explains why the volume of biomass typically reduces at each trophic level.
• Endotherms use internally-generated heat to maintain body temperature independent of external temperature change. A high metabolic demand to keep the body warm leaves a minimum of energy for biomass accumulation.
• Ectotherms rely on external environments for temperature control instead of generating their own body heat.
• Ectotherms are more ecologically efficient than endotherms, passing on up to 15% of residual energy to the next trophic level in comparison to 7% by endotherms. Therefore, food chains with several ectotherms are often longer.