3.5.3 ENERGY + ECOSYSTEMS

• Matter recycled within ecosystem. Energy is not recycled - flows. 

  Plants synthesise organic compounds from atmospheric, or aquatic, carbon dioxide.

  • Most of the sugars synthesised by plants are used as respiratory substrates. The rest are used to make other groups of biological molecules, forming the biomass of the plant.
• Biomass can be measured as mass of carbon or dry mass of tissue per given area. 

  Biomass -  the total mass of living material in a specific area at a given time.
• Dry biomass shows the chemical energy store in an organism and can be measured by the process of calorimetry:

  • A dry sample is weighed and burnt in pure oxygen within a sealed chamber, the temperature increase of the fixed volume of water is used to calculate the energy released. Units kJkg^-1
• Gross primary production (GPP) - the total quantity of chemical energy stored in plant biomass, in a given area or volume. 

  Net primary production (NPP) - the chemical energy store in plant biomass after respiratory losses to the environment have been taken into account.
• NPP = GPP – R   

  R = respiratory loses to the environment

  NPP, GPP & R use units of (kJ m^-2 yr^-1)

  This NPP is available for plant growth and reproduction. Also available to to other trophic levels in ecosystem, such as herbivores and decomposers.
• Net production (N) - the total chemical energy consumers store after energy losses to faeces, urine and respiration have been taken away from the chemical energy store of the ingested plant food.

  Net production of consumers (N) can be calculated as:

  N = I - (F + R)

  N = net production 

  I = total chemical energy store in ingested food

  F = energy lost to environment in faeces and urine 

  R = respiratory losses to environment

  All use units (kJ m^-2 yr^-1) 
• Primary and secondary productivity is the rate of primary or secondary production, respectively. It is measured as biomass in a given area in a given time e.g. kJ ha^–1 year^–1
• % efficiency of energy transfer from one trophic level to another can be calculated as:

  energy available after transfer/energy available before x100
• Energy transferred reduces at each trophic level.

  % of energy transferred from 1 trophic level to next decreases:

  (Producers)

  • Only convert 1-3% of sun's energy available
  • 90% sunlight reflected to space
  • Only certain wavelengths are absorbed
  • Sunlight may miss a chlorophyll
  • Other factors limit photosynthesis rate eg. carbon dioxide

  (Consumers)

  • Undigested food (not all eaten, some not digested)
  • Excretion
  • Heat loss from respiration
• Production is affected by farming practices. They are designed to increase the efficiency of energy transfer to increase yields by:

  • Reducing respiratory losses in a human food chain e.g. reduce movement of animals
  • Simplifying food chains to reduce energy loss to non-human food chains e.g. killing weeds and pest using herbicides and insecticides