Energy and ecosystem

• In any ecosystem, there are producers, also known as autotrophs.
• Photoautotrophs use light energy to synthesise their own food, while chemoautotrophs use inorganic molecules.
• Through photosynthesis, plants synthesise organic compounds such as glucose from atmospheric or aquatic carbon dioxide.
• Most sugars synthesised by plants are used as respiratory substrates.
• Some sugars synthesised by plants are used to make other groups of biological molecules such as cellulose for plant biomass.
• Heterotrophs, such as animals, cannot synthesise their own energy, but must obtain it from autotrophs or other heterotrophs; they act as consumers in food webs.
• Biomass and its stored energy are transferred through trophic levels in a food web inefficiently.
• Biomass can be measured in terms of:

   Mass of carbon  Dry mass of tissue per given area

  1. Sample of organism dried in an oven set to a low temperature.

   Low temperature to avoid combustion (loss of biomass / CO2 )

  2. Sample reweighed at regular intervals e.g. every day

  3. All water removed when mass remains constant

  4. Mass of carbon taken to be 50% of dry mass

   Dry mass more representative because water content of samples varies
• The chemical energy stored in dry biomass can be estimated using calorimetry  Sample of dry biomass burnt  Energy released is used to heat a known volume of water  Change in temperature of water used to calculate the chemical energy
• Gross primary production (GPP)  Chemical energy store in plant biomass, in a given area / volume, in a given time  i.e. the total energy resulting from photosynthesis
• Net primary production (NPP)  Chemical energy store in plant biomass after respiratory losses (R) to the environment have been taken into account  NPP = GPP – R  The NPP is available for plant growth and reproduction  The NPP is also available to other trophic levels in the ecosystem, such as herbivores and decomposers
• Net production of consumers

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  To work out the net production of consumers (N), the formula below can be used:

   N = I – (F + R)

    

  I = the chemical energy store in ingested food F = the chemical energy lost to the environment in faeces and urine R = respiratory losses to the environment
• Rates of productivity are recorded using the units: kJ ha - year- 1

   kJ: a unit for energy  Per unit area (e.g. ha): different environments vary in size; standardizes results so

  environments can be compared  Per year: more representative as takes into account the effect of seasonal variation

  (temperature etc.) on biomass so environments can be compared
• Energy transfer between trophic levels is inefficient

  Sun = producer

  Wrong wavelength of light

  Light strikes non-photosynthetic region e.g. bark

  Light reflected

  Lost as heat
• Energy transfer between trophic levels is inefficient

  Producer = primary consumer = secondary consumer etc.

   Respiratory loss – energy used for metabolism e.g. active transport  Lost as heat  Not all plant/animal eaten e.g. bones  Some food not digested ( faeces)
• Farming practices to increase energy transfer efficiency

  Crops

  Simplifying food webs to reduce energy/biomass losses to non-human food chains:

   Herbicides kill weeds= less competition= more energy to create biomass  Fungicides reduce fungal infections =more energy to create biomass  Pesticides =reduce loss of biomass from crops Fertilisers e.g. nitrates to prevent poor growth due to lack of nutrients
• Farming practices to increase energy transfer efficiency

  Livestock

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    

  Reducing respiratory losses within a human food chain (so more energy to create biomass):

   Restrict movement  Keep warm (especially in winter) Slaughter animal while still growing / young, when most of their energy is used for growth Selective breeding to produce breeds with higher growth rates Treated with antibiotics to prevent loss of energy due to pathogens