Topic 5 B: Energy Transfer and Nutrient Cycles

• an ecosystem includes all the organisms living in a particular areas and all the non-living (abiotic) conditions
• in all ecosystems there are produces - organisms that make their own food:

  • in land-based ecosystems, plants (such as trees, shrubs and grasses) produce their own food through photosynthesis
  • in aquatic ecosystems, plants (such as water lilies and watercress) and algae (such as seaweeds) also produce their own food through photosynth
• • during photosynth plants use energy (from sunlight) and CO2 from the atmosphere in land based ecosystems, or dissolved in water in aquatic ecosystems to make glucose and other sugars
• • some of the sugars produced in photosynth are used in resp to release energy for growth
  • the rest of the glucose is used to make other biological molecules, such as cellulose (component of plant cell walls)
  • these biological molecules make up the plants biomass - mass of living material
• • biomass can be thought of as the chemical energy stored in the plant
• • energy is transferred through the living organisms of an ecosystem when organisms eat other organisms e.g. produces are eaten by primary consumers
  • primary consumers are then eaten by secondary consumers
  • secondary eaten by tertiary
  • food chain
• biomass can be measured in terms of the mass of carbon that an organism contains, or the dry mass of its tissue per unit area
• dry mass is the mass of the organism with the water removed
• • the water content of living tissue varies and so dry mass is used as a measure of biomass rather than wet mass
• to measure the dry mass:

  • a sample of the organism is dried - often in an oven set to a low temp
  • the sample is then weighed at regular intervals (e.g. every day)
  • once the mass becomes constant you know that all the water has been removed
  • the mass of carbon present is generally taken to be about 50% of the dry mass
• • once you have measured the dry mass of a sample, you can scale up the result to give the dry mass (biomass) of the total population or area being investigated
  • typical units might be kgm-2
• you can estimate the amount of chemical energy stored in the biomass by burning the biomass in a colorimeter:

  • the amount of heat given off tells you how much energy is in it
  • energy measured in joules or kilojoules
  • a sample of dry biomass is burnt and the energy released is used to heat a known volume of water
  • the change in temperature of the water is used to calculate the chemical energy of the dry biomass
• Gross primary production (GPP) is the total amount of chemical energy converted from light energy by plants in a given area
• • approx 50% of the GPP is lost to the environment as heat when the plants respire - respiratory loss R
  • the remaining chemical energy is called the net primary production
• NPP = GPP -R
• • often primary production is expressed as a rate - i.e. the total amount of chemical energy (or biomass) in a given area, in a given time
  • typical units might be KI ha-1 yr-1 (kilojoules per hectare per year) or KJ m-2 yr-2 (kilojoules per square meter per year)
  • when primary production is expressed as a rate it is called primary productivity
• the NPP is the energy available to the plants for growth and reproduction - the energy is stored in the plants biomass

  • also the energy available to organisms at the next stage in the food chain (next trophic level)
  • include herbivores and decomposers
• net production in consumers:

  • consumer also store chemical energy in their biomass
  • consumers get energy by ingesting plant material, or animals that have eaten plant material
  • however not all the chemical energy stored in the consumer's food is transferred to the next trophic level
  • around 90% of the total available energy is lost in various ways
• • not all food is eaten (e.g. plant roots, bones) so energy it contains is not taken in
  • of the parts that are ingested:
  • some are indigestible, so are egested as faeces - the chemical energy stored in these parts is therefore lost to the environ
  • some energy is also lost to the environ through respiration or excretion of urine
• • the energy that is left after all this is stored in the consumer's biomass and is available to the next trophic level
  • this energy becomes the consumers' net production
• • net production of consumers can be calculated using:
  • N = I - (F+R)
• • N = net production
  • I = chemical energy in ingested food
  • F = chemical energy lost in faeces and urine
  • R = energy lost through respiration
• the net production of consumers can also be called secondary production or secondary productivity when expressed as a rate
• Efficiency of energy transfer:

  % efficiency of energy transfer = net production of trophic level

  net production of previous trophic level

  X 100
• as you move up the food chain, energy transfer usually becomes more efficient

  e.g. the efficiency of energy transfer from producer to consumer might be 5-10%

  efficiency of energy transfer from consumer to consumer might by 15-20%

  this is bc plants contain more indigestible matter than animals
• plants covert light energy to chemical energy during photosynthesis
• photosynthesis is not 100% efficient - not all of the light energy absorbed by a plant will be converted to chemical energy
• if the organisms in the previous trophic level are producers - use net primary production of previous trophic level rather than net production of previous trophic level