Energy and ecosystems

• Plants in an ecosystem

  • They are the producers in a food web
  • They produce their own carbohydrates from carbon dioxide
  • Known as autotrophs
• Majority of energy lost within each trophic level of a food web

  Lost due to respiration and excretion
• Remaining energy within a food web

  • Forms the biomass of an organism
  • Used to form biological molecules
• Measuring remaining biomass within an organism


  In terms of mass of carbon or dry mass of tissue per given area
• Biomass is worked out per given area to ensure a fair comparison between different sized organisms
• Calculating the dry mass of a plant

  1. Sample of an organism is dried until all water is removed

  2. Placed in an oven at low temperature

  3. Weighed at regular intervals until mass remains constant
• Typical unit for dry mass

  Kg m^2
• Around 50% of dry mass is taken to be carbon
• Estimating the amount of chemical energy in dry biomass

  1. Burning the biomass in a calorimeter

  2. A sample of biomass is burnt and the energy released is used to heat a known volume of water

  3. The change in temperature of water is used to calculate the chemical energy of the dry biomass
• Factors which affect how productive an ecosystem is

  • Abiotic and biotic factors
  • Factors like plenty of water, light, warmth and green plants will increase the rate of photosynthesis
• Increases in photosynthesis increase carbohydrates produced by plants, which increases overall biomass and carbon entering the food chain
• GPP


  Gross primary production - The chemical energy store in plant biomass, given per area or volume, the total energy resulting from photosynthesis, the total energy entering the food chain
• NPP


  Net primary production - The chemical energy store in plant biomass taking into account energy lost due to respiration (R), the NPP is available for plant growth and reproduction, this biomass is then available to the next trophic level in the food web - herbivores and decomposers
• NPP equation


  NPP = GPP - R
• N


  Net production of consumers - Looks at how much energy is left over in consumers to create biomass, taking into account the amount of energy entering the consumer (I) and leaving (F and R)
• N equation


  N= I - F + R
• Rates of productivity units

  KJ ha-1 year-1
• Rates of productivity


  The rate at which biomass is produced by an ecosystem
• KJ


  Unit for energy
• ha-1


  Per unit area/ hectare - 1 hectare = 10,000 square meters - Looking at it per area/ hectare allows for standardization and comparisons with different ecosystems which may vary in size
• year-1


  Per year - Takes into account the impact of seasons - impacts abiotic and biotic factors - rain, light and heat - Provides an annual average to allow for a fair comparison between environments
• Working out the efficiency of energy transferred within an ecosystem

  1. Efficiency = N(net production of consumers) / energy received

  2. Multiplied by 100 to express as percentage
• Calculating the efficiency of biomass transfer from one trophic level to the next

  Efficiency = biomass transferred / biomass intake
• Calculating percentage yields


  1. Percentage yield = actual yield / theoretical yield

  2. Multiplied by 100
• Food chains and webs show how energy is transferred through an ecosystem
• Pests in an ecosystem

  Are organisms which reduce the amount of energy available for crop growth, therefore decrease the overall NPP
• How farmers can remove pests and simplify food webs

  • Using chemical pesticides
  • Using biological agents
  • Using an integrated system
• Using chemical pesticides


  • Using insecticides to kill insect pests that eat and damage crops
  • Using herbicides to kill weeds (unwanted plant species)
• Using biological agents


  • Parasites live in or lay their eggs on pest insects
  • Pathogenic (disease causing) bacteria and viruses can kill parasites
• Using an integrated system of chemical and biological methods can reduce pest numbers even more, increasing NPP even further
• How farmers reduce respiratory losses in livestock

  • Control their environments and conditions they live in
  • Restrict their movement so less energy reduced from respiration
  • Pens are indoors and kept warm so less energy wasted to generate body heat
• More biomass produced, more chemical energy stored, increasing net production - N, increasing efficiency of energy transfer to humans
• Some argue that keeping animals in pens causes them pain, distress and restricts their natural movements
• What happens as farmers reduce respiratory losses?

  • More biomass produced
  • More chemical energy stored
  • Increasing net production - N
  • Increasing efficiency of energy transfer to humans