3.5.3 Energy and ecosystems

• Producers are organisms like plants and algae that convert light energy from the sun into chemical energy via photosynthesis.

  • They are also known as autotrophs and are found at the bottom of the food chain.
• Consumers are organisms that obtain energy by consuming other organisms. They are also known as heterotrophs.
• Primary consumers are herbivores that eat producers.
• Secondary consumers are carnivores that eat herbivores.
• Tertiary consumers are carnivores that eat other carnivores.
• Decomposers are bacteria and fungi that break down dead organic matter, releasing nutrients back into the soil.
• Biomass is the total mass of living material in a specific area at a given time, measured as dry mass or the mass of carbon it contains.
• Biomass measurement:

  • To determine dry mass, organisms are dried (e.g., in an oven at a low temperature) until their mass remains constant.
  • This ensures that water is removed, leaving only organic material.
• The chemical energy store in dry biomass can be estimated using calorimetry.
• Gross Primary Production (GPP) is the chemical energy store in plant biomass, in a given area and time.

  • It is also known as the total amount of chemical energy produced by photosynthesis.
• Net Primary Production (NPP) is the chemical energy stored in plant biomass after respiratory losses (R) to the environment have been taken into account.

  • This is the energy available to consumers.
• Net Primary Production (NPP) can be calculated using the equation:

  NPP = GPP - R

  • where GPP represents the gross primary production and R represents respiratory losses to the environment.
• The net primary production is available for plant growth and reproduction. It is also available to other tropic levels in the ecosystem, such as herbivores and decomposes.
• The net production of consumers (N) can be calculated as:

  N = I - (F + R)

  • I = the chemical energy stored in ingested food.
  • F = the chemical energy lost to the environment in faeces and urine.
  • R = the respiratory losses to the environment.
• 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.
• Each step in the food chain/web is a trophic level. Energy is lost at each trophic level - only about 10% of the energy in one level is transferred to the next.
• How is energy lost at each trophic level?

  • Respiration (heat loss)
  • Excretion
  • Inedible parts (bones, teeth, fur etc)
  • Uneaten material (predators leaving behind carcasses or plants shedding leaves)
  • Inefficiency of photosynthesis
• Photosynthesis can be described as inefficient because only 1-3% of sunlight that hits producers is captured for photosynthesis.

  • Much of the solar energy is lost because:
  • It is reflected by leaves.
  • It is the wrong wavelength for absorption by chlorophyll.
  • It passes through leaves without being absorbed.
• Efficiency of energy transfer between trophic levels is calculated using the equation:

  • (Energy transferred to the next level ÷ Energy available at current level) × 100
• Energy flows through ecosystems in food chains (linear) and food webs (complex interconnections).
• GPP is described as the total amount of chemical energy converted from light energy by plants in a given area in a given time. Suggest suitable units for GPP. (1)

  ​kJ m-2 yr-1
• Food chains are usually shorter than five organisms.

  Explain, using ideas about energy loss, why this is the case. (5)

  • ​Most sunlight energy is not converted to biomass
  • Sunlight can be reflected/absorbed by atmosphere or transmitted
  • ​Approximately 10% of biomass in producers is converted to biomass in consumers
  • ​As above for primary consumers to secondary consumers
  • ​Energy loss as respiratory losses/waste/parts of organisms that are not eaten