OxfordAQA International A Level Biology 3.3.4 Energy Transfer Through Ecosystems Exam Questions

About OxfordAQA International A Level Biology 3.3.4 Energy Transfer Through Ecosystems

Energy transfer through ecosystems in Unit 3 explains why energy transfer between trophic levels is inefficient and how humans manipulate it. You need to interpret food chains and food webs, pyramids of number, biomass and energy, and carry out calculations of efficiency. You must use NPP = GPP - R for producers and N = I - (F + U + R) for consumers. You also need to describe farming practices that increase productivity, including chemical pesticides, biological agents, integrated systems and reducing respiratory losses, and evaluate the economic, environmental and ethical issues involved. It links to photosynthesis, respiration and nutrient cycles, and calculation questions are very common.

What you need to know

• Trophic levels Food chains and webs show feeding relationships. Energy is lost at each level through respiration as heat, through parts not eaten and through egestion and excretion, so food chains rarely have more than four or five levels.
• Pyramids Pyramids of number can be inverted, for example many insects on one tree. Pyramids of biomass show dry mass per unit area. Pyramids of energy show the energy flow over time and are never inverted.
• Primary production Gross primary production (GPP) is the chemical energy in plant biomass in a given area or volume over a given time. Net primary production (NPP) = GPP - R, where R is respiratory losses.
• Consumer production N = I - (F + U + R), where I is the chemical energy in ingested food, F is energy lost in faeces, U is energy lost in urine and R is respiratory loss.
• Efficiency Efficiency of transfer = energy at the higher trophic level / energy at the lower trophic level x 100.
• Farming practices Pesticides, biological control agents and integrated pest management reduce losses to pests. Keeping livestock warm and limiting movement reduces respiratory losses, increasing the proportion of energy converted to biomass, but this raises ethical concerns.

Common mistakes in 3.3.4 Energy Transfer Through Ecosystems

• Saying energy is lost or destroyed. Energy is transferred to the surroundings as heat, mainly through respiration.
• Mixing up NPP and GPP. NPP is what remains after respiration and is available to the next trophic level.
• Forgetting units such as kJ m-2 year-1 when giving production values. Area and time must be included.
• Writing that biological control eliminates the pest. It keeps pest numbers below the level that causes economic damage.

Try three 3.3.4 Energy Transfer Through Ecosystems exam-style questions