1. 1
   An ecologist is studying energy transfer in a grassland ecosystem, in which grass is eaten by rabbits, and rabbits are eaten by foxes. The ecologist measures the amount of energy available at each stage of this food chain to understand how efficiently energy is transferred between organisms.

   (a)

   In a grassland ecosystem, grass converts light energy into chemical energy stored in its tissues. What is the original, ultimate source of this energy?

   [1 mark]

   • AThe Sun
   • BThe soil
   • COther producers
   • DDecomposers

   (b)

   Rabbits eat grass, and foxes eat rabbits. Only a small proportion of the energy stored in the grass eaten by a rabbit ends up stored in the rabbit's own body tissues. Which of the following is NOT a reason for this loss of energy?

   [1 mark]

   • AEnergy is lost as heat from respiration
   • BSome parts of the grass are not eaten or not digested and are lost in faeces (egestion)
   • CEnergy is used for movement and other life processes
   • DAll energy eaten is converted directly into new body mass

   (c)

   Explain why the transfer of energy from grass to rabbits, and from rabbits to foxes, is often not efficient, and use this to explain why food chains usually have fewer than five trophic levels.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A nutritionist is advising a community on the most energy-efficient way to use crop plants grown in the region, comparing a diet based on eating crop plants directly with a diet based on eating beef from cattle that have themselves been fed on the same crop plants.

   (a)

   A nutritionist is comparing two diets: one based mainly on eating crop plants directly, and one based mainly on eating beef from cattle that were fed on crop plants. Which diet makes more efficient use of the energy originally captured by the crop plants?

   [1 mark]

   • AEating the beef from cattle fed on the crops
   • BEating the crop plants directly
   • CBoth diets are equally energy efficient
   • DNeither diet involves any loss of energy

   (b)

   Cattle that are fed on crop plants lose a large proportion of the energy from that food as heat from respiration. This is one reason why eating crop plants directly is more energy efficient. What is another correct reason?

   [1 mark]

   • ACrop plants contain no usable energy for humans
   • BCattle photosynthesise, using extra energy that is then unavailable to humans
   • CSome of the crop plant material eaten by the cattle is not digested and is egested as faeces, representing further energy loss
   • DEating beef always requires more crops to be grown than eating crops directly, for no energy-related reason

   (c)

   Explain, in terms of energy transfer between trophic levels, why it is more energy efficient for humans to eat crop plants directly than to eat beef from cattle that have been fed on the same crop plants.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A group of ecology students is comparing different ways of representing energy and biomass transfer for the same food chain: grass is eaten by grasshoppers, which are eaten by small birds. They construct both a pyramid of biomass and a pyramid of energy for this food chain, and discuss why one type of pyramid may be more useful than another.

   (a)

   Describe, using the terms producer and trophic level, how a pyramid of biomass could be constructed for a food chain consisting of grass, grasshoppers and small birds, and explain what happens to the width of the bars at each successive trophic level in a typical pyramid of biomass.

   [3 marks]

   (b)

   Explain the advantages of using a pyramid of energy, rather than a pyramid of biomass or a pyramid of numbers, to represent this food chain.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   An oceanographic research team is studying energy flow in two very different ecosystems. The first is a marine food chain: phytoplankton, zooplankton, small fish and tuna. The second is a freshwater lake ecosystem in which algae are eaten by small crustaceans, which are eaten by minnows, which are eaten by pike. The team wants to compare the efficiency of energy transfer in both ecosystems and explain why some trophic levels contain far more biomass than others.

   (a)

   A marine food chain consists of phytoplankton (microscopic producers), zooplankton (primary consumers that eat phytoplankton), small fish (secondary consumers that eat zooplankton) and tuna (tertiary consumers that eat small fish). Explain fully how energy flows through this food chain, starting from the Sun, and explain why the total amount of energy available to tuna is very much smaller than the energy captured by the phytoplankton.

   [6 marks]

   (b)

   In the freshwater lake ecosystem, algae are eaten by small crustaceans, which are eaten by minnows, which are eaten by pike. Researchers find that the total biomass of algae in the lake is far greater than the total biomass of pike. Using the concept of trophic levels and energy transfer, explain why biomass typically decreases at each successive trophic level in a food chain like this one, and explain how a pyramid of energy for this food chain would demonstrate this pattern more reliably than a pyramid of numbers.

   [6 marks]

   Total for question 4: 12 marks

End of questions