1. 1
   Deep inside a flooded mine, where no light has ever reached, the walls are coated with orange slime. The slime contains iron-oxidizing bacteria that convert dissolved iron(II) ions (Fe²⁺) in the mine water into iron(III) ions (Fe³⁺), and take in carbon dioxide dissolved in the water. Small worms and crustaceans live in the slime and feed on the bacteria.

   (a)

   What is the source of energy that these bacteria use to make carbon compounds?

   [1 mark]

   • AOxidation of iron(II) ions to iron(III) ions
   • BLight absorbed by pigments in the bacteria
   • CReduction of iron(III) ions to iron(II) ions
   • DCarbon compounds from dead organisms washed into the mine

   (b)

   Which term correctly describes both the bacteria and the worms?

   [1 mark]

   • ABacteria: photoautotrophs; worms: heterotrophs
   • BBacteria: chemoautotrophs; worms: heterotrophs
   • CBacteria: chemoautotrophs; worms: chemoautotrophs
   • DBacteria: heterotrophs; worms: decomposers

   (c)

   Explain why the mine ecosystem is an exception to the generalisation that sunlight is the principal source of energy for ecosystems.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   In a grassland, grasses are eaten by grasshoppers and by field voles. Grasshoppers are eaten by common shrews. Barn owls (Tyto alba) eat both field voles and common shrews. The net primary production of the grassland is about 600 g m⁻² yr⁻¹ of carbon compounds, but the secondary production of all the animals together is much lower.

   (a)

   Which trophic level or levels does the barn owl occupy in this community?

   [1 mark]

   • APrimary consumer only
   • BSecondary consumer only
   • CTertiary consumer only
   • DSecondary consumer and tertiary consumer

   (b)

   Why is secondary production much lower than net primary production?

   [1 mark]

   • AHeterotrophs do not grow or reproduce
   • BEnergy content per gram of animal tissue is much lower than that of plant tissue
   • CMuch of the biomass eaten by heterotrophs is converted to carbon dioxide and water in cell respiration
   • DDecomposers are included in primary production

   (c)

   Explain why there are far fewer barn owls than grasshoppers in this grassland.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   Ecologists measured the energy flowing into each trophic level of a freshwater spring ecosystem over one year. Producers: 87 000 kJ m⁻² yr⁻¹. Primary consumers: 14 000 kJ m⁻² yr⁻¹. Secondary consumers: 1600 kJ m⁻² yr⁻¹. Tertiary consumers: 88 kJ m⁻² yr⁻¹. Decomposers and detritus feeders received 21 000 kJ m⁻² yr⁻¹ from dead organic matter and faeces from all levels.

   (a)

   Calculate the percentage of the energy in primary consumers that passes to secondary consumers, and the percentage of the energy in producers that reaches tertiary consumers. Give both answers to two significant figures.

   [3 marks]

   (b)

   Using the data, explain why this ecosystem does not support a fifth trophic level.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   The concentration of carbon dioxide in the atmosphere has been measured continuously at Mauna Loa, Hawaii, since 1958 (the Keeling Curve). It has risen from about 315 ppm in 1958 to about 421 ppm in 2023. Superimposed on this rise is a regular yearly cycle: the concentration peaks each May and falls to a minimum each September to October, a fluctuation of about 6 ppm. Meanwhile, large areas of peatland have been drained for agriculture; in undisturbed, waterlogged peat bogs, dead plant material accumulates because it decomposes only very slowly.

   (a)

   Explain both the annual fluctuations and the long-term trend in the Keeling Curve.

   [6 marks]

   (b)

   Discuss whether peatlands should be regarded as carbon sinks or carbon sources.

   [6 marks]

   Total for question 4: 12 marks

End of questions