1. 1
   A group of ecology students uses the mark-release-recapture method to estimate the population size of garden snails living in a walled garden. They catch, mark harmlessly and release 40 snails, then a week later catch 50 snails, of which 8 are found to be marked.

   (a)

   Using the mark-release-recapture formula, what is the estimated total snail population?

   [1 mark]

   • A250
   • B90
   • C400
   • D320

   (b)

   Which assumption must be true for this method to give an accurate estimate?

   [1 mark]

   • AAll 40 marked snails must be recaptured
   • BThe marked snails must not affect the snails' survival or behaviour
   • CNo snails may reproduce during the study
   • DThe population must be very small

   (c)

   Explain why the estimated population size would be too high if the mark placed on the snails made them more likely to be eaten by predators before the second catch.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   Ecologists studying a grassland food chain record the following feeding relationships: grass is eaten by grasshoppers, grasshoppers are eaten by shrews, and shrews are eaten by owls. They measure the total biomass at each trophic level over one hectare.

   (a)

   In this food chain, what trophic level does the shrew occupy?

   [1 mark]

   • AProducer
   • BPrimary consumer
   • CSecondary consumer
   • DTertiary consumer

   (b)

   Approximately what percentage of the energy available in the grasshoppers' biomass is transferred to the shrews that eat them?

   [1 mark]

   • AAbout 90%
   • BAbout 50%
   • CAbout 100%
   • DAbout 10%

   (c)

   Explain two reasons why only about 10% of the energy in the grasshoppers is transferred to the shrews, rather than all of it.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A fallen oak tree lies decomposing on a forest floor, its wood broken down over several years by fungi and bacteria, while nearby a colony of wild vetch (a legume) grows, its roots bearing small nodules that scientists find are rich in nitrogen-fixing bacteria.

   (a)

   Describe how carbon from the decomposing oak tree is returned to the atmosphere and can be taken up by other organisms.

   [3 marks]

   (b)

   Explain the role of the nitrogen-fixing bacteria in the root nodules of the vetch plant, and describe one further stage of the nitrogen cycle involving decomposers.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   Environmental scientists monitor a busy shipping port, where large container ships burning heavy fuel oil release sulfur dioxide into the air, and a nearby coastal fish farm, where uneaten fish feed and fish waste accumulate in the surrounding bay.

   (a)

   Explain the biological consequences of the sulfur dioxide released by the ships, and describe how increased carbon dioxide emissions from the same ships contribute to an enhanced greenhouse effect.

   [6 marks]

   (b)

   Explain how the excess fish feed and waste from the fish farm can lead to eutrophication in the bay, and describe the consequences for other organisms living there.

   [6 marks]

   Total for question 4: 12 marks

5. 5
   Ecologists compare the abundance of bracken fern in two areas of a heathland with different soil pH \u2014 one distinctly acidic, the other closer to neutral \u2014 recording the percentage cover of bracken in a series of sample plots in each area.

   (a)

   Soil pH is best described as which type of factor affecting the distribution of the bracken fern?

   [1 mark]

   • AAn abiotic factor
   • BA biotic factor
   • CA trophic factor
   • DA genetic factor

   (b)

   If bracken fern grows more densely in the acidic soil than the neutral soil, which conclusion is best supported?

   [1 mark]

   • ASoil pH has no effect on bracken distribution
   • BBracken is better adapted to grow in acidic soil conditions
   • CAll plants grow better in acidic soil
   • DThe neutral soil area must have more predators

   (c)

   Suggest two other abiotic factors, besides soil pH, that the ecologists should also measure and control for when comparing bracken abundance in the two areas.

   [2 marks]

   Total for question 5: 4 marks

6. 6
   Marine biologists studying a rocky shore record the following feeding relationships: seaweed is eaten by both limpets and periwinkles; limpets and periwinkles are both eaten by crabs; crabs are eaten by gulls.

   (a)

   In this food web, what trophic level do the limpets and periwinkles occupy?

   [1 mark]

   • AProducers
   • BSecondary consumers
   • CPrimary consumers
   • DDecomposers

   (b)

   If a disease wiped out the periwinkle population, what would be the most likely immediate effect on the limpet population?

   [1 mark]

   • ANo effect, because limpets and periwinkles do not interact
   • BLimpets would decrease, because they compete with periwinkles for food
   • CLimpets would also die of the same disease
   • DLimpets may increase, as more seaweed becomes available, though crabs may then prey on limpets more heavily as an alternative food source

   (c)

   Explain why the loss of the periwinkle population could also affect the size of the gull population, even though gulls do not eat periwinkles directly.

   [2 marks]

   Total for question 6: 4 marks

7. 7
   A wildfire, started by a lightning strike, burns through a large area of savanna grassland, converting much of the plant biomass into ash and smoke, before new grass and legume seedlings begin to grow from the blackened soil in the following rainy season.

   (a)

   Explain how the wildfire returns carbon from the savanna's plant biomass to the atmosphere, and how this carbon is later taken up again by the new seedlings.

   [3 marks]

   (b)

   Explain how nitrogen-fixing bacteria in the root nodules of the new legume seedlings help to replace nitrogen compounds lost from the soil during the fire, describing the roles of two other types of bacteria involved in the nitrogen cycle.

   [4 marks]

   Total for question 7: 7 marks

8. 8
   A local council opens an investigation after residents complain about gas escaping from an old landfill site, where buried household waste has been decomposing for years, and about the state of a nearby lake that receives fertiliser runoff from a golf course.

   (a)

   Explain how methane gas is produced in the landfill site, and describe how this and other human activities that release methane contribute to an enhanced greenhouse effect.

   [6 marks]

   (b)

   Explain how fertiliser runoff from the golf course could cause eutrophication in the lake, and describe the effect this has on the fish population.

   [6 marks]

   Total for question 8: 12 marks

End of questions