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Organisation of an ecosystem: exam questions

8 questions, 28 marks. Write your answers on paper, then open each mark scheme.

Spec 4.7.2Questions and mark scheme (PDF)5 pages

Question 1

A student counted the number of dandelions in 8 quadrats: 3, 8, 5, 8, 6, 4, 8, 6. (a) Calculate the mean, and give the mode and the median number of dandelions per quadrat. (3) (b) Each quadrat has an area of 0.25 m². The field has an area of 200 m². Estimate the number of dandelions in the field. (2)

[5 marks]
Show mark scheme for question 1
  • (a) mean = 48 ÷ 8 = 6 (1)
  • mode = 8 (1)
  • median = 6 (1) (middle values 6 and 6)
  • (b) 6 ÷ 0.25 = 24 per m² (1)
  • 24 × 200 = 4800 dandelions (1) allow 4800 with no working for 2 marks; allow ecf from their mean

Question 2

A student wants to investigate the effect of light intensity on the distribution of a plant species in a woodland. The light intensity falls from the edge of the woodland into the middle. Describe a method the student could use. Include how the student would make the results valid and safe.

[6 marks]
Show mark scheme for question 2
LevelMarksWhat the answer does
35–6A clear, logical method using a transect and quadrats at regular intervals, with measurement of light intensity, repeated or random sampling, and results processed. Control variables and a safety point are included.
23–4A method which uses a transect or quadrats and records the number of plants and light intensity, but with some detail or sequence missing.
11–2Simple statements about counting plants or measuring light, with no clear method.

Indicative content

  • lay a tape measure / transect from the edge into the woodland
  • place a quadrat at regular intervals, for example every 2 m
  • count the number of the plant species in each quadrat
  • measure the light intensity at each position with a light meter
  • repeat the transect / use more than one transect and calculate means
  • same size of quadrat, same time of day, same person counting
  • plot a graph of number of plants against distance or light intensity
  • safety, for example wear gloves / avoid harmful plants / wash hands afterwards

Question 3

In a stable community, the numbers of hares and lynx rise and fall in cycles. Each time, the peak in the number of lynx is a short time after the peak in the number of hares. (a) State which animal is the predator. (1) (b) Explain why the number of lynx peaks after the number of hares. (3)

[4 marks]
Show mark scheme for question 3
  • (a) lynx (1)
  • (b) when there are many hares there is more food for the lynx (1)
  • so more lynx survive and breed, and the number of lynx increases (1)
  • breeding takes time, so the rise in lynx lags behind the rise in hares (1)
  • the lynx then eat more hares, so hare numbers fall, and then lynx numbers fall from lack of food (1)
  • Max 3 for (b)

Question 4

Describe how the carbon in a dead leaf can become the carbon in the glucose of a new plant.

[3 marks]
Show mark scheme for question 4
  • microorganisms / decomposers break down the leaf / decay (1)
  • carbon is released as carbon dioxide by respiration (of the microorganisms) (1)
  • carbon dioxide is taken in by a plant / used in photosynthesis to make glucose (1)

Question 5

Explain why the water cycle is important to land plants and animals.

[2 marks]
Show mark scheme for question 5
  • it provides fresh water (1)
  • which plants and animals need to survive / for photosynthesis / for cells (1)

Question 6

Triple only (what this means)Triple only: only in Triple Biology. Combined Science students can skip it. What the labels mean

A student left 60 g of dead leaves to decay. After 20 days, the mass of leaves remaining was 36 g at 25 °C and 52 g at 10 °C. Calculate the mean rate of decay at each temperature, in g per day.

[3 marks]
Show mark scheme for question 6
  • (60 − 36) ÷ 20 or 24 ÷ 20 (1)
  • 25 °C: 1.2 g per day (1)
  • 10 °C: (60 − 52) ÷ 20 = 0.4 g per day (1)

Question 7

Triple only (what this means)Triple only: only in Triple Biology. Combined Science students can skip it. What the labels mean

A gardener wants waste to decay quickly in a compost heap. Explain how the conditions in the heap can be changed to speed up decay.

[3 marks]
Show mark scheme for question 7
  • keep it warm (1) because the decomposers' enzymes work faster / respiration is faster (1)
  • keep it moist / add water (1) because microorganisms need water to grow and be active
  • turn it / mix it to add air / oxygen (1) for aerobic respiration of the decomposers
  • ignore 'heat it' unqualified; do not accept boiling / very high temperatures
  • Max 3

Question 8

Triple only (what this means)Triple only: only in Triple Biology. Combined Science students can skip it. What the labels mean
Higher tier (what this means)Higher tier: only on the Higher tier papers. Foundation students can skip it. What the labels mean

Over 30 years, the mean summer temperature in an area has increased. A butterfly species is now found 50 km further north than it was 30 years ago. Suggest why the distribution of the butterfly has moved north.

[2 marks]
Show mark scheme for question 8
  • the area in the south is now too warm for the butterfly to survive / breed (1)
  • the area further north is now warm enough / has suitable conditions (1)
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