Ecosystems (6.3.1): OCR A Level Biology Exam Questions by Topic

OCRGenetics, Evolution and EcosystemsExam Questions
Written by Tom Whitburn

Ecosystems is where biology meets fieldwork. Spec point 6.3.1 asks you to explain why ecosystems are dynamic, follow biomass along a food chain and calculate the efficiency of each transfer, name the bacteria that recycle nitrogen, describe the carbon cycle, take a habitat from bare rock to a climax community (and explain deflected succession), and choose sampling methods to measure the distribution and abundance of organisms.

The examiners' reports show the same slips year after year. Students drop the minus sign on a nitrite ion, write Azotobacter where Nitrosomonas belongs, say plants turn nitrates into ammonia rather than amino acids, and answer a low-oxygen soil question in terms of respiration instead of denitrification. In the calculations, marks go on units: joules not converted to kilojoules, or a productivity unit with no area, volume or time in it.

Work through the nineteen questions below in order. They start with recall, move on to nitrogen, biomass and succession in new contexts, and finish with sampling methods and calculations. If you'd like to go through questions like these with me each week, my OCR A masterclasses run on Wednesdays at 5.15pm for Year 12 and on Tuesdays at 6.30pm or Thursdays at 5.15pm for Year 13.

What you need from GCSE

Each question on this page assumes you're secure on these GCSE ideas. If any of them feels shaky, look it up before you start.

  • An ecosystem is the community of organisms in an area interacting with each other and with the non-living (abiotic) parts of their environment.
  • Pyramids of biomass get narrower at each trophic level because biomass is lost through respiration, egestion and excretion.
  • Decomposers break down dead organisms and waste, returning carbon dioxide and mineral ions such as nitrates to the environment.
  • The carbon cycle links photosynthesis, respiration, combustion and decay.
  • Quadrats and transects are used to sample organisms, and the mean number per quadrat can be scaled up to estimate a population.

Key terms

These words come up in the questions and mark schemes. Examiners expect you to use them exactly.

  • Biotic and abiotic factors: the living (e.g. predation, competition, disease) and non-living (e.g. light, temperature, pH, water) influences on an ecosystem.
  • Trophic level: a feeding level in a food chain: producers, primary consumers, secondary consumers and so on. Decomposers feed on dead material from every level.
  • Efficiency of biomass transfer: biomass transferred ÷ biomass intake × 100. Biomass is measured as dry mass, or as energy per unit area (or volume) per year.
  • Ammonification: decomposers converting nitrogen-containing compounds in dead organisms and waste into ammonium compounds.
  • Nitrification: Nitrosomonas oxidises ammonium ions (NH₄⁺) to nitrite (NO₂⁻); Nitrobacter oxidises nitrite to nitrate (NO₃⁻). Both need oxygen.
  • Nitrogen fixation: reducing nitrogen gas (N₂) to ammonia, by free-living Azotobacter in the soil or by Rhizobium in the root nodules of legumes.
  • Denitrification: anaerobic bacteria reducing nitrate back to nitrogen gas, which is favoured in waterlogged, low-oxygen soils.
  • Succession: the change in a community over time, through seral stages from a pioneer community on bare ground to a climax community.
  • Deflected succession: succession stopped before the climax community by human activity such as grazing, mowing or burning (heather moorland, for example).
  • Capture–mark–recapture: estimating the size of a mobile population: number in first sample × number in second sample ÷ number marked in the second sample.

What these questions cover (OCR 6.3.1)

The questions test these learning outcomes from the OCR A Level Biology A specification (H420):

  • (a) ecosystems, which range in size, are dynamic and are influenced by both biotic and abiotic factors
  • (b) biomass transfers through ecosystems (including efficiency = biomass transferred ÷ biomass intake × 100)
  • (c) recycling within ecosystems (decomposers, Nitrosomonas, Nitrobacter, Azotobacter and Rhizobium, and the carbon cycle)
  • (d) the process of primary succession in the development of an ecosystem (from pioneer species to a climax community, and deflected succession)
  • (e)(i) how the distribution and abundance of organisms in an ecosystem can be measured
  • (e)(ii) the use of sampling and recording methods to determine the distribution and abundance of organisms in a variety of ecosystems (including transects, quadrats and ACFOR (PAG3))

How to use this page

Write your answer on paper before you open the mark scheme. Allow about 1.2 minutes per mark: the 19 questions are worth 42 marks, so about 50 minutes in total. Then mark your answer strictly against the mark scheme and read the examiners' tip underneath it. If you'd rather print the questions, download the PDF pack.

Recall

Question 1

A level Paper 3 · June 2018 · Q5(c)(i) · 1 mark · Medium

Name the community present at the end of primary succession.

Question: habitat B beside a lake shows ecological succession; state the collective name of the animal and plant populations present at the end of primary succession, 1 mark.
Show the mark scheme Mark scheme: climax community.

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From the examiners' report: Read what the question asks for: the end of succession, not the start. Pioneer species and "climax population" both scored nothing.

Question 2

A level Paper 3 · June 2017 · Q4(a) · 4 marks · Medium

Complete a table of nitrogen cycle bacteria: location, reactant, product, oxidation or reduction.

Table with columns type of bacteria, location, reactant(s), product, and oxidation or reduction of nitrogen, for Rhizobium, Nitrosomonas, Nitrobacter and denitrifying bacteria, with some cells already filled in. 4 marks.
Show the mark scheme Mark scheme: Rhizobium in root nodules of legumes; Nitrosomonas: ammonium ions to nitrite (NO₂⁻), oxidation; Nitrobacter: nitrite to NO₃⁻; denitrifying bacteria in soil: NO₃⁻ to N₂, reduction. One mark per correct column; incorrect formulae or charges not accepted.

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From the examiners' report: Fewer than 1 in 10 scored all four marks. Say Rhizobium lives in root nodules, give NO₂⁻ its negative charge, and remember that nitrification is oxidation and denitrification is reduction.

Question 3

A level Paper 2 · June 2023 · Q21(a) · 6 marks · Hard

Fill six gaps in a passage about the nitrogen cycle.

Passage with six blanks about ammonia formation from dead animals, nitrifying bacteria, the two bacteria converting ammonia to nitrites and nitrites to nitrates, what plants make from nitrates, and the bacteria such as Rhizobium that bring nitrogen gas into the cycle. 6 marks.
Show the mark scheme Mark scheme: ammonification; nitrifying; Nitrosomonas; Nitrobacter; amino acids (protein or nucleic acids allowed); nitrogen-fixing. Phonetic spellings accepted.

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From the examiners' report: Learn the genus names exactly: Nitrosomonas makes nitrite and Nitrobacter makes nitrate, while Azotobacter fixes nitrogen. Plants use nitrates to make amino acids and proteins, not ammonia, and "nitrogen-fixating" was not accepted.

Question 4

A level Paper 2 · November 2020 · Q20(a)(i) · 2 marks · Medium

Identify two processes on a carbon cycle flow chart.

Flow chart of the carbon cycle linking CO₂ in the atmosphere, producers, consumers, dead organisms, decomposers and fossil fuels, with arrows labelled A to H. Asks for the processes at A (fossil fuels to CO₂) and F. 2 marks.
Show the mark scheme Mark scheme: A = combustion (burning allowed); F = respiration. Aerobic or anaerobic ignored.

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Question 5

A level Paper 2 · June 2024 · Q20(b)(iii) · 1 mark · Medium

Suggest what was there before the pioneer community arrived.

Question about mixed woodland that is a climax community formed by succession; suggest what was present in the area before the pioneer community arrived, 1 mark.
Show the mark scheme Mark scheme: bare ground, rock, water, mud, sand, glacier or lava. "Empty land" ignored; soil not credited.

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From the examiners' report: Primary succession starts with no soil at all, so answer bare rock, sand or similar. Soil was marked wrong.

Applying what you know

Question 6

A level Paper 2 · November 2020 · Q21(a)(ii) · 2 marks · Medium

Use a graph of an algal population to explain why a river is a dynamic ecosystem.

Information from earlier in this question:

Earlier part of the same exam question, giving the information this part uses. Question asking how Fig. 21.1, a graph of an algal population in a river through the year, shows that the ecosystem is dynamic. 2 marks.
Show the mark scheme Mark scheme: the idea of changes over time; figures with units to illustrate the population change (a calculated change allowed).

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From the examiners' report: Almost any discussion of change in this context earned a mark. For the second, quote figures with units from the graph.

Question 7

A level Paper 2 · June 2023 · Q21(b) · 2 marks · Medium

Explain why plants grow poorly in soil with little oxygen.

Question: use your knowledge of the nitrogen cycle to suggest and explain why plants do not grow well in soil with a low oxygen content. 2 marks.
Show the mark scheme Mark scheme: more denitrification or less nitrification; conditions favour denitrifying bacteria; more nitrate converted to nitrogen gas, so less nitrate available to plants.

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From the examiners' report: Only about 1 in 10 thought of denitrification, but they usually scored both marks. The question says nitrogen cycle, so don't answer in terms of root respiration or photosynthesis.

Question 8

A level Paper 2 · November 2021 · Q17(a)(i) · 2 marks · Medium

Calculate how much of the light energy falling on heather was not converted into biomass.

Question introducing heather moorland with a food chain heather to red grouse (6% of biomass transferred) to hen harrier (13%); 8.94 × 10⁹ kJ m⁻² of photosynthetically active radiation fell on the heather in a year and 9.08 × 10⁷ kJ m⁻² was converted into biomass. Calculate the energy not converted. 2 marks.
Show the mark scheme Mark scheme: 8.94 × 10⁹ − 9.08 × 10⁷ = 8.85 × 10⁹ kJ m⁻². One mark for 8.8492 × 10⁹ or a correct answer not in standard form.

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From the examiners' report: Most students scored both marks. Subtract with both numbers in the same power of ten and give the answer in standard form.

Question 9

A level Paper 2 · November 2021 · Q17(a)(ii) · 1 mark · Medium

Suggest how to measure the increase in biomass of heather over a year.

Question: suggest how the increase in biomass over time in a plant such as heather could be determined experimentally. 1 mark.
Show the mark scheme Mark scheme: measure changes in dry mass over time, e.g. dry samples of leaves and weigh them, repeating at different times of year.

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Question 10

A level Paper 2 · November 2021 · Q17(a)(iv) · 2 marks · Medium

Explain why less biomass passes from heather to grouse than from grouse to hen harrier.

Question: suggest and explain why the percentage of biomass transferred between heather and grouse is smaller than between grouse and hen harrier. 2 marks.
Show the mark scheme Mark scheme: heather is less easily digested (or the reverse for meat); because of cellulose cell walls or lignin in woody parts. Only part of the plant being eaten is allowed.

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Question 11

A level Paper 2 · November 2021 · Q17(d) · 6 marks · Medium

Outline primary succession and explain why heather moorland is deflected succession (6 marks).

Six-mark extended response: outline the process of primary succession in a newly available area of land and explain why heather moorland is an example of deflected succession.
Show the mark scheme Mark scheme (levels): pioneer community on bare rock, arriving as seeds or spores, with adaptations such as nitrogen fixation; intermediate stages of herbs and grasses, then shrubs and trees; a climax community dominated by a few tree species; each seral stage changes the soil (more organic matter, nitrates and water). Heather moorland is deflected because burning or grazing stops the climax community developing.

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From the examiners' report: This was the easier of the paper's two six-mark questions, but Level 3 needs the seral stages in between, not just the pioneer community. Finish by saying what stops the climax community forming on moorland.

Question 12

A level Paper 2 · June 2017 · Q16(a)(i) · 2 marks · Medium

Suggest why capture–mark–recapture is not suitable for counting jaguars.

Question about estimating the jaguar population of Madidi National Park with camera traps that photograph animals breaking an infrared beam; suggest why it was not appropriate to use capture–mark–recapture. 2 marks.
Show the mark scheme Mark scheme: the estimate would be inaccurate because numbers are low (one more jaguar caught makes a big difference); dangerous for the collector or the jaguar. "Difficult to catch" and "cruel" ignored.

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Question 13

A level Paper 2 · June 2025 · Q19(b)(i) · 1 mark · Medium

Suggest evidence that a puffin burrow is occupied without disturbing it.

Question about puffins on the Farne Islands, estimated from occupied burrows; burrows must not be disturbed and constant monitoring is too expensive. Suggest one other type of evidence that a burrow is occupied. 1 mark.
Show the mark scheme Mark scheme: footprints, feathers, eggshell, fish bones or droppings near the burrow entrance, or predators nearby. Thermal imaging, eggs and nests ignored.

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From the examiners' report: Use the limits in the stem: you can't look inside the burrow and there's no money for cameras, so give evidence found outside the entrance.

Practical and maths

Question 14

A level Paper 3 · June 2018 · Q5(c)(ii) · 3 marks · Medium

Suggest how to sample every stage of succession, avoid bias and sample insects.

Question: a scientist investigates how biodiversity changes from the lake edge across habitat B; suggest how to sample all stages of succession, minimise sampling bias and sample insect biodiversity. 3 marks.
Show the mark scheme Mark scheme: a belt or line transect from the lake edge (or stratified sampling); random selection of transect sites; pooters, sweep nets, pitfall traps, light traps or tree beating for insects.

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From the examiners' report: Use a transect to cross the stages of succession. If you mention random sampling, say exactly what is chosen at random, such as where the transect starts; random quadrats along a transect contradicts itself.

Question 15

A level Paper 3 · June 2018 · Q5(c)(iii) · 1 mark · Hard

Suggest units for primary production in a woodland and in a lake.

Question: the scientist also measured primary production in the woodland and lake habitats; suggest the units for each. 1 mark.
Show the mark scheme Mark scheme: woodland in g or kg m⁻² yr⁻¹ (or kJ m⁻² yr⁻¹) AND lake in g or kg m⁻³ yr⁻¹ (or kJ m⁻³ yr⁻¹). "Per" not allowed.

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From the examiners' report: Only the strongest students scored this. A productivity unit needs mass or energy, an area for land or a volume for water, and a time.

Question 16

A level Paper 2 · June 2022 · Q20(a)(i) · 1 mark · Medium

Name a sampling method for studying the distribution of plants.

Question describing students sampling a 20 m by 20 m grass area using random coordinates and quadrats; asks which type of sampling would be better for studying distribution. 1 mark.
Show the mark scheme Mark scheme: systematic sampling or a transect (described). Stratified ignored.

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From the examiners' report: Distribution means how organisms change across an area, so the answer is a transect or systematic sampling. More quadrats or percentage cover don't answer it.

Question 17

A level Paper 3 · June 2017 · Q4(c)(i) · 2 marks · Medium

Calculate the energy in the producers' trophic level in kJ.

Question: decomposers hold 950 000 J m⁻² yr⁻¹ and the producers' energy is 800% greater; calculate the producers' energy in kJ m⁻² yr⁻¹. 2 marks.
Show the mark scheme Mark scheme: 8550 kJ m⁻² yr⁻¹. One mark for 8 550 000 J, for 9 × 950 000, or for 7600 kJ.

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From the examiners' report: 800% greater means nine times the original, not eight. Then divide by 1000 to give kilojoules, as the answer line asks.

Question 18

A level Paper 3 · June 2017 · Q4(c)(ii) · 1 mark · Medium

Calculate the percentage efficiency of transfer from producers to decomposers.

Question: calculate the percentage efficiency of energy transfer from producers to decomposers, giving the answer to two significant figures. 1 mark.
Show the mark scheme Mark scheme: 11%. Error carried forward from part (i) accepted (7600 kJ gives 13%).

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From the examiners' report: Most scored this. Those who didn't usually ignored the two significant figures the question asked for.

Question 19

A level Paper 3 · November 2020 · Q5(b)(i) · 2 marks · Hard

Use the Lincoln and Chapman formulae to estimate two insect populations.

Information from earlier in this question:

Earlier part of the same exam question, giving the information this part uses. Question: ecologists used capture–mark–recapture for the large heath butterfly and the bog hoverfly; calculate population sizes with the Lincoln formula (n1 × n2 ÷ m) and the Chapman formula ((n1 + 1)(n2 + 1) ÷ (m + 1) − 1) and write them in the table. 2 marks.
Show the mark scheme Mark scheme: large heath butterfly 1405 and 1153; bog hoverfly 30 and 20. Answers rounded to the nearest whole number.

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From the examiners' report: The calculations were well done. The usual slip was not rounding to whole insects.

Where students lose marks (from OCR examiners' reports and mark schemes)

Each point below is followed by wording that gained marks.

  • Nitrite carries a minus sign: Many students wrote NO₂⁺ or NH₃⁺, or left the charge off nitrite altogether, and lost the reactant and product marks.

    ✔ Gains credit: "NH₄⁺ / ammonium ions" → "NO₂⁻ / nitrites" → "NO₃⁻".

  • Plants build amino acids from nitrate: Some students said plants convert the nitrate they absorb into ammonia or nitrogen.

    ✔ Gains credit: "amino acids" (protein or nucleic acids also allowed).

  • Waterlogged soil favours denitrifiers: Asked why plants grow poorly in low-oxygen soil, most students wrote about root respiration or photosynthesis.

    ✔ Gains credit: "conditions favour denitrifying bacteria; (more) nitrate converted to nitrogen gas".

  • Producers' energy ends in kilojoules: Some students left the answer as 7 600 000 or 8 550 000 J, when the answer line asked for kJ.

    ✔ Gains credit: "8550 (kJ m⁻² yr⁻¹)".

  • Land per square metre, water per cubic metre: Very few students built a productivity unit from mass or energy, area or volume, and time.

    ✔ Gains credit: "Woodland = kg m⁻² yr⁻¹ AND Lake = kg m⁻³ yr⁻¹".

  • The end of succession is a climax community: Students answered about the start of succession (pioneer species) or wrote "climax population".

    ✔ Gains credit: "climax community".

  • Seral stages earn Level 3: Many six-mark answers described only the pioneer community and jumped straight to the climax.

    ✔ Gains credit: "Intermediate community: herb species, including grasses, followed by shrubs and trees … climax community is prevented from developing".

  • Look outside the burrow: Students suggested looking for eggs in the burrow or thermal imaging, ignoring that burrows mustn't be disturbed and money is short.

    ✔ Gains credit: "(puffin) footprints / feathers / eggshell / fish bones / droppings (near burrow entrance)".

Download the questions

Download the PDF pack (19 questions, mark schemes at the end)

Keep practising

Questions, mark schemes and examiners' report extracts © OCR.

Tom Whitburn
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