OCR A 6.3.1 · 20 written questions · 36 marks · about 45 minutes · marked as you go
This free quiz has 20 written exam questions on ecosystems, all taken from OCR A Level Biology A papers (A level, 2017 to 2025). Short answers mark themselves. For longer answers you write your answer, then tick off the OCR mark-scheme points you made, so you see exactly where the marks are. This quiz covers the whole of OCR A 6.3.1: why ecosystems are dynamic, how biomass moves between trophic levels, the carbon and nitrogen cycles, primary succession and the fieldwork methods used to measure distribution and abundance.
What the quiz covers (OCR A spec 6.3.1)
Dynamic ecosystems (6.3.1(a)): using population data from a river to explain why an ecosystem is described as dynamic.
Biomass transfer (6.3.1(b)): measuring biomass, explaining differences in transfer efficiency between trophic levels, energy and efficiency calculations, and choosing units for primary production.
Recycling within ecosystems (6.3.1(c)): processes in the carbon cycle, the roles of Rhizobium, Nitrosomonas, Nitrobacter and denitrifying bacteria, and how soil oxygen affects nitrogen availability.
Primary succession (6.3.1(d)): what changes during succession, where it starts and what it ends with.
Measuring distribution and abundance (6.3.1(e)(i)): when capture-mark-recapture is unsuitable, evaluating a camera-trap estimate, indirect evidence of occupation, and comparing the Lincoln and Chapman estimates.
Sampling and recording methods (6.3.1(e)(ii)): transects for distribution, limitations of the ACFOR scale, minimising sampling bias and sampling insects.
How to use it
Write every answer in full, as you would in the exam: the mark for each question is shown, so you know how much to write. When you self-mark, only tick a point if your answer clearly makes it, then read the examiner's tip under each question. Your progress is saved on your device, so you can do it in two sittings, and at the end you can retry just the questions where you dropped marks.
Prefer multiple choice? Try the 19-question Ecosystems and Populations multiple choice quiz first.
Open the Ecosystems quiz in a new tab (easier on a phone)
Where students lose marks (from OCR examiners' reports and mark schemes)
Each point below is followed by wording that gained marks.
Low oxygen favours denitrifying bacteria: Asked why plants grow poorly in soil with little oxygen, most students wrote about root respiration or even photosynthesis, even though the question pointed them to the nitrogen cycle. Only about 1 in 10 mentioned denitrification.
✔ Gains credit: Low oxygen conditions favour denitrifying bacteria, so more nitrate is converted to nitrogen gas and less nitrate is available to plants.
Rhizobium lives in root nodules: Giving the location of Rhizobium as just 'roots' or 'soil' was a common reason for losing the location mark.
✔ Gains credit: Rhizobium lives in the root nodules of leguminous plants.
Get the charge on nitrite right: Students often missed the charge off the nitrite ion, wrote it as positive, or wrote ammonia as a positive ion. OCR does not accept incorrect formulae or charges.
✔ Gains credit: Nitrite is NO₂⁻, nitrate is NO₃⁻ and ammonium is NH₄⁺ (ammonia, NH₃, has no charge).
Nitrifying oxidises, denitrifying reduces: Many mixed up oxidation and reduction when describing what nitrogen cycle bacteria do.
✔ Gains credit: Nitrosomonas and Nitrobacter oxidise nitrogen compounds (ammonium to nitrite to nitrate); denitrifying bacteria reduce nitrate to nitrogen gas.
Climax community, not climax population: When asked to name the organisms present at the end of succession, some wrote 'climax population' or described pioneer species from the start of succession.
✔ Gains credit: The final stable group of plant and animal populations is the climax community.
Lakes need volume in the unit: Very few students could build a unit for primary production. Many didn't realise a lake is measured in three dimensions, and the word 'per' was not accepted.
✔ Gains credit: Woodland: kg m⁻² yr⁻¹ or kJ m⁻² yr⁻¹; lake: kg m⁻³ yr⁻¹ or kJ m⁻³ yr⁻¹ (mass or energy, per area or volume, per time).
Compare the estimates proportionally: When comparing two population estimates, most students only said which one was lower. Few saw that the gap matters more for small populations than large ones.
✔ Gains credit: The difference between the Lincoln and Chapman estimates is proportionally greater for small populations.
Measure biomass as dry mass: Suggesting you weigh a plant to find its biomass, without saying the water must be removed, doesn't score: water content varies.
✔ Gains credit: Measure changes in dry mass over time, e.g. dry samples in an oven and weigh them at different times of year.
Keep practising
Try the Ecosystems and Populations multiple choice quiz, the Photosynthesis multiple choice quiz, the Biodiversity written quiz or the Populations and Sustainability written quiz, or browse all our OCR A Level Biology quizzes. Our OCR A masterclasses (Year 12 on Wednesdays at 5.15pm, Year 13 on Tuesdays at 6.30pm) work through real exam questions on one topic each week.
Questions, mark schemes and examiners' report extracts © OCR.