DSE Biology Practice Paper: Ecology, Energy Flow And Nutrient Cycles

DSE Biology Practice Paper: Ecology, Energy Flow and Nutrient Cycles

2026-11-03

DSE Biology – Ecology Practice

DSE Biology practice paper: 12 questions, 100 marks, 75 minutes. Covers energy flow, productivity, mark-recapture, ecological efficiency, nutrient cycles and eutrophication.

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Paper details　DSE Biology, ecology, energy flow, mark-recapture, eutrophication, 12 questions, 100 marks

Section A (Questions 1 to 4, 26 marks)

Read each question and write the correct answer with an explanation. Show working and units for any calculation.

#	Question	Answer and marking notes
1	A grassland food chain is grass, insects, frogs, snakes and hawks. If the grass fixes 50,000 kJ of energy and only one tenth passes to the next trophic level, how much energy does the hawk obtain?	Answer: 50 kJ. Energy passes along the chain as 50,000, then 5,000, then 500, then 50, all in kJ. Marking: each level multiplied by 0.10 (3); three transfers correct (2); unit (1).
2	The gross primary productivity of an ecosystem is 24,000 kJ per square metre per year and plant respiration uses 9,000 kJ. Calculate the net primary productivity.	Answer: 15,000 kJ m-2 yr-1. Net primary productivity equals gross primary productivity minus plant respiration: 24,000 – 9,000 = 15,000. Marking: formula (2); value (2); unit (2).
3	Six quadrats of 0.25 m2 are placed at random in a 400 m2 grassland. The numbers of a plant in the quadrats are 8, 10, 12, 10, 9 and 11. Estimate the total number of plants in the grassland.	Answer: about 16,000 plants. The six quadrats hold 60 plants, a mean of 10 per quadrat; per square metre this is 10 / 0.25 = 40 plants; the total is 40 x 400 = 16,000. Marking: mean per quadrat (2); conversion to per square metre (2); multiplication by total area (2); random sampling reduces bias (1).
4	40 snails are caught, marked and released. A week later 60 snails are caught and 6 of them carry a mark. Estimate the population size and state one assumption.	Answer: about 400 snails. Population size = (number marked first x number caught second) / number marked in second sample = (40 x 60) / 6 = 400. The marking must not affect survival or the chance of being caught, and marked individuals must mix evenly with the rest. Marking: formula (2); substitution (2); result (2); one assumption (1).

Section B (Questions 5 to 8, 32 marks)

Answer the following questions with working, units and evidence drawn from the data given.

#	Question	Answer and marking notes
5	A primary consumer ingests 200 kJ of energy a day, egests 60 kJ in faeces and uses 100 kJ in respiration. Calculate its assimilation efficiency and its ecological (production) efficiency.	Answer: Assimilation is 200 – 60 = 140 kJ, so assimilation efficiency is 140 / 200 = 70.0 per cent; net production is 140 – 100 = 40 kJ, so ecological efficiency is 40 / 140 = 28.6 per cent. Marking: assimilation (2); assimilation efficiency (2); net production (2); percentages with units (1).
6	In a marine ecosystem the biomass of phytoplankton at one moment is smaller than that of zooplankton, yet the pyramid of energy is still upright. Explain.	Answer: Phytoplankton reproduce quickly and are grazed quickly, so their turnover is rapid; the standing biomass at any moment is small, but the total energy fixed over a year greatly exceeds that of the zooplankton. A pyramid of energy is drawn from rates of energy flow, not from standing biomass, so it remains upright. Marking: turnover concept (3); production versus standing crop (3); basis of the energy pyramid (2).
7	State two processes that move carbon from the atmosphere into living organisms and two that return it to the atmosphere, and name the main form in which carbon cycles.	Answer: Carbon enters living organisms through photosynthesis and chemosynthesis; it returns to the atmosphere through respiration, decomposition and the burning of fossil fuels. Carbon cycles mainly as carbon dioxide, with carbonate and hydrogencarbonate also present. Marking: two input processes (3); two output processes (3); carbon dioxide as the main form (2).
8	Distinguish nitrogen fixation, nitrification and denitrification in the nitrogen cycle, and state the role of root nodules in legumes.	Answer: Nitrogen fixation converts atmospheric nitrogen into ammonia or nitrate, carried out by nitrogen fixing bacteria, blue green algae and lightning; nitrification is the oxidation of ammonia to nitrite and then nitrate by nitrifying bacteria; denitrification reduces nitrate back to nitrogen gas, which returns to the atmosphere. In legumes, symbiotic bacteria in root nodules fix nitrogen into compounds the plant can use. Marking: nitrogen fixation (3); nitrification (3); denitrification (2); root nodule symbiosis (1).

Section C (Questions 9 to 12, 42 marks)

Write fuller answers using correct subject terms and support them with examples from Hong Kong or daily life.

#	Question	Answer and marking notes
9	After heavy nitrogen fertiliser is applied to farmland, a nearby river shows an algal bloom, falling dissolved oxygen and fish deaths. Explain the sequence of events.	Answer: Nitrate from the fertiliser enters the river and its concentration rises; algae and water plants multiply rapidly to form a bloom; when the algae die, decomposers break them down and use up dissolved oxygen; oxygen falls so low that fish and other aerobic organisms die and the water deteriorates further. Marking: nutrient enrichment (2); algal bloom (2); decomposition using oxygen (3); low oxygen and deaths (2); correct sequence (1).
10	Food chains rarely have more than four or five trophic levels. Explain this in terms of energy flow.	Answer: Only about one tenth of the energy at each trophic level passes to the next; the rest is used in respiration, lost as heat or egested. After four or five levels the energy available is too small to support another population, so the length of the chain is limited. Marking: ten per cent transfer (3); where the energy goes (3); energy demand of top consumers (2); conclusion (2).
11	Compare energy flow and net productivity in a forest and in farmland, and explain how human activity affects each.	Answer: A forest has high gross primary productivity, but much of the energy is used in plant respiration and by a large community of consumers, so net production is largely consumed naturally. Farmland has competitors removed and fertiliser added, so more energy is channelled into the crop and net production at harvest is higher, although species numbers are lower. Harvesting exports energy from the system, so nutrients must be replaced by fertiliser and labour. Marking: comparison of productivity (3); fate of the energy (3); effect of human intervention (3); effect on biodiversity (2).
12	Hong Kong protects large areas of woodland as country parks. Design a long term study to assess changes in biodiversity inside a country park.	Answer: Set out permanent transects or quadrats in the park and record plant cover, and the number of species and individuals of insects and birds every season using standardised methods, together with temperature, rainfall and the level of human disturbance. Compare species richness, the Simpson index and abundance between years, and use control plots outside the park to separate natural variation from management effects. Marking: sampling design (3); standardised repeated recording (3); choice of indices (3); controls and analysis (2).

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