S5 Biology Practice Paper: Ecosystems, Energy Flow And Nutrient Cycling (12 Questions, 100 Marks, 75 Minutes, With Answers)

S5 Biology Practice Paper: Ecosystems, Energy Flow and Nutrient Cycling (12 Questions, 100 Marks, 75 Minutes, with Answers)

2026-11-09

HKDSE Biology: Ecosystems, Energy Flow and Nutrient Cycling

12 questions, 100 marks, 75 minutes on food chains, trophic levels, energy flow, pyramids, population estimation, the carbon and nitrogen cycles, decomposition and eutrophication.

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Instructions: This paper has three sections, A, B and C. Answer all questions. Total marks: 100. Time allowed: 75 minutes.

Show all working. Answers without working earn no method marks. Write down a formula before substituting numbers.

Give numerical answers to three significant figures unless stated otherwise, and state the correct units.

Spend about 35 minutes on the ecosystem and energy-flow questions in Section A, then use the remaining time for the population and productivity questions in Section B and the nutrient-cycle, decomposition and eutrophication questions in Section C.

Section A: Ecosystems and Energy Flow

Ecosystem, population and community, roles of producers, consumers and decomposers, energy transfer and energy loss between trophic levels, pyramids of number and biomass

#	Question	Answer and marking notes
1	(a) Explain the terms ecosystem, population and community. (b) Describe the roles of producers, primary consumers and decomposers in a food web. (c) Explain why a food chain usually has only four or five trophic levels.	(a) An ecosystem is a community of organisms interacting with its abiotic environment; a population is all the individuals of one species in an area; a community is all the populations in an area (3 marks). (b) Producers fix light energy into chemical energy by photosynthesis; primary consumers feed on producers; decomposers break down dead bodies and wastes and release nutrients back to the environment (3 marks). (c) Only about one tenth of the energy passes to the next level, the rest being lost in respiration, excretion and uneaten parts, so there is too little energy to support more levels (2 marks).
2	A grassland fixes 20 000 kJ of energy per square metre per year in producers. If only one tenth of the energy passes to the next trophic level, (a) find the energy available to the primary and secondary consumers; (b) find the overall transfer efficiency from producers to secondary consumers; (c) state where the rest of the energy goes; (d) explain why top predators usually have a small biomass.	(a) The primary consumers receive 20 000 x 0.1 = 2 000 kJ and the secondary consumers 2 000 x 0.1 = 200 kJ (2 marks). (b) The efficiency is 200/20 000 = 1 per cent (2 marks). (c) About nine tenths of the energy is lost as heat in respiration, some leaves in excretory products and undigested food, and some remains in uneaten tissue (2 marks). (d) Energy falls by about nine tenths at each level, so top predators obtain the least energy and can support only the smallest numbers and biomass (3 marks).
3	(a) Distinguish between pyramids of number, biomass and energy. (b) Explain why a pyramid of number can be inverted, giving one example. (c) Explain why a pyramid of energy can never be inverted.	(a) A pyramid of number uses the number of individuals, a pyramid of biomass uses total mass, and a pyramid of energy uses the energy content of each level (3 marks). (b) It can be inverted when producers are very large and consumers are numerous, for example one large tree supporting many insects (3 marks). (c) Energy is lost as heat at every transfer, so each level holds less energy than the one below it and the pyramid cannot be inverted (2 marks).
4	Researchers catch 40 snails in a wood, mark them and release them. A week later they catch 50 snails, 8 of which are marked. (a) Estimate the population size using the capture-recapture method. (b) State two assumptions of the method. (c) Explain why the second sample is taken after a short interval.	(a) The population is 40 x 50/8 = 250 snails (4 marks). (b) Marking does not affect survival or the chance of recapture, and there is no birth, death, immigration or emigration between the two samples (2 marks). (c) A long interval allows births, deaths and migration, which break the assumptions and bias the estimate (2 marks).

Section B: Populations and Productivity

Quadrat sampling and population density, capture-recapture, gross and net primary productivity

#	Question	Answer and marking notes
5	A plant is surveyed in a 200 square metre meadow. Each quadrat is 0.5 m by 0.5 m, 20 quadrats are placed at random, and the mean is 6 plants per quadrat. (a) Find the quadrat area and the population density per square metre. (b) Estimate the number of plants in the meadow. (c) Explain the importance of random sampling.	(a) The quadrat area is 0.5 x 0.5 = 0.25 square metres, so the density is 6/0.25 = 24 plants per square metre (3 marks). (b) The meadow holds about 24 x 200 = 4 800 plants (3 marks). (c) Random sampling avoids sampling only where plants are dense, reduces bias and makes the sample representative (3 marks).
6	A forest has a gross primary productivity of 1 200 g per square metre per year and the plants respire 400 g per square metre per year. (a) Find the net primary productivity. (b) Find the net primary productivity as a percentage of the gross value. (c) Explain the significance of net primary productivity to consumers.	(a) The net primary productivity is 1 200 – 400 = 800 g per square metre per year (3 marks). (b) The percentage is 800/1 200, about 66.7 per cent (3 marks). (c) Net primary productivity is the energy available for plant growth and for consumers, so it sets the biomass the ecosystem can support (2 marks).
7	(a) State three main processes of the carbon cycle in an ecosystem. (b) Explain why burning fossil fuels raises the carbon dioxide concentration of the atmosphere. (c) Explain how large-scale deforestation affects the carbon cycle.	(a) Photosynthesis fixes carbon dioxide into organic matter; respiration and decomposition release organic carbon back as carbon dioxide; burning fossil fuels releases stored carbon (3 marks). (b) Fossil fuels hold carbon stored over millions of years; burning combines it with oxygen to form carbon dioxide far faster than the carbon cycle can absorb it (3 marks). (c) Fewer trees means less carbon dioxide removed by photosynthesis and less carbon stored as wood, so atmospheric carbon dioxide accumulates (2 marks).
8	(a) Explain nitrogen fixation, nitrification and denitrification. (b) Explain how leguminous plants improve the nitrogen content of soil. (c) Explain why nitrogen often limits plant growth.	(a) Nitrogen fixation converts atmospheric nitrogen into nitrogen compounds; nitrification converts ammonia to nitrite and then nitrate; denitrification reduces nitrate back to nitrogen gas (3 marks). (b) Legumes have nitrogen-fixing bacteria in root nodules that convert atmospheric nitrogen into compounds the plant can use, and their remains add nitrogen to the soil when decomposed (3 marks). (c) Plants need nitrogen for proteins and nucleic acids, but available nitrogen in soil is often scarce, so it commonly limits growth (2 marks).

Section C: Nutrient Cycling and Human Impact

Carbon cycle, nitrogen cycle, decomposition, eutrophication and ecological succession

#	Question	Answer and marking notes
9	(a) Explain the role of decomposition in nutrient cycling. (b) State two environmental factors that affect the rate of decomposition and explain their effects. (c) Explain why decomposition is faster in tropical rainforests than in boreal forests.	(a) Decomposers break organic matter in dead bodies and wastes into inorganic nutrients that producers can absorb again, so matter is recycled (3 marks). (b) A higher temperature speeds decomposition because enzyme activity is greater; adequate water and oxygen also speed it, while waterlogging or drying slows it (3 marks). (c) Tropical rainforests are hot and wet, so microbial and enzyme activity is high and organic matter decomposes quickly (3 marks).
10	A farm produces 50 000 kJ of plant energy each year. If people eat the plants directly they obtain one tenth of this energy; if the plants are fed to cattle and the beef is eaten, each trophic level transfers one tenth. (a) Find the energy obtained by eating plants directly and by eating beef. (b) Compare the two and explain the difference. (c) State the implication for food supply.	(a) Eating plants directly gives 50 000 x 0.1 = 5 000 kJ, while the beef contains 5 000 x 0.1 = 500 kJ (3 marks). (b) Direct consumption yields ten times the energy because about nine tenths is lost at each trophic level, so the longer the food chain the less energy reaches people (3 marks). (c) A mainly plant-based diet feeds more people from less farmland and makes food supply more efficient (2 marks).
11	(a) Describe the process of eutrophication after a large amount of nitrogen fertiliser enters a lake. (b) Explain why the dissolved oxygen level falls after an algal bloom. (c) Suggest two measures to reduce eutrophication.	(a) Nitrogen and phosphorus from fertiliser enter the lake and algae and water plants grow rapidly, forming a bloom that covers the surface (3 marks). (b) The algae die and are decomposed by bacteria that use up dissolved oxygen, and the bloom shades underwater plants so photosynthesis falls, so oxygen levels drop further (3 marks). (c) Use less chemical fertiliser and more organic manure, keep buffer strips and wetlands to trap nutrients, and improve sewage treatment to remove nitrogen and phosphorus (3 marks). Any two of these measures are acceptable.
12	(a) Explain the meaning of ecological succession and give one example starting from bare ground. (b) Describe the changes in species diversity and biomass during succession. (c) Explain why a late-succession community is more stable.	(a) Succession is the orderly change of a community over time from simple to more complex; for example bare rock is first colonised by lichens and mosses, then grasses, shrubs and finally forest (3 marks). (b) Species diversity increases and biomass and food-web complexity rise (3 marks). (c) A late community has more species and a complex food web with stronger negative feedback, so it resists environmental change better and is more stable (2 marks).

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A TutorZone practice paper; original questions and passages, not publisher material.