Section 9 — Solved Examples
This is the dedicated problem set for Chapter 12 (Ecosystem). The 30 worked examples below are arranged in three tiers — concept checks (definitions, components, the four functions, key terms), application and scenarios (identify-the-trophic-level, identify-the-pyramid, decomposition and food-chain situations) and analytical and numerical (productivity sums using , energy transfer down a chain by the ten per cent law, and multi-concept reasoning that ties the whole chapter together).
Ecosystem questions rarely stop at naming a term. You are usually asked to place an organism at its trophic level, decide whether a pyramid is upright or inverted, or calculate how much energy or biomass survives a transfer. That is exactly the skill this section drills.
How to use this section
- Concept Checks (Q1–Q10): quick recall of structure, productivity, decomposition, energy flow and pyramids. If more than one or two trip you up, revisit Sections 1–8.
- Application and Scenarios (Q11–Q20): the workhorse 2- and 3-mark questions — trophic-level and pyramid identification, food-chain roles, decomposition control.
- Analytical and Numerical (Q21–Q30): the productivity and ten-per-cent-law sums, plus reasoning problems that separate a good answer from a full-mark one.
Total target time: around an hour for a complete revision sweep.
Note: this section is for practice and revision only — there is no quiz at the end. Treat each answer as a model of how much detail an examiner expects.
Memory Capsule — Facts Worth Locking In
Before working through the problems, fix these ten high-yield facts firmly in mind:
| # | Fact | Where it is tested |
|---|---|---|
| 1 | An ecosystem is a functional unit of nature; two categories — terrestrial and aquatic; crop field and aquarium are man-made | Definitions |
| 2 | Four functions: productivity, decomposition, energy flow, nutrient cycling | Structure and function |
| 3 | Two structural features: species composition and stratification (vertical layering) | Structure questions |
| 4 | Productivity relationship — ; NPP is the biomass available to heterotrophs | Numerical sums |
| 5 | Units — production in ; productivity as a rate in | Units and calculations |
| 6 | Annual NPP of the biosphere is about 170 billion tons (dry weight); oceans give only about 55 billion tons despite covering about 70 per cent of the surface | Data recall |
| 7 | Decomposition steps — fragmentation, leaching, catabolism, humification, mineralisation; the first three act simultaneously; largely oxygen-requiring | Process questions |
| 8 | Ten per cent law — only about 10 per cent of energy passes to the next trophic level; this limits chain length | Energy-transfer sums |
| 9 | Pyramid of energy is always upright; pyramids of number and biomass can be inverted (insects on a tree; fish over phytoplankton in the sea) | Identify-the-pyramid |
| 10 | A trophic level is a functional level, not a species — a sparrow is a primary consumer eating seeds, a secondary consumer eating insects | Trophic-level logic |
Pro tip: for a numerical, always write the formula first, substitute the numbers with their units, then state the answer with its unit. For an identify question, name the pyramid or trophic level and give the one-line reason — examiners reward the reason.
Easy — Concept Checks (Q1–Q10)
Q1. Define an ecosystem and name its two basic categories with one example of each.
Answer: An ecosystem is a functional unit of nature in which living organisms interact among themselves and with their surrounding physical environment. The two basic categories are terrestrial (for example, a forest) and aquatic (for example, a pond). A crop field and an aquarium are man-made ecosystems.
Q2. List the four functions that make an ecosystem work as a single unit.
Answer: Productivity, decomposition, energy flow and nutrient cycling. Together these tie the abiotic and biotic components into one working system — input (productivity), transfer (energy flow through food chains and webs) and output (degradation and energy loss).
Q3. What is meant by species composition and stratification?
Answer: Species composition is the identification and enumeration of the plant and animal species present in an ecosystem. Stratification is the vertical distribution of different species at different levels — for example, in a forest trees occupy the top layer, shrubs the second, and herbs and grasses the bottom layers.
Q4. In what units are primary production and primary productivity expressed, and why does productivity carry a time unit?
Answer: Primary production is an amount, expressed as weight () or energy (). Primary productivity is a rate, so it carries a time unit and is written as or . Expressing it as a rate lets us compare different ecosystems on a common footing.
Q5. Write the relationship between gross primary productivity, respiration and net primary productivity, and state which one is available to heterotrophs.
Answer: where GPP is gross primary productivity, R is the respiration losses of the plants, and NPP is net primary productivity. It is the NPP that is available for consumption by heterotrophs — the herbivores and the decomposers.
Q6. Define secondary productivity.
Answer: Secondary productivity is the rate of formation of new organic matter by consumers. Just as producers build biomass through photosynthesis, consumers build their own tissue from the food they assimilate, and the rate at which they do so is the secondary productivity.
Q7. Name the five steps of decomposition and state the raw material on which they act.
Answer: The five steps are fragmentation, leaching, catabolism, humification and mineralisation — of these, fragmentation, leaching and catabolism operate simultaneously on the detritus. The raw material is detritus — dead plant remains such as leaves, bark and flowers, together with dead animal remains, including faecal matter.
Q8. What is humus, and why is it called a reservoir of nutrients?
Answer: Humus is a dark-coloured amorphous substance formed during humification. It is highly resistant to microbial action and decomposes very slowly. Being colloidal, it holds nutrients in the soil and so serves as a reservoir of nutrients, which are freed later by mineralisation.
Q9. What fraction of incident solar radiation is photosynthetically active radiation, and how much of that do plants capture?
Answer: Less than 50 per cent of the incident solar radiation is photosynthetically active radiation (PAR). Of this PAR, plants capture only about 2 to 10 per cent, and yet this small fraction is enough to sustain the entire living world.
Q10. State the ten per cent law of energy transfer.
Answer: The ten per cent law states that only about 10 per cent of the energy present at one trophic level is transferred to the next higher trophic level; the rest is lost, largely as heat. Because so little energy survives each transfer, the number of trophic levels in a food chain is limited.
Medium — Application and Scenarios (Q11–Q20)
Q11. A small pond is studied as a model ecosystem. Assign its main organisms and materials to the abiotic component, the autotrophs, the consumers and the decomposers.
Answer: Abiotic component — the water with its dissolved inorganic and organic substances and the rich soil at the bottom, regulated by solar input and climate. Autotrophs — phytoplankton, some algae and the floating, submerged and marginal plants. Consumers — zooplankton and the free-swimming and bottom-dwelling animals. Decomposers — fungi, bacteria and flagellates, especially abundant at the bottom.
Q12. In which type of ecosystem does the grazing food chain carry most of the energy, and in which does the detritus food chain dominate?
Answer: In an aquatic ecosystem the grazing food chain (GFC) is the major conduit for energy flow. In a terrestrial ecosystem the picture is reversed — a much larger fraction of energy flows through the detritus food chain (DFC), because the fallen leaves, dead wood and animal remains of the forest floor feed a very busy decomposer pathway.
Q13. In the food chain Grass -> Goat -> Man, identify the trophic level and role of each organism.
Answer: Grass is the producer, first trophic level. The goat is the primary consumer (herbivore), second trophic level. Man here is the secondary consumer (primary carnivore), third trophic level. Each organism is placed by the source from which it draws its energy.
Q14. A sparrow eats seeds in the morning and insects in the afternoon. Identify its trophic level in each case and state the principle this illustrates.
Answer: When it eats seeds the sparrow is a primary consumer (second trophic level); when it eats insects it is a secondary consumer (third trophic level). This illustrates that a trophic level is a functional level, not a species — the same species can occupy more than one trophic level at the same time, depending on what it is eating.
Q15. Two leaf litters are compared: litter A is rich in lignin and chitin, litter B is rich in nitrogen and sugars. In which will decomposition be faster, and what two conditions further favour it?
Answer: Decomposition is faster in litter B, because detritus rich in nitrogen and water-soluble substances like sugars breaks down quickly, whereas litter rich in lignin and chitin decomposes slowly. A warm and moist environment further favours decomposition, while low temperature and anaerobiosis inhibit it and cause organic matter to build up.
Q16. A tree is fed upon by thousands of insects, which in turn are eaten by a few birds. Which ecological pyramid does this represent, and what is its shape?
Answer: This is a pyramid of numbers, and it is inverted. A single large producer (the tree) supports a huge number of primary consumers (insects), so the base is narrow and the level above it is broad. It is one of the recognised exceptions to the usual upright pyramid of numbers.
Q17. In the open sea, the biomass of fish greatly exceeds that of the phytoplankton that support them. Which pyramid is described, and is it upright or inverted?
Answer: This is the pyramid of biomass, and in the sea it is generally inverted. The small standing crop of rapidly reproducing phytoplankton supports a much larger standing crop of fish, so the biomass at the higher level exceeds that at the base, giving an inverted shape.
Q18. Why is the pyramid of energy always upright and never inverted, whatever the ecosystem?
Answer: At every transfer of energy from one trophic level to the next, some energy is always lost as heat. A higher trophic level can therefore never contain more energy than the level below it. Because energy only decreases upward, the pyramid of energy is always upright and can never be inverted, unlike the pyramids of number and biomass.
Q19. State two limitations of ecological pyramids and explain why decomposers pose a problem for them.
Answer: Ecological pyramids assume a simple food chain and therefore cannot accommodate a food web, which is what actually exists in nature. They also ignore a species that belongs to two or more trophic levels at once. In addition, they give no place to the saprophytes (decomposers), even though these organisms play a vital role in the ecosystem, because a decomposer cannot be slotted neatly into a single feeding level.
Q20. Why is the standing crop of a trophic level measured as dry weight rather than fresh weight?
Answer: The standing crop is the mass of living material at a trophic level at a given time, measured as biomass or number per unit area. Biomass can be given as fresh or dry weight, but dry weight is more accurate, because the water content of living tissue varies a great deal and would distort any comparison based on fresh weight.
Hard — Analytical and Numerical (Q21–Q30)
Q21. In a grassland, the gross primary productivity is and the plants lose in respiration. Calculate the net primary productivity and state what it represents.
Answer: Using substitute the values: . So the net primary productivity is , and this is the biomass available for consumption by heterotrophs — the herbivores and the decomposers.
Q22. The net primary productivity of a field is . Find the total organic matter fixed each year over a plot of area .
Answer: Total production equals productivity multiplied by area: , which is , or 1 tonne of dry organic matter per year. Multiplying a rate per unit area by the area gives the total for the whole plot.
Q23. Producers in a food chain trap of energy. Applying the ten per cent law, work out the energy available to the herbivores, the primary carnivores and the secondary carnivores.
Answer: Only about 10 per cent passes to each higher level. Herbivores: . Primary carnivores: . Secondary carnivores: . So the energy falls along the chain, which is why food chains are short.
Q24. A secondary carnivore at the fourth trophic level obtains of energy. Assuming the ten per cent law, how much energy did the producers originally fix?
Answer: Working backwards, each lower level holds ten times as much energy. Primary carnivore level: . Herbivore level: . Producer level: . So the producers originally fixed of energy to leave at the top of a four-level chain.
Q25. The annual net primary productivity of the whole biosphere is about 170 billion tons of dry organic matter, yet the oceans contribute only about 55 billion tons despite covering roughly 70 per cent of the surface. Comment on this apparent paradox.
Answer: Although the oceans cover about 70 per cent of the surface, their net primary productivity is only about 55 billion tons, so the great majority of the biosphere's 170 billion tons is produced on land. The mismatch reflects the fact that primary productivity depends on the plant species present, environmental factors, availability of nutrients and the photosynthetic capacity of the producers — and in much of the open ocean the shortage of nutrients keeps productivity low despite the vast area and abundant light.
Q26. A grassland pyramid of numbers shows nearly 6 million plants at the base supporting, through intermediate levels, only about 3 top carnivores. What does this illustrate about energy and pyramid shape?
Answer: The steep fall from 6 million producers to only 3 top carnivores illustrates that the amount of energy decreases sharply at each successive trophic level, following the ten per cent law. Because far fewer individuals can be supported at each higher level, this pyramid of numbers is upright, and the same energetic loss is why only a handful of top carnivores can exist and why chains stay short.
Q27. Distinguish the grazing food chain from the detritus food chain by their starting point, their main organisms and the ecosystem in which each dominates.
Answer: The grazing food chain (GFC) starts with producers (green plants), runs through herbivores and carnivores, and is the main energy route in aquatic ecosystems. The detritus food chain (DFC) starts with dead organic matter (detritus), is made up of decomposers (saprotrophs, mainly fungi and bacteria) that secrete enzymes to break dead matter into simple inorganic substances, and carries a much larger fraction of energy in terrestrial ecosystems. The two are interconnected through shared prey and omnivores such as cockroaches and crows, forming a food web.
Q28. Explain, using the ten per cent law, why grazing food chains in nature rarely have more than four or five trophic levels.
Answer: Because only about 10 per cent of the energy at one level is passed to the next, the energy available shrinks tenfold at every step — for example . After only a few transfers the remaining energy is too small to support another viable trophic level, so the chain cannot extend indefinitely. This energetic limit is the reason natural food chains are short.
Q29. Living systems are highly ordered, yet the universe tends toward disorder. Explain, with reference to the laws of thermodynamics, why an ecosystem needs a constant input of solar energy.
Answer: Energy flow obeys the first law of thermodynamics — energy is neither created nor destroyed, only transformed, so sunlight is converted into the chemical energy of food and passed one way from producers to consumers. Ecosystems also obey the second law: the universe tends toward increasing disorder (entropy). To build and maintain their complex, ordered molecules against this pull, organisms must spend energy continuously. Hence a constant supply of solar energy is required; cut it off and the ordered structure of life quickly breaks down.
Q30. A dead animal falls on a forest floor. Trace the fate of its body and its stored nutrients through decomposition, naming each step.
Answer: The dead body becomes detritus, the raw material for decomposition. In fragmentation, detritivores such as earthworms break it into smaller particles. In leaching, water-soluble inorganic nutrients wash down into the soil and may precipitate as unavailable salts. In catabolism, bacterial and fungal enzymes degrade the detritus into simpler inorganic substances. Humification builds up dark, colloidal humus that stores nutrients, and finally mineralisation degrades the humus to release inorganic nutrients back into the soil for reuse by the producers. Fragmentation, leaching and catabolism proceed simultaneously; humification and mineralisation then take place during decomposition in the soil.
End of Section 9
You have now worked through 30 examples spanning the structure of the ecosystem, productivity and the relationship, decomposition, energy flow and the laws of thermodynamics, trophic levels and standing crop, food chains, food webs and the ten per cent law, and the three ecological pyramids. For every numerical, write the formula, substitute with units, and state the answer with its unit; for every identify question, name the level or pyramid and give the reason.