IGCSE Biology Paper 4 (Theory / Extended)

Topic 20: Human Influences on Ecosystems -- Challenge Exam 3

Duration1 hour 15 minutes

Total Marks80

Questions7

Time Remaining75:00

Syllabus0610

Instructions

Answer all questions in the spaces provided.
Show all working for calculations.
Use appropriate scientific terminology.
Your answers will be automatically graded when you submit.

Question Navigation

This paper covers the whole of Topic 20. Like a real Cambridge paper it ranges across every sub-topic — 20.1 food supply, 20.2 habitat destruction, 20.3 pollution and 20.4 conservation — and it mixes them inside single questions, so one question may begin with a farm and end with a fishery. All three Topic 20 papers do this; they differ in the angle they come at the material from, not in what they cover.

Question 1 — Field P and Field Q

Total: 12 marks

Fig. 1.1 shows two ways of planting the same area of farmland. Field P carries a single crop species. Field Q carries several species and has a hedge along one side. The two fields are the same size and are on the same soil.

(a) [4]

Field P is a large-scale monoculture. Give two advantages and two disadvantages of growing a crop in this way.

Model Answer — 1(a)

advantage: sowing, spraying and harvesting can all be done by machine because the whole field ripens together, so the labour needed per tonne of crop is low [1]

advantage: the whole field can be treated with one fertiliser and one pesticide, which is simpler and gives a high yield per hectare [1]

disadvantage: a pest or a disease of that one species can spread through the entire field, because every plant is suitable for it [1]

disadvantage: few habitats and little variety of food, so biodiversity is low; and the same mineral ions are removed year after year, so more fertiliser is needed [1]

⚠ If you missed marks here: The examiner marks the two halves separately, so an answer that lists four disadvantages scores two marks out of four. A monoculture is not a mistake — it is how most of the world’s grain is grown, for reasons that are worth stating properly before you criticise it.

1 mark — advantage: machinery can be used throughout / low labour per tonne 1 mark — advantage: one treatment for the whole field / high yield per hectare 1 mark — disadvantage: a pest or disease spreads through the whole field 1 mark — disadvantage: low biodiversity, or the same ions removed each year

(b) [3]

Explain how the hedge beside field Q may reduce the amount of insecticide that the farmer needs to use.

Model Answer — 1(b)

the hedge provides a habitat, food and shelter for many more species than the crop alone [1]

these include the natural predators of the insect pests of the crop [1]

the predators feed on the pests, so pest numbers stay lower and less insecticide is needed [1]

⚠ If you missed marks here: Three linked steps: more habitats, therefore predators, therefore fewer pests. An answer that stops at “the hedge increases biodiversity” has not connected anything to the insecticide. Note the honest limit of the argument — the hedge also takes up land that could have grown crop.

1 mark — the hedge provides habitat and food for more species 1 mark — including predators of the crop pests 1 mark — predators reduce pest numbers, so less insecticide is needed

The farmer of field P finds that he has to add more fertiliser every year to obtain the same yield. Explain why, and suggest one change he could make.

Model Answer — 1(c)

the same species is grown every year and removes the same mineral ions from the soil [1]

the crop is harvested and taken away, so those ions are not returned to the soil by decomposition as they would be in a natural ecosystem [1]

he could grow a different crop in alternate years, or grow a legume such as beans or clover, which returns nitrogen-containing compounds to the soil [1]

⚠ If you missed marks here: The reason the soil runs down is that the harvest leaves the field. In a wild grassland the plants die where they grew and decomposers return their ions to the soil; a harvested crop breaks that cycle. Your suggestion has to follow from the explanation — “use more fertiliser” is not a change, it is the problem.

1 mark — the same species removes the same mineral ions each year 1 mark — the crop is removed at harvest, so the ions are not returned 1 mark — a named change: rotating the crop, or growing a legume

(d) [2]

A student writes: “Field Q has greater biodiversity because it contains more plants.” Explain what is wrong with this statement, and write a corrected version of it.

Model Answer — 1(d)

biodiversity is the number of different species in an area, not the number of individual plants; the two fields hold a similar number of plants [1]

corrected: field Q has greater biodiversity because it contains more different species — several crop species, plus the species living in the hedge [1]

⚠ If you missed marks here: Rewriting the sentence is part of the question and carries its own mark, so do not stop at saying that the student is wrong. This precise error — counting individuals when the definition asks for species — appears somewhere in almost every Topic 20 paper.

1 mark — biodiversity counts different species, not individuals 1 mark — a corrected statement referring to the number of different species

Question 2 — A Fishery That Is Running Down

Total: 12 marks

The catch of a coastal fishing community has fallen every year for a decade. The community depends on the fishery for both food and income. Fig. 2.1 shows two nets, R and S, that are available to it.

(a) [3]

The community wants the fishery to become sustainable. Explain what a sustainable resource is, and why the definition has to compare two rates rather than name an amount.

Model Answer — 2(a)

a sustainable resource is one that is produced as rapidly as it is removed from the environment, so that it does not run out [1]

a large catch can be sustainable if the fish reproduce just as quickly [1]

and a small catch can be unsustainable if the fish reproduce more slowly still, so the amount taken means nothing until it is compared with the rate of replacement [1]

⚠ If you missed marks here: The definition is the most quotable sentence in this topic and it is worth learning word for word. The second and third marks reward understanding rather than recall: the same tonnage can be sustainable in one sea and ruinous in another, which is why no number can ever be quoted as a safe catch on its own.

1 mark — produced as rapidly as it is removed, so it does not run out 1 mark — a large catch may be sustainable if replacement is equally fast 1 mark — a small catch may be unsustainable if replacement is slower

(b) [4]

The community must choose between net R and net S, and separately between closing the fishery completely for one year and setting a quota. Choose one option from each pair and justify your choice.

Model Answer — 2(b)

net S, the larger mesh [1]

because the young fish that have not yet bred pass through it and survive to reproduce, replacing the fish that are taken, while the adult catch falls only slightly [1]

a quota, justified: fishing continues so the community keeps some food and income, while the mass taken is held below the rate of replacement [1]

or a closed year, justified: no fish at all are removed, so the stock recovers faster — but the community loses its income entirely for a year and the rule may be broken; either choice earns the mark if the reason is given [1]

⚠ If you missed marks here: Both marks in each pair depend on the justification, not on the choice, and the second option here has no single right answer. Say what your choice achieves biologically and what it costs the people — a choice defended on one of those grounds alone reads as an opinion rather than an evaluation.

1 mark — net S chosen 1 mark — justified by young fish escaping to breed, with little loss of adult catch 1 mark — a quota or a closed year chosen 1 mark — justified with both what it achieves and what it costs the community

Describe what monitoring and education would each contribute to this plan.

Model Answer — 2(c)

monitoring: the mass and the size of the fish caught are recorded regularly over several years [1]

so it can be seen whether the stock is rising or falling, and the quota can be adjusted rather than guessed [1]

education: the fishers understand why the rules exist and what happens without them, so they are far more likely to follow rules that cost them something in the short term [1]

⚠ If you missed marks here: Monitoring is measurement over time, not a single survey, and its purpose is to make the other measures adjustable. Education is easy to dismiss, but every one of these measures depends on people choosing to obey a rule that nobody can enforce at sea.

1 mark — monitoring: regular records of the mass and size of the catch 1 mark — so measures can be judged and quotas adjusted 1 mark — education: people understand the reasons and are more likely to comply

(d) [2]

Suggest why setting up a protected area might not, on its own, conserve a species of fish that moves along the coast during the year.

Model Answer — 2(d)

the fish spend only part of the year inside the protected area [1]

and can be caught freely outside it, so the protection applies for part of the year only and the total removed may still exceed the rate of replacement [1]

⚠ If you missed marks here: A conservation method is not judged in the abstract but against the biology of the species it is meant to protect. A protected area works well for a species that stays put — a coral reef fish, for instance — and much less well for one that does not.

1 mark — the fish are inside the protected area for only part of the year 1 mark — they can be caught outside it, so removal may still exceed replacement

Question 3 — A Permit to Clear a Hillside

Total: 12 marks

A government has been asked for permission to clear the forest from a hillside and use the land for cattle. Fig. 3.1 shows a nearby hillside photographed before clearance and eight years afterwards. In the right-hand panel, A marks the atmosphere above the cleared ground and E is one of the stumps left after felling.

(a) [4]

Discuss the advantages and disadvantages to the country of clearing this hillside. Give at least one of each.

Model Answer — 3(a)

advantage: the land can be used for livestock production, increasing the food supply, and the timber can be sold [1]

advantage: the work provides employment and income, and land is released for housing or for crops [1]

disadvantage: biodiversity falls as habitats are destroyed, and a species living only in that forest may become extinct [1]

disadvantage: the soil is washed off the bare slope and the land becomes poor within a few years, water runs off instead of being taken up by roots so flooding increases downstream, and the carbon dioxide concentration of the atmosphere rises [1]

⚠ If you missed marks here: Both sides again, and the advantages are genuine — a country that clears forest is usually feeding or housing people, not acting carelessly. Notice that one of the disadvantages is also an economic argument: the pasture created here is likely to be poor within a few years because the soil goes with the rain.

1 mark — advantage: land for livestock or crops / increased food supply 1 mark — advantage: timber sold, employment or land for housing 1 mark — disadvantage: reduced biodiversity or extinction of a species 1 mark — disadvantage: loss of soil, flooding, or increased atmospheric carbon dioxide

(b) [3]

On a hillside like the one in Fig. 3.1, soil is lost at 12 tonnes per hectare each year and new soil forms at 0.5 tonnes per hectare each year. Calculate how many times faster the soil is lost than it is formed, and use the definition of a sustainable resource to comment on this use of the land.

Model Answer — 3(b)

12 ÷ 0.5 = 24 times faster [1]

the soil is being removed far faster than it is produced, so it is not being used sustainably [1]

the soil will run out: the depth of soil falls every year, so the land will support less and less growth and the pasture will fail [1]

⚠ If you missed marks here: Soil is a resource like any other and the same definition applies to it. The arithmetic is easy; the mark that is lost is the third one, which asks what the number means for the future of the land rather than simply repeating that it is unsustainable.

1 mark — 24 times faster, with working 1 mark — removal is faster than production, so it is not sustainable 1 mark — the soil will run out / the land will support less growth each year

A newspaper states that clearing the forest is dangerous because “it removes the oxygen from the air so that people cannot breathe”. Explain why this is not the effect on the atmosphere marked A in Fig. 3.1 that you have been taught, and state what that effect is.

Model Answer — 3(c)

the effect described is an increase in the concentration of carbon dioxide in the atmosphere, not a shortage of oxygen [1]

fewer trees photosynthesise, so less carbon dioxide is removed from the air [1]

and burning the felled trees, or their decomposition by decomposers, releases carbon dioxide into it [1]

⚠ If you missed marks here: This is a popular claim and it is not the one you are marked on. Answer the question in terms of carbon dioxide, and give the two-part reason — less removed and more released — because a single reason scores a single mark however well it is written.

1 mark — the named effect is an increase in atmospheric carbon dioxide 1 mark — less photosynthesis, so less carbon dioxide removed 1 mark — combustion or decomposition of the felled trees releases carbon dioxide

(d) [2]

Suggest two conditions the government could attach to the permit that would reduce the undesirable effects.

Model Answer — 3(d)

require an equal area to be replanted, or limit the area that may be cleared in any year [1]

require strips of trees to be left along the river and on the steepest ground, so that roots hold the soil and less water runs off; or set part of the forest aside as a protected area [1]

⚠ If you missed marks here: Suggestions must follow from the effects you named earlier, so tie each condition to the effect it reduces. “Do not allow it” is not a condition on a permit, and an answer that refuses the question rather than answering it scores nothing.

1 mark — replanting required, or a limit on the area cleared 1 mark — trees left on slopes and beside the river, or an area protected

Question 4 — Two Polluted Lakes

Total: 12 marks

Two lakes are the same size and were once the same. Lake 1 now receives untreated sewage from a town. Lake 2 receives no sewage, but a large amount of non-biodegradable plastic waste has been dumped in it over many years.

(a) [3]

Compare the ways in which untreated sewage and non-biodegradable plastic affect an aquatic ecosystem.

Model Answer — 4(a)

sewage supplies organic material and releases ions, so the growth of producers and the number of decomposers both increase [1]

the decomposers respire aerobically and reduce the dissolved oxygen, so organisms that need dissolved oxygen die — the harm is done indirectly, through the oxygen [1]

plastic is not broken down at all; it harms animals physically by filling the gut so they feed less, and by entangling larger animals [1]

⚠ If you missed marks here: The contrast is between an indirect chemical route and a direct physical one. Both are commonly written as “it kills the fish”, which distinguishes nothing. Sewage kills through the oxygen; plastic kills through the gut and by entanglement.

1 mark — sewage supplies organic material and ions, increasing producers and decomposers 1 mark — decomposers use the dissolved oxygen, so organisms needing it die 1 mark — plastic is not broken down and harms animals physically

(b) [3]

Suggest why lake 1 could largely recover within a few months if the sewage discharge were stopped, while lake 2 would not recover even if all dumping stopped.

Model Answer — 4(b)

the sewage already in lake 1 is broken down by decomposers, so once no more arrives the decomposers decline and stop using so much oxygen [1]

oxygen then re-enters the water from the air and from photosynthesis, so the dissolved oxygen concentration rises again and organisms can return [1]

the plastic in lake 2 cannot be broken down by decomposers, so nothing removes it; it stays in the lake and keeps harming animals for as long as it remains [1]

⚠ If you missed marks here: The difference is whether anything can remove the pollutant. Do not write that the plastic will “eventually dissolve” or “break down in the end” — it breaks into smaller pieces, which is not the same thing at all, and those smaller pieces are more easily swallowed.

1 mark — decomposers break down the remaining sewage, so oxygen use falls 1 mark — oxygen re-enters from the air and from photosynthesis 1 mark — plastic cannot be broken down by decomposers, so nothing removes it

Evaluate the claim that, for the survival of the fish living in them, the sewage in lake 1 is the more urgent problem of the two.

Model Answer — 4(c)

in support: the fall in dissolved oxygen can kill a large number of fish in a very short time, by suffocation, whereas the plastic kills individual animals slowly [1]

in support: the sewage also carries the risk of disease to the people who use the lake [1]

against: the effect of the sewage is reversible — stopping the discharge allows the oxygen to recover — while the plastic is permanent and its mass grows every year [1]

against: the plastic affects animals at every stage of the food chain and is found in the soil on land as well, so its effects are wider; a supported overall judgement either way [1]

⚠ If you missed marks here: “Evaluate” asks for both sides and then a decision that follows from them. Answers that give four reasons for one side score half however strong they are. A judgement sentence at the end — naming which argument you find stronger and why — is what turns a list into an evaluation.

1 mark — for: the oxygen fall kills many fish quickly, by suffocation 1 mark — for: a further named effect of the sewage 1 mark — against: the sewage effect is reversible while the plastic is permanent 1 mark — against: the plastic affects the whole chain / a supported overall judgement

(d) [2]

Explain why a fish killed in lake 1 is described as having died of suffocation rather than of poisoning.

Model Answer — 4(d)

nothing toxic was added to the water; the decomposers used the dissolved oxygen for their aerobic respiration [1]

so the fish could not absorb enough oxygen across its gills to respire aerobically, and died from the lack of it [1]

⚠ If you missed marks here: Every year students write that the sewage, the fertiliser or the algae poisoned the fish, and every year it scores nothing. Say what was taken away rather than what was added, and name the process that took it: aerobic respiration by decomposers.

1 mark — nothing toxic added; decomposers used the dissolved oxygen 1 mark — the fish could not obtain enough oxygen at the gills to respire aerobically

Question 5 — Three Species, Three Decisions

Total: 10 marks

Table 5.1 describes three endangered species. A conservation organisation has enough money to fund one method for each of them.

species	description
J	a flowering plant that grows on one mountain and nowhere else; its seeds survive being dried and stored
K	a large mammal hunted for its horn; fewer than 200 remain, all inside one reserve
L	a fish that breeds in a single river during April and May, and is caught there throughout the year

(a) [6]

For each of species J, K and L, choose one conservation method and justify your choice.

Model Answer — 5(a)

J: a seed bank [1] because its seeds survive drying, so they can be stored for many years and used to grow new plants if the wild population on the mountain is lost [1]

K: monitoring and protecting the species and its habitat within the reserve, or a captive breeding programme [1] because the animals are killed by hunters for their horn, so protection inside the reserve keeps the remaining adults alive, while captive breeding with AI and IVF can raise numbers from very few animals [1]

L: a closed season covering April and May, or a protected area covering the river [1] because the fish are then left undisturbed while they breed, so the young are produced and the population can replace the fish taken during the rest of the year [1]

(one method mark and one justification mark for each species)

⚠ If you missed marks here: Each species is described in a way that points to one method, so read the description before choosing: seeds that survive drying, a species inside a reserve, a fish with a two-month breeding season. Any method can earn the marks if the justification fits the description, but a method with no justification earns half.

1 mark — J: seed bank chosen 1 mark — J: justified by seeds surviving storage / a reserve population if the wild one is lost 1 mark — K: protection of species and habitat, or captive breeding, chosen 1 mark — K: justified by hunting pressure or by raising numbers from few adults 1 mark — L: closed season or protected area chosen 1 mark — L: justified by allowing undisturbed breeding, so the population is replaced

(b) [2]

Explain why a captive breeding programme for species K would also require work inside the reserve.

Model Answer — 5(b)

animals bred in captivity must eventually be released into a habitat that still provides food, water and shelter [1]

if hunting continues, or the habitat has been destroyed, the released animals will be killed or will not survive, so the programme will not restore the wild population [1]

⚠ If you missed marks here: The idea being tested is that captive breeding is never enough by itself. Numbers can be raised in a zoo indefinitely without a single wild animal existing; the species is only conserved when there is somewhere safe for it to live.

1 mark — released animals need a habitat with food, water and shelter 1 mark — if hunting or habitat destruction continues, released animals will not survive

The population of species K has been below 200 for thirty years. Explain the risk that remains to it even if hunting is stopped completely.

Model Answer — 5(c)

a population that has stayed small has reduced genetic variation [1]

so there are fewer individuals with features that would allow them to survive a new disease or a change in the environment, and the whole population could still be lost [1]

⚠ If you missed marks here: “It is safe now that the hunting has stopped” is the answer to avoid. Variation lost while the population was small is not restored simply because numbers rise again, and it is variation, not numbers alone, that lets a population survive something new.

1 mark — a small population has reduced genetic variation 1 mark — fewer individuals able to survive a new disease or environmental change

Question 6 — Four Sentences That Lose Marks

Total: 12 marks

Precision matters in this topic: several of its definitions are marked word by word, and the versions below are the ones that appear most often in written work.

(a) [4]

Each statement below is wrong. For each one, explain what is wrong with it. (i) “Biodiversity is the number of animals living in an area.” (ii) “A sustainable resource is one that will never run out.” (iii) “Non-biodegradable means the material takes a very long time to rot.” (iv) “The greenhouse effect is a form of air pollution.”

Model Answer — 6(a)

(i) biodiversity counts the number of different species, not the number of individuals, and it includes plants, fungi and bacteria, not only animals [1]

(ii) it is not a property of the resource but of how it is used: it is produced as rapidly as it is removed, and the same resource is unsustainable if it is removed faster [1]

(iii) it means that decomposers cannot break it down at all, not that decomposition is slow [1]

(iv) the greenhouse effect is natural and keeps the Earth warm enough for life; what causes harm is the increase in carbon dioxide and methane that enhances it [1]

⚠ If you missed marks here: Each of these four is a real mark-loser, and each is wrong in a particular way rather than being vaguely imprecise. Learn the exact wording of the definitions: “different species”, “as rapidly as it is removed”, “decomposers cannot break it down”, “enhanced”.

1 mark — (i) biodiversity is the number of different species, not individuals 1 mark — (ii) sustainability compares removal with production, so it depends on use 1 mark — (iii) non-biodegradable means decomposers cannot break it down at all 1 mark — (iv) the greenhouse effect is natural; the extra gases enhance it

(b) [3]

Explain why the phrase “produced as rapidly as it is removed” cannot be replaced by “only a small amount is taken”.

Model Answer — 6(b)

sustainability is a comparison of two rates, not a statement about an amount [1]

a small amount taken is still unsustainable if the resource is replaced even more slowly than that [1]

and a large amount taken is sustainable if the resource is replaced just as quickly, which is why no single figure can be quoted as safe [1]

⚠ If you missed marks here: Give both directions of the argument — small but unsustainable, and large but sustainable. One direction alone leaves the point half made. This also explains why quotas have to be set separately for every fishery instead of being fixed nationally.

1 mark — sustainability compares two rates, not an amount 1 mark — a small removal can still exceed a slower replacement 1 mark — a large removal is sustainable if replacement is equally fast

In river X, 500 tonnes of fish are taken each year and the stock is falling. In river Y, 900 tonnes are taken each year and the stock is steady. Explain, using the idea of rate, which fishery is being overharvested.

Model Answer — 6(c)

river X is being overharvested [1]

because fish are being removed there faster than the population replaces them by reproduction, which is what overharvesting means [1]

the size of the catch alone does not decide it: river Y takes more, but its fish reproduce quickly enough to replace what is taken, so its stock is steady [1]

⚠ If you missed marks here: The trap is the pair of numbers, which invite you to pick the larger one. The stock behaviour is the evidence: a falling stock means removal is beating replacement, whatever the tonnage. Answers that name Y because 900 is bigger than 500 score nothing.

1 mark — river X identified 1 mark — removal there is faster than replacement by reproduction 1 mark — the size of the catch alone does not decide it, as river Y shows

(d) [2]

State the two ecosystem functions that the syllabus gives as reasons for conservation, and give one example of each.

Model Answer — 6(d)

nutrient cycling — for example decomposers returning carbon to the air and mineral ions to the soil, which keeps the soil fertile [1]

resource provision — food, drugs, fuel and genes; for example genes from a wild plant bred into a crop to make it resistant to a disease [1]

⚠ If you missed marks here: The second of these is the one people forget, and the word “genes” in it is the most valuable item on the list: a wild relative of a crop can carry a form of a gene that no cultivated variety has, and once the wild species is extinct that variation cannot be recovered.

1 mark — nutrient cycling, with an example 1 mark — resource provision of food, drugs, fuel or genes, with an example

Question 7 — One Valley, Thirty Years

Total: 10 marks

A river runs from farmland at the top of a valley, through a lake beside a village, and out to a fishery at the coast. Over the last thirty years the forest on the slopes has been cleared, the mass of fertiliser used on the farmland has doubled, the village has grown and discharges untreated sewage into the lake, and the catch at the fishery has fallen sharply.

(a) [6]

Explain how the changes described could together account for what has happened in the lake and at the fishery. Refer to at least three of the four sub-topics of this unit.

Model Answer — 7(a)

clearing the forest removes the roots that held the soil, so soil is washed downhill into the river and the lake [1]

less water is taken up by roots and less is intercepted by leaves, so more rain runs off the surface, carrying dissolved fertiliser into the water more quickly [1]

the extra nitrate and other ions increase the growth of the producers in the lake [1]

when those producers die, decomposition increases; the decomposers respire aerobically and the dissolved oxygen falls, and the untreated sewage adds more organic material for them to break down [1]

organisms that require dissolved oxygen, including the fish, die in the lake [1]

at the coast, fewer young fish come down the river to replace those caught, so removal at the fishery now exceeds replacement — overharvesting — and the catch falls [1]

⚠ If you missed marks here: Six marks means six separate linked steps, written in order. Work down the valley rather than writing everything you know about each sub-topic in turn: forest, then run-off, then ions, then producers, then decomposers and oxygen, then the fishery. A synthesis question is marked on the links, and a list of facts with no connections between them scores far below what the same facts would earn in sequence.

1 mark — soil washed downhill because roots no longer hold it 1 mark — more surface run-off, carrying fertiliser into the water faster 1 mark — extra ions increase the growth of producers in the lake 1 mark — decomposition and aerobic respiration by decomposers reduce the dissolved oxygen 1 mark — organisms requiring dissolved oxygen die 1 mark — fewer young fish replace those caught, so the fishery is overharvested

(b) [4]

Suggest one measure for this valley aimed at each of: food supply, habitat destruction, pollution and conservation. In each case state what the measure would achieve.

Model Answer — 7(b)

food supply: apply only as much fertiliser as the crop can take up, and apply it when the crop is growing, so that less is left to be washed into the river [1]

habitat destruction: replant trees on the slopes, so that roots hold the soil, less water runs off and the area regains habitats and species [1]

pollution: treat the sewage from the village before it is discharged, so that the organic material is broken down before it reaches the lake and the decomposers there do not use up the dissolved oxygen [1]

conservation: set a quota or a closed season at the fishery and monitor the stock, so that the catch is held below the rate of replacement [1]

⚠ If you missed marks here: One measure per sub-topic, each with its effect — a measure named without a consequence earns nothing. Keep them separated: two measures about fertiliser leave one sub-topic unanswered, however good both are.

1 mark — food supply: match the fertiliser to the crop, so less is washed away 1 mark — habitat: replant the slopes, so roots hold the soil and habitats return 1 mark — pollution: treat the sewage, so decomposers in the lake do not use up the oxygen 1 mark — conservation: quota, closed season or monitoring, so the catch stays below replacement

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