Topic 20: Human Influences on Ecosystems -- Challenge Exam 1
1 hour 15 minutes
80
7
75:00
0610
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 — A Sequence With Three Boxes Missing
Total: 12 marks
A lake receives water that has drained from farmland treated with fertiliser. Fig. 1.1 shows the sequence of events that follows. Boxes 2, 4 and 5 have been left blank and are labelled (i), (ii) and (iii).
(a)[3]
State the event that belongs in each of the blank boxes (i), (ii) and (iii) in Fig. 1.1.
Model Answer — 1(a)
(i) increased growth of the producers [1]
(ii) increased aerobic respiration by the decomposers [1]
(iii) reduction in the dissolved oxygen concentration of the water [1]
⚠ If you missed marks here: The six steps are marked as a sequence, so each one has to name the stage that comes at that point and nothing else. Box 4 is the step most often lost: it is the decomposers respiring, not the algae dying, because the dying was box 3. Write “aerobic respiration” rather than just “respiration” — the whole chain turns on oxygen being used.
(b)[3]
Explain why an increase in the concentration of nitrate ions in the water increases the growth of the producers.
Model Answer — 1(b)
nitrate ions are absorbed by the producers [1]
and are used to make amino acids, which are joined together to make proteins [1]
proteins are needed to make new cells, so the producers can grow and reproduce faster; nitrate was previously the factor limiting their growth [1]
⚠ If you missed marks here: Do not stop at “nitrate is a nutrient they need” — that is the question restated, not an answer. Name what the ion becomes: amino acids, then protein. And say why the extra supply matters: growth was limited by the shortage before the fertiliser arrived.
(c)[3]
Explain fully why the fish in this lake die at stage 6.
Model Answer — 1(c)
the decomposers use the dissolved oxygen in the water for their aerobic respiration [1]
so there is too little dissolved oxygen left for the fish to absorb across their gills [1]
the fish cannot respire aerobically, cannot release enough energy, and die of suffocation — nothing toxic has been added to the water [1]
⚠ If you missed marks here: This is the single most tested idea in the topic and the wrong version is very common: the fish are not poisoned, by the fertiliser or by the algae. Nothing in the lake is toxic. The word the examiner is looking for is suffocation, and the mark before it is for saying who used the oxygen up.
(d)[3]
Untreated sewage released into the same lake produces the same sequence of events. Explain why.
Model Answer — 1(d)
untreated sewage contains a large amount of organic material, which decomposers feed on, so decomposer numbers rise [1]
those decomposers respire aerobically and use up the dissolved oxygen, exactly as at stages 4 and 5 [1]
decomposition of the sewage also releases nitrate and other ions into the water, so the growth of producers increases as well [1]
⚠ If you missed marks here: Sewage joins the sequence at two places, and an answer that finds only one of them scores one mark. It feeds the decomposers directly, and it also releases ions that feed the producers. Note also that it is untreated sewage that does this — treated sewage has had the organic material broken down already.
Question 2 — Eight Years After the Trees Came Down
Total: 12 marks
Fig. 2.1 shows the same area of hillside photographed eight years apart. In the right-hand panel, B is a set of lines falling onto bare ground, C is an arrow drawn along the ground surface pointing down the slope, and D is a shaded area at the foot of the slope.
(a)[4]
Name the undesirable effect of deforestation shown by C, and the one shown by D. In each case explain how the removal of the trees caused it.
Model Answer — 2(a)
C shows loss of soil [1]
because the tree roots that held the soil together have gone, so rain washes the soil down the slope [1]
D shows flooding [1]
because less water is taken up by roots and less is intercepted by the leaves, so more of the rain runs off the surface into the river [1]
⚠ If you missed marks here: Two marks here are for naming and two are for the mechanism, so a bare list of the five effects scores half. The flooding mechanism has two parts worth learning as a pair — uptake by roots and interception by leaves. “The trees used to soak up the rain” is the vague version that examiners will not credit.
(b)[3]
Explain why removing the trees increases the concentration of carbon dioxide in the atmosphere. Two different reasons are required.
Model Answer — 2(b)
there are fewer trees photosynthesising, so less carbon dioxide is removed from the air [1]
and the felled trees are burned, so combustion releases carbon dioxide [1]
or the remains are broken down by decomposers, which respire and release carbon dioxide [1]
⚠ If you missed marks here: Cambridge wants both halves of this answer and marks them separately: less taken out AND more put in. Almost everyone writes the photosynthesis half and stops. Note also that the felled trees do not simply vanish — whether they are burned or left to decompose, their carbon returns to the air.
(c)[3]
State what is meant by biodiversity, and explain why deforestation can cause the extinction of a species rather than only a fall in its numbers.
Model Answer — 2(c)
biodiversity is the number of different species that live in an area [1]
a species found only in that forest loses the whole of its habitat, so it has nowhere else to live [1]
when the last individuals die there are none left anywhere, so the species is lost permanently — that is extinction [1]
⚠ If you missed marks here: Biodiversity is a count of species, never a count of individuals and never “how many animals there are”. For the extinction marks, the argument turns on the species being found nowhere else; a species that also lives in the next valley loses numbers but does not become extinct.
(d)[2]
State two reasons, other than obtaining timber, why humans destroy habitats.
Model Answer — 2(d)
to increase the area available for housing, or for crop plant production, or for livestock production [1]
to allow the extraction of natural resources such as minerals [1]
⚠ If you missed marks here: The syllabus gives a short closed list, so answer from it: land for housing, crops or livestock; extraction of natural resources; and freshwater and marine pollution. Two examples of the same reason — “for houses” and “for towns” — count once between them.
Question 3 — Two Panels, Eighty Years Apart
Total: 12 marks
Fig. 3.1 shows energy entering and leaving the atmosphere in two different years. The circles represent molecules of two gases, G1 and G2. The two panels are eighty years apart.
(a)[3]
Name radiation P and radiation Q, and name the two gases that the syllabus refers to as G1 and G2.
Model Answer — 3(a)
P is short-wave radiation arriving from the Sun [1]
Q is long-wave radiation leaving the Earth’s surface [1]
the two gases are carbon dioxide and methane [1]
⚠ If you missed marks here: Read the arrow directions before naming anything: P points down towards the surface and Q points up away from it. The syllabus names only two gases for this effect, so do not add water vapour or any other gas to the list — extra wrong names can cost you the mark.
(b)[3]
Describe what has changed between year 1 and year 2 in Fig. 3.1, and explain the effect of that change on the Earth’s surface.
Model Answer — 3(b)
there are more molecules of G1 and G2 in the atmosphere in year 2 [1]
the proportion of Q returned to the surface has risen from 3 in every 10 to 6 in every 10, that is from 30 % to 60 %, a doubling [1]
more energy is returned to the surface, so the surface becomes warmer — the greenhouse effect has been enhanced, causing climate change [1]
⚠ If you missed marks here: Use the figures printed on the diagram: “more radiation comes back” without the 3-in-10 and 6-in-10 comparison is worth less than half of this. And use the word enhanced: the greenhouse effect was already there in year 1, which is the whole point of showing you two panels.
(c)[3]
State one source of each of the two gases, and explain why the greenhouse effect itself is not described as a form of pollution.
Model Answer — 3(c)
carbon dioxide is released by the combustion of fossil fuels, and by deforestation [1]
methane is released by cattle, and from flooded rice fields and decomposing waste [1]
the greenhouse effect is natural and keeps the Earth warm enough for life; it is the increase in these two gases that enhances it, and that is what causes climate change [1]
⚠ If you missed marks here: This is a definition trap that catches strong students. The greenhouse effect is not the problem — without it the Earth would be too cold to live on. What has changed is the concentration of two gases. Write “enhanced greenhouse effect” whenever you mean the harmful version.
(d)[3]
Using what you know about the carbon cycle, explain why the concentration of carbon dioxide in the atmosphere stayed roughly constant for a very long time, and why it is now rising.
Model Answer — 3(d)
carbon dioxide is removed from the air by photosynthesis and returned to it by respiration, including the respiration of decomposers [1]
for a long time the rate of removal and the rate of return were about equal, so the concentration did not change [1]
combustion of fossil fuels now returns carbon that had been locked away for millions of years, and deforestation reduces the removal, so more is added than is taken out [1]
⚠ If you missed marks here: The idea being tested is a balance between two rates, so say that the two rates were equal — not that “the cycle worked properly”. The second half needs both sides again: more added by combustion, less removed because of deforestation.
Question 4 — Material W
Total: 12 marks
Fig. 4.1 shows the route taken by pieces of a waste material, W, through a marine food chain. Information about W is given below the diagram.
(a)[2]
State what is meant by non-biodegradable, and quote the evidence given in Fig. 4.1 that W is non-biodegradable.
Model Answer — 4(a)
non-biodegradable means that decomposers cannot break it down [1]
the evidence is that W has been recovered unchanged many years after it was discarded, and that no enzyme has been found that acts on it [1]
⚠ If you missed marks here: “It takes a very long time to rot” and “it does not dissolve” are both refused. The definition is about decomposers and their enzymes, and the evidence in the figure is written in exactly those terms — use it rather than writing what you already knew.
(b)[3]
A sheet of paper dropped into the same sea does not build up in the way that W does. Explain why.
Model Answer — 4(b)
paper is biodegradable: decomposers secrete enzymes that break it down [1]
its carbon is returned to the water and the air as carbon dioxide when the decomposers respire, and its mineral ions are returned to the environment [1]
W does not enter that cycle at all, so none of it is removed and the amount present keeps rising [1]
⚠ If you missed marks here: The contrast is the answer. Anything that decomposers can digest re-enters the carbon cycle; W simply never leaves. Do not write that paper “dissolves” — it is broken down by enzymes, which is a different process.
(c)[3]
Describe the effects that pieces of W may have on the animals shown in Fig. 4.1.
Model Answer — 4(c)
small pieces are swallowed and fill the gut, so the animal feeds less and may starve [1]
larger pieces entangle animals such as seabirds, turtles and seals, so they cannot feed or swim [1]
W is not broken down or absorbed, so it is passed on when a predator eats the prey and is found in animals at every stage of the chain [1]
⚠ If you missed marks here: Give physical effects, not vague harm. “It kills them” earns nothing on its own; “it fills the gut so the animal takes in less food” earns the mark. The third mark is for noticing what the arrows in the figure are telling you — the same material appears at every stage.
(d)[4]
W is also found in the soil of farmland. Describe two effects it may have in a terrestrial ecosystem, and explain why the mass of W in that soil increases every year.
Model Answer — 4(d)
it is eaten by grazing livestock, which may take it into the gut with the grass [1]
it remains in the soil, where it can block drainage and take up space in the habitat [1]
more W is discarded and added to the soil each year [1]
and none of it is removed, because decomposers cannot break it down, so the amount present adds up year after year [1]
⚠ If you missed marks here: The last mark is the one that turns a list into an explanation: the mass rises because addition continues while removal is zero. Say both. Notice too that this is the same material behaving the same way on land as at sea — the syllabus asks about aquatic and terrestrial ecosystems.
Question 5 — Five Ways to Grow More Food
Total: 10 marks
A farmer grows wheat on a large area of land and wants to increase the mass of grain harvested from it.
(a)[5]
State how each of the following increases food production: agricultural machinery; chemical fertilisers; insecticides; herbicides; selective breeding.
Model Answer — 5(a)
machinery allows a much larger area of land to be worked, and does the work faster and more efficiently than people can [1]
chemical fertilisers supply mineral ions such as nitrate to the soil, which improves the yield [1]
insecticides kill insect pests, so less of the crop is eaten or damaged and both quality and yield improve [1]
herbicides kill weeds, so they no longer compete with the crop for light, water and mineral ions [1]
selective breeding: the plants with the highest yield are chosen as parents each generation, so yield improves over many generations [1]
⚠ If you missed marks here: Each mark is for what the method does, not for repeating its name. “Herbicides help the crop” scores nothing; “herbicides kill the weeds that compete for light and mineral ions” scores. Keep insecticides and herbicides apart — insects and weeds are different problems solved by different chemicals.
(b)[3]
Explain how a nitrate ion spread on the soil as fertiliser ends up in the protein of a wheat grain.
Model Answer — 5(b)
the ion is absorbed from the soil by the root hair cells, by active transport, which uses energy from respiration because it moves against the concentration gradient [1]
it is carried up the plant in the xylem in the transpiration stream [1]
it is combined with the carbohydrate made in photosynthesis to make amino acids, which are joined together to form protein stored in the grain [1]
⚠ If you missed marks here: Three named stages: uptake, transport, use. The word most often missing is active — nitrate is usually more concentrated inside the root hair than in the soil, so it cannot arrive by diffusion. And it travels in the xylem, not the phloem.
(c)[2]
Explain why applying more fertiliser than the crop can absorb may damage a river that runs beside the field.
Model Answer — 5(c)
the excess is not absorbed by the crop and is washed off the field, or drains through the soil, into the river when it rains [1]
this raises the concentration of nitrate and other ions in the water, which starts the sequence that ends in a fall in dissolved oxygen and the death of organisms that need it [1]
⚠ If you missed marks here: The link between the two halves of this topic is one sentence: fertiliser that the crop does not take up does not stay put. Do not write that the fertiliser “poisons the river” — it does not; it feeds the producers, and the damage comes several steps later.
Question 6 — A Forest, and What Comes Out of It
Total: 12 marks
A country owns a forest of 40 000 hectares. Each year 1200 hectares are felled for timber and 900 hectares are replanted.
(a)[2]
State what is meant by a sustainable resource.
Model Answer — 6(a)
one that is produced as rapidly as it is removed from the environment [1]
so that it does not run out [1]
⚠ If you missed marks here: The definition is a comparison of two rates, and an answer without both rates in it cannot score both marks. “A resource that lasts a long time” and “a resource we do not use up” are the two versions that lose the first mark.
(b)[3]
Calculate the percentage of the felled area that is replanted each year. Show your working. State, with a reason, whether this use of the forest is sustainable.
Model Answer — 6(b)
900 ÷ 1200 × 100 [1]
= 75 % [1]
it is not sustainable: trees are being removed faster than they are replaced, so the forest shrinks by 300 hectares every year and will eventually run out [1]
⚠ If you missed marks here: Write the division down even when the arithmetic is easy — the working carries its own mark. For the judgement, quote the comparison rather than the percentage on its own: 1200 out is more than 900 back, so removal is faster than replacement. A large forest is not the same as a sustainable one.
(c)[4]
Forests can be conserved by education, protected areas, quotas and replanting. Describe what each of these four does.
Model Answer — 6(c)
education: people learn why the forest matters and how to use it, so they are more likely to follow the rules and to choose timber from managed forests [1]
protected areas: an area is set aside in which felling is not allowed, so a complete habitat and the species in it survive [1]
quotas: a limit is set on the number of trees that may be felled each year, keeping removal at or below the rate of replacement [1]
replanting: new trees are planted to replace those felled, so the area of forest and its ability to take up carbon dioxide are maintained [1]
⚠ If you missed marks here: The mark is for what each method achieves, so a list of the four words scores nothing at all. Quotas and protected areas are easily confused: a quota limits how much is taken, a protected area limits where it is taken from.
(d)[3]
State three reasons why a country runs conservation programmes.
Model Answer — 6(d)
to maintain or increase biodiversity [1]
to reduce extinction, and to protect vulnerable ecosystems [1]
to maintain ecosystem functions: nutrient cycling, and the provision of resources including food, drugs, fuel and genes [1]
⚠ If you missed marks here: The syllabus list is short and specific, and “to save the animals” is not on it. The reason most often forgotten is the practical one: wild species provide food, drugs, fuel and genes, and the genes matter because they can be bred into crops and livestock later.
Question 7 — Forty Animals to Start With
Total: 10 marks
A species of antelope was hunted almost to extinction. Twenty years ago, 40 animals were taken into a captive breeding programme. There are now 260 animals in the programme, and none left in the wild.
(a)[2]
Explain what is meant by overharvesting.
Model Answer — 7(a)
individuals are removed from a population faster than the population [1]
replaces them by reproduction, so the population falls [1]
⚠ If you missed marks here: Again a comparison of two rates, and again the second rate is the one that goes missing. “Taking too many” is not a definition, because it does not say too many compared with what. The comparison is always with the rate of replacement by reproduction.
(b)[4]
Discuss whether this captive breeding programme, on its own, will save the species. Give points both for and against.
Model Answer — 7(b)
in favour: the number of animals has risen from 40 to 260, so the species is no longer about to be lost [1]
in favour: animals bred in captivity can later be released, and artificial insemination and IVF allow many offspring to be obtained from a few adults [1]
against: the animals came from only 40 individuals, so the genetic variation in the population is low and few individuals may survive a new disease or a change in the environment [1]
against: unless the habitat is protected and hunting is stopped, released animals will have nowhere to live and will be killed as before, so monitoring, protection of the habitat and education are needed as well [1]
⚠ If you missed marks here: A four-mark discussion needs both sides; an answer that is entirely positive, or entirely negative, is capped at half. The misconception to avoid is that breeding numbers up is enough on its own — a species restored into a habitat that no longer exists is still lost.
(c)[2]
Describe how artificial insemination (AI) and in vitro fertilisation (IVF) are used in a captive breeding programme.
Model Answer — 7(c)
AI: sperm is collected from a male and placed into the female, so a male and female kept in different places, or unwilling to mate, can still produce offspring [1]
IVF: eggs are collected and fertilised outside the body, and the embryos are placed into females, so many more offspring can be obtained from one valuable female [1]
⚠ If you missed marks here: Give what each technique does for the programme, not just what the letters stand for. The point of both is the same: more offspring from a very small number of remaining adults, and offspring from animals that are not in the same place.
(d)[2]
Explain the risk that remains to this species because its population size fell so low, even if hunting stops completely.
Model Answer — 7(d)
a small population contains reduced genetic variation [1]
so fewer individuals have features that would let them survive a new disease or a change in the environment, and the whole population could be lost [1]
⚠ If you missed marks here: “They are safe now that hunting has stopped” is the answer to avoid. The damage done by a population crash outlasts its cause: variation lost when the population fell does not come back simply because numbers rise again.
Self-Assessment
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C : 32-39
D : 24-31
E : 16-23
U : <16
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