This topic looks like the one where you write about the environment and it is not. Almost every mark is for one of two things: a named mechanism given in the right order, or a balanced answer that gives both sides. Two pieces of it carry more marks than everything else combined. The six-step eutrophication chain must come out in sequence, and the fish at the end of it suffocate — they are not poisoned. And every “advantages and disadvantages” objective is symmetric: a monoculture and an intensive livestock unit both have real advantages, and an answer that only attacks them collects half the marks. Twelve traps, six data-led walkthroughs, six lookalike pairs, a concept map and ten full practice questions below, each aimed at a place where a sensible-sounding sentence earns nothing at all.
Twelve traps that cost marks on Topic 20 challenge papers, spread across all four sub-topics. Every one is an answer that sounds sensible and that mark schemes refuse.
Six challenge-level questions with real figures and data, worked through in the order you should actually think about them. Try each part before revealing the next step.
A river flows past farmland. A drainage pipe discharges into it 4 km from the start of the survey. Dissolved oxygen was measured at intervals along the whole 30 km.
(a) State the dissolved oxygen concentration at 4 km and at 12 km, and calculate the decrease between them. [2] (b) Explain, in terms of the organisms in the water, why the concentration falls between 4 km and 12 km. [4] (c) Suggest why the concentration rises again beyond 12 km. [2]
At 4 km the curve is at about 9.2 mg per dm³; at 12 km it has fallen to about 1.0 mg per dm³. The decrease is therefore about 8.2 mg per dm³. Two habits earn the marks here: read against the gridlines rather than guessing, and always give the unit — a bare “8.2” is not a concentration. Examiners allow a small tolerance on a curve read between gridlines, but they allow none at all on a missing unit.
Notice also what the graph has already told you before any biology starts. The fall does not begin at 0 km; it begins where the pipe discharges. That single observation is worth stating, because it is the evidence that links the cause to the effect.
The caption tells you the pipe drains farmland treated with fertiliser, so the water entering the river carries an increased availability of nitrate and other ions. That is step 1 of the chain, and it is an increase, not a shortage.
Step 2 follows immediately: with more ions available, there is increased growth of producers — algae and water plants multiply in the enriched water. So far nothing has died and nothing has suffocated. If you stop here you have described a green river, not a dead one.
The producers grow, shade one another, and die. That gives increased decomposition after the death of the producers — step 3 — as bacteria and fungi multiply on the dead material.
Those decomposers respire aerobically, and there are now a very great many of them: increased aerobic respiration by decomposers, step 4. They take oxygen out of the water faster than it dissolves back in from the air, which gives step 5, the reduction in dissolved oxygen you can see on the graph. Step 6 is the consequence: death of organisms that require dissolved oxygen. The dashed line shows that most fish cannot survive below 4 mg per dm³, and the curve is below that line from roughly 7.5 km to about 18 km — some ten kilometres of river in which fish would suffocate.
Write suffocate or cannot obtain enough oxygen. Do not write poisoned: nothing in this river is toxic.
Three things happen as the water travels on. The nitrate has been used up and diluted, so there is no longer anything driving extra growth. The dead material has been decomposed or left upstream, so the number of decomposers falls and their oxygen demand falls with it. And all the while oxygen dissolves back into the water from the air, especially where the river is turbulent, while any surviving producers photosynthesise and add more.
This is also the answer to the question that usually follows: a still lake recovers far more slowly, because the polluted water is not carried away, there is much less mixing with the air, and the dead material settles and keeps the decomposers supplied.
A lake next to arable farmland was sampled every month for one year. Curve X is the nitrate concentration, curve Y is the mass of algae and curve Z is the dissolved oxygen concentration, each shown as a percentage of its own highest value.
(a) State the month at which X is highest, the month at which Y is highest and the month at which Z is lowest. [2] (b) Suggest why X rises sharply between months 1 and 3. [2] (c) Explain why X falls between months 3 and 8. [2] (d) Explain why Z reaches its minimum after Y has already passed its peak. [2]
X is highest at month 3, Y is highest at month 5, and Z is lowest at month 6. Write those three numbers down before you attempt any explanation, because the sequence 3, then 5, then 6 is the six-step chain drawn as a graph: ions first, producers next, oxygen last.
A curve question in this topic is nearly always the chain in disguise. If you can match each curve to a step, every remaining part answers itself.
Fertiliser is applied to the fields in spring, and rain washes the soluble nitrate and other ions off the land and into the lake. That is why X climbs steeply through months 2 and 3 rather than staying level all year.
Note the wording of part (b): “suggest”. You are being asked for a sensible cause, not a recalled fact, and any answer combining fertiliser application with drainage or run-off after rain earns both marks. Untreated sewage would be the other acceptable source.
X falls from month 3 onwards because the algae are absorbing the nitrate ions and using them to make amino acids and proteins for growth. That is the same uptake by root hairs and by cell surfaces you met when you studied active transport; the ions are being removed from the water into living material.
You can see the transfer directly: X falls as Y rises, and Y peaks two months after X. The commonest wrong answer here is “the nitrate was washed out of the lake”, which does not explain why the algae grew at exactly the same time.
While the algae are alive and in the light they photosynthesise, so during the day they add oxygen to the water. That is why Z is still fairly high at month 4, when Y is climbing steeply.
The oxygen falls only when the algae die: the dead material is broken down by decomposers, the decomposers multiply, and their aerobic respiration takes dissolved oxygen out of the water. That takes time, which is exactly why Z bottoms out at month 6, one month after Y peaked at month 5. The lag is the evidence that decomposers, and not the algae themselves, are removing the oxygen — and saying so is what turns a description into an explanation.
A farmer owns both fields. Field P is planted with a single crop species; field Q holds several crop species and is bounded by a hedgerow. The farmer is considering managing the whole of the land in the way field P is managed.
(a) State two advantages to the farmer of managing the whole area as field P. [2] (b) State two disadvantages. [2] (c) Explain, in terms of biodiversity, the effect of removing the hedgerow from field Q. [2] (d) Suggest why an insecticide may have to be applied more often to field P than to field Q. [2]
Parts (a) and (b) together are the standard “advantages and disadvantages” objective split into two labelled halves. When it appears as one six-mark question instead, it is still this shape, and the commonest failure in the whole of 20.1 is filling one column and leaving the other empty.
Rule for the exam: the moment you see advantages and disadvantages, rule a line down your rough page and put at least two points in each column before you compose a sentence. Thirty seconds of planning protects half the marks.
Any two of these earn the marks. All the plants can be sown, treated and harvested at the same time by machine, so a large area is farmed efficiently by very few people. The crop can be chosen to suit that particular soil and climate, so it grows well. The yield per hectare is high, so more food is produced from the same land. There is also a simple practical point: one species needs only one set of machinery and one method, which lowers cost.
Say these plainly and without apology. The examiner is checking that you can evaluate rather than disapprove — a farmer feeding a city is solving a real problem, and the marks are for understanding how.
Low biodiversity: only one plant species is present, so the habitats and food sources of other species are removed and fewer different species live there. Pests and disease spread rapidly: a pest that feeds on that crop finds an unbroken food supply across the whole field and multiplies, and a disease of that species has no barrier of resistant plants to stop it. Mineral ions are depleted: the same crop removes the same ions from the soil every year, so more fertiliser must be applied, and that fertiliser may drain into water.
Each disadvantage is a name plus a mechanism. “Pests” alone is one word, not one mark.
(c) Biodiversity is the number of different species living in an area. The hedgerow is a habitat in its own right and holds species that the crop does not — different plants, and the animals that feed and shelter in them. Removing it therefore reduces the number of different species on the farm. Notice that the number of individual plants might well go up when the hedge is replaced by more crop; biodiversity still falls, because it counts species and not individuals.
(d) In field P a pest of that crop has a continuous supply of its food plant and no interruption, so its population grows quickly and the whole field is vulnerable to the same pest. In field Q the pest’s food plants are separated by plants it cannot eat, so it spreads more slowly, and the hedgerow supports predators of the pest. More frequent spraying in P has a further consequence worth a mark in an evaluation question: the insecticide also kills species that were not pests, which alters the food webs on the farm.
The two panels show the same hillside eight years apart. The trees were felled for timber and the land was cleared for grazing. A to E label five features of the right-hand panel.
(a) State the letter that labels (i) rain reaching the ground directly, (ii) soil moving down the slope, (iii) water collecting at the foot of the slope. [3] (b) Explain how the removal of the trees causes the feature labelled C. [2] (c) The concentration of carbon dioxide in the air above this hillside has risen. Give two reasons. [2] (d) Name one further undesirable effect of deforestation that the figure does not show. [1]
(i) rain reaching the ground directly is B — the short blue strokes falling on to open ground where the canopy used to be. (ii) soil moving down the slope is C, the long arrow running downhill across the bare surface. (iii) water collecting at the foot of the slope is D, the shaded band at the bottom of the panel.
Compare the two panels rather than looking only at the right-hand one. In the left-hand panel the same slope carries a thick soil layer under the trees and no water at the bottom, which is the evidence that the features labelled C and D appeared because the trees went.
Trees do two things to rain. Their leaves intercept it, so it does not strike the ground at full force, and their roots take up water and hold the soil particles together. Remove the trees and both stop.
So the rain now hits bare soil directly, loosens it, and runs across the surface instead of soaking in, washing the soil downhill — that is C. The same run-off reaches the river at the bottom far faster than before, so the river rises and floods — that is D. One change, two named effects, and a question that asks for them separately is asking you to give the mechanism twice.
First: with the trees gone, less carbon dioxide is removed from the air by photosynthesis.
Second: the felled timber does not simply vanish. It is burned, and combustion releases carbon dioxide; or it is left to rot, and the decomposers respire as they break it down, releasing carbon dioxide that way. Either route counts, and one of them must appear.
Both marks are available and most candidates take one. Fix the sentence in your memory with the and in the middle: “less photosynthesis removing it, and combustion or decomposition releasing it”.
Reduced biodiversity: fewer different species live in the area, because the trees themselves were species and because the habitats and food they provided for other species have gone.
Extinction: a species found only in that forest and nowhere else loses the whole of its habitat, so the entire species can be lost. Biodiversity falling is a local change; extinction is permanent and global. Examiners accept either as your fifth effect, but they do not accept them as one point written twice.
The two panels show energy entering and leaving the atmosphere above the same place in two years eighty years apart. Each circle represents a molecule of gas G1 or gas G2. The number of downward orange arrows is proportional to the energy returned to the surface.
(a) Name radiation P and radiation Q. [2] (b) Using the figure, describe two differences between year 1 and year 2. [2] (c) G1 and G2 are the two gases named in the syllabus as air pollutants. Name them and give one source of each. [2] (d) Explain why the change shown leads to climate change. [2]
P is drawn coming down from outside the atmosphere and reaching the ground: it is energy radiated from the Sun, arriving at the Earth’s surface. Q points upwards away from the ground: it is energy radiated from the Earth’s surface back out towards space.
Read the arrowheads, not the colours. Every question of this kind is answerable from direction alone, even when the figure is unfamiliar.
First, there are more molecules of G1 and G2 in the atmosphere in year 2 — the concentration of the two gases has increased.
Second, the proportion of Q that is returned to the surface has risen from 3 of every 10 units to 6 of every 10 units, so twice as much energy is returned and correspondingly less escapes. A description question wants the figures quoted, not just the direction of change — “more energy is returned” is one mark, “returned energy has doubled, from 3 in 10 to 6 in 10” is safely two.
Carbon dioxide and methane. Nothing else in the air is on this objective.
Sources of carbon dioxide: combustion of fossil fuels in vehicles, power stations and industry; also the burning and decomposition of felled trees, which links this directly to deforestation. Sources of methane: cattle and other livestock, which is why intensive livestock production appears in a question about the air; rice fields; and the decomposition of waste in tips and landfill.
One source for each gas is all the question asks. Give the clearest one and move on.
The greenhouse effect itself is natural and necessary: without gases returning some of the energy radiated from the surface, the Earth would be too cold for living organisms. Do not write about it as though it were a pollutant.
The change shown is an increased concentration of carbon dioxide and methane, which enhances the effect: a greater proportion of the energy radiated from the surface is absorbed and returned instead of escaping, so the surface and the lower atmosphere become warmer. That warming, and the changes in rainfall and in weather patterns that follow from it, are what is meant by climate change.
The graphs show the mass of fish in two areas of sea, each fished for 20 years by the same method. The nets below are used in the same area by boats of the same size.
(a) Define the term sustainable resource. [2] (b) State which area was fished sustainably, and give the evidence from the graphs. [2] (c) All fishing in the other area stopped at year 20. Suggest why the stock may not recover quickly. [2] (d) Explain how using net S rather than net R helps to conserve a fish stock. [3]
A sustainable resource is one that is produced as rapidly as it is removed from the environment, so that it does not run out. Both halves are needed: the comparison of the two rates, and the consequence.
The related definition is the mirror image. Overharvesting is removing individuals faster than the population can replace them by reproduction. If you can write those two sentences you can answer every graph question in 20.4, because the graphs are simply those sentences drawn.
In area P the stock starts at about 80 thousand tonnes and is still at about 80 thousand tonnes after 20 years; it varies a little from year to year but shows no downward trend. The catch is therefore matching the rate of replacement, so P was fished sustainably.
In area Q the stock falls from about 80 to about 2 thousand tonnes over the same 20 years — a fall of about 78 thousand tonnes, or roughly 97 %. More was removed each year than was replaced, so Q was overharvested. Quote at least one pair of figures; “P stays the same and Q goes down” is a description of the picture and usually scores one of the two marks.
Recovery depends on reproduction, and reproduction depends on how many breeding adults remain. With only about 2 thousand tonnes left, there are very few adults to breed, so few eggs are produced and the stock replaces itself slowly — the smaller the population, the slower the recovery.
There is a second, more advanced reason worth a mark in a Supplement question: a population reduced to a very small size has reduced genetic variation, so it is less likely that any individual carries an allele giving resistance to disease or the ability to survive a change in conditions. Other species may also have taken over the food supply or the space in the meantime.
Net R has a 40 mm mesh and holds both the adults and the young fish. Net S has a 95 mm mesh, so the young fish that have not yet bred pass through and escape. Those fish then survive to reproduce and replace the individuals that were taken, so the rate of replacement keeps pace with the rate of removal and the stock can be harvested year after year without falling. That is three marks: what the mesh does, what the fish then do, and what that means for the stock.
Cambridge names six methods of conserving fish stocks and a full-mark answer explains what each achieves rather than simply listing it: education so that fishermen and buyers understand and accept the limits; closed seasons so that fish are not caught while breeding; protected areas where no fishing is allowed, from which young fish spread out; controlled net types and mesh size, as above; quotas limiting the mass that may be landed; and monitoring, so that the quotas are set from real measurements of the stock.
Six pairs that look almost identical and have different answers. In this topic the distinction is nearly always where the marks live.
Click each node. The whole topic is four stories: how food production was increased, what that did to habitats, what it put into the water and the air, and what can be done about it.
Six answers of the kind that read fluently and score badly. Find the fault before you reveal it.
Ten Cambridge-style challenge questions, each drawing on more than one sub-topic. Write your answer first, then reveal the model answer and the examiner’s notes.
The figure shows the sequence of events in a lake beside farmland. Boxes 2, 4 and 5 have been replaced by (i), (ii) and (iii).
(a) State what should be written in boxes (i), (ii) and (iii). [3] (b) Name the two sources of pollution that could cause the event in box 1. [2] (c) A student writes that the organisms in box 6 were poisoned. Explain why this is incorrect. [2] (d) Suggest one reason why a farmer applies the substance responsible in the first place. [1]
| Measurement | Field P | Field Q |
|---|---|---|
| total crop yield / tonnes per hectare | 9.4 | 6.1 |
| number of plant species recorded | 1 | 23 |
| mass of insecticide applied / kg per hectare | 3.2 | 0.9 |
| mass of fertiliser applied / kg per hectare | 210 | 130 |
(a) Use the table to give one advantage and one disadvantage of managing land as field P is managed. [2] (b) Explain why field P needs more insecticide than field Q. [2] (c) Explain why field P needs more fertiliser than field Q. [2] (d) A stream runs along the edge of both fields. Suggest, with a reason, which field is more likely to cause eutrophication in the stream. [2]
(a) Calculate the percentage of the energy eaten that was stored as new tissue in each group. Show your working. [2] (b) Explain the difference between the two groups. [3] (c) Give two disadvantages of the system used for group A. [2] (d) Suggest one way in which the waste from group A could affect a nearby river. [1]
(a) Define the term biodiversity, and state how the clearance has affected it. [2] (b) Explain why the soil on the slope is now thinner and why the river floods. [3] (c) Explain why the concentration of carbon dioxide in the atmosphere increases as a result of this clearance. [2] (d) State what is likely to have happened to the frog species, and explain why this outcome cannot be reversed. [1]
(a) Explain what is meant by the greenhouse effect, and why it is described as natural. [2] (b) Explain what is meant by the enhanced greenhouse effect. [2] (c) Give one source of the increase in carbon dioxide and one source of the increase in methane, in each case naming the human activity responsible. [2] (d) A student states that the greenhouse effect is a pollutant. Explain why this statement is incorrect. [1]
Material W was manufactured to be light, strong and waterproof. It has been recovered unchanged from the guts of animals at every stage of the chain shown, and from soil on land, many years after it was discarded. No enzyme has been found that acts on it.
(a) Explain what is meant by the term non-biodegradable. [2] (b) Using the information given, explain why W is still present in the habitat many years later. [2] (c) Describe two effects of W on the animals in the chain shown. [2] (d) Suggest one effect of W in a terrestrial habitat, and suggest why W continues to be manufactured despite these effects. [2]
(a) Define the term sustainable resource. [2] (b) Describe how each of the following helps to conserve a forest: replanting, quotas, protected areas. [3] (c) Explain why education is included in the list of methods, even though it does not by itself protect a single tree. [1] (d) Explain, in terms of two named effects, why conserving the forest also benefits the land below it and the atmosphere above it. [2]
(a) Explain what is meant by overharvesting, and explain how the data described suggest that it has occurred. [3] (b) Describe how each of the following conserves a fish stock: closed seasons, controlled mesh size, monitoring. [3] (c) Explain why a protected area in which no fishing is allowed can increase the catch in the surrounding sea. [2] (d) Waste plastic from the fishing boats is discarded at sea. State one effect of this on the fish stock. [1]
(a) Using the information given, state three of the named causes of a species becoming endangered. [3] (b) Describe two ways in which artificial insemination and in vitro fertilisation help a captive breeding programme. [2] (c) Explain why protecting the habitat is more important than increasing the number of animals in captivity. [2] (d) Seed banks are used to conserve endangered plants. State one advantage of a seed bank. [1]
(a) Explain why the lake now contains soil particles. [2] (b) Describe, in order, the sequence of events that led to the death of the fish. [4] (c) Explain what has happened to the biodiversity of the lake, using the correct definition of the term. [2] (d) Suggest one way in which the cattle on the hillside affect the atmosphere. [1]