IGCSE Biology Topic 20: Human Influences on Ecosystems

20.1 Food Supply 20.2 Habitat Destruction & Deforestation 20.3 Pollution 20.4 Eutrophication 20.5 Conservation 20.6 Exam Technique

Hi Tara. This one looks like the essay topic — the one where you write what you think about the environment and hope for the best — and it is nothing of the kind. Read the objectives and you will find that almost every mark in Topic 20 is for one of exactly two things: a named mechanism, written in the right order, or a balanced answer that gives both sides properly. Cambridge awards no marks at all for concern, and it awards a great many for “increased aerobic respiration by decomposers”.

So this guide is built around those two things. 20.1 is food supply: the five named methods, and then the advantages and disadvantages of monocultures and of intensive livestock production — given fairly, because half the mark scheme is on the side most people never write. 20.2 is habitat destruction and deforestation, including the five named effects and the two-reason carbon dioxide answer that comes up almost every year. 20.3 is pollution: sewage, fertiliser, plastics and the two greenhouse gases. 20.4 is eutrophication on its own, because it is a six-step sequence and the order is the mark. 20.5 is conservation and sustainable resources, with a good deal of Supplement material. 20.6 is the vocabulary and the exam technique.

Two sentences carry more marks than anything else here. First: fish in a eutrophic river die of suffocation, not of poisoning — the fertiliser is not toxic and neither are the algae; the decomposers use up the dissolved oxygen. Second: every “discuss” question is marked on both sides, so a monoculture has real advantages and an intensive livestock unit has real advantages, and an answer that only attacks them scores about half. Hold on to those two and a large part of this topic writes itself.

20.1 Food Supply ▼

▶ Watch: Food Supply, Monocultures and Intensive Livestock

Opens on YouTube in a new tab. Watch one, then come back and do the Check Yourself questions — watching without testing yourself feels like learning but is not.

Five Ways, and Only Five

The syllabus objective here is unusually specific. It does not say “describe how food production has increased”; it says “limited to”, and then it names five things. Everything else — irrigation, greenhouses, hydroponics, cloning, genetic engineering — is real, is interesting, and is not what you are being examined on. So the first job is to know the list, and the second is to know what each item on it actually does, because the marks are almost never for the name on its own.

The five named methods. Each box gives the mechanism, which is where the marks are.

1. Agricultural machinery

A tractor, a combine harvester and a seed drill let one person work an area that would once have needed dozens, and each job is finished faster — which matters, because a crop has a narrow window in which it is worth sowing or worth harvesting. The syllabus phrasing is “to use larger areas of land and improve efficiency”, and both halves are worth writing.

There is a cost that follows directly from this, and it is the join between this sub-topic and the next one. Machinery is only efficient in large fields, so fields are made larger, and making a field larger means removing the hedgerow that divided it. A hedgerow is a habitat, so this is habitat destruction arriving by an economic route rather than a deliberate one.

2. Chemical fertilisers

When a crop is harvested and carted away, the mineral ions inside it leave the field with it. Nothing puts them back, because in a farmed field the plants do not die where they grew and decompose in place. Fertiliser replaces those ions — nitrate, phosphate and potassium ions above all.

Be precise about what the plant does with them. Nitrate ions are absorbed by root hairs and used to make amino acids, and from those, proteins, which the plant needs in order to grow and to make enzymes. Magnesium ions are needed to make chlorophyll. None of these ions supplies the plant with energy: the energy comes from photosynthesis, and always has. Calling fertiliser “plant food” is the fastest way to lose this mark.

A named-method answer has two halves

“The farmer used a fertiliser” is worth nothing. “The fertiliser supplies nitrate ions, which the crop uses to make amino acids and proteins, so it grows more and yields more” is worth the mark. Every one of the five methods works like this: name it, then say what it does and what follows.

3. Insecticides

An insecticide kills insect pests. Fewer pests means less of the crop is eaten or damaged, so both the quantity and the quality of the harvest go up — the syllabus mentions quality deliberately, because a bitten apple is still an apple but is worth much less.

The difficulty is that an insecticide is rarely selective. It also kills the insects that pollinate the crop, and the insects that were eating the pest. Remove the predators of a pest and the pest population can rise higher than it did before, which is why a farmer who sprays every year can end up with a worse pest problem than a farmer who does not.

4. Herbicides

A herbicide kills weeds. A weed is simply a plant growing where it is not wanted, and its offence is competition: it takes light, water, mineral ions and space that the crop would otherwise have had. Kill the weeds and the crop has more of all four, so it grows larger.

Herbicide and insecticide are swapped constantly under exam pressure. Herb is a plant word. Insect is an animal word. Ten seconds spent fixing that in your head now will pay for itself.

5. Selective breeding

Choose the individuals with the characteristic you want — the cows that give most milk, the wheat plants with the largest ears — and breed from them. Choose again among the offspring. Repeat. Over many generations the mean value of that characteristic in the population shifts, sometimes dramatically.

Two words earn the marks: parents (you are choosing who reproduces, not treating individuals) and many generations (it is slow). Selective breeding is not genetic engineering, and it is not cloning; it works on variation that is already there.

Large-Scale Monocultures: Both Sides

A monoculture is a large area planted with a single crop species. This is the point in the topic where the examining changes character: you are no longer being asked to recall a mechanism, you are being asked to evaluate. And an evaluation question is marked on both sides.

Read that again, because it is the single most expensive habit in Topic 20. A student who writes six accurate criticisms of monoculture farming and no advantages will typically be capped at about half the marks. Not because the criticisms are wrong — they are all in the mark scheme — but because half the mark scheme is on the other page.

The same land, farmed two ways. Every row of the table is a marking point, and the marks run in both directions.

Large-scale monoculture: advantages	Large-scale monoculture: disadvantages
Every plant is sown, sprayed and harvested at the same time, so the whole job can be done by machine. Labour cost per tonne is low.	Every plant is the same species, so a pest or a disease of that species can spread through the entire field.
One crop needs one fertiliser and one pesticide, so treatment is simple and the yield per hectare is high.	More pesticide is therefore needed, with the effects on non-target insects described above.
A high yield per hectare means more food from less land, which lowers the cost per kilogram and leaves more land unfarmed.	The same mineral ions are removed year after year, so the soil is depleted and more fertiliser must be added, some of which drains into rivers.
The farmer needs one set of machinery and one body of specialist knowledge, rather than several.	Very few habitats and very little variety of food, so biodiversity in and around the field is low, and the natural predators of pests do not survive there.
A large, uniform, predictable harvest makes the food supply more secure and easier to store and transport.	Large fields mean hedgerows are removed, which destroys habitat directly.

Intensive Livestock Production: Both Sides Again

Intensive livestock production means keeping animals at a high density, usually indoors, with the temperature controlled, movement restricted and the diet controlled. Chickens, pigs and dairy cattle are the usual examples.

The biology behind why it works is worth understanding rather than memorising, because it is really an energy-budget question. Think about where the energy in an animal’s food goes. Some is lost in faeces and urine. Some is released in respiration — used for movement, and for keeping the body at its working temperature, with most of it eventually transferred to the surroundings as heat. Only what is left is built into new tissue, and only new tissue is meat.

So: heat the shed, and the animal has to respire less in order to stay warm. Restrict its movement, and less is respired for muscle contraction. Control the diet so that it is exactly what the animal needs, and less is wasted. Each of those pushes a slightly larger fraction of the food energy into growth. The result is more meat, milk or eggs from the same mass of feed — which is what “efficient” means here.

Intensive livestock: advantages	Intensive livestock: disadvantages
Less energy is transferred to the environment as heat and less is used in movement, so more of the energy in the food is converted into new tissue.	Animals are close together, so an infectious disease spreads through the herd or flock quickly.
Far less land is needed per animal, so less habitat has to be cleared for the same amount of meat.	Movement is restricted, which raises real questions about the welfare of the animals.
Animals are inspected daily, so illness is noticed early and veterinary treatment can be given promptly.	Medicines, including antibiotics, may be used routinely rather than only when needed.
The diet is controlled, so growth is fast and predictable and less food is wasted.	Heating, lighting and ventilation all cost energy, which usually means burning fuel.
The animals are sheltered from predators and from bad weather, so fewer are lost.	A great deal of waste is produced in one place. If it drains into a river it adds ions and organic material to the water.
Lower cost per kilogram makes protein affordable to more people.	Large amounts of crop must be grown to feed the animals, and that land has to come from somewhere.

Two things to hold on to

For the five methods: Machinery, Fertilisers, Insecticides, Herbicides, Selective breeding — “My Farm Isn’t Harvested Slowly”.

For the evaluation questions: write the sentence “However, …” at the halfway point. If you have written three advantages, stop, write “However”, and write three disadvantages. That one word is worth roughly three marks a time.

Worked example Discuss the advantages and disadvantages of large-scale monocultures of crop plants. [6]

Step 1 — read the mark allocation as an instruction

Six marks, and a “discuss” command word. Plan for three each way before you write a word. If you run short of time, three and two still scores far better than six and none.

Step 2 — three advantages, each with its consequence

The whole field ripens together, so sowing, spraying and harvesting can be done by machine, which lowers the labour cost per tonne. Only one fertiliser and one pesticide are needed, so the field is simple to manage and the yield per hectare is high. The high yield per hectare means more food is produced from less land, so the cost per kilogram is lower.

Step 3 — three disadvantages, same structure

Every plant is the same species, so a pest or disease can spread through the whole crop. The same mineral ions are removed year after year, so the soil is depleted and more fertiliser is needed, some of which drains into rivers. Very few species can live in the field, so biodiversity is low and the natural predators of the pests are absent.

Step 4 — check the shape, not the content

Six sentences. Each contains a fact and a “so”. Three on each side. No opinion anywhere. That is a full-mark answer, and notice that nothing in it is difficult — the difficulty was structural.

Three advantages and three disadvantages, each stated as one sentence with its consequence attached.

Worked example A farmer keeps 200 pigs indoors in a heated building. A neighbour keeps 200 pigs in an open field. The indoor pigs reach the same mass on 15 % less feed. Explain why. [3]

Step 1 — decide what is being compared

Same species, same number, same final mass. The only differences are temperature and space to move. So the answer has to be about where the energy in the feed goes, not about the feed itself.

Step 2 — the temperature point

In a heated building the pigs lose less heat to their surroundings, so less energy has to be released in respiration to maintain body temperature.

Step 3 — the movement point

The indoor pigs move less, so less energy is released in respiration for muscle contraction.

Step 4 — join the two to the question that was asked

Because less of the energy in the feed is used in respiration, a greater proportion of it is available to be converted into new tissue, so the same growth is achieved from less feed.

Less heat lost and less movement means less respiration, so a greater proportion of the food energy is converted into growth.

Check Yourself: 20.1 Food Supply

12 multiple choice questions. Click an option to check your answer.

Your Score 0 / 12

Question 1

Which list gives the five ways of increasing food production named by the syllabus?

A irrigation, greenhouses, insecticides, cloning and agricultural machinery

B agricultural machinery, chemical fertilisers, insecticides, herbicides and selective breeding

C chemical fertilisers, herbicides, genetic engineering, irrigation and selective breeding

D insecticides, herbicides, antibiotics, hydroponics and agricultural machinery

The syllabus names five and only five, and the objective says “limited to”. Irrigation, greenhouses, hydroponics, cloning and genetic engineering are all real ways of increasing food production, which is exactly why the wrong lists look reasonable — but they are not the five you are examined on here. Learn the list as a list.

Question 2

A farmer sprays a field of wheat with a herbicide. How does this increase the yield of wheat?

A it kills the insects that would otherwise eat the wheat

B it supplies the mineral ions that the wheat needs to make protein

C it kills the weeds, so the wheat is no longer competing with them for light, water and mineral ions

D it kills the fungi growing on the wheat leaves

Herbicide and insecticide are swapped so often that examiners rely on it. A herbicide kills plants; an insecticide kills insects. And notice that the mark is not for “kills weeds” on its own — it is for the consequence, which is reduced competition for light, water, mineral ions and space. The option about supplying mineral ions describes a fertiliser.

Question 3

Why do chemical fertilisers increase crop yields?

A they supply the crop with the energy it needs to grow

B they kill the pests that damage the crop

C they warm the soil so that seeds germinate earlier

D they replace mineral ions such as nitrate, which the crop absorbs and uses to make amino acids and proteins for growth

This tests whether you still believe fertiliser is “plant food”. It is not. A plant makes its own organic nutrients by photosynthesis; what it cannot make is the nitrogen, phosphorus and potassium it needs, so it absorbs those as mineral ions from the soil. Fertiliser supplies raw materials, never energy. Each harvest removes ions in the crop that is carted away, and the fertiliser puts them back.

Question 4

Which description of selective breeding for higher yield is correct?

A the plants or animals with the best characteristic are chosen as parents, and the process is repeated over many generations

B a gene for high yield is taken from one species and inserted into another

C hormones are given to the crop so that each plant grows larger

D the best individual is cloned, so that every plant in the field is identical to it

Two words carry the marks: parents and many generations. Selective breeding works on variation that already exists in the population, one generation at a time, so it is slow. The option about moving a gene between species is genetic engineering, which is a different objective; cloning and hormone treatment are not selective breeding at all.

Question 5

Which of these is a genuine advantage of growing a large-scale monoculture?

A the whole field ripens at the same time, so sowing, spraying and harvesting can be done by machine, which lowers the cost per tonne

B a pest of that crop cannot spread from plant to plant

C biodiversity in the field is high, so natural predators keep the pests in check

D different mineral ions are removed from the soil each year, so less fertiliser is needed

The misconception being tested is “a monoculture has no advantages”, and it costs half the marks on every question of this kind. One species means one sowing date, one ripening date, one machine, one fertiliser, one spray — and therefore a high yield per hectare at a low cost per kilogram. The three wrong options are all advantages of mixed planting, offered in the hope that you will not read carefully.

Question 6

A disease of potatoes appears in one corner of a 200-hectare field planted entirely with potatoes. Why is this more serious than it would be in a field of mixed crops?

A potatoes are more susceptible to disease than other crops

B the machinery used on a monoculture damages the plants and lets the disease in

C every plant in the field is the same species, so the disease can spread through the whole crop

D monocultures cannot be treated with pesticides

A pathogen that can infect one potato plant can infect every plant around it, and the plants are touching. In a mixed planting the species that are not hosts act as a barrier. Nothing about potatoes in particular is being tested — the same answer applies to any monoculture, which is why the option blaming the species is wrong.

Question 7

Cattle raised intensively are kept indoors at a controlled temperature and cannot move far. Why does this produce more meat from the same mass of food?

A the warmth is absorbed by the cattle and converted into new tissue

B less energy is transferred to the environment as heat and less is used in movement, so more of the energy in the food is used for growth

C the cattle eat more food than they would outdoors

D warmth makes the digestive enzymes release more energy from each mouthful

Think of the animal’s energy budget: energy in the food is either respired (and much of that is transferred to the surroundings as heat), lost in faeces and urine, or built into new tissue. Warm the shed and the animal loses less heat, so it needs to respire less to stay warm; restrict movement and less is respired for muscle contraction. The option about heat being “converted into new tissue” has energy flowing backwards — heat transferred to the environment can never be recovered.

Question 8

Which of these is a genuine advantage of intensive livestock production?

A no waste is produced, because the animals are indoors

B the animals are free to move, which improves their welfare

C veterinary medicines are not needed

D many animals are kept in a small space and inspected often, so less land is needed per animal and disease is spotted and treated early

The examinable advantages are practical ones: more animals per hectare, a controlled diet, shelter from weather and predators, and close daily supervision, so that a sick animal is found quickly. Note that “disease spreads easily” and “disease is detected early” are both true of the same system — that is what makes this a balanced-evaluation question rather than a recall one. The other three options are simply false.

Question 9

Which of these is a genuine disadvantage of intensive livestock production?

A a large amount of waste is produced in one place, and if it drains into a river it adds ions and organic material to the water

B the yield of meat or milk per animal is lower than on a free-range farm

C more land is needed for each animal

D the food the animals eat is converted into new tissue less efficiently

The real disadvantages are crowding (so an infectious disease spreads quickly through the herd), restricted movement and the welfare questions that raises, the cost of heating and lighting, and the concentration of waste. The other three options simply state the reverse of the advantages — intensive systems give higher yield per animal, use less land, and convert food into tissue more efficiently, which is the whole reason they exist.

Question 10

A farmer sprays an insecticide every year. After several years the crop is being damaged by insects more than before. Which explanation is most likely?

A the insecticide has been broken down by decomposers into mineral ions that the pests feed on

B the insecticide killed the weeds the pest species preferred, so it moved onto the crop

C the insecticide reduced the light reaching the leaves, so the crop grew weaker

D the insecticide killed insects other than the pest, including the natural predators of the pest, so the pest population is no longer controlled

An insecticide rarely kills only its target. Removing the predators of a pest removes the thing that was holding its numbers down, so the pest can increase — and the same spray also kills pollinators, which can reduce the yield of an insect-pollinated crop directly. The disadvantage listed for fertilisers is run-off into water; do not attach it to insecticides.

Question 11

A question asks: “Discuss the advantages and disadvantages of large-scale monocultures. [6]” A student writes six correct points, all of them disadvantages. What is the most likely outcome?

A full marks, because all six points are correct

B roughly half marks, because the question requires both sides and only one has been given

C no marks, because the question asked for a discussion

D full marks, because six points were asked for and six were given

“Discuss” means both sides, and a six-mark discussion is almost always marked as three points each way with a cap on either side. Six disadvantages therefore hit a ceiling at about three. This is the single most reliable way to lose marks in the whole of Topic 20, and it costs nothing to fix: count your points as you write, and make sure both halves are there.

Question 12

Which statement about agricultural machinery is correct?

A it increases the mineral ion content of the soil

B it kills the pests in the soil as it passes over

C it lets one person work a much larger area and complete each job faster, but large fields are needed, so hedgerows are often removed and habitats are lost

D it removes the need for chemical fertilisers

The objective is “to use larger areas of land and improve efficiency”, so a full answer names both halves. The cost is a change to the landscape: machinery is only efficient in big fields, and enlarging fields means taking out the hedgerows, which are themselves habitats. That is the link between food supply and habitat destruction, and examiners use it to join the two sub-topics together.

20.2 Habitat Destruction and Deforestation ▼

▶ Watch: Habitat Destruction and Deforestation

Opens on YouTube in a new tab. Watch one, then come back and do the Check Yourself questions — watching without testing yourself feels like learning but is not.

Biodiversity, Defined Properly

Cambridge gives you a definition and then marks you on it: biodiversity is the number of different species that live in an area. Five words, and every one of them is doing something.

Number — it is a count. Different — the count is of kinds, not of individuals. Species — not populations, not organisms, not habitats. A wheat field can hold half a million living plants and have a biodiversity close to one; a hedgerow beside it can hold a few thousand organisms belonging to two hundred species. The hedgerow wins, and it is not close.

The wording that will cost you the mark

“Biodiversity is the number of animals in an area.” “Biodiversity is the size of the population.” “Biodiversity is the variety of life.” The first two count the wrong thing. The third sounds like a definition and is not one — it cannot be measured, so it cannot be marked. Write the number of different species that live in an area and move on.

Three Reasons Habitats Are Destroyed

The syllabus names three, and again it says “limited to”.

Increased area needed for housing, crop plant production and livestock production. More people means more houses, and more people means more food, which means more farmland and more grazing. This is the largest cause worldwide, and it is the reason that a question about intensive farming can end up being a question about habitat.
Extraction of natural resources. Mining, quarrying, drilling and the felling of timber all mean clearing whatever was growing on top and, often, removing the ground itself.
Freshwater and marine pollution. This one destroys a habitat without moving any soil. The place is still there, but conditions inside it have changed until the community that lived there can no longer survive — which is exactly what happens to a river below a sewage outfall.

Altering Food Chains and Food Webs

Habitat destruction almost never removes one species tidily. What you already know about food webs is the tool for answering these questions, and the method is the same one you have used before: trace the arrows in both directions, and then sideways.

Upwards. Whatever ate the missing species has lost a source of food, so its numbers fall unless it has an alternative prey.
Downwards. Whatever the missing species ate is no longer being eaten, so its numbers rise, and it may then over-graze the level below it.
Sideways. A predator that has lost one prey species will eat more of its other prey, so those populations fall even though nothing happened to them directly. This is the step that turns a two-mark answer into a four-mark one.

Insects are worth a special note, because they usually occupy more than one place in a web. A pond insect may be food for fish as a larva and a pollinator of flowering plants as an adult. Drain the pond and you remove both roles at once, and the seed production of plants that never touched the water will fall.

Deforestation: Five Named Effects

Here the objective is “explain the undesirable effects of deforestation” and it lists exactly five. Learn them as a list, because the question is normally worth four to six marks with one mark per effect properly explained.

The same hillside before and after. The five numbered effects underneath are the five the syllabus names.

Reducing biodiversity. Fewer different species live in the area. A forest offers a large number of niches — canopy, trunk, leaf litter, dead wood — and clearing it removes almost all of them at once.
Extinction. If a species lives in that forest and nowhere else, then clearing that forest removes every individual of the species. Local loss becomes permanent loss.
Loss of soil. Tree roots bind the soil physically and leaves intercept the rain, taking the force out of it. With both gone, rain strikes bare ground and washes the top layer downhill — and the top layer is the one that holds the mineral ions.
Flooding. Fewer roots absorb water from the soil and fewer leaves intercept the rain, so a far greater proportion of the rainfall runs over the surface and reaches the river at once, rather than seeping in slowly.
Increase of carbon dioxide in the atmosphere. This one has two halves, and they are marked separately. See below.

The two-reason carbon dioxide answer — drill this until it is automatic

“Explain why deforestation increases the carbon dioxide concentration of the atmosphere. [2]”

Half one — less removed. There are fewer trees, so less carbon dioxide is taken out of the air by photosynthesis.

Half two — more released. The felled wood is burned, or it decays and is broken down by decomposers, and both combustion and decomposition release carbon dioxide.

Two marks, two sentences. One sentence gets one mark, every time, no matter how well written it is. This is the most predictable question in the whole topic.

The five effects, in order

Biodiversity, Extinction, Soil, Flooding, Carbon dioxide — “Bare Earth Shows Fewer Creatures”. And when you get to the C, remember it is worth two marks, not one.

Worked example A forested hillside above a village is cleared for cattle grazing. Within three years the village floods twice and the crop yields on the valley floor have fallen. Explain both observations. [4]

Step 1 — the flooding, and it needs two mechanisms

The trees are gone, so there are far fewer roots taking water up out of the soil, and there are no leaves to intercept the rain. A greater proportion of the rainfall therefore runs over the surface and reaches the river quickly, so the river rises faster than it can carry the water away.

Step 2 — the fall in yield, which is the same event seen from below

With no roots to bind it and no canopy to shield it, the soil on the hillside is washed downhill by the rain. The layer lost is the top layer, which contains the most mineral ions, so the soil left behind on the slope supports less growth. Note that this is loss of soil, one of the five named effects.

Step 3 — check you have answered what was asked

Four marks, two observations. Two mechanisms for the flooding, two for the yield (soil removed; mineral ions removed with it). Resist the pull towards carbon dioxide — it is a real effect of deforestation but it explains neither of the two things this question asked about.

Flooding: less uptake by roots and less interception by leaves, so more surface run-off. Yield: soil eroded by rain, and the mineral ions in it lost with it.

Worked example A wetland is drained to build houses. Before draining, dragonfly larvae lived in the water, adult dragonflies fed on flying insects, herons fed on the larvae, and the herons also fed on frogs. Predict and explain the effect on the heron and the frog populations. [4]

Step 1 — draw the web before you write anything

Flying insects → adult dragonflies. Dragonfly larvae → herons. Frogs → herons. Draining the wetland removes the dragonfly larvae, because they develop in water.

Step 2 — upwards first

The herons have lost one of their two food sources, so less food is available and heron numbers are likely to fall.

Step 3 — now sideways, which is where the marks are

The herons that remain will eat a greater proportion of frogs, because that is the food source still available. So the frog population falls even though the frogs did not depend on the wetland larvae at all. A knock-on effect can reach a species that had nothing to do with the change.

Step 4 — a sentence that shows you understand the limits of the prediction

If the herons can find enough frogs, their numbers may not fall much at all — but the frogs will then fall further. Saying which of the two absorbs the change is exactly the kind of reasoned answer a “suggest” or “predict” command word is looking for.

Herons fall because one food source is gone; frogs fall because the surviving herons eat more of them instead.

Check Yourself: 20.2 Habitat Destruction and Deforestation

12 multiple choice questions. Click an option to check your answer.

Your Score 0 / 12

Question 1

What is biodiversity?

A the number of different species that live in an area

B the total number of organisms living in an area

C the size of the largest population in an area

D the number of different habitats in an area

Five words, and they are the whole mark. Biodiversity counts species, not individuals: a field holding half a million grass plants of one species has very low biodiversity. The options about total numbers and about population size are the same misconception in two costumes, and habitats are a different thing again.

Question 2

Field P contains 5000 organisms belonging to 3 species. Field Q contains 900 organisms belonging to 14 species. Which field has the higher biodiversity, and why?

A P, because it contains far more organisms

B they are equal, because biodiversity depends on the area rather than on what lives in it

C P, because larger populations are more stable

D Q, because biodiversity is the number of different species, not the number of individuals

This is the definition asked as an arithmetic question, and it catches people who learned the word but not the sentence. Fourteen species beats three species; the population sizes are irrelevant to the definition. Notice how tempting the “5000 is more than 900” reasoning is when you are working quickly.

Question 3

Which of these is one of the reasons for habitat destruction named by the syllabus?

A the seasonal migration of birds

B extraction of natural resources, such as mining, quarrying and felling timber

C decomposition of leaf litter by fungi

D competition between two species of plant

The three named reasons are: increased area needed for housing, crop production and livestock production; extraction of natural resources; and freshwater and marine pollution. The other options are all ordinary ecological processes that go on with or without people — the objective is specifically about human influences.

Question 4

Cambridge names five undesirable effects of deforestation. Which set is correct?

A reduced biodiversity, extinction, loss of soil, flooding, increase of carbon dioxide in the atmosphere

B reduced biodiversity, extinction, loss of soil, flooding, reduction of oxygen so that people cannot breathe

C extinction, loss of soil, flooding, increase of carbon dioxide, increase in the number of decomposers

D reduced biodiversity, drought, loss of soil, increase of carbon dioxide, loss of the greenhouse effect

Learn the five as a list, because the question is usually “explain the undesirable effects” with a mark for each. The commonest invented sixth effect is the oxygen one — forests do release oxygen, but atmospheric oxygen is not the effect the syllabus names, and writing it instead of the carbon dioxide point loses the mark you were being offered.

Question 5

“Explain why deforestation increases the concentration of carbon dioxide in the atmosphere. [2]” Which answer earns both marks?

A the trees are burned, so carbon dioxide is released

B there are fewer trees, so less carbon dioxide is removed from the air by photosynthesis

C there are fewer trees, so less carbon dioxide is removed by photosynthesis, and burning or decomposition of the felled wood releases carbon dioxide

D the trees release oxygen, so there is less oxygen and therefore more carbon dioxide

Two marks, two ideas, and each of the single-idea options is worth exactly one. The removal side is photosynthesis; the release side is combustion or decomposition. Write both halves every time you see this question — and note that the last option treats the two gases as though one turned into the other, which they do not.

Question 6

Why does deforestation on a hillside cause flooding lower down the valley?

A the trees no longer produce water vapour, so more rain falls

B fewer roots take up water and fewer leaves intercept the rain, so more water runs over the surface and into the river

C the soil left behind is more absorbent than the soil under a forest

D rivers flow faster when there are no trees on the bank

Two mechanisms, both worth stating: interception by the leaves, which slows the rain before it reaches the ground, and uptake by the roots, which removes water from the soil. Take both away and the rain arrives at the river all at once. The option about the soil being more absorbent has it backwards — bare soil is compacted by the rain and takes up less.

Question 7

Why does deforestation lead to loss of soil?

A the soil is carried away in the timber that is removed

B decomposers stop working once the trees have gone, so no new soil is made

C there are no longer any roots to bind the soil, and the rain strikes the bare ground directly and washes it downhill

D the soil dries out and is blown away by the wind before any rain falls

Roots hold soil in place physically; leaves take the force out of the falling rain. Remove both and the top layer, which is the layer holding the mineral ions, is the first to go. Wind erosion is real but it is not the mechanism the mark scheme is looking for, and it is not what a diagram of a rained-on hillside is showing you.

Question 8

Why can clearing one forest cause a species to become extinct rather than simply to decline?

A all species living in a forest are unable to move to another habitat

B extinction always follows any reduction in population size

C the species will have been hunted at the same time

D a species found only in that forest loses the whole of its habitat, so there is nowhere else for any individual of that species to live

The idea doing the work is that the species is found there and nowhere else. If the whole range of a species lies inside the area being cleared, then clearing it removes every individual, and the extinction is global rather than local. The option saying extinction always follows a fall in numbers is the sort of overstatement examiners penalise.

Question 9

A marsh is drained and built on. The insect larvae that developed in it disappear. Which prediction is best?

A the birds that fed on the larvae will decrease, and the plants the adult insects pollinated will produce fewer seeds

B nothing else will change, because only one species has been lost

C the birds that fed on the larvae will increase, because there is now more space

D the plants will increase, because the insects will no longer feed on them

This is the “altering food webs and food chains” objective, and it is answered by tracing the arrows in both directions. Follow the energy upwards to whatever ate the larvae, and follow the insects’ other role sideways to whatever depended on the adults. Habitat destruction rarely removes one species tidily; it removes everything that was connected to it.

Question 10

Which of these is an example of habitat destruction caused by extraction of natural resources?

A a river becoming warmer during a hot summer

B a hedgerow being colonised by a fast-growing shrub

C a population of deer growing until food becomes limiting

D an area of forest cleared and the ground removed to reach the mineral ore beneath it

Extraction means taking materials out of the ground or out of the ecosystem: mining, quarrying, drilling and felling timber. The other options describe natural change, colonisation and a population reaching the limit of its resources — all ecology, none of them human destruction of a habitat.

Question 11

Untreated sewage is discharged into a bay. Fishermen report that the seabed is now bare where there used to be seaweed and shellfish. Why does this count as habitat destruction?

A the seaweed has been eaten by the bacteria in the sewage

B the pollution has changed conditions so that the species that lived there can no longer survive, so the habitat and the community it supported are lost

C the shellfish have migrated to a cleaner bay

D sewage is not a cause of habitat destruction, because it is organic

Freshwater and marine pollution is one of the three named causes of habitat destruction, and this is why: it does not remove the physical place, but it changes conditions until the community that lived there cannot. The option calling sewage harmless because it is organic is the misconception being tested — organic material is exactly what feeds the decomposers that take the oxygen out.

Question 12

Which single change would do most to reduce the loss of habitat caused by livestock production?

A increasing the number of species of livestock kept

B moving livestock to a different field each year

C producing the same mass of meat from a smaller area of land

D feeding livestock on grass rather than on cereals

The habitat is lost because land is cleared, so the amount of land used is the quantity that matters. This is the point at which the two halves of this topic meet: intensive production is criticised on welfare grounds and defended on land-use grounds, and a good evaluation answer says both. Rotating fields and adding species change how the same land is used, not how much of it is needed.

20.3 Pollution: Sewage, Fertiliser, Plastics and the Enhanced Greenhouse Effect ▼

▶ Watch: Pollution, Eutrophication and the Greenhouse Effect

Opens on YouTube in a new tab. Watch one, then come back and do the Check Yourself questions — watching without testing yourself feels like learning but is not.

What This Sub-Topic Covers, and What It Does Not

Pollution is a huge subject and this objective is a narrow slice of it. You are responsible for exactly three things: untreated sewage and excess fertiliser in water; non-biodegradable plastics on land and in water; and methane and carbon dioxide in the air, limited to the enhanced greenhouse effect and climate change. Anything else you may have read about air pollution is outside this syllabus, and writing it costs time without earning marks.

Untreated Sewage and Excess Fertiliser

These two arrive from completely different places and do almost exactly the same thing, which is why they share an objective.

Excess fertiliser is fertiliser the crop did not absorb. Rain washes it off the surface of the field and through the soil into streams, rivers and lakes, carrying dissolved nitrate and other ions with it.

Untreated sewage brings two things at once. It contains a great deal of organic material, which decomposers can feed on immediately, and it also releases nitrate and other ions as that material is broken down. So it starts the same chain as fertiliser and, in addition, feeds the decomposers directly, which can make the oxygen fall faster.

Both end in the same place: a fall in the concentration of dissolved oxygen, and the death of organisms that need it. That whole sequence has a name, eutrophication, and it has its own section next — because it is a six-step chain and reproducing it in the right order is what earns the marks.

Nothing here is a poison

No part of this objective involves toxicity. The fertiliser is not toxic; the sewage is not toxic to fish; the algae are not releasing anything harmful. The harm is done by removing the oxygen. If your answer contains the word “poison”, it is almost certainly wrong.

Non-Biodegradable Plastics

Start with the word, because the word is the mark. Non-biodegradable means decomposers cannot break it down. Not “it does not dissolve”. Not “it rots very slowly”. Not “it takes hundreds of years”. Those last two are especially tempting and they are refused, because they concede that the material eventually re-enters the nutrient cycle, which is the exact opposite of the point.

The reason sits in something you already know about enzymes. Decomposition is enzyme-controlled, and an enzyme has a specific active site that fits a specific substrate. Decomposers have enzymes for cellulose, for starch, for protein, for fat — molecules that living things have been making for a very long time. They have no enzyme whose active site fits a synthetic polymer, so the molecule is never broken down and the carbon it contains never returns to the air as carbon dioxide. It simply stays.

The plastic passes along the chain unchanged, because no decomposer can break it down.

In aquatic ecosystems

Waste plastic is carried to the sea by rivers and by wind. Waves and sunlight break it into smaller and smaller pieces, but the pieces are still plastic.
Small pieces are swallowed by small animals, which are eaten by fish, which are eaten by seabirds. The plastic travels along the food chain because it is inside the organisms that are eaten.
The harm is physical: pieces fill the gut, so the animal feels full and feeds less, and may starve with a full stomach. Larger items — netting, rings, bags — entangle birds, turtles and seals.

In terrestrial ecosystems

The same material persists in soil and in landfill indefinitely, because the same decomposers still cannot break it down.
Grazing livestock swallow pieces from fields; the effect on their gut is the same.
Buried plastic can block drainage, changing how water moves through the soil.

Compare a paper bag. Paper is cellulose, decomposers have cellulase, and within a year the bag has been broken down: its carbon has returned to the air as carbon dioxide and its mineral ions have returned to the soil. The plastic bag lying beside it has done none of those things and will still be there when Tara is an adult. That contrast is the whole objective in one image.

Methane and Carbon Dioxide in the Air

The syllabus names two gases and one effect. Get the mechanism right and the rest follows.

Natural on the left, enhanced on the right. Only the proportion re-radiated back to the surface has changed.

How the greenhouse effect works

Short-wave radiation from the Sun passes through the atmosphere and is absorbed by the Earth’s surface.
The surface warms and re-radiates energy, now as long-wave radiation.
Carbon dioxide and methane in the atmosphere absorb some of that long-wave radiation and re-radiate part of it back towards the surface.
Less energy escapes into space than would otherwise do so, so the surface is warmer.

Notice which radiation is intercepted. The incoming short-wave radiation passes through more or less freely; it is the outgoing long-wave radiation that is absorbed. Answers that have the gases blocking sunlight on the way in are describing something else entirely, and they cannot be given the mark.

The greenhouse effect is not the pollution

The greenhouse effect is natural and it is the reason the Earth is warm enough to live on. Without it the average surface temperature would be far below freezing. What has changed is its size: extra carbon dioxide and methane enhance it, and that is what causes climate change. Write “enhanced greenhouse effect”, and never write that the greenhouse effect should be stopped.

Gas	Sources
Carbon dioxide	combustion of fossil fuels (coal, oil, natural gas) in power stations, vehicles and industry; deforestation, where the felled wood is burned or decays; respiration of all living organisms
Methane	cattle and other livestock, where micro-organisms in the digestive system break down plant material without oxygen; flooded rice fields, where the same happens in waterlogged soil; decomposition of waste in landfill sites

The effect, and the only effect you need, is climate change: rising average global temperature, changing rainfall patterns, melting ice and rising sea levels. In biological terms the consequence is that the conditions a species is adapted to shift or disappear, which is why climate change turns up again in the next-but-one section as one of the six causes of a species becoming endangered.

Worked example Explain why the mass of plastic in an ecosystem tends to increase year after year, while the mass of fallen leaves does not. [4]

Step 1 — the leaves

Leaves are made of cellulose and other organic molecules that decomposers have enzymes for. Decomposers secrete those enzymes, break the molecules down and absorb the products, so the leaf material is removed at roughly the rate at which new leaves fall. The mass present therefore stays about the same.

Step 2 — the plastic

Plastic is non-biodegradable: no decomposer has an enzyme whose active site fits it, so it is not broken down at all. Its rate of removal is effectively zero.

Step 3 — say the comparison out loud

Material accumulates whenever the rate of addition is greater than the rate of removal. For leaves the two rates are similar; for plastic the removal rate is zero, so everything that arrives stays and the total rises every year.

Step 4 — the consequence worth adding

Because it is never broken down, the carbon and other elements in the plastic are not returned to the ecosystem as carbon dioxide or as mineral ions, so it never re-enters the nutrient cycle.

Decomposers have enzymes for cellulose but none for plastic, so leaves are removed as fast as they arrive and plastic is not removed at all.

Worked example A student writes: “Cows cause climate change because they breathe out methane, which blocks the sunlight and heats the Earth.” Identify three errors. [3]

Error 1 — “breathe out methane”

Methane is not a product of respiration. It is produced by micro-organisms in the digestive system of the cow, breaking down plant material without oxygen, and it leaves the animal from the gut.

Error 2 — “blocks the sunlight”

Incoming short-wave radiation passes through. What methane absorbs is the long-wave radiation re-radiated by the Earth’s surface, which it then partly re-radiates back downwards.

Error 3 — the implied claim that this is a new effect

The greenhouse effect is natural and necessary. The correct statement is that extra methane enhances it, and it is that enhancement which causes climate change.

Methane comes from micro-organisms in the gut, not from respiration; it absorbs outgoing long-wave radiation, not incoming sunlight; and it enhances a natural effect rather than creating one.

Check Yourself: 20.3 Pollution

12 multiple choice questions. Click an option to check your answer.

Your Score 0 / 12

Question 1

What does “non-biodegradable” mean?

A it does not dissolve in water

B decomposers cannot break it down

C it breaks down very slowly, over hundreds of years

D it is poisonous to living organisms

One phrase, and Cambridge wants that phrase: decomposers cannot break it down. The “breaks down very slowly” option is the commonest answer given and it is refused, because it concedes that the material does eventually enter the nutrient cycle, and the point is that it does not. Plastic is also not usually poisonous, which is why the harm has to be explained physically.

Question 2

Untreated sewage is released into a river. What is its main effect on the river ecosystem?

A it lowers the temperature of the water, killing the fish

B it removes nitrate from the water, so the producers cannot grow

C it contains organic material that decomposers feed on, and their aerobic respiration reduces the concentration of dissolved oxygen

D it directly poisons the fish, which die within hours

Sewage does two things at once: it supplies organic material for decomposers, and it supplies nitrate and other ions. Both routes end in the same place — less dissolved oxygen. The “poisons the fish” option is the misconception this whole part of the topic is built to catch, and it reappears in the eutrophication questions. Nothing here is toxic; the oxygen is simply used up.

Question 3

Fertiliser is spread on a field. Rain washes some of it into a nearby lake. What is the first effect on the lake?

A the concentration of nitrate and other ions in the water increases

B the concentration of dissolved oxygen falls immediately

C the fish are poisoned by the fertiliser

D the number of decomposers falls, because the water is now cleaner

Getting the order right is what this topic is marked on. The extra ions come first; everything else follows from them, several steps later. Answering with the oxygen fall is not wrong biology, it is the wrong step — and in a sequence question the right idea in the wrong place scores nothing.

Question 4

A plastic bag and a paper bag are both dropped in a hedgerow. A year later the paper bag has gone and the plastic bag has not. Why?

A the paper bag dissolved in the rain

B the plastic bag is heavier, so it sank into the soil

C decomposers have enzymes that break down the cellulose in paper, but no enzyme that will break down the plastic

D the paper bag was eaten by animals and the plastic bag was not

This is the definition of non-biodegradable turned into a scene you can picture. Decomposition is an enzyme-controlled process, and an enzyme has a specific active site: no matching enzyme, no breakdown. That is also why the carbon in the paper returns to the air as carbon dioxide and the carbon in the plastic does not.

Question 5

How does non-biodegradable plastic harm animals in an aquatic ecosystem?

A it releases carbon dioxide into the water as it decomposes

B it dissolves and changes the concentration of the water

C it removes oxygen from the water by respiring

D small pieces are swallowed and fill the gut, so the animal feeds less and may starve, and larger pieces entangle birds, turtles and seals

The harm is physical, not chemical, and an answer that says “it is toxic” usually scores nothing. Two mechanisms are worth naming: blocking the gut and entangling the animal. The options about dissolving and decomposing contradict the meaning of the word non-biodegradable, and plastic, not being alive, does not respire.

Question 6

Which two gases does the syllabus name as air pollutants in this topic?

A carbon monoxide and sulfur dioxide

B carbon dioxide and methane

C nitrogen and oxygen

D methane and ammonia

The objective is explicitly limited to methane and carbon dioxide, and to one effect: the enhanced greenhouse effect and climate change. Other gases are pollutants in the wider world, but they are not on this syllabus, and writing about them uses time that earns nothing.

Question 7

Which pair gives two major sources of methane?

A cattle and flooded rice fields

B car engines and power stations

C photosynthesis and transpiration

D the combustion of coal and the burning of forests

Methane comes from anaerobic conditions where micro-organisms break material down without oxygen: in the digestive systems of cattle, in waterlogged paddy fields, and in landfill sites full of buried waste. Combustion is the classic source of carbon dioxide, so the two options that name burning belong on the other list.

Question 8

Which pair gives two major sources of carbon dioxide released by human activity?

A respiration in cattle and evaporation from lakes

B the decay of plastics and the spraying of insecticides

C photosynthesis in crops and the absorption of ions by roots

D the combustion of fossil fuels and deforestation

Burning coal, oil and gas releases carbon that has been locked away for millions of years, and deforestation counts twice over — the wood is burned or decays, releasing carbon dioxide, and the trees that were removing it have gone. Photosynthesis is the process that takes carbon dioxide out, so the option naming it is the wrong way round.

Question 9

How do carbon dioxide and methane in the atmosphere warm the Earth?

A they absorb the long-wave radiation leaving the Earth’s surface and re-radiate some of it back towards the surface, so less escapes into space

B they trap the Sun’s short-wave radiation before it reaches the ground

C they release heat as they react with oxygen in the atmosphere

D they form a solid layer that stops warm air from rising

Follow the radiation in the diagram: short-wave radiation from the Sun passes through the atmosphere and is absorbed by the surface, which warms and re-radiates energy as long-wave radiation. It is that outgoing long-wave radiation the greenhouse gases absorb. The option about trapping incoming radiation gets the direction wrong, which then makes the whole answer unmarkable.

Question 10

A student writes: “The greenhouse effect is a type of pollution and it should be stopped.” What is the correct version?

A the greenhouse effect is caused only by methane, not by carbon dioxide

B the greenhouse effect happens only over cities

C the greenhouse effect is natural and keeps the Earth warm enough for life; the problem is that extra carbon dioxide and methane enhance it

D the greenhouse effect stopped happening once fossil fuels began to be burned

The word enhanced is in the syllabus for a reason. Without a natural greenhouse effect the Earth’s average temperature would be far below freezing and this ecosystem would not exist. What human activity has changed is the size of the effect, and that is what an answer must say.

Question 11

Which effect of the enhanced greenhouse effect is examinable in this topic?

A a change in the taste of drinking water

B climate change, including rising average temperatures, changing rainfall patterns and rising sea levels

C an increase in the rate of photosynthesis in all crops

D a fall in the oxygen concentration of the atmosphere

The objective stops at “the enhanced greenhouse effect and climate change”, so that is where your answer should stop too. Species are affected because the conditions they are adapted to move or disappear — which is why climate change appears again in the conservation sub-topic as a cause of species becoming endangered.

Question 12

Plastic waste is dumped on land rather than in the sea. Which statement is correct?

A on land it is broken down by soil bacteria within a few months

B plastic causes no problems on land, because terrestrial decomposers are more varied than aquatic ones

C it is harmful only if it is burned

D it remains in the soil indefinitely, and grazing livestock may swallow pieces of it

The syllabus names both aquatic and terrestrial ecosystems, and the reason is the same in each: no decomposer can break the material down, so it stays. The options that credit soil bacteria with digesting plastic assume that a more varied community must include an organism with the right enzyme, and it does not.

20.4 Eutrophication, Step by Step ▼

Supplement

Why This Has a Section to Itself

Eutrophication gets its own section for one reason: it is a six-step mark-scheme sequence, and reproducing it in order is what earns the marks — not understanding it in general, not describing it vividly, but writing the six steps in the order Cambridge writes them.

This is Supplement material, so it is examinable for you. It is also, along with the balanced-evaluation questions in 20.1, the highest-value thing in the topic.

The six steps on the left; what happens to the dissolved oxygen on the right. Each box is one marking point.

The Six Steps, in Order

Supplement

Increased availability of nitrate and other ions. Fertiliser drains off the fields, or untreated sewage enters the water.
Increased growth of producers. Nitrate was the limiting factor; supply it and the algae and water plants multiply rapidly, covering the surface.
Increased decomposition after the death of the producers. The dense growth at the surface shades the producers below, so they cannot photosynthesise and they die. There is now a great deal of dead material.
Increased aerobic respiration by decomposers. Bacteria and fungi multiply on that dead material, and every one of them respires aerobically.
Reduction in dissolved oxygen. The decomposers take oxygen out of the water faster than it dissolves back in from the air.
Death of organisms requiring dissolved oxygen in the water. Fish and mayfly nymphs cannot respire aerobically, and they suffocate.

Read the chain once more and notice the causal joints. Step 2 happens because of step 1. Step 3 happens because of step 2 — and specifically because of shading, which is why the phrase is “after the death of the producers”. Step 4 happens because there is now food for decomposers. Step 5 is the consequence of step 4. Step 6 is the consequence of step 5. If you can say why each arrow points where it does, you cannot write the chain in the wrong order.

Suffocation, not poisoning

The fish die because there is too little dissolved oxygen for aerobic respiration. They do not die because the algae are toxic, or because the fertiliser is toxic, or because there is too much nitrate in the water for them. Nothing in this sequence poisons anything. Examiners test this misconception in almost every version of the question, because it is what most people say.

Six words for six steps

Ions → Growth → Death → Decomposers → Oxygen → Suffocation.

Say it as a sentence: “extra ions cause extra growth, the growth dies, decomposers respire, oxygen falls, fish suffocate”. Nineteen words, six marking points, and it works for fertiliser and for sewage alike.

Reading the Oxygen Graph

The other half of the marks in this sub-topic come from a graph, and it is nearly always the same graph: dissolved oxygen concentration against distance downstream from the point where the pollutant enters. Work through the one in the diagram above.

Before the discharge the oxygen is high — about 10.2 mg per dm³ at 0 km — and roughly constant.
The fertiliser enters at 4 km, and for a short distance afterwards the oxygen barely changes. That delay is important and it is often worth a mark: the algae have to grow and die before the decomposers have anything to work on.
The minimum is at about 12 km, where the oxygen is 1.0 mg per dm³. It is downstream of the discharge, not at it, because the river carries the water along while the chain is running.
The dashed line at 4 mg per dm³ marks the level below which most fish cannot survive. The curve is below it from about 7.5 km to about 18 km — a stretch roughly 10 km long in which most fish will not be found.
Recovery follows once the dead material has been used up: fewer decomposers are respiring, oxygen dissolves back in from the air, and by 30 km the concentration is back to 10.0 mg per dm³.

Worked example Explain how the run-off of fertiliser from a field can lead to the death of fish in a nearby lake. [6]

Step 1 — recognise the question type before you start writing

Six marks, one process, a “how” command. This is the chain, and the six marks are the six steps. Do not improvise; recite.

Step 2 — the six sentences

The fertiliser increases the availability of nitrate and other ions in the water. This causes increased growth of producers such as algae. The producers underneath are shaded, so they die, and decomposition increases after their death. The decomposers multiply and carry out more aerobic respiration. This reduces the concentration of dissolved oxygen in the water. Organisms that require dissolved oxygen, such as fish, then die.

Step 3 — the sentence that protects the last mark

Add: the fish die because they cannot respire aerobically — they suffocate; they are not poisoned. If the mark scheme wants “lack of oxygen” you have said it explicitly, and if a marker is looking for the misconception you have ruled it out.

Step 4 — check the order, not the prose

Number your sentences 1 to 6 in the margin as you go. A step in the wrong place usually scores nothing even when the biology in it is correct, because the mark is for the sequence.

The six steps in order, ending with suffocation rather than poisoning.

A Data Drill

Here is a different presentation of the same process: three measurements taken monthly in one lake over a year, each plotted as a percentage of its own maximum so that all three fit on one set of axes. This is a common trick in data questions — it lets an examiner compare three quantities with completely different units.

Three curves, one lake, twelve months. X is nitrate, Y is the mass of algae, Z is dissolved oxygen.

Worked example Using the graph above: (a) state the month in which each curve reaches its peak or trough; (b) explain what the order of those turning points shows; (c) suggest what happened on the surrounding farmland early in the year. [5]

(a) Read the turning points, and quote the months

Curve X, the nitrate concentration, peaks at month 3. Curve Y, the mass of algae, peaks at month 5. Curve Z, the dissolved oxygen, reaches its lowest value at month 6. Notice that Z is read as a minimum, not a maximum — a question can lose you a mark simply by changing which turning point it wants.

(b) The gaps are the evidence

The three turning points come in the order the eutrophication chain predicts, and they are separated in time. The nitrate rises first. The algae then grow, which takes weeks, so their peak is two months later. The oxygen falls last, because the algae had to die and be decomposed before the decomposers could take the oxygen out — another month again. If all three turned at once you could not say which caused which; because they are staggered in this order, the data support the chain.

Something added nitrate to the lake in the first three months of the year. The obvious suggestion is that fertiliser was applied to the surrounding fields in early spring and was washed into the lake by rain. Untreated sewage entering the lake would be an equally acceptable suggestion for the same data — on a “suggest” question, any biologically sound source of nitrate scores.

One more mark hiding in the graph

The nitrate falls sharply after month 3 while the algae are still rising. Explain that and you have shown you understand the mechanism: the algae are absorbing the nitrate to make amino acids and proteins, so the concentration in the water drops as the algae build it into themselves.

X peaks month 3, Y peaks month 5, Z is lowest month 6. The lag between them is the evidence for the chain, and the likely cause is fertiliser washed off the fields in early spring.

Three things to do with every eutrophication graph

1. Read both axes and their units — distance downstream and time look identical on paper and mean different things. 2. Find where the pollutant entered, and expect the effect to appear after it, in distance or in time. 3. Quote figures with units when you describe: “falls from 10.2 to 1.0 mg per dm³ between 4 and 12 km” beats “the oxygen goes down a lot” by two marks.

Check Yourself: 20.4 Eutrophication, Step by Step

12 multiple choice questions. Click an option to check your answer.

Your Score 0 / 12

Question 1

What is the first step of the eutrophication sequence?

A a reduction in the concentration of dissolved oxygen

B increased availability of nitrate and other ions in the water

C increased aerobic respiration by decomposers

D the death of organisms that require dissolved oxygen

Every step in this chain is a marking point, and they are only worth marks in the right order. The chain starts with the ions arriving — from fertiliser run-off or from untreated sewage. The other three options are all genuine steps, placed first to see whether you learned the sequence or just the vocabulary.

Question 2

Which order is correct?

A more ions → more decomposition → more growth of producers → less oxygen → more respiration by decomposers → organisms die

B less oxygen → more ions → more growth of producers → more decomposition → more respiration by decomposers → organisms die

C more ions → more growth of producers → more decomposition after the producers die → more aerobic respiration by decomposers → less dissolved oxygen → organisms requiring oxygen die

D more growth of producers → more ions → more respiration by decomposers → more decomposition → organisms die → less oxygen

Read each option as a story and ask what causes what. Decomposition cannot increase before there is extra dead material, and the dead material cannot exist before the extra growth. The oxygen cannot fall before something is using it. Only one ordering survives those three tests.

Question 3

Why do the fish die?

A the algae release a poison into the water

B the fertiliser in the water is toxic to fish

C the nitrate concentration is too high for the fish to survive

D the decomposers have used up the dissolved oxygen, so there is too little for the fish to respire aerobically, and they suffocate

Suffocation, not poisoning. One word decides this mark, and three of the four options are the poisoning misconception dressed differently — blaming the algae, the fertiliser or the nitrate in turn. Nothing in the water is toxic. The decomposers simply respire the oxygen away faster than it dissolves back in.

Question 4

Why does the amount of decomposition increase?

A the decomposers reproduce faster in water rich in nitrate

B the producers grow so densely that those underneath are shaded, so they die, and the dead material is broken down by decomposers

C the fish die first, and their bodies are decomposed

D the decomposers begin to feed on living algae

The syllabus wording is “increased decomposition after the death of the producers”, and the words “after the death” are the ones to explain. Algae at the surface block the light from those below; the shaded ones cannot photosynthesise, so they die, and there is suddenly a great deal of dead material. The fish die at the end of the sequence, not the beginning.

Question 5

A student writes: “Eutrophication happens when the water does not contain enough nitrate.” What is wrong?

A nothing — low nitrate starves the producers, so they die

B nitrate has no effect; it is phosphate that causes eutrophication

C eutrophication is caused by too much nitrate, which increases the growth of the producers

D nitrate is only involved when the water is very warm

The whole chain runs on an excess. This looks like a careless slip but it is a common one, because “polluted” and “lacking something” feel similar. Note also that the syllabus says “nitrate and other ions”, so phosphate is included rather than substituted.

Question 6

Which organisms are responsible for the fall in dissolved oxygen?

A the decomposers, which respire aerobically as they break down the dead material

B the algae, which respire at night

C the fish, whose numbers have increased

D the producers, which stop photosynthesising

The decomposer population rises steeply because there is so much dead material to feed on, and every one of those bacteria respires aerobically, taking oxygen from the water. Algae do respire, and photosynthesis does stop in the shaded water, but the marking point Cambridge names is the aerobic respiration of the decomposers.

Question 7

A graph shows the dissolved oxygen concentration in a river downstream of a point where fertiliser drains in at 4 km. The oxygen is 10.2 mg per dm³ at 0 km and 1.0 mg per dm³ at its lowest point. Where is that lowest point?

A at 12 km, some distance downstream of the point where the fertiliser enters

B at 30 km, the far end of the stretch measured

C at 4 km, where the fertiliser enters

D at 0 km, before the fertiliser enters

The minimum is not at the point of entry, and understanding why is worth several marks. The chain takes time to run — the producers must grow, die and be decomposed — and while that is happening the river is carrying the water downstream. So the lowest oxygen is always below the discharge point, and how far below depends on how fast the river flows.

Question 8

On that same graph the oxygen concentration rises again from about 18 km onwards and is back to 10.0 mg per dm³ by 30 km. Which explanation is best?

A the fertiliser has sunk to the riverbed

B the fish have died, so less oxygen is being used

C most of the dead material has been used up, so fewer decomposers are respiring, and oxygen dissolves back into the water from the air

D the river has become deeper, so it holds more oxygen

A recovery needs two halves as well: the demand falls, and the supply continues. Once the dead material is exhausted the decomposer population falls, and oxygen keeps dissolving in from the air at the surface, helped by any photosynthesis in the recovering water. The option about the fish is a real but tiny effect and it explains nothing about the shape of the curve.

Question 9

The same graph has a dashed line at 4 mg per dm³, labelled as the level below which most fish cannot survive. The curve lies below that line from about 7.5 km to about 18 km. What does that tell you?

A there is a stretch roughly 10 km long in which most fish species cannot survive

B no organism of any kind can live between 7.5 and 18 km

C the fish will die everywhere in the river

D the river is unaffected, because the oxygen recovers by 30 km

Read what the line actually says and no more. It marks a threshold for most fish, so other organisms — the decomposers themselves, and invertebrates tolerant of low oxygen — are thriving in exactly that stretch. Quoting the two distances and subtracting them is the sort of precise, figure-based answer that separates a full-mark response from a vague one.

Question 10

Measurements in a lake over one year show nitrate peaking in month 3, the mass of algae peaking in month 5 and dissolved oxygen at its lowest in month 6. What do the time gaps show?

A the three measurements are unrelated

B the oxygen falls first and causes the algae to grow

C the nitrate must have been measured incorrectly, because it should peak last

D each step takes time: the ions arrive first, the producers then grow, and the oxygen falls only after those producers have died and been decomposed

The lag is the evidence. If the three curves rose and fell together you could not tell which caused which; because they are staggered in the order the chain predicts, the graph supports the chain. When a data question gives you three curves, the order in which they turn is usually the answer.

Question 11

Untreated sewage and excess fertiliser have very different origins. Why do they have such similar effects on a river?

A both raise the temperature of the water

B both add ions that increase the growth of producers, and both therefore end in decomposers using up the dissolved oxygen

C both are poisonous to fish

D both prevent light from reaching the producers

One chain, two entrances. Sewage also brings organic material that decomposers can use straight away, so it can lower the oxygen even faster, but the sequence is the same one and you can write the same six steps for either. Learning it once therefore covers both objectives.

Question 12

A river is polluted by fertiliser run-off. Which organisms would you expect to find in the greatest numbers at the point where dissolved oxygen is lowest?

A mayfly nymphs and trout, which need clean water

B no organisms at all

C large fish, which are attracted to the extra food

D bacteria and other decomposers

Low oxygen is fatal to a trout and an opportunity for a decomposer — there is more dead material available there than anywhere else in the river. That reversal is the point: eutrophication does not empty the river, it replaces the community with a different one, which is why species that need plenty of oxygen are used as indicators of clean water.

20.5 Conservation and Sustainable Resources ▼

▶ Watch: Conservation and Sustainable Resources

Opens on YouTube in a new tab. Watch one, then come back and do the Check Yourself questions — watching without testing yourself feels like learning but is not.

Sustainable Resources: a Comparison of Two Rates

Cambridge defines it like this: a sustainable resource is one which is produced as rapidly as it is removed from the environment, so that it does not run out.

The definition is a comparison of two rates, and that is where the mark is. Notice what follows from this: sustainability is not a property of the resource itself, it is a property of how the resource is being used. The same forest is sustainable if you fell 100 trees a year and a hundred grow, and unsustainable if you fell 1000. The same fish stock is sustainable at one catch size and not at another. Nothing is inexhaustible; things are only used at a rate that can be kept up, or not.

The syllabus limits this to two resources: forests and fish stocks. Both are living, and that is exactly why they can be managed sustainably at all — they replace themselves. Coal does not.

The same stock, two catch sizes. On the left the catch matches the rate of replacement; on the right it does not.

The word attached to getting this wrong is overharvesting: removing individuals from a population faster than the population can replace them by reproduction. Again a comparison of two rates. A large catch is not automatically overharvesting and a small one is not automatically safe — what matters is the catch measured against the rate of reproduction.

There is a nasty feedback in the right-hand graph that is worth being able to explain. Each year of over-fishing leaves fewer adults; fewer adults produce fewer eggs; so the rate of replacement falls further, and the same catch becomes proportionally heavier. That is why an over-fished stock can collapse quite suddenly rather than declining smoothly, and why it may not recover even after fishing stops.

Why Species Become Endangered or Extinct

Six named causes. Learn them as a list, because a question asking for three of them is common and each is worth a mark.

Cause	What it does
Climate change	the conditions a species is adapted to shift or disappear, and the species may be unable to move or to adapt fast enough
Habitat destruction	the place the species lives, feeds and breeds in is removed — the largest cause of all
Hunting	individuals are killed deliberately, for food, for materials or for sport
Overharvesting	individuals are removed faster than reproduction replaces them, so the population falls year on year
Pollution	conditions change until the species can no longer survive there, as in a eutrophic river
Introduced species	a species brought to a new area may have no predators there, and may out-compete or prey on native species that have no defence against it

Four Ways to Conserve an Endangered Species

The four Core methods, what each one does, and the reasons behind all of them.

Monitoring and protecting species and habitats. Count the population regularly so that a decline is noticed early and its cause can be identified, and make the habitat a protected area so that the cause can be removed. Note that the species and the habitat are named together in one item — that is deliberate.
Education. Explain to the people who live and work in the area why the species matters and how to avoid harming it. Protection that local people understand and support continues without anyone policing it, which is why this is on the list and not a soft option.
Captive breeding programmes. Breed the species in zoos or reserves, where food is guaranteed and predators absent, and release the offspring into the wild.
Seed banks. Store the seeds of many varieties in cold, dry conditions. Respiration is very slow, so the seeds stay alive for years in a small space, and a plant species can be grown again even after it has been lost in the wild.

Captive breeding alone does not restore a species

If a species became rare because its habitat was destroyed, then breeding 300 of them in zoos and releasing them into that same destroyed habitat will not work — the released animals meet exactly the conditions that removed their ancestors. A full-mark answer always pairs captive breeding with protecting or restoring the habitat and removing the original cause of the decline.

Supplement

Conserving Forests

Four named methods.

Education — so that the people who use the forest understand why it is being managed and support the restrictions.
Protected areas — regions in which felling is not permitted at all, which preserve habitat and provide a source of seed for the surrounding forest.
Quotas — a legal limit on how much timber may be felled in a given period, set so that felling does not exceed the rate at which the forest grows. This is the sustainability definition turned into a rule.
Replanting — new trees planted to replace those removed, which raises the rate of replacement so that a larger harvest can be sustained.

Notice how quotas and replanting attack the same equation from opposite ends: one lowers the rate of removal, the other raises the rate of production. Saying that in an answer shows you have understood the definition rather than memorised the list.

Supplement

Conserving Fish Stocks

Six named methods, and each acts on a different part of the problem.

Method	What it controls	Why it works
Education	understanding	fishing communities that understand why a stock is falling are far more likely to keep to the other five
Closed seasons	when fish may be caught	no fishing during the breeding season, so the fish can reproduce undisturbed and the stock is replaced
Protected areas	where fish may be caught	areas where no fishing is allowed act as a reserve from which fish spread into the fished areas
Controlled net types and mesh size	which fish are caught	a larger mesh lets young fish through, so they survive to breed at least once before they can be caught
Quotas	how many fish may be caught	a legal limit on the mass landed, set at or below the rate of replacement
Monitoring	the information behind all of it	regular surveys of stock size show whether the other measures are working, and allow the quota to be adjusted

Mesh size decides which individuals are caught. Fish that have not yet bred pass through the larger mesh.

Supplement

Why Conservation Programmes Exist

Four named reasons, and the last one is the one most people forget.

Maintaining or increasing biodiversity — keeping the number of different species in an area high.
Reducing extinction — a species lost is lost permanently; there is no route back.
Protecting vulnerable ecosystems — some ecosystems, such as coral reefs and wetlands, are easily damaged and slow to recover.
Maintaining ecosystem functions, limited to nutrient cycling and resource provision including food, drugs, fuel and genes.

That fourth item repays a second look. Nutrient cycling is the work decomposers and other organisms do in returning carbon and nitrogen to circulation — remove them and the cycle stops. Genes means that a wild relative of a crop plant may carry an allele for resistance to a disease that has not appeared yet; lose the wild species and that allele is gone, and no amount of selective breeding can put it back. Conservation therefore has a straightforwardly practical justification as well as an ethical one, and an answer that gives the practical reasons is answering the question that was set.

Supplement

Artificial Insemination and In Vitro Fertilisation

Two techniques used inside captive breeding programmes when ordinary breeding is not enough.

Artificial insemination (AI) is the introduction of semen into the female’s reproductive tract without mating. It solves a set of very practical problems: the male and female may be in zoos on different continents, and moving frozen semen is far easier, cheaper and safer than moving a large animal; a male that is aggressive or simply uninterested can still father offspring; and one male can father offspring in several different populations, which spreads his alleles more widely.

In vitro fertilisation (IVF) goes further: egg cells are collected from the female, fertilised outside the body, and the resulting embryos are placed into a female to develop. This allows several embryos to be produced from one female, and it makes it possible to store embryos or to place an embryo of a rare species into a female of a commoner related species.

Both techniques do the same underlying job: they increase the number of offspring produced from a small number of surviving adults, and they let breeders choose which individuals reproduce together, so that as much of the remaining variation as possible is passed on.

Supplement

Why a Small Population Stays at Risk

Here is a result that surprises people. A species is hunted down to sixty individuals, the hunting is then banned completely, and the species is still in serious danger. Why, if the thing that was killing them has stopped?

Because a population carries a certain amount of genetic variation — the different alleles present among all its members — and a small population carries only a small sample of what the species once had. Thousands of individuals held between them a very large number of alleles. Sixty hold a fraction of them, and the rest are simply gone.

The consequence appears the moment conditions change. Natural selection can only act on variation that already exists: if a new disease arrives, some individuals survive it only if an allele giving resistance happens to be present in the population. In a large, varied population it very often is. In a population of sixty it may not be, and then none of them survives. The species has lost its capacity to adapt.

There is a second, separate problem. In a small population individuals are necessarily closely related, so inbreeding is unavoidable, and inbreeding makes it much more likely that an offspring inherits two copies of a harmful recessive allele and shows the condition.

This is precisely why conservation is not simply a matter of stopping the killing, and why captive breeding programmes go to such trouble to record which individuals are related to which. It is also why a seed bank stores many varieties rather than one.

Worked example A fishery lands 90 000 tonnes of cod a year. Surveys show the stock has fallen from 400 000 to 120 000 tonnes in fifteen years, and that the mean age of the fish caught has fallen from 6 years to 3 years. Explain what is happening and suggest three measures. [6]

Step 1 — name the process, using the definition

The stock is being overharvested: fish are being removed faster than the population can replace them by reproduction. The evidence is direct — the stock has fallen to 30 % of its former size while the catch has continued.

Step 2 — explain the second piece of data, which is the one most people ignore

The mean age has fallen from 6 years to 3, which means fish are now being caught younger — many of them before they have bred. Fewer breeding adults means fewer eggs, so the rate of replacement falls further, and the same catch becomes proportionally heavier each year. This is why the decline accelerates.

Step 3 — three measures, each with its mechanism

Increase the mesh size of the nets, so that young fish pass through and survive to breed at least once. Introduce a quota limiting the mass landed each year, set below the rate at which the stock replaces itself. Introduce a closed season covering the breeding period, so that spawning is undisturbed. (Protected areas and monitoring are equally acceptable, provided you say what each one does.)

Step 4 — the sentence that shows you have understood, not memorised

All of these work by bringing the rate of removal back below the rate of replacement, which is the definition of a sustainable resource. Finishing an answer by tying it back to the definition is a reliable way to pick up the last mark.

Overharvesting, made worse by catching fish before they breed. Larger mesh, a quota below the replacement rate, and a closed season.

Check Yourself: 20.5 Conservation and Sustainable Resources

12 multiple choice questions. Click an option to check your answer.

Your Score 0 / 12

Question 1

What is a sustainable resource?

A a resource that will never run out, however much of it is used

B a resource that is produced as rapidly as it is removed from the environment, so that it does not run out

C a resource that can be replaced by an artificial substitute

D a resource that is used only in small quantities

The definition is a comparison of two rates, and an answer without both rates in it does not score. Sustainability is not a property of the resource itself but of how it is being used: the same fish stock is sustainable at one catch size and unsustainable at another. That is why the first option is wrong — nothing is inexhaustible.

Question 2

What is meant by overharvesting?

A catching organisms out of season

B catching organisms that are too small to sell

C removing individuals from a population faster than the population can replace them by reproduction

D removing every individual of a species from an area

Again a comparison of rates, and again that is what the mark is for. Note that a very large catch is not automatically overharvesting, and a small catch is not automatically safe: what matters is the size of the catch relative to the rate at which the population reproduces.

Question 3

The syllabus limits sustainable management to two named resources. Which are they?

A fresh water and soil

B fossil fuels and metal ores

C forests and fish stocks

D farmland and coral reefs

Forests and fish stocks, and the reason they are the pair chosen is that both are living, so both replace themselves — which is what makes sustainable management possible at all. Fossil fuels and ores are not replaced on any useful timescale, so the word sustainable does not apply to them in the way this objective means.

Question 4

Which list gives the causes of species becoming endangered or extinct named by the syllabus?

A climate change, habitat destruction, hunting, overharvesting, pollution and introduced species

B hunting, disease, competition, predation, drought and flooding

C habitat destruction, hunting, pollution, mutation, competition and drought

D climate change, pollution, natural selection, disease, hunting and inbreeding

Six named causes, and every one of them is a human influence or is made worse by one — which is what the topic is about. Disease, predation and competition are real ecological pressures, but they are the ordinary background against which a population has always lived, and they are not on this list.

Question 5

Which set gives the four ways of conserving an endangered species named at Core level?

A monitoring and protecting species and habitats, education, captive breeding programmes, and seed banks

B captive breeding, cloning, seed banks, and gene editing

C hunting bans, education, zoos, and replanting

D protected areas, quotas, closed seasons, and controlled mesh size

The four are worth memorising as a group. The last option is the fish stock list from the Supplement material, offered here to see whether you can tell the two apart — a good habit is to ask what is being conserved: a species, a forest, or a fish stock. Each has its own list.

Question 6

Why is a seed bank a useful method of conservation?

A seeds continue to grow slowly while in storage, so the population increases

B seeds of many varieties can be stored in cold, dry conditions for years, so a plant species can be grown again even after it has been lost in the wild

C a seed bank replaces the need to protect the habitat

D seeds stored in a bank become resistant to disease

Cold and dry means the seeds are dormant: respiration is very slow, so they survive for years in a small space and at low cost. Storing many varieties matters as much as storing the species, because that is how the genetic variation is preserved. And a seed bank is a backup, not a substitute for the habitat — the seeds still have to be planted somewhere.

Question 7

A captive breeding programme increases the number of a rare antelope in zoos from 40 to 300. Released animals still fail to establish a wild population. Which explanation is most likely?

A the antelope have become a different species while in captivity

B captive breeding always fails

C 300 is still too few individuals for any population to survive

D the habitat the antelope were released into is still damaged, or still contains the cause of the original decline

Captive breeding solves the shortage of animals; it does nothing about the reason they became rare. If the grassland is still being cleared, or the hunting has not stopped, the released animals meet exactly the conditions that removed their ancestors. This is why “monitoring and protecting species and habitats” is written as one item on the list.

Question 8

Supplement Which set gives the four methods of conserving forests named by the syllabus?

A replanting, irrigation, fertilising and fencing

B education, protected areas, quotas and replanting

C closed seasons, quotas, monitoring and protected areas

D captive breeding, seed banks, education and monitoring

Education, protected areas, quotas and replanting. A quota limits how much timber may be felled in a year, so the felling can be held at or below the rate at which the forest grows — the sustainability definition applied to trees. The option listing closed seasons belongs to fish stocks, and the last option is the endangered-species list.

Question 9

Supplement Why does controlling the mesh size of fishing nets help to conserve a fish stock?

A a larger mesh lets young fish escape, so they survive to reproduce and replace the fish that were caught

B a larger mesh catches more fish per trip, so fewer trips are needed

C a smaller mesh is more selective, so only the target species is caught

D the mesh size decides how long the net can be left in the water

Everything in fish conservation comes back to letting individuals breed at least once. A fish caught before it reproduces contributes nothing to the next generation, so a stock fished with fine mesh falls even at a modest catch size. Note that a larger mesh is the conservation measure — the option praising a small mesh has it backwards.

Question 10

Supplement What is the purpose of a closed season?

A to give the fishing fleet time to repair its equipment

B to make the fish easier to count

C to allow the water to clear after a period of pollution

D to prevent fishing at the time of year when the fish are breeding, so that they can reproduce undisturbed

A closed season protects the point in the year when the population replaces itself. Set alongside quotas (how much may be taken), protected areas (where none may be taken), mesh size (which individuals may be taken) and monitoring (how we know), it completes a set of controls that each act on a different part of the same problem.

Question 11

Supplement Which reason for conservation programmes is named by the syllabus?

A to keep zoos supplied with animals for visitors to see

B to prevent any species from ever changing

C to maintain ecosystem functions, including nutrient cycling and the provision of resources such as food, drugs, fuel and genes

D to increase the population size of every species in an ecosystem

Four named reasons: maintaining or increasing biodiversity, reducing extinction, protecting vulnerable ecosystems, and maintaining ecosystem functions. That last one is the one people forget, and the “genes” part of it is worth noticing — a wild relative of a crop may carry an allele for disease resistance that plant breeders will need later.

Question 12

Supplement A population falls from several thousand to about sixty. Hunting is then stopped completely. Why is the species still at risk?

A sixty individuals cannot produce enough offspring to survive one generation

B the survivors will be too closely related to reproduce at all

C the species will now become a different species

D the remaining individuals carry only a small part of the variation the species once had, so if conditions change there may be no allele in the population that gives an advantage, and the species cannot adapt

The risk is a loss of genetic variation, and it outlives the cause of the decline. Sixty animals carry a fraction of the alleles that thousands did; if a new disease arrives, resistance may simply not be present for natural selection to act on. Inbreeding also makes harmful recessive conditions more likely to appear. The stronger-sounding options overstate the case — small populations can breed, and they do recover, which is why conservation works at all.

20.6 Exam Technique and the Vocabulary That Scores ▼

The Sentences That Score

Topic 20 looks like a topic you can write your way through with general knowledge, and it is not. Almost every mark is for one of two things: a named mechanism in the right order, or a balanced advantages-and-disadvantages answer. Here are the sentences that do the work. Each one is short enough to memorise and each one is worth at least a mark.

Biodiversity is the number of different species that live in an area.
A sustainable resource is one produced as rapidly as it is removed from the environment, so that it does not run out.
Non-biodegradable means decomposers cannot break it down.
Overharvesting is removing individuals faster than the population can replace them by reproduction.
Deforestation increases carbon dioxide because less is removed by photosynthesis AND more is released by combustion or decomposition.
The six steps: increased ions → increased growth of producers → increased decomposition after their death → increased aerobic respiration by decomposers → reduction in dissolved oxygen → death of organisms requiring dissolved oxygen.
Fish die of suffocation, not of poisoning.
The greenhouse effect is natural; extra carbon dioxide and methane enhance it.
Intensive livestock production converts food into meat efficiently because less energy is transferred to the environment as heat and less is used in movement.
A small population has less genetic variation, so if conditions change there may be no allele that gives an advantage, and the species cannot adapt.

Words That Cost Marks, and What to Write Instead

Do not write	Write	Why
“the algae poison the fish”	the decomposers use up the dissolved oxygen, so the fish cannot respire and suffocate	nothing in eutrophication is toxic; this is the misconception the objective exists to test
“eutrophication is caused by a lack of nitrate”	caused by increased availability of nitrate and other ions	the whole chain runs on an excess
“plastic dissolves eventually” / “rots slowly”	decomposers cannot break it down	both alternatives concede that it re-enters the nutrient cycle, which is the opposite of the point
“deforestation reduces oxygen so we cannot breathe”	deforestation increases carbon dioxide, for two reasons	the oxygen claim is not one of the five named effects and displaces the one that is
“biodiversity is the number of animals”	the number of different species	counts the wrong thing entirely
“the greenhouse effect is pollution”	the greenhouse effect is natural; it is enhanced by extra carbon dioxide and methane	the syllabus word is “enhanced”, and it is there on purpose
“monocultures are bad”	high yield per hectare and low cost per tonne, but pests spread easily and biodiversity is low	every evaluation question is marked on both sides
“intensive farming is cruel” as a whole answer	restricted movement raises welfare concerns, but less land is used and disease is detected early	opinion is not marked; balanced points are
“fertiliser is plant food”	fertiliser supplies mineral ions, which the plant uses to make amino acids and proteins	ions carry no energy; the energy comes from photosynthesis
“a sustainable resource never runs out”	it is produced as rapidly as it is removed	the mark is for the comparison of two rates
“captive breeding will save the species”	captive breeding plus protecting or restoring the habitat and removing the cause of the decline	animals released into a destroyed habitat meet the same conditions again
“herbicides kill the insects”	herbicides kill weeds; insecticides kill insects	the commonest slip in 20.1, and it is free to avoid

How to Read the Command Word

State / name — the name of the thing and nothing else. “State two ways a fish stock can be conserved [2]” is answered completely by “quotas” and “closed seasons”. Explaining them wastes time you will need later.
Describe — say what happens or what the data show. On a graph that means figures: “falls from 10.2 to 1.0 mg per dm³ between 4 and 12 km”.
Explain — say why. Every sentence needs a “because” or a “so”. A description will not score on an explain question, however accurate it is.
Suggest — apply what you know to something unfamiliar. There is usually more than one acceptable answer, so a sensible biological argument scores even if it is not the one printed on the mark scheme.
Discuss — and this is the one that decides your grade in this topic. Discuss means both sides. Advantages and disadvantages. Benefits and costs. A six-mark discussion is marked as roughly three points each way, with a cap on either side, so an answer that attacks intensive farming for six sentences is capped at about three marks no matter how good the sentences are. The fix takes one word: write “However…” when you are halfway through, and turn round.

A checklist for every graph in this topic

1. Read both axis labels and both units first — oxygen against distance and oxygen against time look identical and mean different things. 2. Find where the pollutant entered, and expect the effect after it. 3. Quote two figures and where they came from. 4. If you calculate, write the division down. 5. Put the unit in the final answer.

Where Topic 20 Meets the Rest of the Course

Challenge papers get their difficulty by reaching backwards, so it is worth knowing exactly where the joins are. Photosynthesis is on both sides of this topic: it is what removes carbon dioxide from the air, and it is what the shaded algae can no longer do. Aerobic respiration is the reason the dissolved oxygen falls — eutrophication is a respiration question in disguise. Decomposers explain why plastic is different from everything else. Enzymes and their specific active sites explain why no decomposer can break plastic down. Active transport in root hairs is how a crop absorbs the nitrate the fertiliser supplied. Energy transfer between trophic levels is why intensive livestock production works, and why less land is needed to produce a kilogram of meat when the animals are warm and still. Selective breeding and variation appear as one of the five ways of increasing food production, and again in the loss of genetic variation in a small population. And food webs are the tool for every habitat-destruction question.

🧬 Apply It: Three Situations Worth Thinking Through

Each of these puts Topic 20 up against something you have already learned. Read it, decide what you would write, and only then open the answer.

Two farms produce the same mass of beef each year. Farm A keeps its cattle outdoors on 400 hectares of pasture. Farm B keeps its cattle indoors on 40 hectares, in heated buildings, feeding them cereals grown on a further 120 hectares of monoculture. A student concludes that Farm A is better for biodiversity because the animals are outside.

Evaluate that conclusion. Which farm uses less land, and what has the student missed?

▼

Do the arithmetic first, because it is the fastest mark on the page

Farm A uses 400 hectares. Farm B uses 40 + 120 = 160 hectares. Farm B produces the same beef from 40 % of the land, which is 240 hectares less. Any question that hands you numbers expects you to use them.

What the student got right

Pasture genuinely does support more species than a shed and a cereal monoculture. On the land each farm occupies, Farm A almost certainly has the higher biodiversity, and the welfare of animals that can move freely is better. Both of those are real marking points and you should say so.

What the student missed — the comparison has to be per unit of food, not per hectare

The question is not “which field is richer” but “which system removes more habitat to feed the same number of people”. Farm B leaves 240 hectares that do not have to be cleared or grazed at all. If that land is left as woodland, the biodiversity across the whole area may be considerably higher than under Farm A, even though Farm B’s own land is poorer.

The balanced conclusion, which is what “evaluate” is asking for

Farm B uses far less land and monitors disease more closely, but restricts movement, concentrates waste in one place where it may drain into rivers, and depends on a monoculture with the pest and soil problems that brings. Farm A supports more species on its own land and allows natural behaviour, but needs two and a half times as much of it. Neither is simply better; the answer depends on what happens to the land that is saved.

Biology Connection

The whole of 20.1 comes down to one habit: ask “compared with what?”. A monoculture is poor compared with a hedgerow and productive compared with the land it replaces having to be doubled. Examiners award the marks to the student who names the comparison, not to the one who picks a side.

A river is sampled at three points. Site 1 is upstream of a farm; site 2 is 3 km below a drainage ditch from the farm; site 3 is 20 km below it. Dissolved oxygen is 9.8, 2.1 and 8.9 mg per dm³. Nitrate is 2, 41 and 9 mg per dm³. Mayfly nymphs are abundant at sites 1 and 3 and absent at site 2. Bacteria are hundreds of times more numerous at site 2 than anywhere else.

Explain every one of those four patterns, and say what a fisherman standing at site 2 would wrongly conclude.

▼

Start with the nitrate, because it is the first step of the chain

Nitrate rises from 2 to 41 mg per dm³ at site 2 — roughly twenty times — because fertiliser has been washed off the farmland into the ditch. By site 3 it has fallen to 9, partly because the producers have absorbed it to make amino acids and proteins, and partly through dilution.

Then the bacteria, which are the mechanism

The extra ions caused a rapid increase in the growth of producers; those shaded underneath died; the dead material is food for decomposers, so the bacterial population increased enormously. This is steps 2, 3 and 4 of the chain, read straight off a data table.

Then the oxygen, which is the consequence

Those bacteria respire aerobically, removing oxygen from the water faster than it dissolves back in, so the dissolved oxygen falls from 9.8 to 2.1 mg per dm³. By site 3 the dead material has been used up, the bacterial population has fallen, and oxygen has dissolved back in from the air, so the concentration has recovered to 8.9.

Then the mayfly nymphs, which are the effect

Mayfly nymphs require a high concentration of dissolved oxygen for aerobic respiration. At 2.1 mg per dm³ there is too little, so they cannot survive at site 2 — which is why they are used as an indicator of clean water. They are present at sites 1 and 3 because the oxygen is high at both.

The fisherman’s mistake

Standing at site 2 he sees dead fish and green water and concludes that the fertiliser has poisoned them. It has not. The fertiliser is not toxic and neither are the algae; the fish and the nymphs suffocated because a vast population of decomposers respired the oxygen away. Suffocation, not poisoning — and notice that the bacteria are thriving in exactly the water that killed the fish, which is impossible if anything there were actually poisonous.

Biology Connection

This is why the six-step chain is worth memorising word for word. Given four numbers you have never seen, the chain tells you which one to explain first, what each of the others must be, and which single word the last mark depends on.

A bird species on one island falls to 34 individuals after its forest is cleared and cats are introduced. A programme removes the cats, replants 200 hectares of forest, and breeds the birds in captivity using artificial insemination, releasing 400 over ten years. The population reaches 480 and stabilises. Twelve years later a fungal disease arrives on the island and 94 % of the birds die within two years, although the same disease kills only about 15 % of a related species on the mainland.

Explain why the programme worked, and why the disease was so much more destructive on the island than on the mainland.

▼

Why the programme worked — it treated the cause, not just the symptom

Three of the six named causes were acting at once: habitat destruction (the forest), an introduced species (the cats), and the small population itself. The programme removed the introduced predator, restored the habitat, and only then added birds. Captive breeding on its own would have failed, because released birds would have met cats and no forest — which is exactly why “monitoring and protecting species and habitats” is one item on the list rather than two.

What artificial insemination contributed

It increased the number of offspring obtained from very few surviving adults, and it let the breeders choose which individuals bred together, so that as much of the remaining variation as possible was passed on rather than lost.

Why the disease was so destructive — this is the mark that separates the top answers

All 480 birds are descended from 34 individuals, so the population carries only the alleles those 34 happened to have. The species’ original genetic variation is largely gone and no amount of breeding brings it back — 480 birds is a big number of individuals but a very small number of different alleles. On the mainland the related species has a large, varied population in which some individuals happen to carry an allele giving resistance to the fungus; those survive and reproduce, so only 15 % die. On the island that allele may simply not be present, so there is nothing for natural selection to act on and almost the whole population is susceptible.

The general principle, stated in one sentence

A population can recover its numbers quickly and its variation not at all. That is why a species that has been through a very small population size stays vulnerable for a long time afterwards, and why conservation aims to act before numbers get that low.

Biology Connection

Everything you learned about natural selection is being used here in reverse. Selection needs variation to act on; take the variation away and the species loses its ability to respond to any change at all. A count of individuals tells you how a population is doing today, and almost nothing about whether it can survive tomorrow.

Check Yourself: 20.6 Exam Technique and Vocabulary

12 multiple choice questions. Click an option to check your answer.

Your Score 0 / 12

Question 1

A question says “Discuss the advantages and disadvantages of intensive livestock production. [6]” What must your answer contain?

A six disadvantages, since that is what the phrase usually means

B a definition of intensive livestock production and then your opinion

C roughly three advantages and three disadvantages, each stated as a separate point

D one long paragraph arguing for whichever side you believe

“Discuss” is a both-sides command word, and a six-mark version of it is almost always capped at about three marks per side. Writing only one side is not a small loss, it is half the question. Your opinion is not marked at all, so a strongly argued one-sided answer scores worse than a flat, balanced list.

Question 2

Which answer would earn the mark for “explain why the fish died” in a river polluted with fertiliser?

A “The fish died because the water was polluted.”

B “The fertiliser killed the fish.”

C “The algae released a toxin, so the fish were poisoned.”

D “Decomposers respired aerobically as they broke down the dead algae, so the concentration of dissolved oxygen fell and the fish could not respire.”

An explanation needs a mechanism and a “so”. The first answer is circular, the second names a cause with no mechanism between it and the effect, and the third is the poisoning misconception. Only the last one takes you from the pollutant to the dead fish by a route the examiner can follow.

Question 3

Which phrase should never appear in an answer about eutrophication?

A “increased growth of producers”

B “the fish were poisoned by the algae”

C “reduction in dissolved oxygen”

D “aerobic respiration by decomposers”

Three of these are the syllabus’s own words for three of the six steps, and they are worth copying into your answers exactly. The remaining one, the poisoning option, is the misconception the entire objective is designed to catch. Suffocation, not poisoning — that is the sentence to carry in.

Question 4

Which definition of biodiversity would score?

A the number of different species that live in an area

B the number of plants and animals in an area

C how healthy an ecosystem is

D the variety of life on Earth

The first is the syllabus definition, word for word. The second counts individuals. The third is a vague impression, not a definition. The fourth sounds impressive and says nothing measurable — and a definition that cannot be measured cannot be marked.

Question 5

Which definition of a sustainable resource would score full marks?

A a resource that is good for the environment

B a resource that can be used again and again

C a resource that is produced as rapidly as it is removed from the environment, so that it does not run out

D a resource that is renewable

The mark is for the comparison of two rates: produced as rapidly as it is removed. “Renewable” and “can be used again” are near-synonyms that describe the resource without ever saying how fast anything is happening, and the first option is an opinion. Whenever a definition in this topic involves a rate, write the rate.

Question 6

“Explain the effect of deforestation on the carbon dioxide concentration of the atmosphere. [2]” What is the safest way to structure the answer?

A describe the appearance of the land before and after

B state that carbon dioxide increases, and give the figure by which it increases

C explain photosynthesis in detail

D write one sentence about less carbon dioxide being removed by photosynthesis, and one about carbon dioxide being released by combustion or decomposition

Two marks, two sentences, and a clear division between them. This is the single most predictable question in Topic 20, and the two-part structure means you can write it correctly even under time pressure. Explaining photosynthesis in detail answers a question that was not asked.

Question 7

Which wording for “non-biodegradable” will always score?

A it takes hundreds of years to rot away

B it does not dissolve

C decomposers cannot break it down

D it is artificial, so nothing in nature can use it

One clause, five words. “Takes hundreds of years” concedes that it does eventually break down, which is the opposite of what the word means; “does not dissolve” describes solubility, which is a different property altogether; and the last option is a generalisation rather than a definition.

Question 8

A question says “State two ways in which a fish stock can be conserved. [2]” How much should you write?

A two short phrases, with no explanation

B two named methods and a full explanation of each

C a paragraph on why conservation matters, then the two methods

D as many methods as you can think of, to be safe

“State” asks for the name of the thing and nothing more — “quotas” and “closed seasons” is a complete two-mark answer. Explaining them wastes time you will need elsewhere. Listing extra methods is riskier than it looks: if a question asks for two and you give four, examiners often mark only the first two.

Question 9

A six-mark question asks for the advantages and disadvantages of large-scale monoculture. Which plan is best?

A write everything you know about farming and let the examiner find the points

B three advantages and three disadvantages, each a separate sentence containing a reason

C one advantage explained in great depth

D a list of six words with no sentences

Marks in this topic come one per developed point, so six short reasoned sentences beat one beautiful paragraph every time. A bare list of words usually fails, because the mark is for the consequence: not “machinery” but “the whole field ripens together, so it can be harvested by machine, which lowers the cost per tonne”.

Question 10

You are given a graph of dissolved oxygen against distance downstream. What should you do first?

A describe the shape of the curve

B calculate the mean oxygen concentration

C look for the lowest point

D read both axis labels and their units, then find the point where the pollutant enters

Axes first, always — a graph of oxygen against distance asks a different question from one against time, and the two look identical at a glance. Locating the discharge point next gives you the reference for everything else you will be asked: how far downstream the minimum lies, and how far it takes to recover.

Question 11

Which earlier part of the course is most directly needed to explain why dissolved oxygen falls during eutrophication?

A aerobic respiration, and the fact that decomposers respire like everything else

B active transport in root hairs

C selective breeding

D the structure of the leaf

Eutrophication is a respiration question wearing a pollution costume. The reason for the fall is that a very large population of decomposers is respiring aerobically, which uses oxygen. Root hairs, leaves and selective breeding all appear elsewhere in this topic — but not here, and an answer built on the wrong earlier idea reads as though you have guessed.

Question 12

You have four minutes left. Question 5(b) asks you to state the six steps of eutrophication in order [3] and question 6(c) asks you to discuss the advantages and disadvantages of intensive livestock production [6]. What should you do?

A answer 6(c) first, because it is worth more marks

B write the six steps, which you know verbatim, then put as many points as you can into 6(c)

C write a plan for 6(c) and leave 5(b)

D check the answers you have already written instead

Marks per minute is the only thing that matters at the end of a paper. A sequence you have memorised is the fastest three marks available anywhere, and it cannot expand to fill the time. A discussion question always can, which is exactly why starting with it is the trap most people fall into.