Biology 0610Chapter 20 of 21

Human influences on ecosystems

Cambridge IGCSE Biology 0610 Topic 20 revision chapter covering human influences on ecosystems, tiered throughout into Core content for all candidates and Supplement content for Extended candidates. Topic 20 is twenty numbered statements across four subtopics, and its tier split is unusually uneven: 20.1 Food supply and 20.2 Habitat destruction are entirely Core with no Supplement content at all, while five of the topic's six Supplement statements sit inside 20.4 Conservation. The chapter is built to follow that shape, running the Core of all four subtopics first in official order and then the six Supplement statements, with each Supplement block opening by naming the Core section it extends. The Core half opens with the subtopic that carries the heaviest load of new content: the five named ways humans have increased food production - agricultural machinery to work larger areas and improve efficiency, chemical fertilisers to replace the mineral ions harvesting removes, insecticides to reduce loss and damage by pests, herbicides to remove the weeds that compete for light, water and mineral ions, and selective breeding, given in a single clause with its full treatment left to Topic 18 where the syllabus puts it. Monoculture and intensive livestock production follow as the two explicitly two-sided statements in the topic, each treated as one property read twice: the uniformity that lets one machine harvest a whole field is the same uniformity that lets one pathogen reach every plant in it, and the restricted movement and controlled warmth that put more of an animal's food energy into growth are the same conditions that raise welfare concerns and let disease spread. Habitat destruction then supplies the definition the rest of the topic measures harm against - biodiversity as the number of different species that live in an area - before the three named reasons for habitat destruction, the statement that humans can damage a habitat by altering its food webs without removing any part of it, and deforestation as the syllabus's worked example, taught as five separate causal chains because the official list names reducing biodiversity and extinction as distinct effects. Pollution gives the Core effects of untreated sewage and excess fertiliser on aquatic ecosystems, of non-biodegradable plastics in both aquatic and terrestrial ecosystems, and of methane and carbon dioxide in the air, held strictly inside the limit the syllabus sets on the last of these. Conservation closes the Core half with the sustainable-resource definition, the six named reasons organisms become endangered or extinct, and the four named ways an endangered species can be conserved. The Supplement half then adds the six statements Extended candidates need: the full six-step eutrophication mechanism, which is where the topic's highest-value answer lives and which exists precisely to explain why the dissolved oxygen falls; the four forest methods and six fish-stock methods with the mechanism attached to each; the four reasons for conservation programmes with the official limit on ecosystem functions enforced; the use of artificial insemination and in vitro fertilisation in captive breeding; and the risk to a species whose population has fallen, taught as the loss of allele variety rather than the loss of individuals. Throughout, the chapter marks where a statement stops: ozone-layer depletion and the bioaccumulation of pesticides along food chains are not part of this syllabus and are treated as supporting context or left out, and every cross-reference to Topics 15, 18, 19 and 21 is a single clause rather than a second lesson. Because Topic 20 names no required practical, the practical section says so plainly and trains the AO3 skill that does reach Papers 5 and 6 from here: reading a pollution profile down a river, describing a trend that falls to a minimum and recovers, calculating a percentage change, and refusing a causal conclusion the data cannot support.Show more

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A summary of this Biology chapter — open a section to read it. The full notes, worked examples and practice questions are in the study modules above.

What you need to be able to do

Core 20.1.1 Describe how humans have increased food production, limited to agricultural machinery, chemical fertilisers, insecticides, herbicides and selective breeding — each with the mechanism by which it raises production.
Core 20.1.2 Describe the advantages and disadvantages of large-scale monocultures of crop plants.
Core 20.1.3 Describe the advantages and disadvantages of intensive livestock production.
Core 20.2.1 Describe biodiversity as the number of different species that live in an area.
Core 20.2.2 Describe the reasons for habitat destruction, including increased area for housing, crop plant production and livestock production; extraction of natural resources; and freshwater and marine pollution.
Core 20.2.3 State that through altering food webs and food chains, humans can have a negative impact on habitats.
Core 20.2.4 Explain the undesirable effects of deforestation as an example of habitat destruction, to include reducing biodiversity, extinction, loss of soil, flooding and increase of carbon dioxide in the atmosphere.
Core 20.3.1 Describe the effects of untreated sewage and excess fertiliser on aquatic ecosystems.
Core 20.3.2 Describe the effects of non-biodegradable plastics, in both aquatic and terrestrial ecosystems.
Core 20.3.3 Describe the sources and effects of pollution of the air by methane and carbon dioxide, limited to the enhanced greenhouse effect and climate change.
Core 20.4.1 Describe a sustainable resource as one which is produced as rapidly as it is removed from the environment so that it does not run out.
Core 20.4.2 State that some resources can be conserved and managed sustainably, limited to forests and fish stocks.
Core 20.4.3 Explain why organisms become endangered or extinct, including climate change, habitat destruction, hunting, overharvesting, pollution and introduced species.
Core 20.4.4 Describe how endangered species can be conserved, limited to monitoring and protecting species and habitats, education, captive breeding programmes and seed banks.
Supplement 20.3.4 Explain the process of eutrophication of water, limited to increased availability of nitrate and other ions, increased growth of producers, increased decomposition after death of producers, increased aerobic respiration by decomposers, reduction in dissolved oxygen, and death of organisms requiring dissolved oxygen in water.
Supplement 20.4.5 Explain how forests can be conserved using education, protected areas, quotas and replanting.
Supplement 20.4.6 Explain how fish stocks can be conserved using education, closed seasons, protected areas, controlled net types and mesh size, quotas and monitoring.
Supplement 20.4.7 Describe the reasons for conservation programmes, limited to maintaining or increasing biodiversity, reducing extinction, protecting vulnerable ecosystems, and maintaining ecosystem functions — limited to nutrient cycling and resource provision, including food, drugs, fuel and genes.
Supplement 20.4.8 Describe the use of artificial insemination (AI) and in vitro fertilisation (IVF) in captive breeding programmes.
Supplement 20.4.9 Explain the risks to a species if its population size decreases, reducing genetic variation. Knowledge of genetic drift is not required.
Read and describe a trend in a table or graph of dissolved oxygen, nitrate concentration, population size or forest area against distance or time, including a trend that is not smooth.
Calculate a percentage change, and distinguish it from a change in percentage points.
Judge what a set of data does and does not establish, and refuse a causal conclusion it cannot support.

Common mistakes to avoid

One more, specific to the greenhouse effect. Do not say that a particular storm, drought or hot summer was caused by global warming — individual weather events cannot be attributed that simply. And the natural greenhouse effect is not a problem: without it the Earth would be far too cold to support life. What this topic is about is the effect of increasing the concentration of those gases.
This section is one half of a two-sided topic, and the other half is coming. Fertilisers raise yield and also cause eutrophication (20.3). Insecticides protect the crop and also kill non-target insects (20.2). Machinery works more land, and working more land is one of the reasons habitats are destroyed (20.2). None of that makes the benefits false. When a question asks how humans have increased food production, give the production answer; when it asks about effects on ecosystems, give the other one. Answering the wrong half of a genuinely two-sided topic is the most common way marks are lost in Topic 20.
Write the welfare point as biology, not as opinion. “It is cruel” is not a mark. “The animals are kept at high density with restricted movement, so they cannot carry out normal behaviours such as moving freely, and this raises welfare concerns” is. The syllabus asks you to describe advantages and disadvantages, which means setting both out accurately — not deciding for the examiner which side wins.
Biodiversity is not “the number of organisms”, “the number of animals” or “the amount of wildlife”. None of the three is the syllabus wording, and none earns the mark. It is the number of different species living in an area. Learn the phrase as it stands.
“Humans destroy habitats” is not a reason. Nor is “deforestation” on its own, which names the act rather than the reason for it. The question is why the land or water is wanted: for housing, for crops, for livestock, for the resources under it, or because waste has been put into it.
Do not turn every pesticide into a catastrophe. Insecticides and herbicides are two of the five named ways food production has been increased (20.1 A), and they work. The statement here is that humans can have a negative impact by altering food webs, and the honest version of the argument keeps both halves: the pest or the weed is controlled, and non-target organisms are affected too. Use the question's own context to decide which side it is asking about.
Two wrong versions to unlearn now. First: “the fertiliser is toxic and kills the fish” — the fertiliser is a nutrient, and it kills nothing directly. Second: “the algae block the light so the fish cannot see and die” — light shortage kills submerged plants, not fish. Both sentences describe something that does not happen, and both lose the marks outright.
Do not claim that all plastics are toxic. The syllabus effects here are physical — blockage, entanglement, contamination, accumulation — and they follow from persistence, not from poisoning. Broad claims about every plastic being poisonous go beyond what this topic supports, and a careful answer says what the plastic does rather than what it supposedly contains.
Three claims to avoid. Do not say that a particular storm, drought or hot summer was caused by global warming — individual weather events cannot be attributed that simply. Do not invent figures: unless a question supplies data, write about the direction of change, not a number of degrees or a percentage. Made-up statistics are an academic error even when the underlying point is right. And do not write that cattle respire methane: aerobic respiration in an animal releases carbon dioxide and water, and the methane comes from microorganisms living in the animal's digestive system, and from its waste.
Conservation is not the same as banning use. A conserved forest can still be logged, and a conserved fish stock can still be fished — at a rate the population can replace. Answers that treat conservation as prohibiting all human activity miss the point of every method in the next three sections, all of which are about managing use rather than ending it.
Renewable does not mean sustainably managed. Fish reproduce, so a fish stock is renewable in principle — and stocks have still been fished to collapse. Timber is renewable, and forests are still cleared faster than they regrow. Renewability says a resource can be replaced; sustainability says it is being replaced as fast as it is taken. Only the second is the definition here.
Do not claim inevitability. “The species will become extinct” is not supportable from overharvesting alone. Populations recover when harvesting is reduced below the replacement rate, which is exactly what quotas, closed seasons and mesh-size rules are designed to achieve. Write may decline, may become endangered, may become extinct. The distinction between endangered and extinct exists precisely because the first is recoverable.
Not every introduced species is invasive. Most fail to establish at all, and many that do have little measurable effect. Wheat, potatoes and honeybees are non-native almost everywhere they are grown or kept. The syllabus wording is deliberately conditional, and an answer claiming that introducing any species always damages an ecosystem is claiming more than the biology supports. The syllabus term is introduced species; non-native species means the same thing and is not penalised.
“Put them in a zoo” is not the answer to everything. Captive breeding is genuinely one of the four named methods, but on its own it protects individuals rather than a species in the wild: the habitat is still being lost, the population in captivity is small, and released animals may not survive. The strong version of the answer pairs it with habitat protection — breed the animals safely and make sure there is somewhere to release them.
Two wrong versions to unlearn. First: “the fertiliser is toxic and kills the fish” — the fertiliser is a nutrient, and it kills nothing directly. Second: “the algae block the light so the fish cannot see and die” — light shortage kills submerged plants, not fish, and even that is only step 4. Both answers omit the decomposers, which is where the oxygen actually goes.
A larger mesh does not prevent every unwanted catch. It lets small fish through, so juveniles of the target species and small non-target species escape — but large non-target animals are still caught. Claiming that mesh size eliminates by-catch overstates it; write that it reduces by-catch and allows young fish to escape.
Genetic variation within a population is a different statement. It is easy to answer this question with “to preserve genetic variation”, and that is a real reason — but it is Supplement 20.4.9, in section 20.4 H, and it is about the risk to a species whose population has shrunk. Statement 20.4.7 asks about the four reasons above. Know which of the two a question is asking for.
IVF is not the same as cloning or genetic modification. Both AI and IVF involve an egg cell and a sperm cell from two parents, and fertilisation produces a zygote with a new combination of alleles exactly as it would after natural mating. Neither technique changes any genes, and neither produces genetically identical offspring.
Three sentences to avoid. “The population mutates to survive” — mutation is random and does not happen because it is needed. “The animals adapt to the disease” — an individual does not adapt during its lifetime; the population changes because different individuals survive at different rates. “Inbreeding causes mutations” — it does not; it makes it more likely that two copies of an existing recessive allele come together. Note also that genetic drift is explicitly not required by this statement, so there is no need to name it.

Examiner tips

Where the tiering bites in Topic 20. It bites in one place above all, and it is worth understanding before you read section 20.3 A. Core 20.3.1 says describe: what happens to a river or lake that receives untreated sewage or excess fertiliser — nutrients rise, producers grow, dissolved oxygen falls, organisms that need it die. A Core answer that gives those effects in order is complete. Supplement 20.3.4 says explain: the process of eutrophication, and the official statement then lists six steps, two of which — decomposition increasing after the producers die, and decomposers respiring aerobically — are the reason the oxygen falls at all. So the mechanism lives in the Supplement run, section 20.3 D, and section 20.3 A stops before it without leaving the Core candidate with anything wrong.
Three things that reliably cost marks in this topic. Stopping a chain one link short — the marks are in the middle, not at the end. Answering the wrong half of a two-sided question, because the topic sounds like it has a right answer. And inventing figures: unless a question supplies data, write about the direction of a change, never a number of degrees or a percentage.
Two pairs that are constantly confused. Endangered and extinct — the first is a population at risk, the second is a species that is gone. And renewable and sustainable — renewable says a resource can be replaced, sustainable says it is being replaced as fast as it is taken. Only the second is the definition this syllabus examines.
Five methods, and a closed list. The statement says limited to, which cuts both ways. You are not expected to discuss irrigation, greenhouses, hydroponics or genetic modification in answer to this statement — and an answer that offers them instead of the five named methods is answering a question that was not asked. Genetic modification of crops is real and is examined, but it belongs to Topic 21.
Say why, not just that. “Monoculture reduces biodiversity” is one mark. “Monoculture reduces biodiversity because one species has replaced a community of many, so there are far fewer different habitats and food sources for other organisms” is two or three. The same is true of the pest point: the mark is for unlimited food supply for that pest, in a stand of closely-spaced similar plants, not for the word “disease”.
The statement says including, not limited to. That is a deliberate difference from 20.1.1, and it matters: the three groups above are the ones you must know, and a further genuine reason offered alongside them — a dam flooding a valley, a road cutting a forest in two — is not wrong. What loses marks is offering an extra example instead of the named three.
Follow the arrows, do not jump to the end. The reliable method for any web question is the one used in Topic 19: ask what the changed organism ate, what ate it, and what competed with it, and write one sentence for each. An answer that goes straight from “the weeds were sprayed” to “the birds declined” has missed the two marks in the middle.
“Habitats are destroyed” is one mark at most. The examiner is counting linked steps. Adding “so the animals that lived in those trees lose their food and shelter, and their populations fall” turns one mark into three. If a question is worth four or more marks on a single consequence, it wants the whole chain, not four different consequences.
Most real cases are two or three of these at once. A rainforest primate is usually endangered because its forest is being cleared and it is hunted; a coral reef fish because the water is warming and polluted and overfished. If a question gives you a context, look for more than one cause in it, and give each one with its mechanism.
The list is limited to these four. Ways of conserving forests and fish stocks — quotas, closed seasons, mesh sizes, replanting — are separate Supplement statements (20.4 D and 20.4 E), not items on this list. If the question says endangered species, answer with these four; if it says forest or fish stock, use the other list.
Pair every method with a mechanism. “Quotas” scores nothing. “Quotas limit the number of trees felled each year, so trees are removed no faster than new ones grow to replace them” scores. If a question asks for three methods, that is three method-plus-mechanism pairs, not three nouns. The official verb here is explain, and it is not decoration.
Stay inside the limit on (d). Pollination of crops, stabilising of soil by roots and regulation of the climate are all genuine ecosystem services, and none of them is on this syllabus's list. The list is nutrient cycling and resource provision: food, drugs, fuel and genes. Writing about pollination instead of those will not earn the mark the question is offering.
The verb is describe, so keep the biology at that level. What is wanted is: what AI is, what IVF is, and what they are used for in a captive breeding programme. Hormone treatments, embryo culture media and the detail of human fertility treatment are not part of this statement. The reproductive biology behind fertilisation belongs to Topic 16.
Step 3 is the statement; step 4 is the mark. Saying “a small population has less genetic variation” restates the question. What 20.4.9 asks you to explain is the risk: what happens to a population with little variation when something changes. Get to a named change — a disease, a rise in temperature, a new predator — and say that there may be no individual able to survive it.
Watch which list a question is asking for. Three of the six Supplement statements come with a named list, and the lists are different: forests get four methods, fish stocks get six, and conservation reasons get four with a further limit inside reason (d). The fastest way to lose marks in the Extended half of Topic 20 is to give a correct list in answer to a question about the other one.
Four conclusions that lose marks. “Ban all fishing” — ignores that conservation manages use rather than ending it. “Education will solve it” — education changes understanding, not the removal rate, on its own. “Quotas always work” — they work only when set correctly and enforced. “Replanting restores the ecosystem” — it restores tree cover; biodiversity is a separate measurement. Each of these states an outcome without the mechanism or the evidence.

How Human influences on ecosystems is examined

Topic 20 reaches every paper you sit. Its statements are chains, so they suit extended-answer questions on the theory papers, and it reaches Papers 5 and 6 through data rather than through a practical of its own.
Six statements use it, and all six want a chain: deforestation (20.2.4), eutrophication (20.3.4), endangerment (20.4.3), conserving forests (20.4.5), conserving fish stocks (20.4.6) and genetic variation (20.4.9). Count the marks, then count your linked steps. They should match.
Most of the topic. Give the named items from the official list, each with the short mechanism attached. A bare list of nouns is usually half the marks at best.
20.2.3 and 20.4.2 only. Here the short answer is the answer, and padding it with an essay wastes time you need elsewhere.
Applied to an unfamiliar context. Give the benefit with its mechanism, the limitation with its mechanism, and a conclusion that names the condition it depends on. Do not assert a verdict without the evidence that would support it.
Three things that reliably cost marks in this topic. Stopping a chain one link short — the marks are in the middle, not at the end. Answering the wrong half of a two-sided question, because the topic sounds like it has a right answer. And inventing figures: unless a question supplies data, write about the direction of a change, never a number of degrees or a percentage.

Frequently asked questions

Is biodiversity the same as the number of organisms?

No. Biodiversity is the number of different species living in an area. A wheat field contains millions of organisms and has very low biodiversity, because nearly all of them are one species.

What exactly kills the fish when fertiliser reaches a river?

Lack of dissolved oxygen. The fertiliser is a nutrient and kills nothing directly. Producers grow rapidly and then die, decomposers feeding on them multiply, and those decomposers respire aerobically and use up the oxygen dissolved in the water.

Do Core candidates need the eutrophication steps?

No. Core 20.3.1 asks you to describe the effects of untreated sewage and excess fertiliser on aquatic ecosystems, which you can do without naming the decomposers. The six-step mechanism is Supplement 20.3.4 and is examined only on Papers 2 and 4.

Are insecticides and herbicides good or bad?

Both, and the syllabus examines both. They are two of the five named ways food production has been increased (20.1.1), and they are also an example of humans altering food webs (20.2.3). Answer the half the question is asking about.

How many undesirable effects of deforestation are there?

Five: reducing biodiversity, extinction, loss of soil, flooding, and increase of carbon dioxide in the atmosphere. Biodiversity and extinction are listed separately, so treating them as one point loses a mark.

Is a monoculture simply bad?

No, and the statement asks for advantages and disadvantages. It allows the whole area to be worked by machinery in one operation with one set of growing conditions, which is why it feeds so many people. It also has very low biodiversity, lets a pest spread through the whole field, and depletes the soil of the same mineral ions year after year.

Is the greenhouse effect the same as the hole in the ozone layer?

No. The greenhouse effect involves thermal radiation leaving the Earth's surface; ozone depletion involves ultraviolet radiation arriving from the Sun. Ozone is not examined anywhere in this syllabus, and 20.3.3 is explicitly limited to the enhanced greenhouse effect and climate change.

Is the natural greenhouse effect a problem?

No — without it the Earth would be far too cold to support life. The problem is the increase in the concentration of greenhouse gases, which raises the proportion of thermal radiation retained.

What is the difference between renewable and sustainable?

Renewable says a resource can be replaced. Sustainable says it is being replaced as fast as it is removed. Fish stocks are renewable and have still been fished to collapse. Only the second is the definition this syllabus examines.

Which resources can be conserved and managed sustainably?

The syllabus limits statement 20.4.2 to forests and fish stocks, and those are the two that reappear as Supplement statements with their own method lists.

Does conservation mean stopping all use?

No. Conservation manages human use of a resource. A conserved forest can still be logged and a conserved fish stock can still be fished — at a rate the population can replace. Every method in 20.4 is about managing use, not ending it.

Is every introduced species harmful?

No. Most fail to establish, and many that do have little measurable effect — wheat, potatoes and honeybees are non-native almost everywhere they are grown or kept. The syllabus wording is conditional, and so should your answer be.

Will an over-harvested species definitely become extinct?

No. The population declines while removal exceeds replacement, and it recovers when the catch falls below it. That is exactly what quotas, closed seasons and mesh-size rules are for. Write may become endangered, may become extinct.

If a captive breeding programme increases the numbers, is the species safe?

Not by itself. Captive breeding protects individuals from the threats in the wild, but it does nothing about the reason the species became endangered. If the habitat is still being destroyed there is nowhere to release the animals, which is why the syllabus lists monitoring and protection of habitats alongside it.

Do I need to know about genetic drift?

No — statement 20.4.9 says so explicitly. What you need is the chain: fewer individuals, so fewer different alleles, so less genetic variation, so fewer individuals able to survive a new disease or a change in conditions.

Syllabus reference and sources

Written against: Cambridge IGCSE Biology (0610), syllabus for 2026, 2027 and 2028 (version 2, published December 2025), Subject Content, Topic 20: Human influences on ecosystems, subtopics 20.1 Food supply, 20.2 Habitat destruction, 20.3 Pollution and 20.4 Conservation.

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