Cambridge IGCSE Biology 0610. Topic 19/20: Organisms and their environment / Human influences on ecosystems

Human Influences on Ecosystems

By the IGCSE Biology Team · reviewed for accuracy by our Biology specialists · last reviewed June 2026 · about us

In short

Most marks here are for cause-and-effect sequences in the right order. Eutrophication runs: excess nitrate → algal bloom → light blocked → plants die → decomposers respire and use up oxygen → fish die. The step students skip is that it is the decomposers respiring, not the algae, that removes the dissolved oxygen.

The exam angle

This topic is examined as cause-and-effect sequences, and the marks are in the order of the steps, not the keywords. Eutrophication is the classic: students name algae and dead fish but skip the decomposers-using-oxygen step that actually carries the marks. Learn three sequences cold, eutrophication, the greenhouse effect, deforestation consequences, and this becomes one of the most predictable topics on the paper.

Eutrophication: get the sequence in the right order

The chain, in the order the mark scheme awards:

Excess nitrate (from fertiliser run-off) or sewage enters the water.
It causes rapid growth of algae and water plants. An algal bloom.
The bloom blocks light, so plants below cannot photosynthesise and die.
Decomposers (bacteria) feed on the dead plants and algae, multiply, and respire, using up oxygen.
The dissolved oxygen concentration falls, so fish and other aquatic animals die.

The step that separates a full-mark answer from a half one is step 4: it is the decomposers respiring that removes the oxygen, not the algae 'using up oxygen' directly. Name nitrate as the ion responsible, 'fertiliser' alone is weaker.

The greenhouse effect and climate change

The mechanism: greenhouse gases. Mainly carbon dioxide (from burning fossil fuels and deforestation) and methane (from cattle and rice fields/landfill). Absorb and re-emit heat (long-wave radiation) that would otherwise leave the Earth, so the atmosphere warms. Consequences examiners accept: climate change, melting ice caps, rising sea levels, changing rainfall and more extreme weather affecting habitats. Be precise that the gases trap heat/long-wave radiation: 'CO2 traps the sun's rays' is too loose. The link to deforestation is double: cutting trees releases stored carbon AND removes the trees that would absorb CO2 in photosynthesis.

Deforestation and pollution

Deforestation consequences to list with reasons: loss of habitat and reduced biodiversity (species lose where they live); soil erosion (no roots to bind soil) and flooding; less photosynthesis so more CO2 remains, adding to the greenhouse effect; disruption of the water cycle. Plastic pollution in water: plastics are non-biodegradable, persist for a long time, and harm aquatic animals (entanglement, blocked guts when eaten). Water pollution from sewage links back to eutrophication and to spreading pathogens. For each, the exam wants the pollutant, the mechanism of harm, and the effect on organisms. Three linked parts, not a single word.

Conservation and sustainable resources

The positive half of the topic. Sustainable development means meeting present needs without stopping future generations meeting theirs. Sustainable resource use examples: replanting forests (reforestation) and quotas/protected areas for fish. Reasons for conserving species and habitats that score: maintaining biodiversity, protecting food chains, potential medicines and genetic resources, and keeping ecosystems functioning. Recycling and reducing fossil-fuel use connect to the carbon dioxide and pollution sections. Questions often ask you to 'suggest how a named resource could be managed sustainably'. Give a specific method (e.g. fishing quotas, restocking, net mesh size to let young fish through) rather than 'don't overuse it'.

Process diagram Eutrophication, step by step The marks are in the order. Step 4 (decomposers respiring) is the one students skip.

Excess nitrate (fertiliser run-off) enters the water
Rapid algal growth → algal bloom blocks light
Plants below cannot photosynthesise and die
Decomposers feed on dead matter, multiply and respire
Dissolved oxygen falls → fish and animals die

Worked exam question (6 marks)

Fertiliser from a field drains into a nearby river. Explain how this can lead to the death of fish in the river.

Show the mark-scheme answer

fertiliser adds nitrate (ions) to the water; causes rapid growth of algae / algal bloom; algae block light so plants below die; decomposers / bacteria feed on dead plants and algae and increase in number; decomposers respire and use up (dissolved) oxygen; oxygen concentration in the water falls so fish die;

Each point separated by a semicolon is worth one mark. This is how real Cambridge mark schemes are written.

Common mistakes that cost marks

Skipping the decomposer step in eutrophication. It is bacteria respiring that removes the oxygen, not algae directly.
Writing 'fertiliser' instead of naming nitrate as the ion responsible.
Saying greenhouse gases 'trap the sun's heat' loosely. They absorb and re-emit long-wave radiation/heat leaving the Earth.
Listing deforestation effects without reasons (e.g. 'soil erosion' with no 'no roots to hold the soil').
Confusing biodegradable and non-biodegradable when discussing plastics.
Answering conservation questions with 'use less' instead of a specific management method (quotas, restocking, reforestation).

How to phrase the 6-mark answer on this topic

Almost every six-marker here is a sequence, so write numbered or clearly ordered steps and never leave a gap in the chain. For eutrophication, the examiner is literally ticking: nitrate in → algal bloom → light blocked → plants die → decomposers respire → oxygen falls → fish die. For the greenhouse effect, go: gas named → source → absorbs/re-emits long-wave radiation → atmosphere warms → named consequence. Naming the specific agent at each step (nitrate, decomposers, methane, long-wave radiation) is what lifts the answer from 4 to 6.

Learn the full 6-mark technique →

For students in Malaysia

This is the most locally relevant topic on the syllabus. Deforestation for oil palm, river pollution, transboundary haze and coral/marine impacts are all real Malaysian issues that examiners have used as contexts. You will have strong background knowledge; the exam skill is translating it into the precise Cambridge sequences (especially the decomposer/oxygen step) rather than writing a general essay about the environment.

Frequently asked questions

What is eutrophication and what causes fish to die?

Eutrophication is over-enrichment of water with nutrients. Excess nitrate causes an algal bloom that blocks light, so plants die; decomposers feed on the dead matter, multiply and respire, using up the dissolved oxygen, so fish die. It is the decomposers respiring that removes the oxygen.

How do greenhouse gases cause climate change?

Greenhouse gases such as carbon dioxide and methane absorb and re-emit heat (long-wave radiation) that would otherwise leave the Earth, so the atmosphere warms. This leads to climate change, melting ice caps and rising sea levels.

What are the effects of deforestation?

Deforestation reduces biodiversity by destroying habitats, causes soil erosion and flooding (no roots to bind the soil), and increases atmospheric carbon dioxide because fewer trees photosynthesise and stored carbon is released.

Exam guides for this topic

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Human Influences on Ecosystems

Eutrophication: get the sequence in the right order

The greenhouse effect and climate change

Deforestation and pollution

Conservation and sustainable resources

Worked exam question (6 marks)

Common mistakes that cost marks

How to phrase the 6-mark answer on this topic

For students in Malaysia

Frequently asked questions

Frequently asked questions

Related topics

Exam guides for this topic