IGCSE Biology: Human Influences on Ecosystems Practice Questions
Human activity affects ecosystems through habitat destruction, pollution and over-exploitation of resources. Eutrophication, deforestation and plastic pollution are the three examples Cambridge tests most often, and each has a sequence of consequences that must be given in order.
Topic 20 of Cambridge IGCSE Biology 0610 is marked on sequences. Eutrophication in particular is a five step chain, and skipping a step loses the mark for it even if the conclusion is right. The questions below train each sequence in the order the mark scheme expects.
What you need to know for Human Influences on Ecosystems
- EutrophicationExcess nitrate from fertiliser runs off into water, causing rapid algal growth. Algae block light, so submerged plants die. Decomposers multiply as they feed on the dead material, and their aerobic respiration removes oxygen from the water, so fish and other organisms die.
- DeforestationCauses loss of habitat and reduced biodiversity, extinction of species, soil erosion and flooding because roots no longer bind the soil, and increased atmospheric carbon dioxide because fewer trees photosynthesise and burning releases carbon.
- Plastic pollutionPlastics are non-biodegradable, so they persist for a very long time. Marine animals become entangled or eat plastic that fills the gut, and microplastics accumulate along food chains.
- Sustainable resourceA resource that is produced as rapidly as it is removed, so that it does not run out. Forests and fish stocks can be managed sustainably through quotas, protected areas, closed seasons, net mesh size limits and replanting.
- Conservation reasonsMaintaining biodiversity, protecting a gene pool that may hold useful alleles for medicine or crop breeding, keeping food chains and nutrient cycles intact, and preserving resources for future generations.
- RecyclingReduces the extraction of finite raw materials, reduces energy use and reduces the volume of waste going to landfill.
IGCSE Biology Human Influences on Ecosystems questions and answers
4 exam-style questions written to the 0610 syllabus. Try each one on paper first, then open the worked answer to check your method against the marks.
Fertiliser containing nitrate runs off farmland into a lake. Several weeks later large numbers of dead fish are found. Explain the sequence of events that caused this.
Show the worked answer
- Nitrate ions from the fertiliser enter the lake and act as a mineral supply for algae.
- The algae grow and reproduce rapidly, forming an algal bloom that covers the surface.
- The bloom blocks light from reaching the submerged plants below, so they cannot photosynthesise and they die.
- Decomposing bacteria feed on the dead plants and algae and multiply rapidly.
- The decomposers respire aerobically, using up the dissolved oxygen in the water. Fish and other aerobic organisms cannot respire and die.
Describe three effects of large scale deforestation on the environment.
Show the worked answer
- Effect 1: habitats are destroyed, so species lose their food and shelter. Populations fall and some species become extinct, reducing biodiversity.
- Effect 2: tree roots no longer bind the soil and the canopy no longer intercepts rain, so heavy rain washes the topsoil away. Soil erosion follows, along with increased flooding and silting of rivers.
- Effect 3: fewer trees means less photosynthesis, so less carbon dioxide is removed from the atmosphere. Burning the felled trees releases stored carbon as carbon dioxide, contributing to the enhanced greenhouse effect.
- A further effect: leaching of mineral ions from bare soil reduces its fertility.
Explain how a fishery can be managed so that fish stocks are used sustainably.
Show the worked answer
- Set quotas limiting the mass of fish each boat may catch, so the number removed does not exceed the number replaced by breeding.
- Impose a minimum net mesh size, so young fish escape and survive to reproduce at least once before they can be caught.
- Introduce closed seasons during the breeding period, so fish are not removed while spawning.
- Create protected areas where fishing is banned entirely, allowing populations to recover and restock surrounding waters. Fish farming can also reduce pressure on wild stocks.
Explain why plastic waste is a particular problem in marine ecosystems.
Show the worked answer
- Most plastics are non-biodegradable, meaning decomposers cannot break them down, so they remain in the environment for hundreds of years.
- Marine animals such as turtles and seabirds become entangled in discarded netting and packaging, which restricts movement and can cause drowning or injury.
- Animals mistake plastic for food and eat it. It fills the gut without providing nutrition, so the animal starves.
- Plastic also breaks into microplastics that are ingested by small organisms and passed along the food chain, accumulating in larger predators.
Common mistakes in this topic
- Saying algae use up the oxygen in eutrophication. It is the decomposers.
- Omitting a step in the eutrophication sequence. Each step is a separate mark.
- Confusing biodegradable with recyclable.
- Describing deforestation only in terms of animals losing homes. Include soil and atmospheric effects.
- Writing that sustainable means using less. It means replacing resources as fast as they are removed.
Exam tips
- Write eutrophication as five numbered steps in order. It is one of the most predictable five mark questions on the paper.
- For any pollution question, name the pollutant, the source, the organism affected and the mechanism.
- Define sustainable resource precisely before applying it. The definition is often worth a mark.
- Link every conservation argument to at least one practical benefit for humans, such as future medicines from a preserved gene pool.
- Distinguish the greenhouse effect, which is natural and necessary, from the enhanced greenhouse effect caused by additional emissions.
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Human Influences on Ecosystems FAQs
What causes eutrophication?
Excess nitrate, usually from fertiliser run-off or sewage, enters a body of water and causes rapid algal growth. The algal bloom blocks light so submerged plants die, decomposing bacteria multiply as they feed on the dead material, and their aerobic respiration removes dissolved oxygen so fish and other aerobic organisms die.
What are the main effects of deforestation?
Loss of habitat leading to reduced biodiversity and extinction, soil erosion and flooding because roots no longer bind the soil and the canopy no longer intercepts rain, leaching of mineral ions from exposed soil, and increased atmospheric carbon dioxide from reduced photosynthesis and from burning the felled trees.
What does sustainable mean in IGCSE Biology?
A sustainable resource is one that is produced as rapidly as it is removed from the environment, so that supply is maintained indefinitely. Forests and fish stocks can be used sustainably through replanting, quotas, closed seasons, minimum net mesh sizes and protected areas.
Why is plastic pollution such a persistent problem?
Most plastics are non-biodegradable, so decomposers cannot break them down and they remain in the environment for centuries. Animals become entangled in them or eat them, filling the gut without nutrition, and microplastics are ingested by small organisms and accumulate along food chains.
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Written to the published Cambridge IGCSE Biology (0610) syllabus. Check your school entry code and syllabus year, because Core and Extended candidates are assessed on different content. Last reviewed 2026-08-12.