Example 1: capture–recapture
50 beetles are marked and released. Later, 60 are caught, and 12 are marked. Estimate the population.
- Population = (first catch × second catch) ÷ marked recaptured.
- (50 × 60) ÷ 12 = 3000 ÷ 12 = 250 beetles.
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IB DemystifiedMYP Sciences
Human impact on ecosystems and biodiversity
Forests become farms, rivers carry fertiliser, and a pesticide sprayed on crops ends up in birds' eggs. Human activity is reshaping every ecosystem on Earth, and the same science that shows the damage also shows how to measure it and repair it.
Recommended for MYP 4 · eAssessment priority · About 3 lessons · Criteria A, B, C and D
On this page
- Learning objectives
- Before you start
- Key vocabulary
- Understanding the ideas
- Measuring and protecting biodiversity
- Biodiversity in the real world
- Worked examples
- In the eAssessment
- Check your understanding
- Practice questions
- Investigation
- Criterion-linked questions
- Challenge questions
- Topic check
- Review your mistakes
- Your progress
Learning objectives
By the end of this topic you should be able to:
- explain biodiversity and the ecosystem services it provides
- explain how habitat loss, pollution, invasive species, overexploitation and climate change threaten biodiversity
- explain eutrophication and bioaccumulation
- estimate populations using quadrats and capture–recapture
- evaluate conservation methods and sustainable use
- discuss decisions that balance human needs and biodiversity
Before you start
You will use these skills. If any feel shaky, review them first.
- food chains, food webs and ecosystems
- photosynthesis and respiration
- climate change (see Climate change and the greenhouse effect)
Key vocabulary
- Biodiversity
- The variety of life: species, genetic variety within species, and variety of ecosystems.
- Ecosystem services
- Benefits people get from ecosystems, such as pollination, clean water and flood protection.
- Eutrophication
- Excess nutrients in water causing algal growth, oxygen loss and death of animals.
- Bioaccumulation
- Build-up of a persistent chemical in organisms, becoming more concentrated up a food chain.
- Invasive species
- A species introduced to a new area where it spreads and harms native species.
- In-situ / ex-situ conservation
- Protecting species in their natural habitat / away from it, such as in zoos or seed banks.
Understanding the ideas
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What is it?
Biodiversity is the variety of life at every level. Human activities, including clearing land, pollution, introducing species, overharvesting and changing the climate, are reducing it faster than at any time in human history.
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Why does it happen?
People convert habitats to meet needs for food, housing and income, and chemicals used to grow food can travel through water and food chains. Many of these effects are indirect: nutrients cause eutrophication, persistent pesticides bioaccumulate, and fragmented habitats leave small populations at risk.
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How do we know?
Ecologists measure biodiversity with quadrats, transects and capture–recapture, and repeat surveys over years. Rachel Carson's book Silent Spring (1962) drew attention to DDT harming birds, and later studies measured DDT increasing up food chains. Global assessments combine thousands of such studies.
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Why does it matter?
Ecosystems provide food, clean water, medicines, pollination, flood and storm protection and climate regulation. Losing biodiversity makes ecosystems less stable and harms people, especially those who depend directly on nature.
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What does it connect to?
This topic links to food webs, evolution and genetics in biology; to fertilisers and pollution in chemistry; to climate change in Earth science; and to economics and ethics.
Measuring and protecting biodiversity
- Quadrats, placed at random, estimate the number of species or the abundance of plants and slow-moving animals.
- Capture–recapture: population ≈ (first catch × second catch) ÷ marked in second catch.
- In-situ conservation: protected areas, habitat restoration, hunting bans, wildlife corridors.
- Ex-situ conservation: seed banks, zoos and captive breeding with gene exchange.
- Sustainable use: fishing quotas, mesh sizes, sustainable forestry, careful use of fertilisers and pesticides.
Biodiversity in the real world
Pakistan has run large mangrove-replanting programmes in the Indus Delta and protects the endangered Indus river dolphin, whose numbers have slowly recovered. Water hyacinth clogs lakes and canals in many warm countries. Around the world, bans on DDT allowed birds of prey to recover.
Worked examples
Example 2: explaining eutrophication
Explain why fish die after heavy fertiliser runoff into a pond.
- Nutrients cause an algal bloom that blocks light.
- Plants below die; bacteria decompose them and respire, using up oxygen.
- Fish cannot get enough oxygen and die.
In the eAssessment
Ecology questions combine sampling data, sequences of events and real decisions. Expect:
- Process sampling data: means, percentages and capture–recapture estimates.
- Explain sequences such as eutrophication and bioaccumulation step by step.
- Evaluate sampling methods and conservation plans.
- Discuss trade-offs between development and conservation using ecological, economic and social points.
Common ways to lose marks: saying fertiliser poisons fish directly; missing the role of bacteria and oxygen in eutrophication; confusing in-situ and ex-situ conservation; and forgetting to place quadrats at random.
Check your understanding
Quick questions on the ideas above. Try each one before using a hint.
Practice questions
Investigation: how does trampling affect plants beside a path?
Partially guided investigation · about 60 minutes · small groups, outdoors
- Research question
- How does the number of plant species per quadrat change with distance (0, 1, 2, 3, 4 and 5 m) from a well-used footpath across school grassland?
- Scientific background
- Trampling compacts soil and damages plants, so only tough species survive close to paths. A belt transect places quadrats at regular distances along a line to show how a community changes across an area.
- Hypothesis
- Write your own prediction, with a scientific justification.
- Variables
- Identify your independent, dependent and control variables, and explain how you will control them.
- Apparatus
- Tape measure, 0.25 m² quadrats, plant identification sheet or app, clipboard, ruler to measure plant height (optional), soil thermometer or compaction probe (optional).
- Method
- Lay a tape at right angles to the path, starting at its edge.
- Place a quadrat at every metre along the tape, up to 5 m.
- Count the number of different plant species in each quadrat (and optionally measure plant height).
- Repeat along two more transects at least 5 m apart, and calculate the mean at each distance.
Safety. Stay away from roads and water, wear gloves or wash hands after handling soil and plants, avoid plants that sting or cause allergies, and follow your teacher's boundaries. Leave the area as you found it.
Then evaluate: could anything other than trampling explain your pattern, such as shade, drainage or mowing?
Criterion-linked questions
Criterion B: inquiring and designing
Criterion C: processing and evaluating
Criterion D: reflecting on the impacts of science
Challenge questions
Harder problems in unfamiliar contexts. Plan before you calculate.
Topic check
Five questions picked at random from the whole topic. Take a new set whenever you like.
Review your mistakes
Questions you got wrong on this device appear here so you can try them again. Answer one correctly and it leaves the list.
Your progress
Tracked separately for each skill, on this device only.
| Skill | Correct | Status |
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