Overview
What you’ll learn
- Define population, community and ecosystem, and order a food chain.
- Run the quadrat practical: random placement, counts, mean, habitat estimate.
- Classify adaptations as structural, behavioural or functional.
- Describe how decomposers feed the water and carbon cycles.
- Explain how land use, deforestation, peat loss and pollution cut biodiversity.
- Evaluate conservation methods and design a two-site quadrat comparison.
Specification checklist (AQA Trilogy 8464 · Edexcel 1SC0 · OCR Gateway J250)
Statements are paraphrased from the shared DfE combined science subject content and grouped by tier. Foundation content is studied by every student; Higher statements are examined on Higher-tier papers only. This chapter covers only what Combined Science (double award) examines — separate-science extras live on the GCSE Biology, Chemistry and Physics hubs. QuizQuip is not affiliated with AQA, Pearson Edexcel or OCR.
Foundation — every student
- 10.1Describe a population, a community and an ecosystem, and levels of organisation in food chains: producers making glucose by photosynthesis, primary, secondary and tertiary consumers, and the role of decomposers.
- 10.2Carry out the required practical: measure the population size of a common species using quadrats placed randomly, and estimate the whole-habitat population from the mean count per unit area.
- 10.3Describe organisms’ adaptations to their environments as structural, behavioural or functional, including extremophiles living in extreme conditions.
- 10.4Describe the water cycle (evaporation and transpiration, condensation, precipitation) and the carbon cycle (photosynthesis, respiration, combustion, decomposition), including the role of decomposers in returning nutrients.
- 10.5Describe biodiversity as the variety of organisms in an area and explain how human impacts — land use, deforestation, peat destruction, pollution and fertiliser run-off — reduce it.
- 10.6Describe conservation programmes such as protected habitats, breeding programmes and replanting that maintain biodiversity.
Key points
- 1Population = one species in a habitat; community = all species there; ecosystem = community + interactions with the environment.
- 2Food chains run producer → primary consumer → secondary consumer → tertiary consumer; arrows point the way energy flows.
- 3Quadrats: random placement avoids bias; population = mean count per m² × total habitat area.
- 4Adaptations are structural (bodies), behavioural (actions) or functional (internal chemistry); extremophiles thrive where little else can.
- 5The water cycle: evaporation + transpiration → condensation → precipitation. The carbon cycle: photosynthesis locks carbon in; respiration, combustion and decomposition release it.
- 6Decomposers break down dead material and waste, returning nutrients to the soil for producers to reuse.
- 7Biodiversity = the variety of organisms in an area; land-use change, deforestation, peat destruction, pollution and fertiliser run-off all cut it.
- 8Eutrophication: fertiliser run-off → algal bloom → decay → oxygen stripped → water life suffocates.
- 9Conservation tools: protected habitats, breeding programmes, hedgerows, replanting, peat preservation.
Notes · 1 of 6
Communities, ecosystems and food chains
Who lives where, who eats whom, and how sunlight pays for everything.
A population is all the organisms of one species in a habitat. A community is every population living there together. An ecosystem is the community plus its interactions with the non-living surroundings — soil, water, air, light. Organisms in a community depend on each other and compete: plants compete for light, water and minerals; animals compete for food, mates and territory.
| Level | Role | Meadow example |
|---|---|---|
| Producer | Makes glucose by photosynthesis | Grass |
| Primary consumer | Eats producers | Rabbit |
| Secondary consumer | Eats primary consumers | Fox |
| Tertiary consumer | Eats secondary consumers | Top predator (e.g. eagle) |
| Decomposers | Break down dead material and waste at every level | Bacteria and fungi |
Arrows in a food chain point from the food to the feeder — they show the direction energy travels. Food webs weave many chains together, so a change that hits one species ripples through the others: lose the rabbits and foxes lean harder on voles.
Notes · 2 of 6
Required practical: estimating population with quadrats
A square frame, random throws, and multiplication.
- 11. Frame it
Use a 1 m² (or 0.25 m²) quadrat — a square frame laid on the ground.
- 22. Place randomly
Generate random coordinates and lay the quadrat at each point; never choose “nice-looking” spots, or you bias the sample.
- 33. Count
Count the target species inside each quadrat (decide a rule for edge organisms first).
- 44. Repeat
Sample at least 8–10 quadrats, then calculate the mean count per quadrat.
- 55. Scale up
Multiply the mean count per m² by the total habitat area to estimate the population.
Exam tips
- Worked example — scaling up a mean. Ten 1 m² quadrats in a meadow give daisy counts of 4, 6, 5, 7, 4, 8, 6, 5, 5 and 6. The mean is (4+6+5+7+4+8+6+5+5+6) ÷ 10 = 56 ÷ 10 = 5.6 daisies per m². The meadow measures 40 m by 25 m, an area of 1000 m², so the estimate is 5.6 × 1000 = 5600 daisies. Quote units at every step — marks live in the method as much as the number.
- Worked example — comparing two sites. A second survey on an unmown strip gives a mean of 8.2 daisies per m² against 2.4 per m² on the mown lawn. The unmown strip holds 8.2 ÷ 2.4 ≈ 3.4 times as many daisies — mowing suppresses them. Comparisons like this are exactly why the quadrat method exists: same method, two sites, one clear conclusion.
| Element | Detail |
|---|---|
| Independent variable | The site or condition being compared (e.g. mown vs unmown) |
| Dependent variable | Population size (count per quadrat, scaled to the habitat) |
| Why random placement | Removes choice bias so the sample represents the whole habitat |
| Why repeat | A single quadrat is unrepresentative; a mean smooths patchy distributions |
| Key error | Non-random “friendly spot” sampling inflates or deflates the estimate |
Notes · 3 of 6
Adaptations and extremophiles
Bodies, behaviours and chemistry tuned to a home.
Adaptations are features that make an organism suited to its environment. Structural adaptations are body parts: a penguin’s fat layer, a cactus’s spines, an Arctic fox’s white coat. Behavioural adaptations are actions: migration, nocturnal hunting, basking. Functional adaptations are internal chemistry: desert animals producing concentrated urine to save water, or enzymes that work at extreme temperatures.
Extremophiles push the limits: bacteria in deep-sea vents thriving above 100 °C, salt-loving archaea in brine pools. They show life’s chemistry can be tuned almost anywhere — a favourite example when questions ask how organisms survive extreme conditions.
Notes · 4 of 6
Decomposers and the water and carbon cycles
Nothing is wasted — matter loops through life, air, soil and sea.
Decomposers — bacteria and fungi — break down dead material and waste, returning mineral nutrients to the soil for producers to absorb. They are the recycling crew of every ecosystem and a named step in both material cycles.
| Cycle | Key processes | One-line story |
|---|---|---|
| Water | Evaporation and transpiration → condensation → precipitation | Water leaves land and sea as vapour, clouds form, rain returns it |
| Carbon | Photosynthesis removes CO₂; respiration, combustion and decomposition release it | Carbon shuttles between air, plants, animals, fuels and soil |
Both directions matter in the carbon cycle. Photosynthesis pulls carbon dioxide out of the air and builds it into glucose; respiration in every living thing releases CO₂; combustion of wood and fossil fuels releases it instantly; decomposition returns carbon from dead matter to the soil and air. Plants sit at the hinge — taking carbon in by day and releasing some by respiration around the clock.
Notes · 5 of 6
Biodiversity and human impacts
The variety of life, and the many ways we shrink it.
Biodiversity is the variety of all the different organisms in an area — species, and the genetic variety within them. High biodiversity makes ecosystems stable and resilient. Human activity trims it in several ways: land use (building and farming replace habitats), deforestation (forests felled for timber and farmland), peat destruction (drained bogs release stored carbon and lose unique species), pollution (sulfur dioxide causing acid rain, fertilisers and sewage in water), and fertiliser run-off driving eutrophication.
Eutrophication runs as a chain: fertiliser washes from fields into rivers → algae bloom in the enriched water → the algae die and are decomposed → decomposition strips oxygen from the water → fish and other aquatic life suffocate. Each link is a marking point.
Worked example — a population in decline
A hedgerow survey estimates 5600 daisies in a meadow one year. After the meadow is partially ploughed, the same method estimates 4480. The fall is 5600 − 4480 = 1120 plants; as a percentage, 1120 ÷ 5600 × 100 = 20%. Pairing a quadrat estimate with a percentage change is a common higher-mark combination — show the subtraction and the division separately.
Notes · 6 of 6
Conservation
Rebuilding variety — on purpose, with evidence.
Conservation programmes defend biodiversity from several angles: protected habitats such as nature reserves and national parks; breeding programmes for endangered species in captivity before release; replanting forests and restoring hedgerows; keeping peat bogs waterlogged so their stored carbon stays put; and reducing pollution at source. Conservation also has human pay-offs — healthy ecosystems protect food chains, pollination and the medicines of the future.
Revise
Key terms
The words examiners expect you to use precisely.
- Population
- All the organisms of one species living in a habitat.
- Community
- All the populations of different species living and interacting in a habitat.
- Ecosystem
- The interaction of a community with the non-living parts of its environment.
- Producer
- An organism that makes its own glucose by photosynthesis — the start of every food chain.
- Quadrat
- A square frame used to sample and count organisms in a habitat.
- Extremophile
- An organism adapted to extreme conditions such as deep-sea vents or salt lakes.
Show 6 more key termsShow fewer key terms
- Decomposer
- A bacterium or fungus that breaks down dead material, returning nutrients to the soil.
- Transpiration
- The evaporation of water from plant leaves, driving water up the stem.
- Biodiversity
- The variety of different organisms in an area.
- Eutrophication
- Fertiliser run-off feeding algal blooms whose decay strips oxygen from water.
- Deforestation
- The permanent clearing of forests, destroying habitats and releasing stored carbon.
- Conservation
- Protecting and restoring habitats and species to maintain biodiversity.
Revise
Common mistakes
Where students most often lose marks in this chapter — and what to do instead.
Mistake: Mixing up population, community and ecosystem
They are nested, not synonyms.
Instead: Population = one species; community = all species; ecosystem = community + environment.
Mistake: Drawing food chain arrows from eater to eaten
Arrows show energy flow, which runs from food into the feeder.
Instead: Grass → rabbit → fox, always pointing toward the consumer.
Show 10 more mistakesShow fewer mistakes
Mistake: Starting a food chain with a consumer
Every chain begins with an organism that makes its own food.
Instead: Producers photosynthesise; consumers arrive later.
Mistake: Choosing pretty quadrat spots
Deliberate placement biases the sample.
Instead: Random coordinates make the sample representative of the habitat.
Mistake: Multiplying the mean by the number of quadrats
That estimates the population of the sampled patch only.
Instead: Population = mean per m² × TOTAL habitat area.
Mistake: Forgetting the edge rule
Counting edge organisms inconsistently changes counts between quadrats.
Instead: Decide and state a rule (e.g. count if over half inside) before counting.
Mistake: Saying animals adapt because they want to
Individuals cannot wish new bodies into being.
Instead: Adaptations arise over generations by natural selection.
Mistake: Calling migration a structural adaptation
Moving is a behaviour, not a body part.
Instead: Structural = body features; behavioural = actions; functional = internal chemistry.
Mistake: Saying photosynthesis releases carbon dioxide
Photosynthesis absorbs CO₂; respiration releases it.
Instead: Plants take CO₂ in for photosynthesis and release some through respiration.
Mistake: Equating biodiversity with tree count
Biodiversity covers every organism, not one species.
Instead: Biodiversity is the variety of ALL organisms, including microbes, insects and fungi.
Mistake: Reordering the eutrophication chain
Marks depend on the sequence.
Instead: Fertiliser in → algal bloom → death and decay → oxygen used up → aquatic life dies.
Mistake: Leaving transpiration out of the water cycle
Land plants are a major water source for the atmosphere.
Instead: Evaporation and transpiration both load water vapour into the air.
Check yourself
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Practise
Topical questions
12 exam-style questions with model answers and mark schemes.
A set of structured and calculation questions matched to this chapter, in three tiers, each with a model answer, mark scheme and examiner tip. Higher-tier questions are tagged.
Practise topical questionsExam practice
Practice paper
A 30-mark paper in the style of a Combined Science paper, with a separate mark scheme. Give yourself 40 min.
Question paper
Chapter 10 Practice Paper
30 marks · 40 min · 5 pages
6 multiple-choice questions and structured questions with answer lines. Communities and ecosystems, food chains, the quadrat required practical, adaptations and extremophiles, decomposers, the water and carbon cycles, biodiversity, human impacts and conservation.
Mark scheme
PROChapter 10 Practice Paper — mark scheme
Answers · Marking points · 3 pages
Model answers with a mark for each creditworthy point — for marking at home or in class.
How does this chapter fit the real exams?
Biology Paper 1
1 h 15 · 70 marks (Trilogy)
Cell biology, organisation, infection and response, bioenergetics (chapters 1–7)
Biology Paper 2
1 h 15 · 70 marks (Trilogy)
Homeostasis, inheritance variation and evolution, ecology (chapters 8–10)
Tiers
Foundation (grades 1-1 to 5-5) · Higher (4-3 to 9-9)
Two grades are awarded together; the 21 required practicals are examined on the written papers
Want a full paper? Try the Biology Paper 1-style (Foundation), Biology Paper 1-style (Higher), Biology Paper 2-style (Foundation), Biology Paper 2-style (Higher), Chemistry Paper 1-style (Foundation), Chemistry Paper 1-style (Higher), Chemistry Paper 2-style (Foundation), Chemistry Paper 2-style (Higher), Physics Paper 1-style (Foundation), Physics Paper 1-style (Higher), Physics Paper 2-style (Foundation), Physics Paper 2-style (Higher), Required practicals practice sets.
Questions
Frequently asked questions
How is Combined Science ecology different from triple Biology?
Combined covers communities, quadrats, adaptations, the water and carbon cycles and biodiversity. Triple adds the decay practical, biomass transfer between trophic levels and food security, none of which appear in Combined papers.
Which maths skills appear in this chapter?
Means from quadrat counts, scaling up (mean per m² × habitat area), ratios of means between sites, and percentage change in population size.
What is the classic quadrat calculation?
Mean count per quadrat → mean per m² (if the quadrat is not 1 m²) → multiply by total habitat area. Show every step; the area step is usually worth its own mark.
Which cycles do I need to know?
The water cycle (evaporation, transpiration, condensation, precipitation) and the carbon cycle (photosynthesis in, respiration and combustion out), including decomposers as recyclers.
How do I remember the eutrophication sequence?
Food for algae, feast, then famine of oxygen: fertiliser in → algal bloom → bloom dies and decays → oxygen stripped → fish die.
Up next
Chapter 11: Atoms and the periodic table