What you'll be able to do by the end
- Use ecological vocabulary precisely — population, community, ecosystem, habitat, niche
- Describe and evaluate sampling — quadrats and transects
- Calculate population size from quadrat data, and calculate mean, median and mode
- Describe the carbon cycle and the water cycle
- Explain the role of decomposers and the factors affecting decay
- Explain the causes and consequences of global warming, deforestation and pollution
- Define biodiversity and explain why it matters
- Describe and evaluate methods of maintaining biodiversity
- Interpret pyramids of biomass and calculate energy transfer efficiency
- Evaluate methods of food production and food security
Part 1 — The vocabulary of ecology
| Term | Meaning | Example |
|---|---|---|
| Population | All the organisms of one species in a habitat | All the hedgehogs in a park |
| Community | All the populations of all species in a habitat | Every living thing in that park |
| Ecosystem | The community plus the non-living surroundings | The park — organisms, soil, water, air |
| Habitat | The place where an organism lives | Hedgerow, pond, leaf litter |
| Niche | The role an organism plays — what it eats, when it's active, where it lives | Nocturnal insectivore of the woodland floor |
The difference is one word: living. A community is only the organisms; an ecosystem adds the abiotic parts — soil, water, light, temperature. If a question mentions rainfall or soil pH, it's talking about the ecosystem, not the community.
Part 2 — Sampling
You cannot count every organism, so you sample and scale up. A quadrat is a square frame (usually 0.25 m² or 1 m²) placed on the ground to count what's inside — used for plants and slow-moving animals, not anything that runs away.
Random sampling — and why it must be random
- Lay out two tape measures at right angles to form a grid
- Use a random number generator to produce pairs of coordinates
- Place the quadrat at each coordinate and count the organisms inside
- Repeat many times and calculate a mean per quadrat
- Scale up: mean per quadrat × (total area ÷ quadrat area)
Worked example. A student places a 0.25 m² quadrat 10 times in a field of 40 m × 25 m and counts 68 daisies in total. Estimate the total daisy population.
Step 1 — mean per quadrat: 68 ÷ 10 = 6.8. Step 2 — quadrat-areas in the field: 40 × 25 = 1000 m²; 1000 ÷ 0.25 = 4000. Step 3 — scale up: 6.8 × 4000 = 27,200 daisies.
Students forget the quadrat is 0.25 m², not 1 m². Dividing by 0.25 makes the number bigger, not smaller. Sense-check: a smaller quadrat means more of them fit in the field, so the total must be larger.
Why random? If you choose where to put the quadrat, you unconsciously pick interesting patches — that's bias. Random coordinates remove your judgement from the process.
Transects
A transect is a line across a habitat, with quadrats at regular intervals — used to investigate how distribution changes (from a hedge into open ground, up a beach, from a path into woodland). A quadrat survey is random and estimates how many; a transect is systematic and shows how distribution changes with a factor.
Mean, median and mode
For the data 4, 7, 7, 9, 13: Mean = (4+7+7+9+13) ÷ 5 = 8; Median (middle value) = 7; Mode (most common) = 7. With an even number of values, the median is the mean of the middle two — e.g. 3, 5, 8, 10 → (5+8) ÷ 2 = 6.5.
Part 3 — Cycling materials
The Earth has a fixed amount of carbon, nitrogen and water. Life depends on recycling it.
The carbon cycle
Enters living things: photosynthesis removes CO₂ from the atmosphere to make glucose. Moves through: feeding — animals take in carbon compounds. Returns to the atmosphere: respiration (all organisms release CO₂), decomposition (microorganisms respire), combustion (burning fossil fuels or wood). Locked away: dead organisms compressed over millions of years form fossil fuels; carbon in the ocean and sedimentary rock.
Almost every carbon-cycle question is answered by identifying which of these four processes is involved.
The water cycle
Evaporation (the Sun evaporates water) → transpiration (from plant leaves) → condensation (vapour cools into clouds) → precipitation (rain, snow, hail) → run-off and drainage (back to the sea or into the ground). It produces fresh water from salt water — every land organism depends on it.
Decomposition
Decomposers — bacteria and fungi — break down dead organisms and waste, releasing the materials inside back into the ecosystem. Without them, nutrients would stay locked in dead bodies and life would stop.
| Factor | Effect on decay rate |
|---|---|
| Temperature | Higher increases enzyme activity and decay — up to a point; too hot denatures enzymes and decay stops |
| Oxygen | Most decomposers respire aerobically and need oxygen |
| Water | Decomposers need moisture; dry conditions slow or stop decay |
| Number of decomposers | More microorganisms means faster decay |
Compost heaps are designed around these factors — turned to add oxygen, kept moist, and they warm up as microorganisms respire. Food preservation works by removing them: freezing lowers temperature, drying removes water, canning removes oxygen, salting draws water out by osmosis. Same biology, opposite goals.
Part 4 — Biodiversity
Biodiversity is the variety of all the different species of organisms on Earth, or within an ecosystem.
A great biodiversity ensures the stability of ecosystems, because species depend on each other for food, shelter and maintaining their environment. If an ecosystem has many species and one is lost, others can fill the gap; if it has few, losing one may cause collapse. Human survival depends on it too — food, medicine, materials, clean water and breathable air all come from functioning ecosystems.
| Threat | How it reduces biodiversity |
|---|---|
| Habitat destruction | Building, farming and quarrying remove where organisms live |
| Deforestation | Removes habitat and reduces CO₂ uptake |
| Pollution | Kills organisms directly or damages the habitat |
| Global warming | Species cannot survive changed conditions or migrate fast enough |
| Introduced species | Outcompete natives — grey squirrels, Japanese knotweed, signal crayfish |
| Over-hunting / over-fishing | Removes species faster than they reproduce |
Part 5 — Human impact
Pollution
| Where | From | Effect |
|---|---|---|
| Water | Sewage, fertiliser, toxic chemicals | Kills aquatic organisms; fertiliser causes eutrophication |
| Air | Smoke, sulfur dioxide, nitrogen oxides | Causes acid rain, damaging trees and acidifying lakes |
| Land | Landfill, toxic chemicals, pesticides | Poisons soil organisms; pesticides accumulate through food chains |
Land use & peat bogs
Humans reduce land for other organisms through building, quarrying, farming (monoculture supports very few species) and dumping waste. Peat bogs are a specific case: peat is decayed plant material accumulated over thousands of years in waterlogged, low-oxygen conditions. Draining bogs for compost destroys a rare habitat and its species, releases stored CO₂ as the peat decays in air, and cannot be reversed on any human timescale.
Global warming
Cause: increasing greenhouse gases — carbon dioxide and methane — absorbing heat radiated from Earth. Sources: burning fossil fuels (CO₂), deforestation (less CO₂ absorbed), livestock and landfill (methane).
| Consequence | Explanation |
|---|---|
| Loss of habitat | Melting ice removes polar habitat; rising seas flood low-lying land |
| Changes in distribution | Species move towards the poles or to higher altitudes |
| Changes in migration | Birds and insects arrive and leave at different times |
| Reduced biodiversity | Species unable to move or adapt fast enough become extinct |
In many turtle and some lizard species, incubation temperature determines sex — warmer nests produce more females. On some green turtle beaches, over 99% of hatchlings are now female: no shortage of eggs, but a coming shortage of males. That's global warming affecting a population through a mechanism nobody would predict from "it gets hotter."
Deforestation
Reasons: clearing land for cattle, rice and biofuel crops. Consequences: less CO₂ absorbed (fewer trees); more CO₂ released (burning and decomposing wood); loss of habitat and biodiversity (tropical forests hold most of Earth's species); soil erosion (roots no longer hold soil); disruption of the water cycle (less transpiration, less local rainfall).
Part 6 — Maintaining biodiversity
| Method | How it works | Limitation |
|---|---|---|
| Breeding programmes for endangered species | Captive breeding increases numbers; may reintroduce | Expensive; small gene pool; captive-bred may not survive wild |
| Protecting/regenerating rare habitats | Nature reserves, SSSIs, replanting | Needs land and money; conflicts with development and farming |
| Field margins and hedgerows | Uncultivated strips create habitat | Reduces the farmer's usable land |
| Reducing deforestation & emissions | Government targets and international agreements | Requires global cooperation; economic pressure to develop |
| Recycling rather than landfill | Reduces waste and habitat destruction | Needs public participation and infrastructure |
Exam questions ask why biodiversity programmes are hard to implement. The answer is always competing interests: farmers need to make a living; developing countries argue for the growth developed countries already had; conservation costs money; local communities may depend economically on the damaging activity. An answer that only says "people don't care" will not score — the pressures are real.
Part 7 — Biomass and energy transfer
Biomass is the mass of living material. A pyramid of biomass shows the biomass at each trophic level, drawn to scale.
| Level | Name | Example |
|---|---|---|
| 1 | Producer | Grass, algae |
| 2 | Primary consumer (herbivore) | Rabbit, slug |
| 3 | Secondary consumer (carnivore) | Hedgehog, fox |
| 4 | Tertiary consumer (top carnivore) | Badger, owl |
Only about 10% of the biomass from each trophic level is transferred to the level above. The other 90% is lost because not all is eaten (roots, bones, fur), not all is digested (egested as faeces), energy is used in respiration and released as heat, and energy is lost in excretion (urea). Mammals and birds lose most — they are endothermic, spending a large part of their energy maintaining body temperature.
This is why reptiles need so much less food than mammals of the same size — an ectothermic bearded dragon spends almost nothing maintaining body temperature. The same fact explains why food chains rarely have more than four or five levels: after four transfers at 10%, only 0.01% of the original energy remains.
Calculating efficiency
Efficiency = (biomass transferred ÷ biomass available) × 100.
Worked example. Producers contain 20,000 kJ; the rabbits eating them contain 1,600 kJ. Calculate the efficiency.
(1600 ÷ 20000) × 100 = 8%
Part 8 — Food security and food production
Food security means having enough food to feed a population. Threats: rising population; changing diets (demand for meat rises as countries develop, and meat is far less efficient than crops); new pests and pathogens; environmental change (drought and flooding); cost of farming inputs; and conflict.
Because only ~10% transfers between levels, eating plants is far more efficient than eating meat — the same land feeds many more people growing crops than raising cattle. Intensive farming increases efficiency by limiting the energy animals lose: restricting movement, and keeping animals warm. The trade-off is welfare — animals may be unable to express natural behaviours, a genuine ethical objection.
For: more food from less land; cheaper food; more people fed; less land cleared so more habitat preserved. Against: poor animal welfare; disease spreads quickly; heavy antibiotic use drives resistance; high energy costs. Judgement required — either conclusion scores if argued from the points made.
Fishing and sustainability
Fish stocks are declining. If fish are caught faster than they reproduce, the population collapses and does not recover. Methods to maintain stocks: fishing quotas (limits per species) and net-size controls (larger mesh lets young fish escape to breed).
Exam-style questions
Answer all questions. Marks are shown in brackets. Total: 49 marks.
Question 1
A student estimated the dandelions in a field of 50 m × 30 m, using a 0.25 m² quadrat placed randomly 12 times. She counted 54 dandelions in total.
(Total 8 marks)
Question 2
(Total 11 marks)
Question 3
| Trophic level | Energy (kJ) |
|---|---|
| Grass | 50,000 |
| Rabbits | 4,000 |
| Foxes | 350 |
(Total 9 marks)
Question 4
In many turtles, the sex of hatchlings is determined by incubation temperature; warmer temperatures produce more females. On one beach, 99% of green turtle hatchlings are now female.
(Total 7 marks)
Question 5
(Total 8 marks)
Question 6 — extended response
Intensive farming increases efficiency but has been criticised on welfare and environmental grounds. Evaluate the use of intensive farming methods for producing meat. (6)
(Total 6 marks)
TOTAL FOR PAPER: 49 MARKS
Mark scheme
Question 1
(a) 1 — 54 ÷ 12 = 4.5.
| (b) 3. Field area = 50 × 30 = 1500 m² | 1 |
| Quadrat areas = 1500 ÷ 0.25 = 6000 | 1 |
| 4.5 × 6000 = 27,000 dandelions | 1 |
Allow error carried forward from (a). A common wrong answer is 6,750 — from multiplying by 1500 instead of 6000 (forgetting the quadrat is smaller than 1 m²).
| (c) 2. Random placement avoids bias — can't unconsciously choose areas with more/fewer plants | 1 |
| Makes the sample representative, so the estimate is more accurate | 1 |
(d) 2 — any two: more quadrats; repeat and take a mean; random number generator rather than throwing; same time of year / similar conditions.
Question 2
(a) 1 — Photosynthesis. (b) 3 — respiration; combustion/burning; decomposition (accept decay by microorganisms).
| (c) 3. Decomposers break down dead organisms and waste | 1 |
| They respire, releasing CO₂ back into the atmosphere | 1 |
| Without them, carbon would remain locked in dead material, unavailable to others | 1 |
(d) 4 — two suggestions, two marks each (action + reason): turn the heap/add air holes (more oxygen for aerobic respiration); add water if dry (decomposers need moisture); keep warm/insulate (higher temperature speeds enzyme activity); shred material (more surface area).
Question 3
(a) 2 — (4000 ÷ 50000) × 100 = 8% (method + answer). (b) 2 — (350 ÷ 4000) × 100 = 8.75% (accept 8.8%).
(c) 3 — any three: not all eaten (bones, roots, fur); not all digested (egested as faeces); respiration releases heat; excretion (urea).
| (d) 2. Only ~10% of energy is transferred at each level | 1 |
| After several levels, too little energy remains to support another population | 1 |
Question 4
| (a) 2. Average temperatures have risen due to global warming | 1 |
| Warmer incubation produces more female hatchlings | 1 |
| (b) 3. Very few males are being produced | 1 |
| Sexual reproduction needs both sexes, so too few males to fertilise eggs | 1 |
| Fewer fertilised eggs → population declines and could go extinct despite large hatchling numbers now | 1 |
(c) 2 — any one action + link to producing more males: shade/plant vegetation over nests (lowers temperature); relocate eggs to cooler beaches; artificially incubate some at male-producing temperatures.
Question 5
(a) 1 — the variety of different species on Earth, or within an ecosystem.
| (b) 3. Species depend on one another for food, shelter and maintaining the environment | 1 |
| If one is lost, others can fill its role / alternative food sources exist | 1 |
| Low biodiversity means losing one species can cause collapse | 1 |
(c) 4 — two consequences, two marks each: loss of habitat (rare habitat + its species, biodiversity falls); release of CO₂ (carbon stored for thousands of years released as peat decays, contributing to global warming); cannot be replaced (peat forms over thousands of years — effectively permanent).
Question 6 — extended response (6 marks, levels-marked)
Level 3 (5–6): balanced evaluation, explained not listed, with a justified conclusion. Level 2 (3–4): both sides with limited explanation, or one side developed and the other acknowledged. Level 1 (1–2): relevant points, largely one-sided.
Indicative — for: restricting movement/keeping warm reduces energy lost, so more food becomes biomass; more food from less land (cheaper, more people fed); less land needed so more habitat left; food security improves. Against: welfare (no natural behaviours); crowding spreads disease; antibiotic use drives resistance; high energy costs; meat is inefficient regardless of method. Conclusion: any justified position.
Common mistakes in this topic
Forgetting the quadrat is not 1 m². Dividing by 0.25 makes the number larger — sense-check the direction.
Confusing community and ecosystem. Community is living only; ecosystem includes soil, water, light.
Saying "10% of energy is transferred" without saying where the rest goes. The mark is usually for the losses, not the number.
Writing that global warming "kills animals." Too vague — name the mechanism: habitat loss, changed distribution, disrupted migration, skewed sex ratios.
One-sided answers on farming and conservation. These are always evaluate questions — both sides plus a judgement.
Saying people "don't care" about conservation. The difficulty is competing legitimate interests — livelihoods, development, cost. That's what earns the mark.
Teaching notes
The quadrat calculation needs practising until it's automatic. Mean per quadrat → how many quadrat-areas fit → multiply. Students who understand it conceptually still get the arithmetic wrong under pressure; ten quick examples with different quadrat sizes fixes it.
Do the fieldwork if you possibly can. A transect from a hedge into open ground with a light meter takes forty minutes and makes Part 2 concrete in a way no diagram does. Even a school field works.
The turtle example in Question 4 is the best hook in the unit. It links global warming to sex determination to population collapse and surprises students — nobody predicts "warmer beaches, no males." It connects directly to the sex-determination content in the inheritance unit.
Part 8 is where students have opinions. Intensive farming, meat consumption and fishing quotas produce genuine debate. Let it run, then show them how to turn an argument into a levels-marked answer — the skill is structuring a view, not having one.
Handle food security carefully. Some students will have direct experience of food poverty; keep the discussion about systems and land use, not individual families.
This unit connects to almost everything else — photosynthesis and respiration, adaptation, natural selection, sex determination, antibiotic resistance. Flag those links explicitly; it helps students see biology as one subject.