Your revision route
What you’ll learn
- Describe ecological levels and explain interdependence, competition and stable communities.
- Explain how abiotic and biotic factors affect populations and distribution.
- Relate structural, behavioural and functional adaptations to environmental conditions.
- Use quadrats and transects to estimate abundance and investigate distribution safely.
- Interpret food chains, predator–prey cycles and carbon- and water-cycle diagrams.
- Explain decomposition and environmental change, including separate-Biology detail.
- Explain human effects on biodiversity and evaluate conservation strategies.
- Calculate biomass-transfer efficiency and interpret pyramids of biomass for separate Biology.
- Evaluate biological threats and possible responses affecting food security.
Build the big picture
Key ideas
An ecosystem is a web, not a guest list
Species share food, shelter, pollinators and conditions, so removing one strand can tug on the whole community.
- A population is one species in a habitat; all populations form a community; the community interacting with abiotic conditions forms an ecosystem.
- Interdependence means species rely on others for resources and services such as food, shelter, pollination and seed dispersal.
- In a stable community, species and environmental factors remain in balance, so population sizes stay fairly constant.
The punchline: When one population changes, trace at least one consequence for another species.
Competition begins when a resource runs short
Two organisms can need the same limited resource; obtaining more of it can improve survival or reproductive success.
- Plants often compete for light and space, plus water and mineral ions from soil.
- Animals often compete for food, mates and territory.
- State the limited resource and link access to growth, survival or reproduction rather than merely naming competition.
The punchline: Name what is limited and why it matters.
Communities answer to living and non-living pressures
A population graph is the visible result; the ecological mechanism is the part that earns the explanation mark.
- Abiotic factors include light, temperature, moisture, soil pH and minerals, wind, plant carbon dioxide and oxygen for aquatic animals.
- Biotic factors include food availability, new predators, new pathogens and a competitor reducing a species until too few remain to breed.
- Use data to connect the changed factor to a resource, process, survival or reproduction, then to population size or distribution.
The punchline: Abiotic means non-living; biotic means caused by living organisms.
An adaptation solves a specific environmental problem
A useful feature only makes sense when you name the condition it helps an organism survive or reproduce in.
- Structural adaptations are physical features; behavioural adaptations are actions; functional adaptations are internal processes.
- Explain the mechanism: feature → effect on exchange, temperature, water, feeding or reproduction → advantage in that environment.
- Extremophiles live in conditions such as high temperature, pressure or salt; some deep-sea-vent bacteria are examples.
The punchline: Do not list a feature—explain how it works under the stated condition.
Ecologists count a sample and confess the uncertainty
A quadrat turns an impossible census into an estimate, provided the sample is random, repeated and representative.
- Use random quadrat positions to estimate abundance across a habitat and reduce selection bias.
- Use a belt transect or quadrats along a line to investigate how distribution changes across an environmental gradient.
- Increase sample size, calculate a mean and identify anomalies; measure the relevant abiotic factor at the same positions.
- Scale mean density to habitat area only when quadrat size and total area use matching units.
The punchline: Random quadrats estimate abundance; transects test change with position.
Four ecological zoom levels
Ecology keeps changing scale. Each level includes the one before it, until living and non-living parts share the frame.
- OrganismOne individual living thing, such as a single oak tree.
- PopulationAll organisms of one species living in the same habitat.
- CommunityAll the populations of different species living and interacting in a habitat.
- EcosystemThe community interacting with the abiotic parts of its environment.
Food-chain arrows follow biomass transfer
The arrow points from dinner to diner: it shows the direction in which biomass is transferred.
- Every food chain begins with a producer, usually a green plant or alga that makes glucose by photosynthesis.
- Primary consumers eat producers; secondary consumers eat primary consumers; tertiary consumers may eat secondary consumers.
- Predators kill and eat prey. In a stable community, their populations often rise and fall in linked cycles.
- On a graph, prey usually increase first; predator increase follows because extra food supports survival and reproduction.
The punchline: Read an arrow as ‘is eaten by’ and explain any cycle with a time lag.
Matter cycles because tomorrow's life needs today's atoms
Carbon and water move between living organisms and the environment; microorganisms reopen locked stores.
- Photosynthesis moves atmospheric carbon dioxide into plant biomass; feeding transfers carbon through food chains.
- Respiration by plants, animals and microorganisms returns carbon dioxide to the atmosphere.
- Decomposers break down dead material and waste, returning carbon dioxide and mineral ions for reuse.
- Evaporation and transpiration move water to the atmosphere; condensation, precipitation and drainage return fresh water.
The punchline: A cycle answer needs named processes and the stores they connect.
Separate Biology: decay is enzyme-controlled recycling
A compost heap is a managed microbial reactor—warm, moist and aerated enough for decomposers to work quickly.
- Increasing temperature speeds enzyme-controlled decay up to an optimum; excessive heat can reduce enzyme activity by denaturation.
- Water supports microbial reactions and oxygen enables efficient aerobic respiration, so moist, aerated waste usually decays faster.
- Anaerobic decay produces methane; biogas generators capture it as a fuel.
- Compost returns mineral ions and organic matter to soil, but rate claims should be supported by measurements over time.
The punchline: Explain decay rate through microorganisms, enzymes, water, oxygen and temperature.
Separate Biology Higher Tier: distributions move when conditions move
A species occupies places where its tolerances and interactions allow it—not every place it could physically reach.
- Temperature, water availability and atmospheric-gas composition can shift where a species survives and reproduces.
- Changes may be seasonal, geographical or caused by human activity.
- Evaluate distribution data by considering sampling, timescale, other changing factors and whether a mechanism supports the association.
The punchline: A changed distribution can support a cause, but correlation alone does not prove it.
Make the model move
Interactive checkpoint
Touch the science. Change a state, build a route or test a relationship.
Change the lens
Build an ecosystem from one organism
Arrange the levels from one living individual to living communities interacting with non-living conditions.
Choose the first step below.
One organism belongs to a population; different populations form a community; the community plus abiotic interactions forms an ecosystem.
Sample, then scale
Estimate a population from quadrats
Adjust the mean count per quadrat and the number of quadrat-sized areas in the habitat to see the population estimate.
estimated population = mean count per quadrat × number of quadrat areas
Estimated population1,000 organisms
Random, representative quadrats provide a mean density. Multiplying by the number of equal quadrat areas in the habitat estimates total abundance.
Follow the consequence
One human action, several ecological effects
Select an action and trace the mechanism that changes biodiversity or atmospheric gases.
Species lose habitat; decay or burning of peat releases carbon dioxide. Cheap compost is a benefit, but peat-free alternatives reduce damage.
1 of 4 states explored
Strong evaluation follows an action through a mechanism to a biological consequence, then weighs a realistic benefit or response.
Biodiversity is ecological redundancy with living detail
When many species perform overlapping roles, an ecosystem is less dependent on any single ecological thread.
- Biodiversity is the variety of species on Earth or within an ecosystem.
- Greater biodiversity can improve ecosystem stability by reducing dependence on one species for food, shelter or environmental maintenance.
- Human survival depends on maintained ecosystems, yet many human activities reduce biodiversity.
The punchline: Link biodiversity to stability, then name the mechanism by which an action changes it.
Waste causes damage when ecosystems receive more than they can handle
Population growth and rising resource use generate larger waste streams; poor handling turns them into ecological stress.
- Water pollution can come from sewage, fertiliser and toxic chemicals; air pollution includes smoke and acidic gases.
- Land pollution includes landfill and toxic chemicals.
- Pollution can kill organisms, disrupt food webs and reduce biodiversity; trace the route from pollutant to biological effect.
The punchline: Name the pollutant, where it travels and the organism or process it affects.
Peat bogs are habitats and carbon stores
Digging peat removes a rare home and exposes stored carbon to decay or burning—a double ecological cost.
- Building, quarrying, farming and waste disposal reduce habitat area for other species.
- Destroying peat bogs reduces their biodiversity; decay or combustion of peat releases carbon dioxide.
- Evaluation should weigh cheap compost or food-production benefits against habitat loss and greenhouse-gas emissions.
The punchline: Keep the two peat consequences separate: habitat loss and carbon dioxide release.
Deforestation changes both habitat and atmosphere
Removing tropical forest makes space for cattle, rice or biofuel crops while stripping away a vast biological network.
- Deforestation removes habitats, reduces biodiversity and can release carbon dioxide through burning and decay.
- Fewer trees also means less carbon dioxide removed by photosynthesis.
- Rising atmospheric carbon dioxide and methane contribute to global warming, which can alter habitats, migration, distribution and extinction risk.
- Scientific consensus rests on systematic reviews of thousands of peer-reviewed studies; complex forecasts still carry uncertainty.
The punchline: Uncertainty about size or timing does not mean there is no evidence for the mechanism.
Conservation works by repairing particular pressures
A vague promise to ‘save nature’ becomes scientific only when the action matches the cause of decline.
- Breeding programmes support endangered populations; habitat protection and regeneration preserve the conditions species need.
- Field margins and hedgerows restore habitat beside monocultures; recycling reduces landfill and demand for new resources.
- Governments can reduce deforestation and carbon dioxide emissions, but policies create economic and social trade-offs.
- Evaluate each method using mechanism, likely scale, evidence, cost, timescale and competing pressures.
The punchline: Match conservation action to threat before weighing benefits and drawbacks.
Carbon keeps changing address
Carbon atoms cycle between atmospheric carbon dioxide, living biomass, dead material and fuels. They are reused, not used up.
- PhotosynthesisPlants and algae take carbon dioxide from the atmosphere and build carbon compounds.
- FeedingCarbon compounds move through food chains as organisms eat biomass.
- RespirationPlants, animals and microorganisms release carbon dioxide while transferring energy.
- Death and wasteCarbon compounds enter dead material and waste when organisms die or egest.
- DecompositionMicroorganisms break material down and respire, returning carbon dioxide and mineral ions.
- CombustionBurning biomass or fossil fuels transfers stored carbon back to atmospheric carbon dioxide.
A food chain runs on a shrinking biomass budget
Only part of one trophic level becomes new biomass at the next, so food chains support fewer organisms near the top.
- Producer biomassPlants and algae make organic molecules; about 1% of incident light energy is transferred into biomass.
- IngestedConsumers eat only some available material, and not every part swallowed is absorbed.
- Respired and excretedGlucose supports respiration; carbon dioxide, water and urea leave rather than becoming growth.
- New biomassApproximately 10% is typically transferred to the next trophic level as available biomass.
Separate Biology: trophic levels form a biomass staircase
Each feeding step is numbered by distance from the producer, and each step usually holds less biomass.
- Producers occupy trophic level 1, primary consumers level 2, secondary consumers level 3 and tertiary consumers level 4.
- An apex predator has no predators; decomposers secrete enzymes and absorb small soluble products from dead material.
- A pyramid of biomass shows relative biomass at each level, with producers at the base; bar widths must be drawn to scale.
- Approximately 10% of biomass typically transfers upwards, but calculate from the data when values are supplied.
The punchline: Trophic level describes feeding position, not body size or importance.
Separate Biology: most ingested biomass takes another route
A consumer is not a perfect biomass conveyor: some material is never absorbed, and much absorbed glucose powers life.
- Some material is not eaten; some ingested material is not absorbed and leaves as faeces.
- Respiration transfers energy from glucose and releases carbon dioxide and water; urea and water leave in urine.
- Only biomass built into new tissue is available to the next trophic level, limiting population size higher in food chains.
The punchline: Distinguish egestion, excretion and respiration when explaining transfer loss.
Separate Biology: food security is a biological systems problem
Enough food depends on populations, pathogens, climate, resources, conflict and the choices built into farming.
- Threats include rising population, changing diets, new pests and pathogens, failed rainfall, input costs and conflict affecting food or water access.
- Limiting livestock movement and controlling temperature reduce respiratory energy transfer; high-protein feed can increase growth.
- Intensive methods may increase yield but can raise animal-welfare, resource-use and pollution concerns.
- Interpret population and production data before judging whether a method is sustainable.
The punchline: An evaluation needs yield, resource use, welfare, environmental effect and evidence.
Separate Biology: sustainable supply needs several tools
The cleverest food system keeps breeding populations intact and turns microbial growth into useful biomass.
- Fishing quotas limit catches and suitable net sizes let younger fish escape, helping stocks remain large enough to reproduce.
- Fusarium grows aerobically on glucose syrup to make protein-rich mycoprotein suitable for vegetarians; its biomass is harvested and purified.
- GM crops such as golden rice may increase food amount or nutrition; insulin from GM bacteria is harvested, purified and used to treat diabetes.
- Benefits must be evaluated alongside ecological, economic, social and ethical concerns in the supplied context.
The punchline: Explain how each intervention changes reproduction, yield or resource efficiency.
Words worth knowing
Key definitions
- population
- All organisms of one species living in the same habitat.
- community
- All the populations of different species living and interacting in a habitat.
- ecosystem
- A community of organisms interacting with the abiotic parts of its environment.
- interdependence
- The dependence of species on other species for resources or services such as food, shelter, pollination and seed dispersal.
- abiotic factor
- A non-living environmental factor that can affect a community.
- biotic factor
- A living factor, or one caused by living organisms, that can affect a community.
- adaptation
- A structural, behavioural or functional feature that helps an organism survive and reproduce in its normal environment.
- quadrat
- A frame of known area used to sample the abundance or distribution of organisms in a habitat.
- transect
- A line or strip across a habitat along which organisms and environmental factors are sampled to investigate distribution.
- arithmetic mean
- The total of all measured values divided by the number of values.
- median
- The middle value after a data set has been arranged in numerical order.
- mode
- The value that occurs most often in a data set.
- producer
- An organism, usually a plant or alga, that makes biomass from simple substances using photosynthesis.
- trophic level
- An organism's feeding position in a food chain, numbered from producers at level 1.
- decomposer
- A microorganism that secretes enzymes onto dead material and absorbs the small soluble products.
- biodiversity
- The variety of different species on Earth or within an ecosystem.
- biomass
- The dry mass of living material in an organism or trophic level.
- food security
- Having enough food to feed a population.
- extremophile
- An organism adapted to live in extreme conditions such as high temperature, pressure or salt concentration.
Calculate with confidence
Equations
Arithmetic mean abundance
mean = total number counted / number of samples
Summarises repeated quadrat counts before estimating abundance across a habitat.
| Symbol | Meaning | Unit |
|---|---|---|
| total counted | sum of the organism counts from all quadrats | organisms |
| samples | number of quadrats counted | quadrats |
| mean | average organism count per quadrat | organisms per quadrat |
Exam tip: Include every repeat unless there is a justified reason to exclude an anomaly, and keep sensible precision.
Estimated population from a sample
estimated population = organisms counted / area sampled × total habitat area
Scales a representative sample density to the full habitat area.
| Symbol | Meaning | Unit |
|---|---|---|
| organisms counted | total individuals recorded in all sampled quadrats | organisms |
| area sampled | combined area of all quadrats | m² |
| habitat area | total area being estimated | m² |
| estimated population | predicted total abundance in the habitat | organisms |
Exam tip: Use matching area units and label the result as an estimate; random sampling does not remove all uncertainty.
Separate Biology: relative rate of milk decay
relative rate = 1 / time to the same pH endpoint
Compares milk-decay rates when every tube is timed to the same indicator colour or pH.
| Symbol | Meaning | Unit |
|---|---|---|
| t | time taken to reach the fixed pH endpoint | s |
| relative rate | reciprocal of the endpoint time | s⁻¹ |
Exam tip: Use time in seconds before taking the reciprocal. A shorter endpoint time gives a larger relative rate.
Separate Biology: biomass-transfer efficiency
efficiency (%) = biomass transferred to next level / biomass available at previous level × 100
Calculates the percentage of biomass at one trophic level incorporated into the next.
| Symbol | Meaning | Unit |
|---|---|---|
| next level biomass | biomass gained by the higher trophic level | g, kg or another mass unit |
| previous level biomass | biomass available at the lower trophic level | the same mass unit |
| efficiency | fraction transferred expressed as a percentage | % |
Exam tip: Put the smaller transferred biomass in the numerator and multiply the fraction by 100.
Follow it step by step
Processes to remember
How to estimate abundance with quadrats
- Measure or obtain the total habitat area and the quadrat area in matching units.
- Use random coordinates to choose positions, then place the quadrat without choosing attractive patches.
- Count the target species using one clear boundary rule at every position.
- Repeat across enough positions to represent the habitat and calculate a mean count or density.
- Scale the sample density to the total area and report an estimated population.
- State uncertainty, possible patchiness and how more samples would improve reliability.
Exam tip: Random placement reduces selection bias; a larger sample reduces the influence of an unusual quadrat.
How to explain a predator–prey graph
- Identify which population rises first; this is normally the prey.
- Explain that more prey provides more food, so predator survival and reproduction increase after a time lag.
- As predator numbers rise, predation increases and the prey population falls.
- Less prey then reduces predator survival and reproduction, so predator numbers fall later.
- Use values from the graph and avoid claiming a perfectly fixed cycle.
Exam tip: The lag between peaks is evidence for the feeding link; correlation alone still requires a plausible mechanism.
How to trace carbon through an ecosystem
- Start with carbon dioxide in the atmosphere.
- Use photosynthesis to move carbon into plant or algal biomass.
- Use feeding to transfer carbon compounds through consumers.
- Use respiration by plants, animals and microorganisms to return carbon dioxide.
- Move carbon into dead material and waste, then use decomposition to recycle it.
- Add combustion when biomass or fossil fuels are burned.
Exam tip: Name the carbon store on both sides of each process; arrows alone rarely explain the cycle.
How to evaluate an ecological intervention
- Identify the environmental pressure and the population, habitat or process it affects.
- Explain how the proposed action changes that pressure.
- Use supplied data to judge the likely size and reliability of the benefit.
- Consider cost, timescale, unintended effects and stakeholders with competing needs.
- Reach a conditional conclusion that follows from the evidence.
Exam tip: A balanced answer is not a list of opposites; compare their importance in the stated context.
See the thinking
Worked example
Worked example: estimate a daisy population
Five 0.25 m² quadrats contain 4, 6, 5, 7 and 3 daisies. The field is 50 m². Estimate the daisy population.
- Add the counts: 4 + 6 + 5 + 7 + 3 = 25 daisies.
- Calculate sampled area: 5 × 0.25 m² = 1.25 m².
- Find density: 25 / 1.25 m² = 20 daisies/m².
- Scale to the field: 20 daisies/m² × 50 m² = 1000 daisies.
Answer: The estimated population is 1000 daisies.
The calculation assumes the random quadrats represent the field. More well-distributed samples would reduce the influence of patchy growth and improve reliability.
Protect the marks
Common mistakes
Watch out: Calling a community an ecosystem.
Do this instead: A community includes all living populations; an ecosystem includes the community interacting with abiotic conditions.
Watch out: Mixing up abiotic and biotic factors.
Do this instead: Abiotic factors are non-living conditions; biotic factors involve living organisms, such as predators, pathogens or competitors.
Watch out: Listing an adaptation without explaining it.
Do this instead: Link the feature to a mechanism and then to improved survival or reproduction in the stated environment.
Watch out: Choosing convenient quadrat positions.
Do this instead: Use random coordinates to reduce selection bias when estimating abundance across a habitat.
Watch out: Drawing food-chain arrows towards the organism being eaten.
Do this instead: Arrows show biomass transfer from the organism eaten to the consumer.
Watch out: Saying energy is recycled in ecosystems.
Do this instead: Materials such as carbon and water cycle; energy enters, is transferred and eventually dissipates to the surroundings.
Watch out: Claiming a correlation proves an environmental factor caused distribution.
Do this instead: A mechanism and control of other changing factors are needed before making a causal claim.
Watch out: Saying peat damage only releases carbon dioxide.
Do this instead: Destroying peat bogs also removes a distinctive habitat and reduces biodiversity.
Watch out: Writing that all biomass eaten reaches the next trophic level.
Do this instead: Some material is not absorbed, and much absorbed glucose is used in respiration rather than stored as new biomass.
Watch out: Treating a population estimate as an exact count.
Do this instead: It is an estimate whose reliability depends on representative random sampling, adequate repeats and accurate area measurements.
Plan it like the exam
Required practicals
Investigate population size and species distribution in a habitat
AQA Biology Required Practical 9; Combined Science: Trilogy Required Practical 7
Aim: Use quadrats to estimate a common species population and a transect to investigate how an environmental factor affects distribution.
Method
- Define the habitat boundary, identify the target species reliably and measure or obtain the total area.
- Generate random coordinate pairs, locate each position with tape measures and place a quadrat of known area.
- Count the target species using the same rule for organisms touching the boundary; repeat at many random positions.
- Calculate mean abundance or density and scale it to the habitat area to estimate population size.
- For distribution, lay a tape across an environmental gradient and place quadrats at regular distances to form a belt transect.
- At each distance, record target-species abundance and measure the relevant abiotic factor with suitable equipment.
- Repeat with parallel transects where practical, calculate means and plot abundance and the factor against distance.
- Look for a pattern, quote data and discuss other variables before suggesting a causal explanation.
Variables
- Independent
- position along the transect or the measured environmental factor associated with position
- Dependent
- abundance, frequency or percentage cover of the target species
- Controls
- quadrat dimensions and boundary-count rule
- distance between transect samples
- species-identification method
- time of day and weather where practical
- instrument and measurement technique
- total sampling effort at each location
Analysis: Calculate means and an estimated population with units. Plot suitable graphs and describe correlation, but use biological reasoning and control of other factors before claiming cause.
Safety
- Complete the school's fieldwork risk assessment; wear suitable footwear and clothing and avoid steep, unstable, roadside or deep-water sites.
- Do not touch unknown plants, fungi, animals, waste or sharp objects; cover cuts and wash hands after fieldwork.
- Keep the group together, follow site boundaries and weather guidance, and carry equipment so tapes and quadrats do not become trip hazards.
- Minimise disturbance: replace moved objects, avoid trampling outside the sample and release any organism promptly if observation is permitted.
Improvements
- Increase the number of random quadrats and sample across the whole defined habitat.
- Use several parallel transects instead of relying on one possibly unusual line.
- Calibrate probes, repeat abiotic measurements and calculate means.
- Use a photograph or gridded quadrat to estimate percentage cover consistently.
Separate Biology: investigate temperature and fresh-milk decay
Separate Biology only — AQA Required Practical 10
Aim: Investigate how temperature affects the rate of decay of fresh milk by timing a fixed pH change.
Method
- Label tubes for several temperatures; add equal volumes of fresh milk and sodium carbonate for the same alkaline start, then the same amount of Cresol red, to each.
- Prepare equal portions of the same lipase solution, which models decay by breaking milk fat into fatty acids that lower pH.
- Place each milk mixture and lipase portion in its water bath until both reach the target temperature; verify with a thermometer.
- Add the lipase to the milk, mix in the same way and start the timer immediately.
- Stop timing at one defined indicator-colour or pH endpoint and record the time in seconds.
- Repeat at each temperature, calculate a mean endpoint time and calculate relative rate = 1 / mean time.
- Plot mean relative rate against temperature and describe the pattern within the measured range.
Variables
- Independent
- temperature of the milk and lipase mixture in the water bath
- Dependent
- time to a fixed pH endpoint and calculated relative rate, 1 / time
- Controls
- milk volume and starting batch
- lipase volume and concentration
- sodium carbonate volume and concentration
- indicator identity and volume
- starting pH and chosen endpoint
- equilibration time and mixing method
Analysis: For each temperature, calculate a mean time and its reciprocal in s⁻¹. A larger relative rate means faster pH change; identify anomalies and do not extrapolate beyond the data.
Safety
- Wear eye protection. Lipase, sodium carbonate and indicator can irritate skin or eyes; use prepared solutions, avoid aerosols and rinse splashes as instructed.
- Use thermostatically controlled water baths, heatproof mats and test-tube racks; take care with hot water and glassware.
- Use fresh pasteurised milk; check school allergy information, avoid skin contact, never taste materials, clean spills and follow the disposal procedure.
- Carry out the practical under trained school supervision and follow the reagent-specific risk assessment.
Improvements
- Use a calibrated pH probe or data logger for an objective, repeatable endpoint.
- Measure the reaction mixture temperature rather than relying only on the water-bath setting.
- Repeat each temperature, investigate anomalies and calculate a mean.
- Use a narrower temperature interval around any apparent optimum.
Try it before you move on
Quick check
Say your answer first, then open the card to check it.
What extra ingredient turns a community into an ecosystem?
Answer: Interactions with the abiotic parts of the environment.
A community contains the living populations; an ecosystem includes their non-living conditions and interactions.
Name two resources plants commonly compete for.
Answer: Any two of light, space, water or mineral ions from the soil.
The resource must be limited for competition to affect growth, survival or reproduction.
Why use random coordinates for quadrats?
Answer: To reduce selection bias and make the sample more representative of the habitat.
Choosing visually interesting patches could systematically overestimate or underestimate abundance.
In grass → rabbit → fox, what does the first arrow mean?
Answer: Biomass is transferred from grass to rabbit when the rabbit eats the grass.
Food-chain arrows point from the organism eaten towards the consumer.
Which process removes carbon dioxide from the atmosphere?
Answer: Photosynthesis by plants and algae.
Respiration, decomposition and combustion return carbon dioxide to the atmosphere.
Give two ecological effects of destroying a peat bog.
Answer: Habitat and biodiversity are lost, and decay or burning of peat releases carbon dioxide.
Peat is both a specialised habitat and a long-term carbon store.
Separate Biology: why does biomass decrease up a food chain?
Answer: Not all material is eaten or absorbed, and absorbed biomass is lost through respiration and waste rather than becoming new tissue.
Only new consumer biomass is available for transfer to the next trophic level.
How can a larger legal net-mesh size help a fish stock recover?
Answer: Smaller, younger fish can escape and survive long enough to reproduce.
Maintaining breeding individuals supports future recruitment into the stock.
Good questions, clear answers
Frequently asked questions
What is the difference between habitat and ecosystem?
A habitat is the place where an organism lives. An ecosystem is a community interacting with the abiotic parts of its environment.
When should I use a quadrat or a transect?
Use random quadrats to estimate abundance across a habitat. Use quadrats along a line or belt transect to investigate how distribution changes across a gradient.
Why must quadrat sampling be random?
Random positions reduce the investigator's selection bias. Repeating many positions then makes the estimate more representative and less sensitive to patchiness.
Do predator and prey populations always cycle perfectly?
No. The model predicts linked rises and falls with a time lag, but weather, disease, migration, competitors and other food sources can disturb the pattern.
Are climate-change predictions uncertain because scientists disagree that warming occurs?
No. Scientific consensus is based on systematic review of extensive evidence. Uncertainty mainly concerns the exact size, timing and local effects in a complex system.
Why is dry biomass often used?
Water content varies between organisms and conditions. Removing water makes comparisons of living material between trophic levels more valid.
Which Ecology content is separate Biology only?
AQA labels decomposition, trophic levels, biomass transfer, food security, farming, fisheries and biotechnology as separate Biology; environmental-change impact is also Higher Tier.
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