OCR GCSE Biology A: Ecosystem organisation, abiotic and biotic factors, trophic levels and biomass transfer (J247) – Revision Notes

Quick recall for OCR GCSE (9-1) Gateway Science Biology A (J247), Version 4.0 (August 2026), first assessment in 2018, statements B4.1e to B4.1j. No statement in this unit is marked Higher Tier only. The study guide has the full explanations and worked examples; this page condenses it. Then try the practice questions, tick off topics on the printable checklist, see the course hub and take the free diagnostics.

Key definitions

Term	Meaning
Organism	One individual living thing
Population	Every individual of a single species living in one area
Community	The populations of every species living together in one area
Ecosystem	The community plus the non-living parts of the area
Abiotic factor	A non-living factor, such as temperature, light intensity, moisture or soil pH
Biotic factor	A factor caused by living organisms, such as predators or food supply
Producer	An organism that makes its own food by photosynthesis (plants, algae)
Consumer	An organism that gets biomass by eating other organisms
Trophic level	A feeding level in a food chain
Interdependence	Organisms in a community relying on each other
Competition	Organisms needing the same limited resource
Pyramid of biomass	A diagram showing the dry mass of each trophic level in an area
Efficiency of biomass transfer	Biomass passed to the next level as a percentage of the biomass at the level below

Levels of organisation (B4.1e)

Order from smallest to largest: organism, population, community, ecosystem.

• “All the frogs in a pond” is a population.
• “All the frogs, fish, plants and bacteria in a pond” is a community.
• “The pond including its water, light and temperature” is an ecosystem.

Abiotic and biotic factors (B4.1f)

Factor	Type	Effect on a community
Temperature	Abiotic	Controls enzyme activity, so growth and reproduction. Extreme values reduce which species can live there
Light intensity	Abiotic	Limits photosynthesis, so producer numbers, so the food supply for consumers
Moisture level	Abiotic	Affects germination, water uptake and survival of plants and animals that dry out easily
Soil pH	Abiotic	Affects mineral availability and root growth, so which plants can grow
Predators	Biotic	Remove prey, so more predators usually means fewer prey
Food	Biotic	More food allows a population to grow. A shortage reduces numbers

Method for an explain answer:

1. Name the factor and say whether it increases or decreases.
2. Say what this does to individual organisms (growth, survival, reproduction).
3. Say what happens to the population or the community.

Interdependence and competition (B4.1g)

Relationship	Who benefits	Example
Predation	Predator gains, prey is killed	Ladybird and aphids
Mutualism	Both species	Nitrogen-fixing bacteria in clover root nodules: sugars for the bacteria, nitrogen compounds for the plant
Parasitism	Parasite gains, host is harmed	Tapeworm in the gut, flea on a mammal

Competition:

• Plants: light, water, mineral ions, space.
• Animals: food, water, shelter, territory, mates.
• Within a species it is often stronger, because individuals need identical resources.
• If one species wins a resource, the other declines.

Predator-prey graph: the prey peak comes first and the predator peak follows. Prey numbers fall when predators are plentiful, and predator numbers then fall because food is scarce.

Trophic levels (B4.1h)

Position	Name	Feeds on
First	Producer	Makes its own food
Second	Primary consumer	Producers
Third	Secondary consumer	Primary consumers
Fourth	Tertiary consumer	Secondary consumers

• Arrows show the direction of biomass transfer: from the food to the feeder.
• An apex predator has no natural predators.
• Removing producers affects every level. Removing a top predator can allow its prey to increase.

Pyramids of biomass and biomass loss (B4.1i)

• Use dry mass per unit area, with producers at the bottom.
• Each bar is drawn to the same scale and the bars are centred.
• The pyramid narrows upwards because biomass is lost at each transfer.

Loss	What happens	Example
Egestion	Undigested material leaves as faeces	Cellulose, bone, shell, hair. Parts not eaten at all, such as roots
Excretion	Metabolic waste is removed	Urea in urine, carbon dioxide from the lungs
Respiration	Glucose is broken down to release energy, and carbon leaves as carbon dioxide	Movement, keeping warm

Egestion is not excretion. Egestion is leaving the gut unabsorbed. Excretion is waste made by cells.

Calculating efficiency (B4.1j)

efficiency (%) = biomass transferred to the next level / biomass at the level below × 100

Method in steps:

1. Choose the two consecutive levels. The lower level is the denominator.
2. Check both values use the same unit and the same area and time.
3. Divide and multiply by 100.
4. Give the answer with a % sign and sensible rounding.

Worked reminder: a producer level makes 450 kg of biomass, the primary consumers gain 36 kg and the secondary consumers gain 3.24 kg.

Producer to primary: 36 / 450 × 100 = 8%. Primary to secondary: 3.24 / 36 × 100 = 9%. Overall producer to secondary: 3.24 / 450 × 100 = 0.72%.

If 8% is transferred, 100 - 8 = 92% is not. It is lost as egested material, excreted waste and respiration.

Working through several levels at about 10% each: multiply by 0.1 for each step up, divide by 0.1 for each step down.

Food chains are short, usually four or five levels, because so much is lost at each step that too little biomass remains to support another level.

Small worked reminders

Reading a factor from data. A table shows the mean number of one plant species per quadrat on soils of increasing acidity. If numbers fall as pH decreases, say so with figures, name pH as the abiotic factor, and suggest a reason such as poor mineral availability or root damage. Then note that a correlation does not prove that pH is the only cause.

Reading a pyramid. If the bars read 800, 90 and 7 (g per m²), the biomass falls by roughly a factor of ten at each step. That fits the idea that most biomass is lost between levels. You can turn it into efficiency: 90 / 800 × 100 = 11.25%, and 7 / 90 × 100 = 7.8%.

Explaining a loss. Use the pattern: name the route, say what leaves the animal, and say why it cannot become new tissue. For respiration: glucose is broken down to release energy, carbon leaves as carbon dioxide, so that carbon is not added to the animal’s body.

Making a pyramid of biomass: pick a scale so the widest bar fits the page, draw each bar at the same height, centre every bar on the same vertical line, put the producer at the bottom and label each bar with the organism and its biomass.

Command words for this unit

Command word	What to write
Describe	Say what happens or what the diagram shows, using data if given
Explain	Give a reason linked with because, so or therefore
Calculate	Show the working and give the unit or % sign
Suggest	Apply what you know to an unfamiliar situation

Must-know distinctions

• Population against community: one species against many.
• Community against ecosystem: living only against living and non-living.
• Abiotic against biotic: non-living against living.
• Mutualism against parasitism: both gain against one is harmed.
• Egestion against excretion: undigested food against metabolic waste.
• Biomass transferred against biomass lost: use the denominator carefully.

Quick self-test

1. A group of 60 owls of one species lives in a forest. Is this an organism, a population, a community or an ecosystem?
2. Is soil pH abiotic or biotic?
3. Name the type of relationship in which a tick feeds on a deer and harms it.
4. Give two resources that plants compete for.
5. In the chain algae → water flea → minnow, which organism is the primary consumer?
6. State the direction an arrow points in a food chain.
7. Name the three ways in which biomass is lost between trophic levels.
8. Why does a pyramid of biomass use dry mass?
9. A secondary consumer level has 2400 kg of biomass. The tertiary consumers gain 288 kg. What is the percentage efficiency?
10. At 10% transfer, how much producer biomass supports 30 kg of tertiary consumers?
11. Why do food chains rarely have more than five trophic levels?

Answers

1. A population, because they are one species in one area.
2. Abiotic.
3. Parasitism.
4. Any two of light, water, mineral ions, space.
5. The water flea.
6. From the organism that is eaten to the organism that eats it.
7. Egestion, excretion and respiration.
8. Water content varies, so dry mass measures the actual amount of tissue.
9. 288 / 2400 × 100 = 12%.
10. 30 / 0.1 / 0.1 / 0.1 = 30 000 kg.
11. So much biomass is lost at each transfer that too little is left to support another level.

Where marks are usually lost

• Writing “community” for “population”, or leaving the non-living part out of a definition of ecosystem.
• Giving an effect without a consequence, such as “less light” without saying fewer producers.
• Confusing predation, parasitism and mutualism. Say who benefits and who is harmed.
• Drawing arrows the wrong way round in a food chain.
• Saying “energy is lost as heat” and not naming biomass loss through egestion, excretion or respiration.
• Treating egestion as excretion.
• Using the wrong level as the divisor in an efficiency calculation.
• Quoting an answer with no working or no % sign.
• Describing a graph without using the numbers, or ignoring a time lag in predator-prey data.

Official syllabus

OCR GCSE (9-1) Gateway Science Biology A (J247), Version 4.0 (August 2026), first assessment 2018, published by OCR. Topic B4.1, statements e to j.