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

This study guide follows the OCR GCSE (9-1) Gateway Science Biology A (J247) specification, Version 4.0 (August 2026), for first assessment in 2018. It covers statements B4.1e to B4.1j of topic B4, Community level systems: levels of organisation, abiotic and biotic factors, interdependence and competition, trophic levels, pyramids of biomass and the efficiency of biomass transfer. None of these statements is marked as Higher Tier only in the specification text, so all of them are for both tiers. The cycling of materials and decomposition (B4.1a to B4.1d) are taught separately.

Related pages: revision notes, practice questions, the OCR GCSE Biology hub, the printable checklist and the free diagnostics.

What you need to be able to do

Statement	What you must be able to do
B4.1e	Describe the levels of organisation in an ecosystem, from individual organisms to the whole ecosystem
B4.1f	Explain how abiotic and biotic factors affect communities (temperature, light intensity, moisture level, soil pH, predators, food)
B4.1g	Describe the importance of interdependence and competition, including predation, mutualism and parasitism
B4.1h	Describe the differences between trophic levels, using the terms producer and consumer
B4.1i	Describe pyramids of biomass and explain, with examples, how biomass is lost between trophic levels (egestion, excretion, respiration)
B4.1j	Calculate the efficiency of biomass transfers between trophic levels and explain how this affects the number of trophic levels in a food chain

B4.1e Levels of organisation

An ecosystem is all the living organisms in an area together with the non-living parts of it. Each level contains the one before it.

Level	Meaning	Example (a freshwater pond)
Organism	One individual living thing	One pond snail
Population	All the organisms of one species living in an area	All the pond snails in the pond
Community	The populations of all the different species living in an area	Snails, algae, pondweed, frogs, dragonfly larvae and bacteria together
Ecosystem	The community plus the non-living (abiotic) parts: water, light, temperature, dissolved minerals, sediment	The whole pond

Two points are often lost. A population is one species only, so “all the animals in the pond” is a group of populations, not a population. A community is living things only, so it leaves out the water and the light.

B4.1f Abiotic and biotic factors

An abiotic factor is a non-living factor of the environment. A biotic factor is one that comes from living organisms. The specification names these:

• Abiotic: temperature, light intensity, moisture level, pH of soil.
• Biotic: predators and food (and, as the next section shows, competitors, parasites and so on).

The effect of a factor is always on the size or distribution of a population, and so on the make-up of the community. To explain it, say what the factor does to individuals and then what that does to numbers.

• Light intensity limits photosynthesis, so less light means fewer producers and less food for everything above them.
• Moisture: dry soil limits germination and water uptake, so only drought-adapted plants survive.
• Soil pH changes which minerals are available and can damage roots, so each plant species grows best in a certain range.
• Temperature affects enzyme activity, so it controls the rate of growth and reproduction. Cold-blooded animals are more affected than mammals and birds.
• Predators remove prey. More predators usually mean fewer prey.
• Food supply: more food allows a population to grow, and a shortage lowers it.

Worked example: interpreting sampling data

A student counted plants of one species in 1 m² quadrats at three places in a wood. Each place had six quadrats. Light intensity was measured at each place.

Place	Light intensity (arbitrary units)	Plants per quadrat
A (dense shade)	12	2, 3, 1, 4, 2, 2
B (partial shade)	45	9, 11, 8, 12, 10, 10
C (open clearing)	90	5, 6, 4, 7, 5, 3

Mean for A = (2 + 3 + 1 + 4 + 2 + 2) / 6 = 14 / 6 = 2.3 plants per quadrat. Mean for B = 60 / 6 = 10.0. Mean for C = 30 / 6 = 5.0.

The species is most abundant at moderate light. Too little light limits photosynthesis in A. The fall at C does not follow from light alone, because an open clearing is probably also drier and hotter, so moisture or temperature may be the limiting factor. A good conclusion links the pattern to the factor and admits that other factors vary too.

B4.1g Interdependence and competition

Organisms in a community depend on each other. The specification asks for three kinds of interdependence.

• Predation: a predator kills and eats prey. A ladybird eating aphids is an example.
• Mutualism: both species benefit. Nitrogen-fixing bacteria live in root nodules of a leguminous plant such as clover. The bacteria receive sugars. The plant receives nitrogen compounds it can use to make protein.
• Parasitism: the parasite benefits and the host is harmed. A tapeworm in a gut takes digested food from its host, and a flea feeds on blood.

Competition happens when organisms need the same limited resource.

• Plants compete for light, water and mineral ions, and for space.
• Animals compete for food, water, shelter, territory and mates.
• Competition within one species (intraspecific) is often fiercest, because members need exactly the same things.
• Competition between different species (interspecific) happens where their needs overlap.

Stable communities have predator and prey numbers that rise and fall in step. When the prey increase, there is more food, so the predators increase a little later. More predators eat more prey, so prey numbers fall. Then the predators starve, and their numbers fall. The predator peak lags behind the prey peak.

Worked example: a predator-prey table

Month	Aphids per plant	Ladybirds per plant
1	40	2
2	130	3
3	300	6
4	190	14
5	60	9
6	35	4

The aphids peak in month 3 and the ladybirds peak in month 4. Ladybirds are the predator because their peak comes after the prey peak and their numbers follow the aphids. Aphid numbers fell after month 3 because the growing ladybird population was eating them, and ladybird numbers fell after month 4 because there was less food.

B4.1h Trophic levels

A trophic level is a feeding level in a food chain. Arrows show the direction in which biomass is transferred, so they point from the organism being eaten to the organism that eats it. They do not point from predator to prey.

• Producers are plants and algae. They make their own food by photosynthesis and are the first trophic level.
• Primary consumers eat producers. They are herbivores.
• Secondary consumers eat primary consumers.
• Tertiary consumers eat secondary consumers. An apex predator has no predators of its own.

Consumers get their biomass by eating other organisms. A consumer can be a carnivore, a herbivore or an omnivore. An omnivore may sit at more than one level in a food web.

Example chain: wheat → field mouse → barn owl. Wheat is the producer, the mouse is the primary consumer and the owl is the secondary consumer. If the wheat were removed, every consumer after it would lose its food, which is why a loss of producers is more damaging than a loss of one top predator.

B4.1i Pyramids of biomass and biomass loss

A pyramid of biomass shows the dry mass of living material at each trophic level for a given area. Each bar is drawn to scale, the producers go at the bottom and the bars are centred. Dry mass is used because water content varies and does not tell you how much tissue is there.

Sample data for a seagrass bed (dry mass per m²):

Trophic level	Organism	Biomass (g)
1	Seagrass	640
2	Snails	70
3	Wrasse	8

Wrasse     |■|
Snails   |■■■■■|
Seagrass |■■■■■■■■■■■■■■■■|

Biomass falls at each step because not all of the biomass eaten becomes new biomass in the consumer. It is lost in three main ways:

• Egestion: material that is eaten but not digested leaves as faeces, such as cellulose, bone, hair and shell. Some parts are not eaten at all, such as roots or bones.
• Respiration: cells break down glucose to release energy, and the carbon leaves the body as carbon dioxide. The energy is used for movement and keeping warm, and ends up as heat.
• Excretion: waste products such as urea in urine are removed. Carbon dioxide is also an excretory product.

Only what is left is used for growth and so adds to the next trophic level.

Worked example: where did the biomass go?

A cow eats 500 kg of dry mass of grass. 150 kg is egested, 240 kg is lost through respiration, 20 kg is lost in urine and 90 kg becomes new cow tissue.

Check: 150 + 240 + 20 + 90 = 500 kg. Percentage used for growth = 90 / 500 × 100 = 18%. Percentage lost by respiration = 240 / 500 × 100 = 48%.

The largest loss is respiration. Warm-blooded animals such as cows lose a lot of biomass this way because they use energy to keep warm.

B4.1j Efficiency of biomass transfer

efficiency of biomass transfer (%) = biomass transferred to the next trophic level / biomass available at the previous level × 100

Worked example: a wheat field

Per square metre per year, the wheat produces 2500 g of biomass, the mice eating it gain 215 g and the owls eating the mice gain 17.2 g.

Wheat to mice: 215 / 2500 × 100 = 8.6%. Mice to owls: 17.2 / 215 × 100 = 8.0%. Overall, wheat to owls: 17.2 / 2500 × 100 = 0.688%, or 0.69%.

Transfers of about 10% are typical, though the exact values vary. The overall efficiency is much lower than either single transfer, because each step multiplies the previous losses.

Why food chains are short

Because about 90% is lost at each step, very little biomass remains at the top. Start with 100 000 kg of producer biomass and assume 10% transfer:

Level	Biomass (kg)
Producer	100 000
Primary consumer	10 000
Secondary consumer	1000
Tertiary consumer	100
Quaternary consumer	10
Fifth-level consumer	1

Too little biomass is left to support a population large enough to survive, so food chains rarely have more than four or five trophic levels.

Working backwards

How much producer biomass is needed to support 12 kg of tertiary consumers if each transfer is 10%? The tertiary consumers are three steps above the producers, so the producers must be 12 / 0.1 / 0.1 / 0.1 = 12 000 kg.

This is also why eating plants directly gives humans more food from the same land than eating animals that fed on plants. There is one less step, so one less round of losses.

Common errors

• Arrows drawn from predator to prey. They show the flow of biomass.
• Saying energy is “used up”. Say that biomass is lost as carbon dioxide, water, waste and uneaten parts.
• Describing a population as several species.
• Calling egestion and excretion the same. Egestion is undigested food leaving the gut. Excretion is the removal of metabolic waste.
• Dividing by the wrong level in an efficiency calculation. The denominator is the level you start from.
• Stating that a factor “affects the organism” without saying what happens to numbers or distribution.

Next steps

Test yourself with the practice questions, condense this with the revision notes and take a diagnostic to find weak spots.

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.