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No living thing exists on its own. A rabbit depends on grass for food, on burrows in the soil for shelter, and on other rabbits to breed; the grass depends on the rabbit's droppings to return minerals to the soil, on bees and the wind for pollination, and on the Sun for light. Pull on any one thread in a habitat and the whole web shifts. This lesson opens Topic B4 (Community-level systems) of OCR Gateway Science A by building the vocabulary you need to talk about whole communities — the levels of organisation from a single individual up to an entire ecosystem — and by sorting the things that affect organisms into the living (biotic) and non-living (abiotic) factors that shape where they can survive. Getting these ideas secure now makes everything that follows in B4 (food webs, competition, sampling, the recycling of materials) far easier to understand.
By the end of this lesson you should be able to define the terms individual, population, community and ecosystem, distinguish biotic from abiotic factors and give examples of each, and explain what interdependence means and why stable communities matter.
Ecologists describe the living world at several levels of organisation, each one larger and more complex than the last. Just as a single cell builds up into a tissue, organ and whole organism (Topic B1), a single organism builds up into populations, communities and ecosystems.
Working from the smallest to the largest:
So an ecosystem is the biggest of these ideas because it includes both the living community and its non-living surroundings. A useful test in the exam: if a definition mentions only living things, it is describing a population or community; if it brings in the soil, water, air and climate as well, it is describing an ecosystem.
Exam Tip: Two definitions are very often confused. A population is one species; a community is all the species together. The quickest way to keep them apart is the phrase "population = one species, community = many species". An ecosystem then adds the abiotic environment to the community.
A habitat is the place where a particular organism lives — the part of an ecosystem that provides what it needs. A rocky shore, an oak woodland, a freshwater pond and a hedgerow are all habitats. Within one ecosystem there may be many habitats: a single woodland contains the leaf litter on the floor, the rotting log, the tree canopy and the stream, each home to different species. The conditions in a habitat — how light, how warm, how wet — decide which organisms can live there, and that brings us to the factors that shape every community.
Everything that affects whether an organism can survive in a habitat falls into one of two groups.
Learning which is which, with examples, is worth reliable marks because OCR frequently asks you to classify factors or to suggest how a named factor affects a population.
| Biotic factor | What it means | Example of its effect |
|---|---|---|
| Food availability | How much food there is for a species | If grass is plentiful, the rabbit population can grow |
| New predators | Animals that hunt and eat a species | More foxes means fewer rabbits survive to breed |
| Competition | Organisms competing for the same limited resource | If a new species competes better for food, the original species may decline |
| New pathogens (disease) | Disease-causing microorganisms | A new disease can rapidly reduce a population with no resistance |
| Abiotic factor | What it means | Example of its effect |
|---|---|---|
| Light intensity | How much light reaches the habitat | Plants need light for photosynthesis; shaded plants grow poorly |
| Temperature | How warm the habitat is | Affects the rate of plant growth and where animals can survive |
| Water / moisture | How much water is available | In dry conditions, plant growth and animal survival fall |
| Soil / mineral content | The amount of mineral ions (e.g. nitrate) in the soil | Low nitrate limits plant protein synthesis and growth |
| Oxygen / carbon dioxide levels | The amount of these gases available | Aquatic animals need dissolved oxygen; plants need CO₂ for photosynthesis |
The carbon dioxide and oxygen levels matter especially in water. A fast-flowing, cool stream holds plenty of dissolved oxygen, so it can support oxygen-demanding animals such as mayfly nymphs; a warm, still, polluted pond holds little dissolved oxygen and supports far fewer species.
Exam Tip: A simple memory hook: biotic = bio = life, so any factor that is a living thing (food, predators, disease, competitors) is biotic; everything physical (light, temperature, water, minerals, gases) is abiotic. If a question asks for "a biotic factor", do not offer "temperature" — that is abiotic.
A pond becomes warmer and more crowded with fish over one summer. Identify one abiotic and one biotic factor that have changed, and suggest one effect of each.
Step 1 — pick out the abiotic change. "Warmer" is a change in temperature, which is part of the non-living environment, so it is abiotic. A possible effect: warmer water holds less dissolved oxygen, so fish may struggle to get enough oxygen.
Step 2 — pick out the biotic change. "More crowded with fish" is a change in population size of a living species, so the increased competition between the fish is a biotic factor. A possible effect: the fish compete more strongly for food, so some may not get enough to survive or breed.
Answer: Abiotic — temperature has risen, lowering dissolved oxygen; biotic — competition between fish has increased, reducing the food available to each.
Within a community the species are not just neighbours; they depend on one another. This mutual dependence is called interdependence. Organisms rely on each other for food, shelter, pollination and the dispersal of seeds, among other things. A few examples make the idea concrete:
Because everything is connected, a change to one species ripples out to others. If a disease wiped out the rabbits in a meadow, the foxes that eat them would have less food and might decline, while the grass the rabbits grazed might grow taller and shade out smaller plants. This sensitivity to change is the heart of B4.
flowchart LR
A["Plant<br/>(producer)"] -->|"food"| B["Rabbit<br/>(consumer)"]
B -->|"food"| C["Fox<br/>(predator)"]
B -->|"droppings return minerals"| D["Decomposers<br/>(in soil)"]
C -->|"dead body"| D
D -->|"mineral ions"| A
E["Bee"] -->|"pollination"| A
A -->|"nectar"| E
The arrows show that material and dependence flow in many directions at once — a community is a web of relationships, not a simple line.
A stable community is one in which the species and the environmental conditions are in balance, so that population sizes stay roughly constant over time. The numbers of each species do go up and down a little, but in a stable community these changes are small and the community keeps the same overall make-up year after year. Mature woodlands, established coral reefs and old grasslands are examples of stable communities.
Stability matters because it means the community can support all its species reliably and can withstand small changes without collapsing. The interdependence described above is what creates this balance: predators keep prey numbers from exploding, competition stops any one species taking over, and decomposers keep recycling the minerals that plants need. When a community is disturbed too much — by the loss of a key species, a new disease, or a change in an abiotic factor such as temperature — that balance can be lost, and the whole community may change.
Exam Tip: A "stable community" is defined by population sizes staying roughly constant over time because biotic and abiotic factors are in balance — not by "nothing ever changing". Use the phrase "in balance, so populations stay roughly constant" and you will hit the marking points.
| Misconception | The correct idea |
|---|---|
| "A population is all the living things in an area" | A population is all the organisms of one species; all species together is a community |
| "An ecosystem is just the living organisms" | An ecosystem is the community plus the non-living environment (soil, water, air, light, temperature) |
| "Temperature is a biotic factor because it affects living things" | Temperature is abiotic — it is part of the physical environment; biotic factors are living things |
| "Interdependence means animals helping each other on purpose" | Interdependence simply means species rely on one another (for food, pollination, shelter); it is not deliberate cooperation |
| "A stable community never changes" | Populations stay roughly constant because factors are in balance; small fluctuations still occur |
| "A habitat and an ecosystem are the same thing" | A habitat is the place an organism lives; an ecosystem is the whole community plus its environment, and can contain many habitats |
Question (6 marks): A meadow contains grass, rabbits, foxes and earthworms. Using examples from the meadow, explain the terms population, community and ecosystem, and describe how one biotic and one abiotic factor could affect the rabbit population.
Mid-band response: "A population is one type of animal, like the rabbits. A community is all the animals and plants, like the rabbits, grass and foxes. An ecosystem is everything including the soil and air. A biotic factor is foxes eating the rabbits. An abiotic factor is the temperature."
Examiner-style commentary: This gains marks for the basic definitions and for naming one biotic factor (predation by foxes) and one abiotic factor (temperature). It is held back by loose wording ("one type of animal" misses plants and the idea of a species) and by not saying how the factors affect the rabbits. To climb a band, sharpen the definitions and explain the effect of each factor.
Stronger response: "A population is all the individuals of one species in an area, for example all the rabbits in the meadow. A community is all the populations of the different species living there — the rabbits, grass, foxes and earthworms together. An ecosystem is the community plus the non-living environment, such as the soil, water and air. A biotic factor is predation: more foxes would eat more rabbits, so the rabbit population would fall. An abiotic factor is temperature: in a cold winter there is less plant growth, so there is less food and fewer rabbits survive."
Examiner-style commentary: A clear, well-organised answer that defines all three levels accurately with examples and explains the effect of both a biotic and an abiotic factor. To reach the top band, it could use the word "interdependence" and note that the ecosystem includes the community interacting with its environment.
Top-band response: "A population is all the individuals of a single species living in an area at one time — for example, all the rabbits in the meadow. A community is all the populations of all the different species living and interacting there: the rabbits, the grass, the foxes and the earthworms together. An ecosystem is that whole community plus the abiotic (non-living) environment — the soil, water, air, light and temperature — with which the organisms interact, so it is the largest of the three. The species in the meadow show interdependence: the foxes depend on the rabbits for food, and the rabbits depend on the grass. A biotic factor affecting the rabbits is predation — an increase in the fox population would reduce the number of rabbits surviving to breed, lowering the rabbit population. An abiotic factor is temperature — a cold winter slows grass growth, so less food is available and fewer rabbits survive. Because the species are interdependent, a change in the rabbit population would also affect the foxes and the grass."
Examiner-style commentary: Full marks. The three levels are defined precisely with meadow examples, the ecosystem is correctly described as the community plus its abiotic environment, and both a biotic and an abiotic factor are explained with their effect on the population. The use of "interdependence" and the closing note that change ripples through the community show exactly the connected understanding examiners reward.
This content is aligned with OCR Gateway Science A GCSE Biology (J247), Topic B4 Community-level systems. Refer to the official OCR specification document for the exact wording.