The big idea: A species is not found everywhere — it lives only where conditions suit it.
Two kinds of factor decide where an organism can live:
Abiotic factors — the non-living, physical and chemical features of the environment (for example temperature, light, water and pH).
Biotic factors — the living features, meaning the organism's interactions with other living things (for example food, competition and predators).
Together these factors set the distribution of the species — the places where it is found.
| Type of factor | What it is | Examples |
|---|---|---|
| Abiotic factor | A NON-living, physical or chemical feature of the environment | Temperature, light intensity, water / rainfall, pH, salinity, soil mineral nutrients, dissolved oxygen |
| Biotic factor | A LIVING feature — an interaction with other organisms | Food supply, competition, predation, disease, availability of mates, plants for shelter |
- Abiotic factor
- A non-living, physical or chemical feature of the environment, such as temperature, light intensity, water availability, pH or salinity.
- Biotic factor
- A living feature of the environment — an interaction with other organisms, such as competition, predation, disease or food supply.
- Distribution (of a species)
- The range of places where a species is found — where individuals of that species actually live.
- Habitat
- The particular place or type of environment in which a species normally lives.
- Range of tolerance
- The range of values of an abiotic factor (for example temperature) within which an organism can survive; outside this range it cannot live.
Telling them apart: Abiotic = a-BIOTIC = NOT living. If it is a physical or chemical condition (heat, light, water, pH, salt, oxygen), it is abiotic.
Biotic = living. If it involves other organisms (food, competitors, predators, disease), it is biotic.
Each species can only survive within a certain range of each abiotic factor — its range of tolerance.
Where the conditions fall inside that range, the species can live there. Where any factor is pushed beyond the organism's limits of tolerance, the species is absent.
Range of tolerance: For an abiotic factor such as temperature, an organism does best in a middle optimum range.
On either side of the optimum there is a zone of stress where the organism survives but struggles — growth and reproduction fall.
Beyond the lower and upper limits of tolerance, the factor is too extreme and the organism cannot survive there at all — so it is absent.
This is why a single abiotic factor can decide whether a species is present or absent in a place.
An organism thrives across its optimum range, struggles in the zones of stress on either side, and is absent beyond its lower and upper limits of tolerance — so an abiotic factor sets where the species can live.
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Limiting factors: A limiting factor is the factor that is in shortest supply (or is the most extreme), so it is the one holding back growth or survival in that place.
If you increase only that factor, the population responds; increasing any of the others has little effect until the limiting one is relieved.
Example: in the open ocean, iron is often scarce, so even with plenty of light and nutrients the growth of phytoplankton is limited by iron — add iron and they bloom.
Biotic factors also limit distribution: It is not only the physical conditions. Living factors set distribution too:
a consumer cannot live where there is too little food; a species can be excluded by strong competition from a better-adapted species; or kept out by heavy predation or disease.
So a place may be physically suitable, yet a species is still absent because of a biotic factor.
| Factor | Type | How it limits where a species lives |
|---|---|---|
| Temperature | Abiotic | Too cold or too hot is outside the tolerance range; enzymes and growth slow or stop, so the species is absent |
| Water / rainfall | Abiotic | Too little water means a land plant or animal cannot survive there; deserts exclude species needing moisture |
| Light intensity | Abiotic | Plants need enough light to photosynthesise; deep shade or deep water excludes many plants and algae |
| Soil mineral nutrients | Abiotic | Few nutrients (e.g. nitrogen) limits plant growth, so only tolerant species survive |
| Food supply | Biotic | Without enough food a consumer cannot survive in an area, so its distribution is restricted |
| Predation / competition | Biotic | Heavy predation or strong competition from another species can exclude an organism from otherwise suitable habitat |
Abiotic factors
- Non-living physical / chemical conditions
- Temperature, light, water, pH, salinity, oxygen, nutrients
- Each species survives only within its range of tolerance
- Often the limiting factor that holds growth back
Biotic factors
- Living interactions with other organisms
- Food supply, competition, predation, disease, mates
- Can exclude a species even where conditions are suitable
- Together with abiotic factors, they set distribution
A memory hook: Distribution = where a species lives. To explain it, list both kinds of factor: abiotic (the conditions) and biotic (the other organisms).
The limiting factor is simply the one in shortest supply — relieve it and growth increases.
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How this is tested: A common Paper 3 task is to list factors (for example three) that influence where a species is distributed — give separate scoring points and mix abiotic with biotic.
An Identify question can ask for the abiotic factors that limit growth in a named biome such as tundra (low temperature, short growing season, little liquid water, few nutrients).
On a Paper 1B data question you may be asked to suggest which factor is limiting and propose a hypothesis — for example, why a region of ocean shows unexpectedly high phytoplankton growth when iron normally limits it.
Vary your answer to the command term: List wants short separate items, Outline wants a short cause-and-effect, and Suggest wants a reasonable factor that fits the data.
IB-style question — list factors that set distribution
List three factors that could influence where a species of animal is distributed. [3]
How to score all three marks
- Give an abiotic factor. Temperature — the animal can only survive within its range of tolerance for temperature.
- Give a second factor (mix in a biotic one). Food supply (a biotic factor) — the animal cannot live where there is not enough food.
- Give a third, distinct factor. Water availability (abiotic) — or predation / competition (biotic). (Award 1 mark for each distinct, correct factor, up to 3 — they may be abiotic or biotic.)
Final answer
Any three distinct factors, e.g. temperature (abiotic), food supply (biotic) and water availability (abiotic) — or competition / predation.
✓ Why this scores full marks: Each item is a separate, distinct factor, and the answer mixes abiotic and biotic — showing you know both types.
For a 3-mark List you need three different factors, not the same idea written three ways (e.g. 'too hot', 'too cold' and 'temperature' would score once).
IB-style question — suggest the limiting factor from data
In one region of ocean, adding iron causes a large bloom of phytoplankton, yet light and other nutrients were already plentiful. Suggest which factor was limiting phytoplankton growth, and explain your reasoning. [2]
Model answer
- Name the limiting factor. Iron was the limiting factor.
- Justify it with the data. Because light and other nutrients were already plentiful but growth only increased when iron was added, iron must have been the factor in shortest supply holding growth back — so it was limiting. (Mark 1: iron is limiting. Mark 2: growth rose only when iron was added / iron was in shortest supply.)
Final answer
Iron — growth only increased when iron was added, even though light and other nutrients were plentiful, so iron was the factor in shortest supply (the limiting factor).
✓ Reading a limiting-factor data question: The trick is to find the one factor whose change made a difference. If everything else was already plentiful and growth only rose when iron was supplied, iron was limiting.
A 'Suggest' answer must be a reasonable factor supported by the data given — name it and link it back to the result.
| Factor | Type | How it limits where a species lives |
|---|---|---|
| Temperature | Abiotic | Too cold or too hot is outside the tolerance range; enzymes and growth slow or stop, so the species is absent |
| Water / rainfall | Abiotic | Too little water means a land plant or animal cannot survive there; deserts exclude species needing moisture |
| Light intensity | Abiotic | Plants need enough light to photosynthesise; deep shade or deep water excludes many plants and algae |
| Soil mineral nutrients | Abiotic | Few nutrients (e.g. nitrogen) limits plant growth, so only tolerant species survive |
| Food supply | Biotic | Without enough food a consumer cannot survive in an area, so its distribution is restricted |
| Predation / competition | Biotic | Heavy predation or strong competition from another species can exclude an organism from otherwise suitable habitat |