The big idea: A species is not found everywhere — it lives only where the conditions suit it.
The non-living, physical and chemical conditions of a habitat are called abiotic factors — things like temperature, water, light, pH and salinity.
Each abiotic factor has a range of values that a species can tolerate. Where those conditions are met, the species can live; where they are not, it is absent. This is why abiotic factors determine the distribution of a species.
- Abiotic factor
- A non-living, physical or chemical feature of a habitat — for example temperature, water availability, light intensity, pH or salinity.
- Biotic factor
- A living feature of a habitat that comes from other organisms — for example predators, competitors, food or disease. (Contrast with abiotic.)
- Distribution
- Where a species is found across an area — the places it does and does not live.
- Range of tolerance
- The span of values of an abiotic factor over which a species can survive; outside this range the species cannot live.
- Limiting factor
- The one factor in short (or excess) supply that holds back a species and stops it living in a particular place.
Abiotic vs biotic: Abiotic = Absence of life — the physical/chemical conditions (temperature, water, light, pH, salt, oxygen, wind, soil minerals).
Biotic = the living influences (predators, competitors, food, disease).
This micro is about the abiotic ones, but real distributions are usually shaped by both together.
For any abiotic factor, a species does best at some best (optimum) value in the middle.
As the factor moves away from that optimum, the species is put under more and more stress, until it reaches a limit beyond which it can no longer survive — so it is absent from places where the factor is too high or too low.
Optimum, stress, and limits: Plot how well a species does against an abiotic factor and you get a range of tolerance:
• an optimum range in the middle where the species is most abundant;
• a zone of stress on each side where it survives but struggles;
• a lower limit and an upper limit — beyond these, the species is absent.
So an abiotic factor sets a band of conditions in which the species can live — and that band decides its distribution.
An organism thrives in its optimum range, struggles in the zones of stress, and is absent beyond its lower and upper limits of tolerance — so each abiotic factor sets a band of conditions in which the species can live.
Interactive diagram
Explore the labelled diagram, charts and maps for this topic in full study mode.
Each factor works through the organism's biology: An abiotic factor never affects distribution by magic — it works through the organism's biology:
Temperature sets the rate of enzyme-controlled reactions, and extremes denature proteins.
Water is needed for cell processes — too little dehydrates the organism, too much can drown roots.
Light drives photosynthesis in plants, so deep shade or deep water limits where they grow.
When you explain a distribution, always name the factor and the biological reason.
| Abiotic factor | Why it affects where a species lives | Example effect on distribution |
|---|---|---|
| Temperature | Sets the rate of enzyme-controlled reactions; extremes denature proteins or freeze tissues | Tropical species are absent from cold mountain tops or polar regions |
| Water / moisture | Needed for cell processes; too little causes dehydration, too much can drown roots or reduce oxygen | Cacti live in dry deserts; rushes live in waterlogged marshes |
| Light | Drives photosynthesis in plants and algae; sets activity in animals | Shade-loving ferns live on a forest floor; few plants grow in deep, dark water |
| pH (acidity) | Affects enzyme activity and the availability of mineral ions in soil or water | Some plants only grow in acidic peat; some fish cannot survive in acidified lakes |
| Salinity (salt) | Affects water movement into and out of cells by osmosis | Only salt-tolerant plants live in estuaries and on coastal mudflats |
Inside the range of tolerance
- Conditions suit the species
- Near the optimum it is most abundant
- Enzymes and cell processes work well
- The species is present here
Outside the range of tolerance
- The factor is too high or too low
- The species is stressed, then cannot survive
- It passes a limiting value
- The species is absent here
A memory hook: Think 'Goldilocks': not too hot, not too cold — just right.
A species lives where every abiotic factor is within its range of tolerance. Push any one factor past a limit and the species disappears from that spot.
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How this is tested: These questions almost always sit on a data figure — a distribution graph, a transect or a table.
A 2-mark Describe question gives you one factor (say water depth) and asks for two more abiotic factors that could affect the distribution — each with a reason.
A Suggest question asks you to propose an abiotic factor that could explain a measured difference (for example why soil moisture is higher in winter than summer).
The marks come in pairs: name the factor, then give the reason for its effect. A bare list of factors with no reasons scores half marks.
IB-style question — two abiotic factors, with reasons
A study of seaweed on a rocky shore shows that one species lives only in the shallowest water. Besides water depth, describe two abiotic factors that could affect where this seaweed is found. Give a reason for each. [4]
How to score all four marks
- Factor 1 — light intensity. Light intensity is lower in deeper water. Seaweed needs light for photosynthesis, so it can only live where enough light reaches it — keeping it in the shallows.
- Factor 2 — exposure / drying out (or temperature). Higher up the shore the seaweed is exposed to air for longer at low tide and can dry out (dehydrate), so it cannot survive too high up.
- Pair each factor with its reason. Notice every factor is followed by a 'because…': light → needed for photosynthesis; exposure → causes drying out. (Award 1 mark per correctly named factor and 1 mark per matching reason, max 4.)
Final answer
Light intensity (lower in deep water; needed for photosynthesis, so it stays shallow) and exposure to air / drying out (higher up the shore it is exposed at low tide and dehydrates).
✓ Why this scores full marks: Each of the two factors is named and linked to a clear biological reason — that is what makes four scoring points.
The most common way to lose marks here is to list factors without reasons (e.g. just 'light and temperature'), which scores only the naming marks.
| Part of the answer | What to write | Worked example |
|---|---|---|
| The factor | Name ONE specific abiotic factor | Light intensity |
| The link (reason) | Say HOW that factor affects the organism | Light is needed for photosynthesis, |
| The effect on distribution | Say WHERE it can or cannot live as a result | so the plant only grows where it is not heavily shaded |