The big idea: Every organism lives in an environment made of two kinds of factor: abiotic and biotic.
Abiotic factors
- The non-living physical and chemical parts of the environment
- Things like temperature, light, water, pH, oxygen and salinity
Biotic factors
- The living parts
- Other organisms acting as predators, competitors, parasites or food
Tolerance and biomes: An organism can only survive where every abiotic factor stays inside its range of tolerance. Across the whole planet, regions that share a characteristic climate (and so a characteristic community) are called biomes.
- Abiotic factor
- A non-living, physical or chemical feature of the environment — for example temperature, light intensity, water availability, pH, oxygen concentration or salinity.
- Biotic factor
- A living feature of the environment — the effect of other organisms, such as predators, competitors, parasites, disease or food supply.
- Range of tolerance
- The range of an abiotic factor over which an organism can survive. It performs best in an optimum range in the middle and is absent beyond its upper and lower limits.
- Limiting factor
- The single abiotic factor that is furthest from an organism's optimum, and so the one that restricts where it can live.
- Biome
- A large geographical region with a characteristic climate (its abiotic conditions) and a characteristic community of organisms — for example hot desert, tropical rainforest or tundra.
Living or non-living — that is the whole test: To sort a factor: ask is it alive?
If it is a non-living physical or chemical condition (heat, light, water, pH), it is abiotic.
If it is the effect of another organism (a predator, a competitor, a parasite), it is biotic.
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A species is not equally happy at every level of an abiotic factor.
Moving away from the optimum
1
Optimum range
For something like temperature, there is an optimum range in the middle where the organism performs best.
2
Zone of stress
As you move away from the optimum in either direction, it passes through a zone of stress where it survives but does poorly.
3
Limit of tolerance
Finally it reaches a limit of tolerance beyond which it cannot survive at all and is absent.
Optimum, then stress, then the limit: absent.
Range of tolerance — read it off the curve: Plotting an organism's performance (or abundance) against an abiotic factor gives a bell-shaped tolerance curve.
Reading the curve
- The peak is the optimum range
- The shoulders are zones of stress
- The points where the curve reaches zero are the lower and upper limits of tolerance
- Beyond those limits the organism is absent — so the curve tells you exactly where a species can and cannot live for that one factor
An organism's range of tolerance for an abiotic factor: it performs best in the optimum range, struggles through a zone of stress on each side, and is absent beyond the lower and upper limits of tolerance.
Interactive diagram
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The limiting factor decides the niche: An organism must be inside its tolerance range for every abiotic factor at once. The factor that is furthest from its optimum is the limiting factor — it is the one holding the organism back, even if every other condition is fine.
Lab versus wild: This is why a species often grows faster in a controlled tank or mesocosm than in the wild: in the lab the limiting factor (say, low light or a competitor) has been removed, so conditions sit closer to its optimum.
Abiotic factor
- A non-living physical / chemical condition
- Temperature, light, water, pH, oxygen, salinity
- Sets the conditions an organism must tolerate
- Each one has a range of tolerance
Biotic factor
- A living part of the environment
- Predators, competitors, parasites, food supply
- Acts through interactions with other organisms
- Can also limit where a species lives
Biomes — characteristic abiotic conditions: A biome is a large region defined by its climate — mainly its temperature and its rainfall (water availability). The abiotic conditions of a biome decide which organisms can tolerate living there.
Three biomes
- A hot desert is characterised by very high daytime temperatures and very low, scarce rainfall.
- A tropical rainforest has high temperature and very high rainfall all year.
- A tundra is very cold with little available water.
| Biome | Temperature | Water / rainfall |
|---|---|---|
| Hot desert | Very high by day, often cold at night (large daily range) | Very low — rainfall is scarce and unreliable |
| Tropical rainforest | High and steady all year | Very high rainfall all year |
| Tundra | Very cold for most of the year | Low — much water is locked up as ice |
A memory hook: Tolerance = the range you can take; limit = where it kills you. The limiting factor is the one nearest a limit.
For a biome, lead with its two headline abiotic conditions: how hot and how wet.
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How this is tested: Abiotic factors are tested in three ways, from a 1-mark choice to a data question.
| Question | What it asks | How to score |
|---|---|---|
| Paper 1A (1 mark) | A list of examples: select the abiotic factor | Pick the non-living physical/chemical one (temperature, light, pH), not a predator or competitor |
| Paper 2 2-mark State | Abiotic factors that characterise a named biome | For example a hot desert: high temperature and low rainfall |
| Data questions (a Paper-1B / Paper-2 favourite) | Tolerance ranges or a graph | Deduce where a species can live, or suggest why it grows faster in a controlled mesocosm than in its natural environment |
IB-style question — state the abiotic factors of a biome
State two abiotic factors that characterise a hot desert biome. [2]
How to score both marks
- Give the temperature condition. A hot desert has a very high daytime temperature (often with cold nights — a large daily temperature range).
- Give the water condition. A hot desert has very low rainfall / scarce water — it is extremely dry. (Award 1 mark for each distinct abiotic factor. Both must be non-living conditions; naming the plants or animals would not score.)
Final answer
High temperature (very hot by day) and very low rainfall / little available water.
Why this scores full marks: Both answers are abiotic (non-living) and both are characteristic of the named biome.
A common slip is to name cacti or camels — those are organisms, not abiotic factors, so they would score zero on a 'state the abiotic factors' question.
| Feature | Abiotic factor | Biotic factor |
|---|---|---|
| What it is | A non-living, physical or chemical part of the environment | A living part of the environment — an effect of other organisms |
| Living or non-living? | Non-living | Living |
| Examples | Temperature, light intensity, water / rainfall, pH, oxygen, salinity, soil minerals | Predators, competitors, parasites, disease, food supply, prey |
| Effect on a species | Sets the physical conditions it must tolerate to survive | Affects it through interactions with other organisms |