The big idea: An organism's ecological niche is its role in an ecosystem — not just where it lives, but how it lives.
A niche has three parts: the abiotic conditions the organism can tolerate, how it obtains its food and energy, and its interactions with other species.
To survive, an organism must live somewhere that provides the conditions for survival its niche requires — for example the right temperature, enough light, and enough oxygen.
- Ecological niche
- An organism's role in its ecosystem: the abiotic conditions it can tolerate, how it obtains its food and energy, and its interactions with other species.
- Habitat
- The physical place where an organism lives. (Its niche is what it DOES there — a habitat is the address, the niche is the job.)
- Conditions for survival
- The environmental conditions an organism needs to stay alive and function — for example a suitable temperature, light, water and, for many organisms, oxygen.
- Abiotic factor
- A non-living, physical or chemical feature of the environment, such as temperature, light intensity, pH, salinity or dissolved oxygen.
- Biotic factor
- A condition created by other living organisms, such as food supply, predators or competitors.
| Type of condition | What it means | Examples |
|---|---|---|
| Abiotic | A non-living, physical or chemical feature of the environment | Temperature, light intensity, pH, dissolved oxygen, salinity, water availability |
| Biotic | A condition created by other living organisms | Availability of food, predators, competitors, mates, disease |
Habitat vs niche: A useful hook: the habitat is the address (where it lives), and the niche is the job (what it does and what conditions it needs).
Two species can share the same habitat but still have different niches, because they use the conditions and resources differently.
Every species can only survive within a certain range of each abiotic factor. Too little or too much, and the organism cannot function.
Biologists describe this as the range of tolerance: the organism does best in an optimum range, struggles in a zone of stress on each side, and dies (is absent) beyond the limits of tolerance.
The range of tolerance: For any one abiotic factor — say temperature — an organism's performance follows a bell shape:
in the optimum range it is most abundant and active; in the zone of stress either side it survives but is stressed; and beyond the lower and upper limits of tolerance it cannot survive at all.
An organism can only live where every abiotic factor stays within its tolerable range.
An organism's range of tolerance for one abiotic factor: it does best in the optimum range, struggles in a zone of stress on each side, and is absent beyond the lower and upper limits of tolerance.
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Oxygen — a condition many organisms cannot do without: Dissolved oxygen is one of the most important conditions for survival for aquatic organisms.
An obligate aerobe (such as many seaweeds and fish) must have oxygen to respire — without it, it cannot release enough energy and it dies.
An obligate anaerobe is the opposite: oxygen is toxic to it, so it can only survive where oxygen is absent (for example deep in waterlogged mud).
Obligate aerobe
- Must have oxygen to survive
- Uses oxygen for aerobic respiration
- Found where oxygen is available (e.g. well-aerated water)
- Dies if oxygen runs out
Obligate anaerobe
- Oxygen is toxic to it
- Releases energy without oxygen
- Found only where oxygen is absent (e.g. waterlogged mud)
- Dies if oxygen is present
| Part of an organism's niche | What it describes | Example for a rock-pool seaweed |
|---|---|---|
| Abiotic conditions it tolerates | The range of physical/chemical conditions it can survive in | It needs dissolved oxygen, a suitable temperature and salinity |
| How it obtains energy / food | Whether it makes or takes its food, and from where | It is a producer — it photosynthesises using light |
| Interactions with other species | Its relationships with other living organisms | It competes for light and space; it is grazed by snails |
Why this matters for distribution: Because each species can only tolerate a certain range of conditions, the niche helps explain where an organism is found.
If a place lacks even one required condition (too cold, too acidic, no oxygen), the organism is absent there — no matter how good the other conditions are.
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How this is tested: On Paper 1B a short State / Describe question often gives an organism (such as an obligate-aerobe seaweed) and asks you to name one environmental condition it needs to survive — the strongest answer is dissolved oxygen, because the organism cannot respire without it.
A common Paper 1A point asks you to distinguish abiotic from biotic conditions, or to define the ecological niche as an organism's role.
On Paper 1B / Paper 2 a data question may show a tolerance graph and ask you to deduce whether an organism could survive at a stated value of an abiotic factor.
IB-style question — a condition an obligate aerobe needs
A species of seaweed living in a coastal rock pool is an obligate aerobe. State one environmental condition the seaweed needs to survive, and explain why. [2]
How to score both marks
- State the condition. The seaweed needs dissolved oxygen in the water.
- Explain why. As an obligate aerobe, it must carry out aerobic respiration, which requires oxygen to release energy — without oxygen it cannot respire and would die. (Mark 1: (dissolved) oxygen. Mark 2: needed for aerobic respiration / cannot survive without it.)
Final answer
Dissolved oxygen — as an obligate aerobe the seaweed must respire aerobically, and aerobic respiration requires oxygen to release the energy it needs to survive.
✓ Why this scores full marks: The answer names a specific abiotic condition (oxygen) and then links it to the organism's biology (it is an obligate aerobe, so it needs oxygen for respiration).
A vague answer like 'water' or 'a good environment' would not score — the question wants a named condition tied to why the organism needs it.
Exam Tips:
- Read the organism's description: 'obligate aerobe' is a strong hint that the required condition is OXYGEN.
- Always pair the condition with a reason (oxygen → for aerobic respiration) when the command term is 'explain'.
- An abiotic condition is non-living (oxygen, temperature, light); food and predators are biotic — don't mix them up.