Habitat is your address. Niche is your job. Two species can share an address quite happily — but if they try to do exactly the same job in the same place, one of them will lose.
📚 What you need to know
A niche is the role of a species within its habitat. Learn that wording exactly.
The role covers what it eats, what eats it, when it is active, exactly where it feeds, and the abiotic and biotic factors acting on it.
No two species can occupy the same niche in the same habitat. One will out-compete the other and it will die out there.
Species that look like they share a niche always differ in some detail — feeding height, food source or time of day.
Obligate anaerobes can only respire anaerobically and cannot tolerate oxygen.
Facultative anaerobes mainly respire aerobically but can switch fully to anaerobic. Obligate aerobes need oxygen and cannot manage without it.
Habitat, then niche
The place where a species lives is its habitat. Within that habitat, each species occupies a specific niche.
Learn this word for word
A niche is the role of a species within its habitat.
“Role” is doing a lot of work in that sentence, so here is what it actually covers:
What it eats — the food source it depends on
Which other species depend on it for food — what preys on or parasitises it
What time of day it is active — day, night or dawn and dusk
Exactly where in the habitat it lives and feeds — not just “the woodland”, but which layer of it
The abiotic factors acting on it — temperature, pH, oxygen, light
The biotic factors acting on it — its competitors, predators and partners
If your answer names a place, you have described a habitat. If it names an activity — what the organism does, eats, or is eaten by — you have described a niche. Examiners set this trap constantly.
Why two species cannot share a niche
If two species genuinely occupied the same niche in the same habitat, they would need exactly the same resources at exactly the same time and place. That puts them in direct competition for every single one of those resources.
Competition like that has only one ending. One species will be slightly better at getting the resources, so it grows and reproduces faster. The other gets steadily less, and eventually dies out in that habitat. It is not a stalemate — it is a slow squeeze.
So when species look like they share a niche, look harder. There is always a difference: they feed at a different height, on a different part of the plant, at a different time of day, or on a slightly different food source.
The warblers in a spruce tree
This is the classic example, and it is worth knowing because it shows the principle rather than just stating it. Several warbler species live in the same conifer forest, eat insects, and nest in the same trees. On the face of it, identical niches.
Watch them closely and the picture changes. Each species feeds in a different part of the tree.
Feeding location is only one way niches can differ. Timing, prey size and nesting height can separate species just as effectively.
🤔 Why does splitting the tree up work?
Because it turns one contested resource into three separate ones. As long as each species takes most of its food from a band the others barely touch, none of them is fighting the others for every meal.
That is why the differences can look trivial to us and still matter enormously to the birds. A few metres of height, in a resource they all depend on, is the difference between coexisting and being excluded.
Oxygen and the niche
Niche is not only about food. Abiotic factors are part of it too, and for microorganisms the biggest one is oxygen. Whether a species can tolerate oxygen decides where on Earth it can live at all.
A quick respiration recap
All living organisms respire. Aerobic respiration requires oxygen; anaerobic respiration does not.
Aerobic respiration
glucose + oxygen → carbon dioxide + water
Anaerobic respiration in muscle
glucose → lactic acid
Anaerobic respiration in yeast
glucose → ethanol + carbon dioxide
Three ways of living with oxygen
The facultative anaerobe grows everywhere but is densest at the top, because aerobic respiration releases far more energy per glucose molecule.
Oxygen-free places: lower soil layers, deep water, inside the bodies of other organisms
Facultative anaerobe
Mainly aerobic, but switches fully to anaerobic when oxygen is absent, with no harm done
Both kinds of environment; brewer’s yeast Saccharomyces cerevisiae and the bacterium Escherichia coli
Obligate aerobe
Relies on aerobic respiration; may manage a few seconds of anaerobic in some cells, but the damage is too great to continue
Oxygenated environments; most animals, most fungi (not yeast), and some bacteria such as Mycobacterium tuberculosis
🤔 Why are obligate anaerobes now stuck in corners?
Early bacteria were obligate anaerobes, and that was no handicap at all — the atmosphere of the early Earth had essentially no oxygen in it. They were perfectly suited to the world as it was.
Then photosynthesis appeared and released oxygen into the atmosphere. What had been a neutral gas became a poison to them, and their available habitat shrank to the few places oxygen cannot reach. Their niche did not change; the planet did.
Worked examples
WORKED EXAMPLE
Define the term niche, and distinguish it from the term habitat. [3 marks]
Step 1: the definition
A niche is the role of a species within its habitat.
Step 2: the other term
A habitat is the place where an organism lives.
Step 3: the actual distinction
A habitat is only a location, whereas a niche also includes what the species eats, what feeds on it, when it is active, and the abiotic and biotic factors acting on it.
Definition + definition + a comparison sentence“Distinguish” always needs that third sentence. Two definitions alone will not get all three marks.
WORKED EXAMPLE
Three warbler species live in the same spruce forest and all eat insects. Explain how they are able to coexist. [3 marks]
Step 1: state the principle
No two species can occupy the same niche in the same habitat, so if all three survive there, their niches must differ.
Step 2: give the actual difference
Each species feeds in a different part of the tree: one at the branch tips near the top, one in the middle, and one low down and at the bases of the middle branches.
Step 3: the consequence
They are therefore not in direct competition for the same insects, so none of them out-competes the others and all three can persist.
Different feeding zones = different niches = no direct competition
WORKED EXAMPLE
A bacterium is grown in a nutrient tube and forms a dense band only at the very bottom. Identify the type of organism and justify your answer. [2 marks]
Step 1: read the oxygen gradient
Oxygen dissolves in at the surface, so the bottom of the tube has the least oxygen.
Step 2: match the growth to the type
Growth only where oxygen is absent means the organism cannot tolerate oxygen and can only respire anaerobically.
An obligate anaerobeGrowth spread through the whole tube but densest at the top would be a facultative anaerobe instead.
💡 Exam tip
Learn the niche definition word for word. Definition marks are all-or-nothing.
When you describe a niche, name at least two or three parts of the role, not just what it eats.
For coexistence questions, always state the principle first and then the specific difference.
Use “cannot tolerate oxygen” for obligate anaerobes. “Does not need oxygen” is weaker and often not credited.
Facultative means “has the option”. Obligate means “has no choice”. That holds for anaerobes, aerobes and mixotrophs alike.
Yeast is the exception among fungi — it is a facultative anaerobe, while most fungi are obligate aerobes.
⚠ Common mix-up
Describing a place when asked for a niche. “It lives in the forest” is a habitat answer.
Saying two species share a niche. By definition they cannot, in the same habitat.
Thinking facultative anaerobes are harmed by switching. They are not — that is exactly what “facultative” means.
Mixing up obligate anaerobe and obligate aerobe. Read the middle of the word, not the ends.
Assuming anaerobic respiration is a fallback for everyone. Obligate aerobes can only do it for a few seconds before the damage is too great.
Forgetting abiotic factors are part of the niche. Oxygen tolerance is as much a part of a niche as diet is.
Up next: Methods of Nutrition — the single biggest thing that separates one niche from another is how an organism gets its food, so that is where we go next.
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