Habitat is an address. Niche is a job description. Three warbler species can all live in the same spruce tree without ever getting in each other’s way, because each one works a different part of it. Once you see niche as a job rather than a place, competition and exclusion stop being confusing.
📘 What you need to know
A habitat is where a species lives. A niche is the role of a species within its habitat.
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 fill the same niche in a habitat. If they try, one out-competes the other.
Species that look like they share a niche always differ in something subtle — feeding time, or food source.
Obligate anaerobes can only respire anaerobically and cannot tolerate oxygen.
Facultative anaerobes mainly respire aerobically but can switch fully to anaerobic respiration.
Obligate aerobes cannot survive without oxygen.
What a niche actually includes
Definition
A niche is the role of a species within its habitat.
“Role” is doing a lot of work in that sentence, so unpack it. A species’ niche takes in all of the following:
What it eats.
Which other species depend on it for food.
What time of day it is active.
Exactly where in the habitat it lives and feeds.
The abiotic factors acting on it.
The biotic factors acting on it.
Notice that two of those six are about location. That is why niche and habitat get confused. The difference is that a habitat is only a place, while a niche is a place plus a job plus a timetable plus a set of relationships.
One species per niche
Here is the rule that makes the whole topic hang together:
No two species can fill the same niche within a habitat.
If two species ever do, they are in direct competition for the same resources.
One will out-compete the other, and the loser dies out in that particular habitat.
You will often see species that look like they are doing the same job. Look closer and there is always a difference — they feed at different times of day, or they take different food sources, or they work a different part of the same structure.
Feeding location is one of the easiest features to use when you want to show two species have different niches. Feeding time works just as well.
Respiration and the oxygen niche
One of the abiotic factors in a niche is oxygen concentration, and organisms differ enormously in what they can put up with. Start with the two kinds of respiration.
Anaerobic respiration — no oxygen needed
in muscle: glucose → lactic acid
in yeast: glucose → ethanol + carbon dioxide
Every living organism carries out some form of respiration. What differs is which forms an organism can use, and that splits organisms into three groups.
Group
What it can do
Examples
Obligate anaerobes
Only anaerobic respiration. They cannot tolerate oxygen at all.
Bacteria in the lower layers of soil, in deep water, and inside the bodies of other organisms
Facultative anaerobes
Mainly aerobic, but can switch fully to anaerobic respiration when oxygen runs out, with no ill effects.
Cannot survive without oxygen. Some cells manage anaerobic respiration for a few seconds, but the damage is too great to keep it up.
Most animals, most fungi (but not yeast), some bacteria such as Mycobacterium tuberculosis
This is a real laboratory test. Grow an unknown microbe in a tube and see where it settles, and you have learned something about its niche.
A bit of history worth knowing
Early bacteria were obligate anaerobes, and they did fine, because the atmosphere of early Earth had no oxygen in it. Photosynthesis then pumped oxygen into the atmosphere, which was catastrophic for them. Today obligate anaerobes are pushed into the few places oxygen does not reach: deep soil, deep water, and the insides of other organisms.
A neat link. Photosynthetic organisms changed the abiotic conditions of the whole planet, and that shrank the niche available to obligate anaerobes. Living things do not just fit into their environment — sometimes they remake it.
Worked examples
WORKED EXAMPLE
Distinguish between the habitat and the niche of a species. [2]
Define both, using the right wordsHabitat: the place where the species lives.Niche: the role of the species within that habitat.Add what “role” covers, to show you mean itThe niche includes what it eats, when it is active, where it feeds, and the factors acting on it“distinguish” means both sides must appear. Do not define only one.
WORKED EXAMPLE
Three warbler species live in the same spruce trees and eat the same kind of insect. Explain how all three can survive there. [3]
State the rule first
No two species can occupy the same niche in a habitat.
Now show they do notEach species feeds in a different part of the tree, so their niches differ.Say what that avoidsThey are not in direct competition for the same resources, so none is out-competedthe marks are for the difference plus its consequence, not for describing the birds.
WORKED EXAMPLE
A bacterium is grown in a tube of nutrient broth and forms a layer only at the very bottom. Deduce its type of respiration. [2]
Work out where the oxygen isOxygen diffuses in from the surface, so the bottom of the tube has the least oxygen.So what does growing only there tell you?It is an obligate anaerobe — it respires anaerobically and cannot tolerate oxygena facultative anaerobe would have grown all the way up the tube.
💡 Exam tip
Learn the niche definition word for word: the role of a species within its habitat.
If asked what a niche includes, give at least three of the six features. Do not stop at “what it eats”.
For “how can these species coexist”, always name the specific difference in their niches.
Use obligate and facultative precisely. Obligate means no choice; facultative means it has one.
Remember yeast is the odd one out among fungi — it is a facultative anaerobe, while most fungi are obligate aerobes.
⚠ Common mix-up
Using niche to mean a small place. It is a role, not a nook.
Saying niche and habitat are the same. Many species share a habitat; only one can hold a given niche.
Thinking anaerobes do not respire. They respire; they just do not use oxygen.
Thinking obligate anaerobes simply “prefer” no oxygen. Oxygen is toxic to them.
Assuming facultative anaerobes are harmed by switching. The switch has no negative effects for them.
Forgetting that humans do anaerobic respiration too. We are still obligate aerobes — we cannot keep it up.
Up next: Methods of Nutrition — the different ways organisms get hold of the organic molecules they need to respire in the first place.
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