To sort an interaction, ask who gains and who loses. To separate habitat from niche, ask “where does it live?” for the habitat and “what does it do?” for the niche.
This lesson closes the SPM Biology ecosystems chapter. It builds on constructing food chains and webs.
What do the interactions look like?
| Interaction | Organism A | Organism B | Example |
|---|---|---|---|
| Predation | Gains (food) | Loses (eaten) | A snake eating a frog |
| Competition | Loses some | Loses some | Two plants sharing light and space |
| Mutualism | Gains | Gains | Nitrogen-fixing bacteria in root nodules of a legume |
| Commensalism | Gains | Not affected | An orchid growing on a tree trunk |
| Parasitism | Gains | Harmed | A tick on a dog |
Worked example: sorting pairs
A clownfish and a sea anemone. The fish gains protection and the anemone gains cleaning and defence. Both gain, so this is mutualism.
A tapeworm in a human gut. The worm takes nutrients, and the human is harmed. This is parasitism.
A fern growing on a tree branch. The fern gains a place in the light. The tree is not affected, so this is commensalism.
What is the difference between a habitat and a niche?
A habitat is the place where an organism lives, such as a mangrove swamp. The niche is the organism’s role there: what it eats, what eats it, and how it affects other organisms.
Two different organisms can share a habitat but have different niches. A mudskipper and a mangrove crab both live in the mangrove mud, but they feed and move in different ways.
The mistake that costs marks
The common slip is to call every close relationship mutualism, or to write “niche” when the question means the place where an organism lives.
| Wrong | Right | |
|---|---|---|
| Orchid on a tree | Mutualism | Commensalism, because the tree is not affected |
| Tick on a dog | Commensalism | Parasitism, because the dog is harmed |
| Niche | The mangrove swamp | The role in the mangrove (what it eats and is eaten by) |
The two-question test, “who gains and who loses?”, settles every pair.
Check yourself
Name the interaction between a cattle egret that follows grazing cattle to catch the insects they disturb, where the cattle are not affected. Then state the habitat and niche of a mangrove crab in one sentence each.
Answer
The egret gains food and the cattle are not affected, so this is commensalism.
A mangrove crab’s habitat is the mangrove mud. Its niche is its role there, such as feeding on fallen leaves and being eaten by larger animals.
What to study next
Test all four lessons in the ecosystems practice set, and read about ecological evidence and population relationships. Practise with the biological data interpretation trainer, and log slips in the mistake log and paper-error review.
If you would like a teacher to test your sorting on new pairs, see online one-to-one Biology tuition.