Explain mutualism, commensalism and parasitism, with one example of each.

1. 1 · Introduce. Populations of different species that live together interact in ways that may help or harm each other.
2. 2 · Mutualism. Both gain: in a lichen the alga makes food, and the fungus gives water, minerals and shelter.
3. 3 · Commensalism. One gains, the other is unaffected: a sucker fish rides on a shark for protection.
4. 4 · Parasitism. One gains and harms the host: a tapeworm in the human intestine.
5. 5 · Real life. Bee boxes near crops help both bees and plants; deworming removes parasites.
6. 6 · Close in one line. So close associations, together called symbiosis, link the populations of an ecosystem.

Scoring tip: For each kind, say who gains and who loses; that is exactly what the examiner marks.