Interactions between populations
Populations of different species affect each other. In mutualism both gain; in commensalism one gains and the other is unaffected; in parasitism one gains and harms the host.
Kinds of interaction
| Interaction | Species 1 | Species 2 | Example |
|---|---|---|---|
| Mutualism | Gains | Gains | Lichen: alga and fungus |
| Commensalism | Gains | Not affected | Hermit crab living in an empty gastropod (snail) shell; sucker fish riding on a shark |
| Parasitism | Gains (parasite) | Harmed (host) | Tapeworm in the human intestine |
Lichen: a partnership that feeds both
A lichen looks like one living thing, but it is an alga and a fungus living together. The fungus forms the main body. The alga makes food by photosynthesis for itself and for the fungus. In return, the fungus gives the alga water, minerals and shelter. Both gain, so this is mutualism.
Symbiosis: living together
Symbiosis literally means living together: a close association between two kinds of organisms. Your NIOS lesson uses it as a name that covers parasitism, commensalism and mutualism. Its example is bees and flowers: bees get nectar, and the flowers get cross-pollinated. Plants pollinated by insects make less pollen than plants that depend on the wind. The bacteria in the root nodules of pulses, which you met in the nitrogen cycle, are another close partnership.
Commensalism vs mutualism
Your NIOS lesson uses symbiosis for any close living-together of two species, and its example, bees and flowers, benefits both. If asked how it differs from commensalism, say: in commensalism only one partner gains and the other is unaffected, while in a close symbiotic partnership like bees and flowers both gain.
Spot the interaction
- Bees visiting mustard flowers: both gain (symbiosis, mutual benefit).
- Hermit crab in an empty snail shell: commensalism.
- Sucker fish clinging to a shark: commensalism.
- Tapeworm in the intestine: parasitism; the person is the host.
- Rhizobium in the root nodules of gram: both gain; the bacteria get food and shelter, the plant gets nitrogen.
Many Indian farmers keep bee boxes near mustard and sunflower fields: the bees get nectar and the crops get pollinated, so both gain. Deworming tablets given to children remove parasitic worms that live in the gut.
1 marks: “Name the interaction in which one partner gains and the other is not affected.” Commensalism.
2 marks: “Why is a lichen called an example of mutualism?” Both partners gain: the alga makes food for itself and the fungus, and the fungus gives the alga water, minerals and shelter.
3 marks: “Distinguish between mutualism, commensalism and parasitism, giving one example of each.” Mutualism: both gain (lichen). Commensalism: one gains, the other unaffected (sucker fish on shark, hermit crab in shell). Parasitism: one gains and harms the host (tapeworm in man).
Mutual = both win; Commensal = one wins, one shrugs; Parasite = one wins, one loses.