Population interactions: commensalism, mutualism and amensalism
In commensalism (+, 0) one species gains and the other is unaffected. In mutualism (+, +) both gain. In amensalism (−, 0) one is harmed, the other unaffected.
Commensalism vs amensalism
Predation, parasitism and commensalism share one feature: the interacting species live closely together.
More examples of commensalism
| Example | Who gains, and how | The other partner |
|---|---|---|
| Cattle egret and grazing cattle | Egrets catch insects that the moving cattle stir up from the grass | Cattle: no apparent effect |
| Clown fish among sea anemone tentacles | The fish is protected, since predators avoid the stinging tentacles | Sea anemone: no apparent benefit |
Mutualism: both partners gain
Mutualism benefits both species. Lichens are a close partnership between a fungus and a photosynthesising alga or cyanobacterium: the alga makes food, and the fungus gives shelter and absorbs water and minerals. Mycorrhizae are associations between fungi and the roots of higher plants. The fungus helps the plant absorb essential nutrients from the soil, and the plant gives the fungus energy-yielding carbohydrates. In the root nodules of legumes such as pea and groundnut, the bacterium Rhizobium fixes atmospheric nitrogen for the plant and gets food and shelter in return.
Plant-animal mutualism and co-evolution
Plants need animals to pollinate their flowers and disperse their seeds. They pay 'fees': pollen and nectar for pollinators and juicy, nutritious fruits for seed dispersers. The system must also be guarded against cheaters, such as animals that steal nectar without pollinating. So a flower and its pollinator often co-evolve: their evolution is tightly linked. In many fig species, each fig has one partner wasp species. The female wasp lays eggs in the fig fruit and pollinates the fig inflorescence while searching for egg-laying sites. In return, the fig gives some of its developing seeds as food for the wasp larvae. Not all orchids give a reward. The Mediterranean orchid Ophrys uses sexual deceit: one petal looks like a female bee in size, colour and markings. A male bee pseudocopulates (tries to mate) with it, gets dusted with pollen, and carries the pollen to the next flower. If the female bee's colours change even slightly during evolution, the orchid must co-evolve to keep the likeness, or pollination success falls.
Activity: spotting lichens, root nodules and Cuscuta
In the practical exam you identify each specimen and write short comments. For each, write the **identification**, the **type of interaction with its symbols**, and **two features** you can see.
- 1Lichen on a piece of bark or rock: a crusty, leafy or branched grey-green growth. Identify it as mutualism (+, +) between a fungus and an alga or cyanobacterium. Comment: the alga makes food; the fungus gives shelter and absorbs water and minerals. Lichens do not grow in air polluted with SO₂, so they indicate clean air.
- 2Root nodules on the roots of a legume such as pea, gram or groundnut: small, rounded swellings on the roots. Identify them as mutualism (+, +) between the plant and the bacterium Rhizobium. Comment: Rhizobium fixes atmospheric nitrogen for the plant; the plant gives it food and shelter. A cut nodule looks pink because of leghaemoglobin.
- 3Cuscuta on its host: yellow, leafless, thread-like stems twining round a green host plant. Identify it as parasitism (+, −). Comment: Cuscuta has no chlorophyll and no leaves; it sends haustoria (sucking roots) into the host's stem to draw food and water.
- 4★Record: draw a labelled sketch of each specimen, and under it write the identification, the interaction with symbols, and two visible features.
★ key step
Farmers grow pulses such as moong and groundnut in rotation with cereals because the Rhizobium in their root nodules adds nitrogen to the soil. The white cattle egret walking beside a grazing buffalo in a paddy field is commensalism you can watch any day.
2 marks: “Define commensalism and amensalism with their signs. Give one textbook example of commensalism, showing which partner gains.” Commensalism (+, 0): one species benefits and the other is unaffected, e.g. the cattle egret gains insects stirred up by grazing cattle, which are unaffected. Amensalism (−, 0): one species is harmed and the other is unaffected.
3 marks: “Explain the mutualism between fig trees and wasps. Why is it called co-evolution?” Each fig species is pollinated only by its partner wasp. The female wasp lays eggs in the fruit and pollinates the inflorescence while looking for egg-laying sites; the fig gives some developing seeds to the larvae. Their evolution is tightly linked, so a change in one needs a matching change in the other.
3 marks: “What is 'sexual deceit' in the orchid Ophrys? How does it show co-evolution?” A petal resembles a female bee in size, colour and markings; the male bee pseudocopulates and carries pollen to another flower. If the bee's colour pattern changes, the orchid must change too, or pollination success falls.
Commensalism: one comfortable, one unbothered (+, 0). Amensalism: one aching, one unbothered (−, 0). Mutualism: mutual gain (+, +).