BiologyCBSE Class 12 · Organisms and Populations

Predation, Parasitism, Commensalism and Mutualism

Updated 15 October 2026 · Fact-checked

Population interactions are classified by their effect on each species: mutualism (+/+), competition (−/−), predation and parasitism (+/−), commensalism (+/0) and amensalism (−/0). Predators control prey and maintain diversity; parasites are adapted to live on or in hosts; in commensalism one benefits without affecting the other; mutualists such as lichens, mycorrhiza and fig-wasp both benefit.

Read each row: the signs show the effect on A and B, so predation and parasitism are both +/− while mutualism is +/+.

Illustration drawn for QuestionBuddy.

Understand Predation, Parasitism and Mutualism

The interaction table.

Species A	Species B	Name
+	+	Mutualism
−	−	Competition
+	−	Predation
+	−	Parasitism
+	0	Commensalism
−	0	Amensalism

(+ benefit, − harm, 0 no effect). In predation, parasitism and commensalism the species live closely together.

Predation. Nature's way of passing energy fixed by plants to higher trophic levels; herbivores are 'predators' of plants. Roles of predators:

Keep prey populations in check. Introduced (exotic) species become invasive when their natural predators are absent: the prickly pear cactus introduced into Australia in the early 1920s spread over millions of hectares and was controlled only after a cactus-feeding moth from its native habitat was introduced. Biological pest control is based on this.
Maintain species diversity by reducing competition among prey: when the starfish *Pisaster* was removed from an intertidal area of the American Pacific coast, more than 10 invertebrate species became extinct within a year due to interspecific competition.
Predators are 'prudent': if a predator over-exploits its prey, the prey and then the predator could go extinct.

Prey defences: camouflage (cryptic colouring of some insects and frogs); being poisonous; the Monarch butterfly is distasteful to birds because of a chemical it acquires as a caterpillar by feeding on a poisonous weed.
Plant defences (about 25 per cent of insects are phytophagous): thorns (Acacia, cactus) as morphological defence; chemicals that sicken, deter feeding or digestion, disrupt reproduction or kill: *Calotropis* makes poisonous cardiac glycosides, so cattle and goats never browse it. Nicotine, caffeine, quinine, strychnine and opium are produced by plants as defences against grazers and browsers.

Parasitism. Gives free lodging and meals, so it has evolved in many groups. Many parasites are host-specific, and host and parasite tend to co-evolve. Adaptations: loss of unnecessary sense organs, adhesive organs or suckers, loss of digestive system, and high reproductive capacity. Life cycles can be complex: the human liver fluke needs two intermediate hosts (a snail and a fish); the malarial parasite needs a vector (mosquito). Most parasites harm the host, reducing its survival, growth and reproduction and making it more vulnerable to predators.

Ectoparasites feed on the outside: lice on humans, ticks on dogs, copepods on marine fish; *Cuscuta* (a plant on hedges) has lost its chlorophyll and leaves and draws food from its host. The female mosquito is not considered a parasite though it needs our blood for reproduction.
Endoparasites live inside (liver, kidney, lungs, RBCs); their body is simplified while reproductive potential is emphasised.
Brood parasitism: a parasitic bird lays eggs in the host's nest, and the host incubates them; the cuckoo (koel) lays eggs in the crow's nest. The parasite's eggs have evolved to resemble the host's in size and colour.

Commensalism (+/0). An orchid growing as an epiphyte on a mango branch; barnacles on a whale; cattle egret foraging near grazing cattle, which flush out insects; clown fish living among the stinging tentacles of a sea anemone (protected from predators; the anemone gains nothing apparent).

Mutualism (+/+).

Lichens: fungus + photosynthesising alga or cyanobacterium.
Mycorrhiza: fungus helps the root absorb nutrients; the plant gives the fungus carbohydrates.
Plant-animal: plants reward pollinators with pollen and nectar and seed dispersers with fruits.
Fig and wasp: a fig species is pollinated only by its partner wasp species. The female wasp lays eggs in the fruit and pollinates the fig while searching for egg-laying sites; the fig gives some developing seeds as food for wasp larvae.
Orchids: many floral patterns attract the right pollinator. The Mediterranean orchid *Ophrys* uses sexual deceit: a petal resembles a female bee, so a male bee pseudocopulates with it and carries pollen to the next flower. If the female bee's pattern changed, the orchid would have to co-evolve to stay successful.

Flow chart (deciding the interaction): Does A gain? Does B gain, lose or stay unaffected? → +/+ mutualism; +/− predation (A kills and eats B) or parasitism (A lives on/in B, usually without killing quickly); +/0 commensalism; −/0 amensalism; −/− competition.

Draw and label: interaction chart. Checklist: a 3 × 3 grid with rows and columns labelled +, 0, −; fill cells: (+,+) mutualism, (+,−) predation/parasitism, (+,0) commensalism, (−,0) amensalism, (−,−) competition; add one example in each cell.

Predation, Parasitism and Mutualism: key facts

Interaction signs: $+ / + mutualism; - / - competition; + / - predation, parasitism; + /0 commensalism; - /0 amensalism$; Signs show effect on each species.
Roles of predators: $prey control + energy transfer + species diversity (less competition)$; Pisaster removal: more than 10 species lost in a year.
Parasite adaptations: $suckers + loss of sense organs, digestive system + high reproduction$; Liver fluke: snail and fish as intermediate hosts.
Plant chemical defences: $C a l o t r o p i s \to cardiac glycosides; nicotine, caffeine, quinine, strychnine, opium$; Thorns of Acacia and cactus are morphological defences.
Classic mutualisms: $lichen (fungus + alga), mycorrhiza (fungus + root), fig + wasp, O p h r y s + bee$; Ophrys uses sexual deceit (pseudocopulation).

How to answer Predation, Parasitism and Mutualism questions

Interaction questions give a pair of organisms or a situation. Decide the effect on each partner, then name and justify.

1For each species, ask: benefits (+), harmed (−) or unaffected (0)?
2Match the sign pair to the interaction name using the table.
3Separate predation from parasitism: a predator kills and eats prey; a parasite lives on or in the host and draws food, usually without killing it quickly.
4Name an NCERT example and give the reason (e.g. clown fish gets protection; anemone gains nothing: commensalism).
5For defences or adaptations, give the type (morphological, chemical, camouflage) with an example.
6For co-evolution questions, explain that a change in one partner forces a matching change in the other (fig-wasp, Ophrys-bee, host-parasite).

Predation, Parasitism and Mutualism in real life

The idea, met in an everyday situation. Answer the question in your own words, then compare with the model answer. The one-line hook is what to remember.

Predators maintain diversity

Suppose a coastal village removes all starfish from a rocky shore because they eat mussels the villagers collect. A year later, mussels cover most rocks and several snail, barnacle and limpet species have disappeared.

Explain what happened using the role of predators, and predict what would happen if starfish were brought back.

Try it first, then show the solutionSolution

Starfish (like Pisaster) prey on the dominant competitor (mussels), keeping it in check.
Without the predator, mussels out-competed other invertebrates for space; interspecific competition eliminated several species (as when more than 10 species vanished within a year in the Pisaster experiment).
Reintroducing starfish would reduce mussels, free up space and allow other species to return, restoring diversity.

Answer: Removing the predator let the best competitor take over, reducing diversity; bringing starfish back would reduce competition and restore species.

Memory hook: Remove the predator, lose the neighbours.

Tricks to remember Predation, Parasitism and Mutualism

Short memory aids. Say each one aloud once, then use it on a question the same day.

M-C-P-P-C-A
Mutualism ++, Competition −−, Predation and Parasitism +−, Commensalism +0, Amensalism −0.
Use it when: Interaction table.
Cuckoo cheats the crow
Koel lays in the crow's nest: brood parasitism.
Use it when: Example of brood parasitism.
Fig needs its wasp
One fig, one wasp: pollination for seeds.
Use it when: Mutualism example.

Predation, Parasitism and Mutualism: solved examples

Ordered from board level to HOTS. Try each one before you open the solution.

Example 1: Name the interaction · 2 marks · Very short answerBoard level

Name the interaction: (a) clown fish and sea anemone (b) Cuscuta and hedge plant (c) cattle egret and grazing cattle (d) lichen.

Try it first, then show the solutionSolution

(a) Commensalism; (b) parasitism (ectoparasite) (1 mark).
(c) Commensalism; (d) mutualism (fungus + alga or cyanobacterium) (1 mark).

Answer: (a) commensalism (b) parasitism (c) commensalism (d) mutualism.

Example 2: Roles of predators · 3 marks · Short answerBoard level

Explain three ways in which predators are important in nature, with an example of each.

Try it first, then show the solutionSolution

They transfer energy fixed by plants to higher trophic levels (1 mark).
They keep prey populations in check: the prickly pear cactus in Australia was controlled only after a cactus-feeding moth was introduced (1 mark).
They maintain species diversity by reducing competition among prey: removing the starfish Pisaster led to extinction of more than 10 invertebrate species within a year (1 mark).

Answer: Energy transfer, prey control (prickly pear), diversity (Pisaster).

Example 3: Parasite adaptations · 3 marks · Short answerHarder board level

Describe the adaptations of parasites to their mode of life. Why do many have complex life cycles?

Try it first, then show the solutionSolution

Loss of unnecessary sense organs; adhesive organs or suckers to cling to the host (1 mark).
Loss of digestive system (they absorb ready food) and high reproductive capacity (1 mark).
Intermediate hosts or vectors help them reach the primary host: the human liver fluke uses a snail and a fish; the malarial parasite uses a mosquito (1 mark).

Answer: Suckers, reduced sense organs and gut, high reproduction; intermediate hosts or vectors aid transmission.

Example 4: Brood parasitism · 2 marks · Very short answerHarder board level

What is brood parasitism? Explain the evolutionary significance of the parasite's eggs resembling the host's.

Try it first, then show the solutionSolution

A parasitic bird (cuckoo/koel) lays eggs in the host's (crow's) nest and lets the host incubate them (1 mark).
Eggs resembling the host's in size and colour reduce the chance that the host detects and ejects them; natural selection favoured such resemblance (1 mark).

Answer: Laying eggs in a host's nest; egg mimicry avoids rejection.

Example 5: Ophrys and co-evolution · 3 marks · Short answerHarder board level

How does the orchid Ophrys get pollinated, and why is this an example of co-evolution?

Try it first, then show the solutionSolution

One petal of Ophrys resembles a female bee in size, colour and markings (sexual deceit) (1 mark).
A male bee pseudocopulates with the flower, is dusted with pollen, and transfers it when it pseudocopulates with another flower (1 mark).
If the female bee's colour pattern changed during evolution, pollination success would fall unless the orchid's petal co-evolved to match: the two evolve together (1 mark).

Answer: Petal mimics a female bee; male bee pseudocopulates and carries pollen; orchid must track changes in the bee: co-evolution.

Example 6: Why Calotropis is left alone · 3 marks · Short answerHOTS / competency

Suppose a goat herder notices that his goats graze every plant in an abandoned field except one milky weed. Identify the likely weed, explain the plant's defence, and name two commercially used plant chemicals that also evolved as defences.

Try it first, then show the solutionSolution

The weed is likely Calotropis, common in abandoned fields (1 mark).
It produces highly poisonous cardiac glycosides, so cattle and goats avoid browsing it: a chemical defence against herbivores (1 mark).
Other defensive chemicals: nicotine, caffeine, quinine, strychnine, opium (any two) (1 mark).

Answer: Calotropis; cardiac glycosides deter herbivores; e.g. nicotine and quinine.

Where students lose marks in Predation, Parasitism and Mutualism

The slips examiners see most often, and what to write instead.

Mistake: Calling clown fish-anemone mutualism
Close association is assumed to be mutual.
Wrong working
Clown fish and sea anemone show mutualism.
Right working
Commensalism: the fish gets protection; the anemone gains no apparent benefit (as per NCERT).
Do this: Check whether both partners clearly benefit.
Usually costs: 1 mark.
Mistake: Treating predation and parasitism as identical
Both are +/−.
Wrong working
Lice on humans are predators.
Right working
Lice are ectoparasites: they live on the host and feed without killing it.
Do this: Predator kills and eats; parasite lives on/in host.
Usually costs: 1 mark.
Mistake: Missing the reason in examples
Students only name examples.
Wrong working
Cattle egret and cattle: commensalism.
Right working
Commensalism: cattle flush out insects that egrets eat; cattle are unaffected.
Do this: Give who gains what.
Usually costs: ½-1 mark.
Mistake: Wrong intermediate hosts
Parasite life cycles are mixed.
Wrong working
Liver fluke uses mosquito as intermediate host.
Right working
Liver fluke: snail and fish; malarial parasite: mosquito vector.
Do this: Learn each with its hosts.
Usually costs: 1 mark.

Exam tips

Draw the sign table in interaction questions; it earns marks quickly.
Use NCERT examples exactly: Pisaster, prickly pear, Monarch, Calotropis, Cuscuta, koel-crow, clown fish-anemone, fig-wasp, Ophrys.
Always explain who benefits and how.
Mention co-evolution for host-parasite, fig-wasp and Ophrys-bee pairs.
Competition (−/−) is covered in detail in its own topic page.

Practice questions on Predation, Parasitism and Mutualism

Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.

Q1 · 1 mark · MCQBoard level
In which interaction does one species gain while its partner is left unaffected?
(a) mutualism (b) amensalism (c) commensalism (d) parasitism
Hint
+/0.
Answer
(c) commensalism.
Q2 · 1 mark · Assertion-reasonHarder board level
Assertion (A): The female mosquito is not considered a parasite.
Reason (R): The female mosquito needs human blood for reproduction.
(a) Both A and R are true and R is the correct explanation of A.
(b) Both A and R are true but R is not the correct explanation of A.
(c) A is true but R is false.
(d) A is false but R is true.
Hint
Does needing blood make it a parasite?
Answer
(b). Both are true, but R does not explain A; it is not a parasite although it needs blood (it does not live on the host).
Q3 · 2 marks · Very short answerBoard level
Explain the mutualism between figs and wasps. What does each partner gain?
Hint
Pollination vs food.
Answer
A fig species is pollinated only by its partner wasp species. The female wasp lays eggs in the fruit and, while searching for egg-laying sites, pollinates the fig inflorescence. The fig gains pollination; the wasp gains an egg-laying site and some developing seeds as food for its larvae.
Q4 · 3 marks · Short answerHOTS / competency
Suppose a new insect herbivore is introduced to an island and multiplies rapidly, damaging crops. Using the prickly pear example, explain why it spread so fast and suggest a control strategy with one precaution.
Hint
Natural predators absent.
Answer
Introduced species become invasive because the new area lacks their natural predators, so nothing keeps their numbers in check (as with the prickly pear cactus in Australia). Strategy: introduce a natural predator or parasite of the insect from its native habitat (biological control), as the cactus-feeding moth controlled prickly pear. Precaution: test that the control agent is specific to the pest and will not itself become invasive or harm native species.
Q5 · 4 marks · Case-basedHarder board level
Read and answer.
Suppose a student lists: (i) monarch butterfly avoided by birds; (ii) thorns on Acacia; (iii) leaf-coloured insects; (iv) lice on a child's head; (v) orchid on a mango branch.
(a) Which are prey or plant defences? Classify each. (1)
(b) Why is the monarch butterfly distasteful, and when does it acquire this? (1)
(c) Name the interactions in (iv) and (v) with their sign pairs. (2)
Hint
Chemical, morphological, camouflage.
Answer
(a) (i) chemical defence (distasteful), (ii) morphological defence (thorns), (iii) camouflage (cryptic colouring). (b) It contains a special chemical that makes it distasteful to birds, acquired during its caterpillar stage by feeding on a poisonous weed. (c) (iv) ectoparasitism (+/−); (v) commensalism (+/0).
Q6 · 2 marks · Very short answerHarder board level
Why are predators said to be 'prudent'?
Hint
Over-exploitation.
Answer
If a predator were too efficient and over-exploited its prey, the prey could go extinct, and then the predator would also die out for lack of food. Predators in nature therefore do not wipe out their prey; this restraint is described as prudence.

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

1.Sign pair for commensalism?Show answer
+/0.
2.Which starfish's removal caused extinction of over 10 species?Show answer
Pisaster.
3.What poison does Calotropis produce?Show answer
Cardiac glycosides.
4.Name the two intermediate hosts of the human liver fluke.Show answer
A snail and a fish.
5.Which bird lays eggs in the crow's nest?Show answer
Cuckoo (koel).
6.Which orchid uses sexual deceit?Show answer
Ophrys.

Predation, Parasitism and Mutualism: frequently asked questions

What are the types of population interactions?

Mutualism (+/+), competition (−/−), predation (+/−), parasitism (+/−), commensalism (+/0) and amensalism (−/0).

How do predators maintain species diversity?

By feeding on dominant competitors, they reduce competition among prey species; removing Pisaster starfish led to the loss of over 10 species.

What is brood parasitism?

A parasitic bird such as the cuckoo lays eggs in a host's (crow's) nest, and the host incubates them; the eggs mimic the host's eggs.

Give examples of commensalism.

Orchid on a mango branch, barnacles on a whale, cattle egret with grazing cattle, and clown fish among sea anemone tentacles.

What is mutualism? Give examples.

An interaction that benefits both species, such as lichens, mycorrhiza, fig and wasp, and plant-pollinator relationships.