Predation, Parasitism and Mutualism: ISC Class 12 Biology
Updated 15 October 2026 · Fact-checked
Population interactions are scored + (benefit), − (harm) or 0 (neutral). Mutualism is +/+, competition −/−, predation and parasitism +/−, commensalism +/0 and amensalism −/0. Predators transfer energy and control prey; parasites live on (ecto) or in (endo) a host, or use its nest (brood); mutualists such as fig and wasp both gain.
Understand Predation, Parasitism and Mutualism
The interaction table. No species lives alone; interactions between two species can be beneficial (+), detrimental (−) or neutral (0).
| Species A | Species B | Interaction | Example |
|---|---|---|---|
| + | + | mutualism | lichen (fungus + alga/cyanobacterium); mycorrhiza; fig and wasp |
| − | − | competition | flamingoes and fishes for zooplankton |
| + | − | predation | tiger and deer; sparrow eating seeds |
| + | − | parasitism | lice on humans; tapeworm |
| + | 0 | commensalism | orchid on mango branch; barnacles on whale; cattle egret and cattle; clown fish and sea anemone |
| − | 0 | amensalism | one species harmed, the other unaffected (e.g. Penicillium inhibiting bacteria around it) |
Predation, parasitism and commensalism share one feature: the species live closely together.
1. Predation. Nature's way of transferring energy fixed by plants to higher trophic levels. Herbivores eating plants are, broadly, predators too.
Roles of predators: (a) conduits for energy across trophic levels; (b) keep prey populations under control; without them prey can explode and destabilise the ecosystem; invasive exotic species spread because the new land lacks their predators (the prickly pear cactus in Australia was controlled only after a cactus-feeding moth was introduced); (c) biological control in agriculture; (d) maintain species diversity by reducing competition among prey (removing the starfish Pisaster from an intertidal area led to extinction of more than 10 invertebrate species within a year). Predators are 'prudent': over-exploiting prey would make both extinct.
Prey defences. Animals: camouflage (cryptic colour in some insects and frogs); poison or distastefulness: the Monarch butterfly is distasteful to birds because of a chemical it acquires as a caterpillar by feeding on a poisonous weed. Plants (cannot run away; about 25% of insects are phytophagous): thorns (Acacia, cactus); chemicals: Calotropis makes highly poisonous cardiac glycosides, so cattle and goats do not browse it; nicotine, caffeine, quinine, strychnine and opium are plant defences against grazers.
2. Parasitism (+/−). The parasite gets free lodging and food; the host is harmed. Many parasites are host-specific and co-evolve with the host.
Adaptations of parasites: loss of unnecessary sense organs; adhesive organs or suckers; loss of digestive system; high reproductive capacity; complex life cycles with intermediate hosts or vectors (human liver fluke needs a snail and a fish; malarial parasite needs a mosquito).
Effects on host: reduce survival, growth, reproduction and population density; make the host weak and more vulnerable to predation.
Types.
| Type | Where | Examples |
|---|---|---|
| Ectoparasite | on the external surface | lice on humans, ticks on dogs, copepods on marine fish, Cuscuta (no chlorophyll or leaves; feeds on hedge plants) |
| Endoparasite | inside the body (liver, kidney, lungs, RBCs) | tapeworm, liver fluke, malarial parasite; simplified body, huge reproductive potential |
| Brood parasite | uses the host's nest | cuckoo (koel) lays eggs in the crow's nest; its eggs resemble the host's in size and colour so they are not ejected |
The female mosquito is not considered a parasite: it takes blood only for reproduction and does not live on the host.
3. Mutualism (+/+). Both species benefit.
- Lichens: fungus + photosynthesising alga or cyanobacterium.
- Mycorrhizae: fungi help roots absorb nutrients; the plant gives carbohydrates.
- Plant-animal mutualism: plants reward pollinators with pollen and nectar and seed dispersers with fruits; they must guard against 'cheaters' (nectar thieves). This leads to co-evolution.
- Fig and wasp: a one-to-one relationship; the female wasp pollinates the fig inflorescence while searching for egg-laying sites, and the fig gives some developing seeds as food for wasp larvae.
- Ophrys orchid and bee: 'sexual deceit': one petal resembles the female bee; the male bee pseudocopulates, is dusted with pollen and pollinates the next flower. If the female bee's pattern changes, the orchid must co-evolve to keep pollination.
Commensalism (+/0) in detail. Orchid as an epiphyte on mango (support and light for the orchid; mango unaffected); barnacles on a whale; cattle egret feeding on insects flushed out by grazing cattle; clown fish protected among the stinging tentacles of a sea anemone.
Predation, Parasitism and Mutualism: key facts
- Sign code
Mutualism both gain (+ / +) Competition both lose (− / −) Predation predator gains, prey harmed (+ / −) Parasitism parasite gains, host harmed (+ / −) Commensalism one gains, other unaffected (+ / 0) Amensalism one harmed, other unaffected (− / 0) - Learn as a table.
- Parasite types
Ectoparasite (outside) lice, ticks, Cuscuta Endoparasite (inside) tapeworm, liver fluke, malarial parasite Brood parasite (nest) cuckoo (koel) in the crow's nest - Lice/ticks/Cuscuta; tapeworm/liver fluke; cuckoo-crow.
- Roles of predators
Energy transfer carry energy to higher trophic levels Prey control prickly pear cactus checked by a moth in Australia Biological control pests in agriculture Species diversity Pisaster removal led to loss of more than 10 species - Pisaster experiment for diversity.
- Mutualisms to quote
Fig and wasp pollination in return for seeds as larval food Ophrys and bee pollination by sexual deceit (pseudocopulation) Lichen fungus with alga or cyanobacterium Mycorrhiza fungus with plant roots - Pollination for food or deceit.
How to answer Predation, Parasitism and Mutualism questions
ISC asks interaction names from examples, definitions with + / − / 0 signs, types of parasitism with examples, roles of predators, prey and plant defences, and mutualism with co-evolution.
- 1Decide what each partner gains or loses, write the signs, then name the interaction.
- 2Spell it right: commensalism (not 'communalism'), amensalism.
- 3For parasitism, define it (parasite benefits, host harmed, depends for food and shelter), then give each type with its example.
- 4Never give commensal examples (orchid on mango) as parasites.
- 5For predators' roles, give at least three with an example (prickly pear, Pisaster).
- 6For mutualism, name both partners and what each gains.
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 village removes all the snakes from its fields because people fear them, and within two years rats multiply and destroy much of the stored grain.
Explain this using the role of predators, and suggest a better approach.
Try it first, then show the solutionSolution
- Snakes are predators that keep rodent populations in check.
- Without predators, rats grew rapidly, as prey populations do when predation is removed.
- The rat explosion damaged grain and may spread disease, an ecosystem imbalance.
- Better: protect predators (snakes, owls), store grain in rat-proof bins and educate people about harmless snakes.
Answer: Removing predators released the prey (rats) from control; conserving predators is natural pest control.
Memory hook: No predator, prey explodes.
Tricks to remember Predation, Parasitism and Mutualism
Short memory aids. Say each one aloud once, then use it on a question the same day.
Co-mensal = same table
Commensal comes from 'eating at the same table': one eats, the other is unaffected (+/0).
Use it when: Spelling and meaning of commensalism.
Ecto-Endo-Brood
Ecto = exterior, Endo = inside, Brood = baby-sitting by the host.
Use it when: Parasite types.
A-men-salism: harmed alone
Amensalism: one harmed (−), the other zero (0).
Use it when: Telling amensalism from commensalism.
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 · Short answerBoard level
Name the interaction and give its sign code: (i) cattle egret and grazing cattle, (ii) fungus and alga in a lichen, (iii) Cuscuta on a hedge plant, (iv) flamingoes and fish feeding on zooplankton.
Try it first, then show the solutionSolution
- (i) Commensalism (+/0); (ii) mutualism (+/+) (1 mark).
- (iii) Parasitism (+/−), ectoparasite; (iv) competition (−/−) (1 mark).
Answer: Commensalism; mutualism; parasitism; competition.
Example 2: Three parasites, three types · 3 marks · Short answerBoard level
A tick on a dog, a tapeworm in a human intestine and a koel egg in a crow's nest are all examples of parasitism. Define parasitism, then name the type shown by each and state what each parasite gains from its host.
Try it first, then show the solutionSolution
- Parasitism: an interaction in which the parasite benefits by obtaining food and shelter from the host, which is harmed (+/−) (1 mark).
- Tick: ectoparasite, feeding on the dog's blood from the outside; tapeworm: endoparasite, living inside the intestine and absorbing digested food (1 mark).
- Koel: brood parasite; the crow incubates the egg and rears the chick, so the koel saves the cost of parental care (1 mark).
Answer: Parasite +, host −; tick ectoparasite (food), tapeworm endoparasite (food and shelter), koel brood parasite (incubation and rearing).
Example 3: Why predators matter · 3 marks · Short answerHarder board level
Give three roles of predators in an ecosystem with an example of each.
Try it first, then show the solutionSolution
- They transfer energy fixed by plants to higher trophic levels (tiger eating deer) (1 mark).
- They keep prey populations under control; prickly pear cactus spread over millions of hectares in Australia until 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 (1 mark).
Answer: Energy transfer, prey control (prickly pear), diversity (Pisaster).
Example 4: Plant defences · 3 marks · Short answerHarder board level
Plants cannot run away from herbivores. Describe two morphological or chemical defences with examples, and explain why cattle avoid Calotropis.
Try it first, then show the solutionSolution
- Morphological: thorns in Acacia and cactus (1 mark).
- Chemical: substances that make herbivores sick, inhibit feeding or digestion, disrupt reproduction or kill, e.g. nicotine, caffeine, quinine (1 mark).
- Calotropis produces highly poisonous cardiac glycosides, so cattle and goats do not browse it (1 mark).
Answer: Thorns; defensive chemicals; Calotropis has cardiac glycosides.
Example 5: Fig and wasp · 3 marks · Short answerHarder board level
Explain the mutualism between fig trees and wasps. Why is it called co-evolution?
Try it first, then show the solutionSolution
- A given fig species is pollinated only by its partner wasp species: a one-to-one relationship (1 mark).
- The female wasp pollinates the fig inflorescence while searching for egg-laying sites in the fruit; the fig lets some developing seeds feed the wasp larvae (1 mark).
- The flower and the pollinator are so tightly linked that a change in one requires a matching change in the other, so they evolve together (1 mark).
Answer: Pollination for seeds as larval food; tightly linked evolution.
Example 6: Is it parasitism? · 4 marks · Short answerHOTS / competency
Suppose a student lists the following as parasites: (i) an orchid growing on a mango branch, (ii) a female mosquito biting a human, (iii) a cuckoo chick raised by crows, (iv) a tick on a dog. Evaluate each and give the correct interaction.
Try it first, then show the solutionSolution
- (i) Not a parasite: the orchid only gets support and light; the mango is unaffected: commensalism (1 mark).
- (ii) Not considered a parasite: the female mosquito takes blood for egg production and does not live on or depend on the host for shelter (1 mark).
- (iii) Yes: brood parasitism; the crow spends effort raising the cuckoo chick (1 mark).
- (iv) Yes: ectoparasitism (1 mark).
Answer: (i) Commensalism; (ii) not a parasite; (iii) brood parasite; (iv) ectoparasite.
Where students lose marks in Predation, Parasitism and Mutualism
The slips examiners see most often, and what to write instead.
Mistake: Writing 'communalism'
In 2024 examiners reported 'communalism' for commensalism, and vague answers such as positive interaction, symbiosis or mutualism for the orchid on a mango tree.
Do this: Spell commensalism and give the sign +/0.
Usually costs: 1 mark.
Mistake: Types of parasitism without examples
In 2025 examiners noted types explained without examples and no proper definition of parasitism.
Do this: Define first, then give each type with an example.
Usually costs: 1 mark per type.
Mistake: Wrong examples of parasites
Examiners (2025) saw orchids on mango, or fungi and bacteria in general, given as parasites, and commensalism and amensalism named as types of parasitism.
Do this: Use lice, ticks, Cuscuta, tapeworm, liver fluke, cuckoo.
Usually costs: 1 mark.
Mistake: Mixing amensalism and commensalism
Examiners (2024) suggested contrasting the two together because candidates confused them.
Do this: Commensalism +/0; amensalism −/0.
Usually costs: 1 mark.
Exam tips
- Draw the + / − / 0 table when a question asks for several interactions.
- Use ecto = outer and endo = inner to remember parasite types.
- Name both partners and what each gains in mutualism answers.
- Quote the Pisaster and prickly pear examples for predator roles.
- Remember that Cuscuta lacks chlorophyll and leaves.
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
Barnacles attached to a whale's skin show:
(a) mutualism (b) commensalism (c) parasitism (d) amensalismHint
Whale unaffected.
Answer
(b) commensalism.
Q2 · 2 marks · Short answerBoard level
Give two adaptations of parasites to their mode of life.
Hint
Loss and gain.
Answer
Loss of unnecessary sense organs and of the digestive system; adhesive organs or suckers; high reproductive capacity (any two).
Q3 · 1 mark · Assertion-reasonHarder board level
Assertion (A): The Monarch butterfly is rarely eaten by birds.
Reason (R): It acquires a distasteful chemical by feeding on a poisonous weed during its caterpillar stage.
(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
Chemical defence.
Answer
(a).
Q4 · 2 marks · Short answerHarder board level
How does the Ophrys orchid get pollinated without offering any reward?
Hint
Sexual deceit.
Answer
One petal resembles the female bee in size, colour and markings. The male bee pseudocopulates with the flower, is dusted with pollen and transfers it when it pseudocopulates with another flower.
Q5 · 2 marks · Short answerHarder board level
Why are predators described as 'prudent'?
Hint
Over-exploitation.
Answer
If a predator were too efficient and over-exploited its prey, the prey could become extinct and the predator would then die out for lack of food; predators in nature do not wipe out their prey.
Q6 · 2 marks · Short answerHarder board level
Why are the life cycles of endoparasites complex? Give an example.
Hint
Intermediate hosts.
Answer
Because of extreme specialisation they often need one or two intermediate hosts or vectors to reach their primary host. The human liver fluke depends on a snail and a fish as intermediate hosts.
Q7 · 3 marks · Short answerHOTS / competency
Suppose a goat species is introduced onto an island where a native tortoise browses the same plants, and the tortoise declines rapidly. (a) Name the interaction. (b) Which real case does this resemble? (c) Suggest one way the tortoise might have survived if the competition were less intense.
Hint
Galapagos; resource partitioning.
Answer
(a) Interspecific competition (−/−, with the goat far superior). (b) The Abingdon tortoise of the Galapagos became extinct within a decade after goats were introduced, apparently because goats browsed more efficiently. (c) By resource partitioning: using different food plants, feeding at different times or in different places, reducing overlap with the goats.
Q8 · 3 marks · Case-basedHOTS / competency
Read and answer.
Suppose a park ranger records: a koel's eggs in a crow's nest; lice on a monkey; clown fish among sea anemone tentacles; fungal threads wrapped round the roots of a pine tree, helping it absorb phosphorus.
(a) Name each interaction. (2)
(b) Why have the koel's eggs evolved to look like the crow's? (1)Hint
Brood parasitism.
Answer
(a) Brood parasitism; ectoparasitism; commensalism; mutualism (mycorrhiza). (b) Eggs resembling the host's in size and colour reduce the chance that the crow detects and ejects them, so selection favoured such eggs.
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 code of amensalism?Show answer
− / 0.
2.Give an example of brood parasitism.Show answer
Cuckoo (koel) laying eggs in a crow's nest.
3.Which plant makes cardiac glycosides as a defence?Show answer
Calotropis.
4.Name a parasitic plant.Show answer
Cuscuta.
5.Which orchid uses sexual deceit?Show answer
Ophrys.
6.What controlled the prickly pear cactus in Australia?Show answer
A cactus-feeding moth (a predator) introduced from its natural habitat.
Predation, Parasitism and Mutualism: frequently asked questions
What is the difference between predation and parasitism?
Both are +/−; a predator kills and eats its prey, while a parasite lives on or in its host, takes food and shelter, and usually harms without killing at once.
What are the types of parasites?
Ectoparasites (lice, ticks, Cuscuta), endoparasites (tapeworm, liver fluke) and brood parasites (cuckoo).
Is the orchid on a mango tree a parasite?
No. It is an epiphyte in commensalism: the orchid benefits and the mango is unaffected.
What is mutualism?
An interaction in which both species benefit, such as lichens, mycorrhizae and fig-wasp pollination.
How do prey avoid predators?
By camouflage, by being poisonous or distasteful (Monarch butterfly), and in plants by thorns and defensive chemicals.