Population Interactions II (Predation, Competition, Amensalism)

Attempt the quiz above first, then review every question with its correct answer and a short explanation below.

1. Q1.In the rocky intertidal communities of the American Pacific Coast, the starfish Pisaster is an important predator. When all starfish were experimentally removed from an area, more than 10 species of invertebrates became extinct within a year due to:

   • A.Severe disease outbreak
   • B.Interspecific competition among the prey species✓ Correct Answer
   • C.Loss of primary producers
   • D.Co-extinction with parasites

   Explanation: NCERT Class 12 Biology Chapter 13 notes: 'If a predator is too efficient... then the prey might become extinct. Predators also help in maintaining species diversity in a community, by reducing the intensity of competition among competing prey species. In the rocky intertidal communities... starfish Pisaster is an important predator. When all starfish were removed... more than 10 species of invertebrates became extinct... because of interspecific competition.'

2. Q2.In 1920s Australia, the invasive Prickly Pear Cactus spread rapidly across millions of hectares, causing havoc. How was this invasive weed successfully controlled?

   • A.By spraying chemical herbicides over the entire continent.
   • B.By introducing a co-evolved cactus-feeding moth from its natural habitat.✓ Correct Answer
   • C.By manual weeding using intensive human labor.
   • D.By flooding the fields with seawater.

   Explanation: NCERT documents: 'The prickly pear cactus introduced into Australia in the early 1920's caused havoc... Finally, the invasive cactus was brought under control only after a cactus-feeding predator (a moth) from its natural habitat was introduced into the country.' This shows the predator's role in controlling prey populations.

3. Q3.Why do cattle and goats avoid browsing on the common poisonous weed Calotropis growing in abandoned agricultural fields?

   • A.It has long sharp thorns.
   • B.The plant produces highly toxic cardiac glycosides that make the animals sick.✓ Correct Answer
   • C.The leaves are too tough to chew.
   • D.It is camouflaged as dry wood.

   Explanation: NCERT Chapter 13 notes Calotropis defense: 'You might have seen the weed Calotropis growing in abandoned fields. The plant produces highly poisonous cardiac glycosides and that is why you never see any cattle or goats browsing on this plant.'

4. Q4.The Monarch Butterfly is highly distasteful to its predator birds because it contains a toxic chemical in its body. How does the butterfly acquire this chemical?

   • A.It synthesizes the chemical internally during adulthood.
   • B.It acquires the chemical during its caterpillar stage by feeding on a poisonous weed.✓ Correct Answer
   • C.It absorbs the chemical directly from the air.
   • D.It receives the chemical from male butterflies during mating.

   Explanation: NCERT Class 12 Biology states: 'The Monarch butterfly is highly distasteful to its predator (bird) because of a special chemical present in its body. Interestingly, the butterfly acquires this chemical during its caterpillar stage by feeding on a poisonous weed.'

5. Q5.When the Abingdon tortoise in the Galapagos Islands became extinct within a decade after goats were introduced on the island, it represented which type of ecological interaction?

   • A.Predation by goats on tortoises
   • B.Interference competition leading to competitive exclusion due to greater browsing efficiency of goats✓ Correct Answer
   • C.Mutualism gone wrong
   • D.Amensalism where tortoises were unaffected

   Explanation: NCERT Class 12 Biology states: 'The Abingdon tortoise in Galapagos Islands became extinct within a decade after goats were introduced on the island, apparently due to the greater browsing efficiency of the goats.' This shows resource/interference competition.

6. Q6.Connell's elegant field experiments on the rocky sea coast of Scotland showed that the larger superior barnacle Balanus dominates the intertidal area and excludes the smaller barnacle Chthamalus. This ecological phenomenon is called:

   • A.Competitive Release
   • B.Competitive Exclusion✓ Correct Answer
   • C.Resource Partitioning
   • D.Mutualistic Coexistence

   Explanation: Connell's experiment demonstrates competitive exclusion under natural conditions. The dominant barnacle Balanus physically excludes the smaller Chthamalus from the active zones of the intertidal rocks (NCERT Chapter 13).

7. Q7.If two closely related species competing for the exact same resources co-exist by developing different feeding times or foraging patterns, it is an example of:

   • A.Gause's Exclusion
   • B.Resource Partitioning✓ Correct Answer
   • C.Competitive Release
   • D.Allelopathy

   Explanation: NCERT Chapter 13 notes: 'If two species compete for the same resource, they could avoid competition by choosing... different times for feeding or different foraging patterns. MacArthur showed that five closely related species of warblers living on the same tree were able to avoid competition and co-exist due to behavioural differences in their foraging activities.' This is resource partitioning.

8. Q8.Which of the following principles states that two closely related species competing for the same limiting resources cannot coexist indefinitely, and the inferior one will be eliminated eventually?

   • A.Paul Ehrlich's Rivet Hypothesis
   • B.Gause's Competitive Exclusion Principle✓ Correct Answer
   • C.Allen's Rule of Extremes
   • D.MacArthur's Coexistence Rule

   Explanation: Gause's Competitive Exclusion Principle states that two closely related species competing for the same resources cannot co-exist indefinitely and the competitively inferior one will be eliminated eventually, provided resources are limiting (NCERT Chapter 13).

9. Q9.The interaction where one species is harmed, while the other is neither benefited nor harmed (neutral), is defined as:

   • A.Parasitism
   • B.Amensalism✓ Correct Answer
   • C.Commensalism
   • D.Competition

   Explanation: Amensalism is a (-, 0) interaction where one species is inhibited or harmed (-) while the other species remains completely unaffected (0). E.g. Penicillium releasing penicillin that kills bacteria.

10. Q10.Penicillium block the growth of bacteria by releasing the chemical penicillin, but the fungus itself derives no benefit or harm from this inhibition. This is a classic example of:

    • A.Commensalism
    • B.Amensalism✓ Correct Answer
    • C.Parasitism
    • D.Mutualism

    Explanation: This is a classic example of Amensalism (-, 0). The bacteria are harmed/killed (-) by the antibiotic, while the fungus Penicillium is unaffected (0) by the death of the bacteria.

11. Q11.What is the biological term for the chemical defense mechanism where a plant releases toxic chemicals into the soil to inhibit the germination and growth of neighboring competitor plants?

    • A.Mycorrhiza
    • B.Allelopathy✓ Correct Answer
    • C.Eutrophication
    • D.Biomagnification

    Explanation: Allelopathy is a biological phenomenon where an organism produces one or more biochemicals (allelochemicals) that influence the growth, survival, and reproduction of other organisms. Black Walnut tree releasing juglone is a classic allelopathic amensalism.

12. Q12.Herbivores are technically predators because they consume living plant tissue. Which of the following is a chemical defense produced by plants to discourage herbivory?

    • A.Sweet nectar production
    • B.Production of chemical compounds like nicotine, caffeine, quinine, and strychnine✓ Correct Answer
    • C.Absence of cell wall
    • D.Root nodules formation

    Explanation: NCERT Class 12 Biology states: 'A wide variety of chemical substances that we extract from plants on a commercial scale (nicotine, caffeine, quinine, strychnine, opium, etc.,) are produced by them actually as defences against grazers and browsers.'

13. Q13.Approximately what percentage of all insect species are estimated to be phytophagous (feeding on plant sap and other parts of plants)?

    • A.Less than 5 percent
    • B.Nearly 25 percent✓ Correct Answer
    • C.Exactly 50 percent
    • D.More than 90 percent

    Explanation: NCERT Chapter 13 documents: 'Nearly 25 per cent of all insects are known to be phytophagous (feeding on plant sap and other parts of plants).' This highlights the massive predatory/herbivore pressure on plants.

14. Q14.When a species distributed in a restricted area because of a superior competitor expands its range when the competitor is experimentally removed, the process is called:

    • A.Competitive Exclusion
    • B.Competitive Release✓ Correct Answer
    • C.Resource Partitioning
    • D.Character Displacement

    Explanation: NCERT states: 'Competitive Release: A species whose distribution is restricted to a small geographical area because of the presence of a competitively superior species, is found to expand its distributional range dramatically when the competing species is experimentally removed.'

15. Q15.In South American lakes, visiting flamingos and resident fishes compete for the same food resource, which is:

    • A.Phytoplankton
    • B.Zooplankton✓ Correct Answer
    • C.Small frogs
    • D.Water lilies

    Explanation: NCERT Chapter 13 notes: 'Firstly, totally unrelated species could also compete for the same resource. For instance, in some shallow South American lakes, visiting flamingos and resident fishes compete for their common food, the zooplankton in the lake.'

16. Q16.In plants, morphological defenses like thorns are commonly found in which of the following genera to prevent grazing?

    • A.Acacia and Cactus✓ Correct Answer
    • B.Calotropis and Neem
    • C.Banyan and Mangrove
    • D.Pine and Spruce

    Explanation: NCERT Chapter 13 states: 'Plants have evolved an astonishing variety of morphological and chemical defences against herbivores. Thorns (Acacia, Cactus) are the most common morphological means of defence.'

17. Q17.Competition is best defined in ecology as a process in which:

    • A.Both species benefit mutually (+, +).
    • B.The fitness of one species is significantly lower in the presence of another species (- , -).✓ Correct Answer
    • C.One species is unaffected, while other is harmed.
    • D.Energy transfer is maximized.

    Explanation: In ecology, competition is a (-, -) interaction. Even if resources are abundant, the presence of a competitor reduces the feeding efficiency or reproductive rate (fitness) of both species, leading to mutual harm.

18. Q18.In interspecific competition, which type of competition occurs when individuals of different species interfere directly with each other's foraging, even if resources are not limiting?

    • A.Exploitation competition
    • B.Interference competition✓ Correct Answer
    • C.Apparent competition
    • D.Scramble competition

    Explanation: NCERT Class 12 Biology states that competition can occur even when resources are unlimited. In interference competition, the feeding efficiency of one species might be reduced because of the interfering and inhibitory presence of the other species.

19. Q19.When a predator acts as a 'conduit' in an ecosystem, what does this refer to?

    • A.It purifies the water in the ecosystem.
    • B.It transfers energy fixed by plants to higher trophic levels.✓ Correct Answer
    • C.It causes soil erosion.
    • D.It decomposes dead litter.

    Explanation: NCERT states: 'Besides acting as "conduits" for energy transfer across trophic levels, predators play other important roles...'. By eating herbivores, carnivores transfer energy from producers up the food chain.

20. Q20.Which of the following is a behavioral defense mechanism exhibited by frogs and insects to avoid being detected by predators?

    • A.Production of chemical toxins
    • B.Camouflage (cryptic coloration)✓ Correct Answer
    • C.Growing sharp thorns
    • D.Aesthetic display

    Explanation: NCERT Chapter 13 notes: 'Some species of insects and frogs are cryptically-coloured (camouflaged) to avoid being detected easily by the predator.' This is a classic behavioral and morphological defense.