Section 13 — Board Exam-Pattern Questions (Class 12 Biology · Chapter 11)
Chapter 11 (Organisms and Populations) is one of the most dependable scoring chapters in the Class 12 Biology Board paper. Its parent unit (Ecology, about 10 marks) leans on it heavily — questions spread across MCQs, 2- and 3-mark short answers, and frequently a 5-mark question on growth models or population interactions.
This section curates 24 high-yield Board exam-pattern questions (modelled on the CBSE style — not year-tagged official questions) in the mark-wise format used in Board papers:
| Marks | Number of Qs | Indicative frequency in papers |
|---|---|---|
| 1 mark | 7 questions (Q1–Q7) | 2–3 / paper |
| 2 marks | 6 questions (Q8–Q13) | 2 / paper |
| 3 marks | 6 questions (Q14–Q19) | 1–2 / paper |
| 5 marks | 5 questions (Q20–Q24) | 1 / paper (very high probability) |
| Total | 24 questions | 62 marks of practice |
How to use: Cover the answer, attempt it yourself scaled to the marks, then check against the model answer and its keyword notes.
No quiz at the end: Pair this set with the Solved Examples and the NEET practice section.
What the Board Keeps Asking
Most questions on Chapter 11 fall into five recurring themes:
1. Population attributes (1- or 2-mark) — attributes a population has but an individual does not, density and its measures, sex ratio, age pyramids.
2. The four processes and the density equation (2- or 3-mark) — natality, mortality, immigration, emigration, and calculating the new density.
3. Growth models (3- or 5-mark) — exponential vs logistic, the equations, J- and S-shaped curves, the meaning of and .
4. Population interactions (2-, 3- or 5-mark) — the sign table, predation and prey defences, competition and competitive exclusion, parasitism, commensalism and mutualism with examples.
5. Applied ecology (2- or 3-mark) — biological control, invasive species, resource partitioning, brood parasitism and co-evolution.
1-Mark Questions (Q1–Q7)
Q1. [1 mark] Name any two attributes that a population has but an individual organism does not.
Answer: Birth rate and death rate (also acceptable: sex ratio, age distribution).
Keyword note: any two population-level attributes earn the mark.
Q2. [1 mark] What is population density represented by, and give one measure other than total number.
Answer: It is represented by ; an alternative measure is per cent cover or biomass.
Q3. [1 mark] Name the two processes that increase population density.
Answer: Natality (births) and immigration.
Q4. [1 mark] What does the letter stand for in the logistic growth equation?
Answer: Carrying capacity — the maximum population the habitat can support.
Q5. [1 mark · Assertion-Reason]
Assertion (A): A lichen represents mutualism. Reason (R): In a lichen both the fungus and the alga derive benefit from the association.
Options: (a) Both A and R true and R explains A. (b) Both true but R does not explain A. (c) A true, R false. (d) A false, R true.
Answer: (a) — a lichen is mutualism (+ +), and it is precisely because both partners benefit that it qualifies, so R correctly explains A.
Q6. [1 mark] Which growth model produces a J-shaped curve?
Answer: The exponential (geometric) growth model, described by .
Q7. [1 mark] Name the interaction in which one species is harmed and the other is unaffected.
Answer: Amensalism (sign combination minus, zero).
2-Mark Questions (Q8–Q13)
Q8. [2 marks] A pond had 20 lotus plants and gained 8 new plants through reproduction in a year. Calculate the birth rate and state its unit.
Answer: Birth rate . So the birth rate is 0.4 offspring per lotus per year.
Keyword note: correct value and the per-capita, per-time unit each carry a mark.
Q9. [2 marks] Distinguish between exponential and logistic growth on the basis of resources and curve shape.
Answer: Exponential growth occurs when resources are unlimited; it has no ceiling and gives a J-shaped curve. Logistic growth occurs when resources are limiting; growth slows as the population nears the carrying capacity , giving an S-shaped (sigmoid) curve.
Q10. [2 marks] Write the equation for the change in population density over time and name each of its four components.
Answer:
Here B = births (natality), I = immigration add to density, while D = deaths (mortality), E = emigration reduce it.
Q11. [2 marks] Define carrying capacity and the intrinsic rate of natural increase.
Answer: Carrying capacity () is the maximum number of individuals a habitat can support with its resources. The intrinsic rate of natural increase () is the inherent capacity of a population to grow, equal to the per-capita birth rate minus death rate ().
Q12. [2 marks] An orchid grows as an epiphyte on the branch of a mango tree. Name and define this interaction.
Answer: This is commensalism (+ 0) — an interaction in which one species benefits (the orchid gains support and better light) while the other is neither harmed nor benefited (the mango tree is unaffected).
Q13. [2 marks] Name two morphological and two chemical defences that plants have evolved against herbivory.
Answer: Morphological: thorns (as in Acacia and cactus) and spines/hard structures. Chemical: poisonous substances such as the cardiac glycosides of Calotropis, and defence compounds like nicotine, caffeine, quinine or strychnine that deter or sicken grazers.
3-Mark Questions (Q14–Q19)
Q14. [3 marks] Draw a conclusion about population growth from the four basic processes, using the density equation, and state the condition under which density increases.
Answer: Density changes through natality, mortality, immigration and emigration, summarised as
Natality and immigration raise density; mortality and emigration lower it. Density increases whenever . Under normal conditions births and deaths are the dominant factors, while immigration and emigration matter mainly in special situations such as a newly colonised habitat.
Keyword note: the equation, the four terms correctly grouped, and the increase condition each score.
Q15. [3 marks] Explain the age pyramid and describe the three shapes it can take for a human population.
Answer: An age pyramid plots the proportion of individuals (males and females) in each age group, and its shape reflects the population's growth status. A broad-based, triangular pyramid indicates a growing (expanding) population with many young. A bell-shaped pyramid with straight sides indicates a stable (stationary) population. A pyramid narrow at the base indicates a declining population with fewer young than adults.
Q16. [3 marks] Derive the integral form of the exponential growth equation and define its terms.
Answer: Starting from , integration gives where = population density after time , = density at time zero, = intrinsic rate of natural increase, and = the base of natural logarithms (2.71828). This describes unimpeded, resource-unlimited growth and yields a J-shaped curve.
Q17. [3 marks] How does predation help to (a) transfer energy, (b) control prey numbers, and (c) maintain species diversity?
Answer: (a) Predation acts as a conduit for energy transfer to higher trophic levels — the energy fixed by plants passes to predators. (b) Predators keep prey populations under control; without them prey could reach very high densities and destabilise the ecosystem. (c) By preying on competitively dominant prey, predators reduce the intensity of competition and thus help maintain species diversity, as shown by the starfish Pisaster in intertidal communities.
Q18. [3 marks] State the Competitive Exclusion Principle and explain, with an example, how competing species can still co-exist.
Answer: Gause's Competitive Exclusion Principle states that two closely related species competing for the same limiting resources cannot co-exist indefinitely, and the inferior competitor is eliminated. However, species can co-exist through resource partitioning — using the resource differently. MacArthur's five warbler species on the same tree avoided competition by foraging in different zones and behaviours, allowing co-existence rather than exclusion.
Q19. [3 marks] Describe three adaptations seen in parasites in relation to their mode of life.
Answer: Parasites show (1) loss of unnecessary organs — for example the digestive system and some sense organs are reduced; (2) adhesive organs or suckers to cling to the host; and (3) very high reproductive capacity to offset the low odds of reaching a new host. Many are also host-specific and have complex life cycles with intermediate hosts or vectors, such as the human liver fluke using a snail and a fish.
5-Mark Questions (Q20–Q24)
Q20. [5 marks] Compare exponential and logistic population growth. Give the equation for each, describe the curve, and explain why the logistic model is considered more realistic.
Answer:
Exponential growth happens when resources (food and space) are unlimited. It is described by whose integral form is Plotting against time gives a J-shaped curve that keeps steepening, since growth is proportional to the current size with no ceiling.
Logistic growth happens when resources become limiting, forcing competition. It passes through a lag phase, then acceleration and deceleration, and finally an asymptote at the carrying capacity , giving an S-shaped (sigmoid) curve. It is described by This is the Verhulst-Pearl logistic growth.
Why more realistic: because no habitat has unlimited resources, real populations sooner or later meet limits, so the logistic model, which builds in the carrying capacity, describes nature better than the exponential model.
Keyword note: both equations, J vs S curve, as ceiling, and the realism point each score.
Q21. [5 marks] Classify interspecific interactions using the sign convention, and give one example of mutualism, commensalism, parasitism, predation and competition.
Answer:
Interactions between two species are labelled with + (benefit), - (harm) or 0 (neutral) for each partner:
- Mutualism (+ +) — both benefit; example: a lichen (fungus with alga), or the fig-wasp partnership.
- Commensalism (+ 0) — one benefits, the other unaffected; example: an orchid growing on a mango branch, or the cattle egret and grazing cattle.
- Parasitism (+ -) — the parasite benefits, the host is harmed; example: Cuscuta on a host plant, or brood parasitism by the cuckoo.
- Predation (+ -) — the predator benefits, the prey is killed; example: a tiger eating a deer, or a sparrow eating a seed.
- Competition (- -) — both suffer; example: flamingoes and fishes competing for zooplankton in shallow lakes.
(Amensalism (- 0) — one harmed, other unaffected — completes the table.)
Q22. [5 marks] Explain predation as an ecological process. Describe its roles and the defences that prey species have evolved against it.
Answer:
Predation is an interaction (+ -) in which a predator kills and feeds on prey; herbivores eating plants are predators in a broad sense.
Roles of predation: (i) it acts as a conduit for energy transfer to higher trophic levels; (ii) predators keep prey populations in check, preventing overpopulation and instability; (iii) predators help maintain species diversity by lowering competition among prey, as with the starfish Pisaster; (iv) predators underpin biological pest control, as when a moth controlled the prickly pear cactus in Australia.
Prey defences: many insects and frogs are cryptically coloured (camouflaged); some are poisonous or distasteful, such as the Monarch butterfly, which stores a chemical acquired as a caterpillar. Plants, being unable to flee, evolve thorns (Acacia, cactus) and chemical defences such as the cardiac glycosides of Calotropis and compounds like nicotine and quinine.
Q23. [5 marks] Describe mutualism in plant-animal relationships, using fig-wasp and Ophrys orchid-bee examples, and explain co-evolution.
Answer:
Mutualism (+ +) benefits both partners, and its most striking cases are plant-pollinator relationships in which plants pay pollinators with pollen and nectar.
Fig and wasp: many figs have a one-to-one relationship with a specific wasp. The wasp pollinates the fig while seeking egg-laying sites, and the fig, in return, provides some developing seeds as food for the wasp larvae. A given fig can be pollinated only by its partner wasp.
Ophrys orchid and bee: the orchid uses sexual deceit — one petal mimics a female bee in size, colour and markings. The male bee pseudocopulates with the flower, gets dusted with pollen, and carries it to the next flower.
Co-evolution: because success depends on a precise match, the flower and its pollinator evolve in tight step. If the female bee's colours changed, pollination would fail unless the orchid co-evolved to keep the resemblance — showing how the two lineages are evolutionarily linked.
Q24. [5 marks] Explain competition in nature. Define it, describe interference competition and competitive release, and discuss resource partitioning as a route to co-existence.
Answer:
Competition (- -) is a process in which the fitness of one species (its ) is significantly lowered in the presence of another. It need not involve closely related species — unrelated flamingoes and fishes compete for the same zooplankton — and resources need not even be scarce.
Interference competition: the feeding efficiency of one species may be reduced by the inhibitory presence of another, even when food and space are abundant.
Competitive exclusion and release: Gause's principle says the superior competitor eliminates the inferior when resources are limiting; the Abingdon tortoise was lost after goats were introduced. Competitive release is the reverse — a species confined by a superior competitor expands its range when that competitor is removed, as shown by Connell's barnacles Balanus and Chthamalus.
Resource partitioning: rather than exclude each other, species may divide the resource by feeding at different times or in different ways. MacArthur's warblers co-existed on one tree through such behavioural differences, showing that competing species can evolve mechanisms that promote co-existence.
End of Section 13
You have now worked through 24 high-yield Board questions spanning population attributes, the four processes, exponential and logistic growth, and the full range of population interactions. If you can answer the five 5-markers (Q20–Q24) from memory with their key points, you have effectively secured this chapter's contribution to your paper.
Final tip: On exam day, draw and label diagrams wherever allowed — the age pyramids, the J- and S-shaped growth curves, the fig-wasp relationship — examiners give marks for clear labelled structure. Tabulate comparison answers (exponential vs logistic, the interaction sign table) and always write the growth equation before substituting numbers.