Organisms and Populations Class 12: Concepts, Growth Numericals and Board Answers

What Organisms and Populations is about

Weightage: This chapter is part of Unit X, Ecology and Environment, which carries 10 of the 70 theory marks together with Ecosystem and Biodiversity and Conservation.Source: CBSE Biology (044) syllabus 2026-27, course structure, Unit X Ecology and Environment

In real board papers: asked in 7 of the 8 board papers we hold (2023 to 2026), on average 3.3 marks per paper. See the questions

Key concepts

Population and its attributes

NCERT section 11.1.1 Population Attributes

Population: individuals of a species living in a well-defined geographical area, sharing or competing for similar resources and potentially interbreeding. (Groups produced by asexual reproduction are also treated as populations in ecology.) Examples: all the cormorants in a wetland, rats in an abandoned house, teak trees in a forest tract, bacteria in a culture plate, lotus plants in a pond.

Why population ecology matters: an individual copes with a changed environment, but natural selection operates at the population level; so population ecology links ecology to population genetics and evolution.

Attributes a population has but an individual does not:
1. Birth rate (natality rate): per capita births. Example: a pond had 20 lotus plants; 8 new plants were added: birth rate = 8/20 = 0.4 offspring per lotus per year.
2. Death rate (mortality rate): per capita deaths. Example: 4 of 40 fruit flies died in a week: death rate = 4/40 = 0.1 individuals per fruit fly per week.
3. Sex ratio: e.g. 60% females, 40% males.
4. Age distribution: the per cent of individuals in each age group; plotted as an age pyramid.

Age pyramids (for humans, males and females shown side by side): the shape shows whether the population is

• (a) expanding/growing: broad base (many pre-reproductive individuals), triangular;
• (b) stable: pre-reproductive and reproductive groups about equal, bell-shaped;
• (c) declining: narrow base (fewer young than reproductive adults), urn-shaped.

Age groups are pre-reproductive, reproductive and post-reproductive.

Worked example

A pond had 50 frogs at the start of a year. During the year, 30 tadpoles grew into new frogs and 10 frogs died. Calculate the birth rate and death rate, and the population at the year's end if there was no migration. (2 marks)

Try it first, then show the solutionSolution

1. Birth rate = 30/50 = 0.6 frogs per frog per year; death rate = 10/50 = 0.2 frogs per frog per year. (1)
2. End population = 50 + 30 − 10 = 70. Check with rates: 50 × (1 + 0.6 − 0.2) = 50 × 1.4 = 70. (1)

Answer: Birth rate 0.6, death rate 0.2 per frog per year; 70 frogs.

Population density and its measurement

NCERT section 11.1.1 Population Attributes (population density)

Population size, technically population density (N), tells us a lot about the status of a population. Any ecological process (competition, predation, pesticide effect) is judged by change in population size. Sizes range from under 10 (Siberian cranes at Bharatpur wetlands in a year) to millions (Chlamydomonas in a pond).

Ways to measure density:

• Total number: usually the most appropriate, but sometimes meaningless or hard.
• Per cent cover or biomass: better when individuals differ hugely in size. Example: 200 carrot grass (Parthenium hysterophorus) plants and one huge banyan tree: saying banyan density is low underestimates its enormous role; per cent cover or biomass is more meaningful.
• For huge populations (e.g. a dense bacterial culture in a petri dish), counting is impossible; another measure (such as biomass or cover) is used.
• Relative density: absolute numbers are not always needed; e.g. number of fish caught per trap is good enough for a lake.
• Indirect estimates: e.g. the tiger census in national parks and tiger reserves is often based on pug marks and faecal pellets.

Worked example

Suggest the most suitable measure of population density for (a) one banyan tree among 200 Parthenium plants, (b) fish in a large lake, (c) tigers in a reserve. Give a reason for each. (3 marks)

Try it first, then show the solutionSolution

1. (a) Per cent cover or biomass: numbers would wrongly make the huge banyan seem unimportant. (1)
2. (b) Relative density, e.g. number of fish caught per trap: counting every fish is impossible, and a relative measure is enough. (1)
3. (c) Indirect estimate from pug marks and faecal pellets: tigers are rarely seen and cannot be counted directly. (1)

Answer: (a) Cover or biomass (b) catch per trap (c) pug marks and faecal pellets.

Population growth: the four processes

NCERT section 11.1.2 Population Growth

Population density changes with food availability, predation pressure and weather, through four basic processes:

• Natality (B): number of births in a period, added to the initial density.
• Mortality (D): number of deaths in the period.
• Immigration (I): individuals of the same species that came into the habitat from elsewhere.
• Emigration (E): individuals that left the habitat for elsewhere.

Natality and immigration increase density; mortality and emigration decrease it.

Equation: if Nₜ is the density at time t, then
Nₜ₊₁ = Nₜ + [(B + I) − (D + E)]

The population grows if (B + I) > (D + E).

Which factors matter most? Normally births and deaths; immigration and emigration matter only in special conditions. For example, when a new habitat is being colonised, immigration may add more than births.

Worked example

A deer population of 500 had 60 births, 30 deaths, 10 immigrants and 20 emigrants in a year. Calculate the population at the end of the year and state whether it is growing. (2 marks)

Try it first, then show the solutionSolution

1. Nₜ₊₁ = 500 + [(60 + 10) − (30 + 20)] = 500 + (70 − 50) = 520. (1)
2. Check by stepping through: 500 + 60 = 560; − 30 = 530; + 10 = 540; − 20 = 520. Since B + I (70) > D + E (50), the population is growing. (1)

Answer: 520 deer; growing.

Exponential growth

NCERT section 11.1.2 Population Growth (exponential growth)

Condition: unlimited resources (food and space). Each species can then realise fully its innate potential to grow (as Darwin noted), and the population grows exponentially (geometrically).

Equation: if b = per capita birth rate and d = per capita death rate,
dN/dt = (b − d) × N; let (b − d) = r, so dN/dt = rN.

• r = intrinsic rate of natural increase, an important parameter for assessing the impact of any biotic or abiotic factor on growth.
• Examples of r: Norway rat 0.015, flour beetle 0.12, human population in India in 1981: 0.0205.

Integral form: Nₜ = N₀e^(rt), where Nₜ = density after time t, N₀ = density at time 0, e = 2.71828.

Curve: plotting N against time gives a J-shaped curve.

How fast? Any species growing exponentially can reach enormous numbers quickly. Darwin showed that even slow-breeding elephants could reach huge numbers without checks. NCERT's chessboard story: a king agrees to place 1 grain on square 1, 2 on square 2, 4 on square 3, doubling each time; by half the board, all the wheat in his kingdom is not enough. Similarly a Paramecium doubling every day by binary fission would reach a mind-boggling number in 64 days (if food and space were unlimited).

Doubling time and r: if a population doubles in time T, then 2 = e^(rT), so r = ln 2 ÷ T ≈ 0.693 ÷ T.

Worked example

A population growing exponentially doubles in size every 5 years. Find its intrinsic rate of increase, r, and the time it takes to become 8 times its present size. (2 marks)

Try it first, then show the solutionSolution

1. Nₜ = N₀e^(rt); doubling in 5 years gives e^(5r) = 2, so 5r = ln 2 = 0.693 and r = 0.693 ÷ 5 ≈ 0.139 per year. Check: e^(0.139 × 5) = e^(0.693) ≈ 2. (1)
2. 8 times = 2³, i.e. three doubling times: 3 × 5 = 15 years. Check: e^(0.1386 × 15) = e^(2.079) ≈ 8. (1)

Answer: r ≈ 0.139 per year; 15 years to grow eightfold.

Logistic growth and carrying capacity

NCERT section 11.1.2 Population Growth (logistic growth)

Reality: no population has unlimited resources. Limited resources cause competition among individuals; the 'fittest' survive and reproduce. (Many governments have likewise put restraints on human population growth.)

Carrying capacity (K): the maximum number a habitat's resources can support, beyond which no further growth is possible.

Phases of logistic growth (plot of N against time is sigmoid, S-shaped):
1. Lag phase: slow start.
2. Acceleration: rapid growth.
3. Deceleration: growth slows as resources become limiting.
4. Asymptote: density levels off at K.
This is Verhulst–Pearl logistic growth.

Equation: dN/dt = rN × (K − N)/K

• When N is small compared with K, (K − N)/K ≈ 1, so growth is almost exponential.
• As N approaches K, (K − N)/K approaches 0, so growth stops.
• Growth rate dN/dt is highest at N = K/2.

Why it is more realistic: resources for most animal populations are finite and become limiting sooner or later.

Exponential versus logistic:

• Resources: unlimited versus limited.
• Curve: J-shaped versus S-shaped (sigmoid).
• Limit: none versus carrying capacity K.
• Equation: dN/dt = rN versus dN/dt = rN(K − N)/K.

Worked example

A population has r = 0.5 per year and carrying capacity K = 1000. Calculate dN/dt when N = 200 and when N = 900. Explain the difference. (3 marks)

Try it first, then show the solutionSolution

1. N = 200: dN/dt = 0.5 × 200 × (1000 − 200)/1000 = 100 × 0.8 = 80 per year. (1)
2. N = 900: dN/dt = 0.5 × 900 × (100/1000) = 450 × 0.1 = 45 per year. Check: rN alone would be 100 and 450; the factor (K − N)/K cuts them to 80 and 45. (1)
3. Near K, resources are almost used up, so the brake (K − N)/K is strong; at 200 the population is far from K and grows nearly exponentially. (1)

Answer: 80 per year at N = 200; 45 per year at N = 900; growth slows as N approaches K.

Life history variation

NCERT section 11.1.3 Life History Variation

Populations evolve to maximise their reproductive fitness, also called Darwinian fitness (a high r value), in their habitat. Under a particular set of selection pressures, organisms evolve the most efficient reproductive strategy.

Variation in strategies:

• Breed once in a lifetime: Pacific salmon fish, bamboo.
• Breed many times: most birds and mammals.
• Many small offspring: oysters, pelagic fishes.
• Few large offspring: birds, mammals.

Which is best? Ecologists suggest life history traits evolve to suit the limits set by the living and non-living parts of each habitat. This is an active area of research.

Worked example

Give one example each of an organism that breeds only once in its lifetime and one that produces a large number of small offspring. What do both strategies aim to maximise? (2 marks)

Try it first, then show the solutionSolution

1. Breeds once: Pacific salmon (or bamboo). Many small offspring: oysters (or pelagic fishes). (1)
2. Both aim to maximise reproductive (Darwinian) fitness, a high r, under the constraints of their habitat. (1)

Answer: Pacific salmon/bamboo; oysters/pelagic fishes; maximising Darwinian fitness.

Population interactions: predation

NCERT section 11.1.4 Population Interactions (table; predation)

No habitat holds just one species; even a plant needs soil microbes (to return nutrients) and animals (for pollination). Interspecific interactions can be beneficial (+), detrimental (−) or neutral (0):

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

Predation, parasitism and commensalism share one feature: the species live closely together.

Predation: nature's way of transferring energy fixed by plants to higher trophic levels. A sparrow eating seeds is a predator too; herbivores are, broadly, predators of plants.

Roles of predators:
1. Conduits for energy across trophic levels.
2. Keep prey populations in check; without predators, prey may reach very high densities and destabilise ecosystems. Exotic species become invasive where their natural predators are absent: the prickly pear cactus introduced into Australia in the early 1920s spread over millions of hectares of rangeland and was controlled only after a cactus-feeding moth from its native habitat was introduced. Biological pest control is based on this predator–prey regulation.
3. Maintain species diversity by reducing competition among prey. When the starfish Pisaster was removed from an enclosed rocky intertidal area of the American Pacific coast, more than 10 invertebrate species became extinct within a year because of interspecific competition.
4. Predators are 'prudent': an over-efficient predator could wipe out its prey and then itself.

Prey defences:

• Camouflage (cryptic colouring) in 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 against herbivores (nearly 25% of insects are phytophagous; plants cannot run away):

• Morphological: thorns (Acacia, cactus).
• Chemical: chemicals that sicken herbivores, inhibit feeding or digestion, disrupt reproduction or kill. Calotropis produces highly poisonous cardiac glycosides, so cattle and goats never browse it. Nicotine, caffeine, quinine, strychnine, opium are actually plant defences against grazers and browsers.

Worked example

Explain with the Pisaster experiment how a predator helps maintain species diversity. (2 marks)

Try it first, then show the solutionSolution

1. In rocky intertidal communities of the American Pacific coast, the starfish Pisaster preys on many invertebrates, reducing the intensity of competition among them. (1)
2. When all Pisaster were removed from an enclosed area, more than 10 invertebrate species became extinct within a year because of interspecific competition: so the predator maintained diversity. (1)

Answer: By keeping competing prey in check; removing Pisaster led to the extinction of over 10 prey species.

Competition

NCERT section 11.1.4 Population Interactions (competition)

Darwin saw interspecific competition as a potent force in evolution ('struggle for existence', 'survival of the fittest').

Common belief and its corrections:
1. Competition is not only between closely related species: unrelated species can compete. In some shallow South American lakes, visiting flamingoes and resident fishes compete for zooplankton.
2. Resources need not be limiting: in interference competition, one species' feeding efficiency is reduced by the interfering, inhibitory presence of another, even when food and space are plentiful.

Best definition: competition is a process in which the fitness of one species (measured by r) is significantly lower in the presence of another species.

Evidence:

• Gause and others showed in the laboratory that with limited resources the superior competitor eventually eliminates the other; evidence in nature is less conclusive.
• The Abingdon tortoise of the Galapagos became extinct within a decade after goats were introduced, apparently because goats browsed more efficiently.
• Competitive release: a species restricted to a small area by a superior competitor expands dramatically when the competitor is removed. Connell's experiments on the rocky coasts of Scotland: the larger, superior barnacle Balanus dominates the intertidal zone and excludes the smaller Chthamalus (spelt Chathamalus in NCERT) from it.
• Herbivores and plants seem more affected by competition than carnivores.

Gause's Competitive Exclusion Principle: two closely related species competing for the same resources cannot co-exist indefinitely; the inferior one is eventually eliminated. This may hold when resources are limiting, but not otherwise.

Resource partitioning: species may evolve ways to co-exist, e.g. feeding at different times or using different foraging patterns. MacArthur showed that five closely related species of warblers on the same tree co-existed because of behavioural differences in foraging.

Worked example

State Gause's competitive exclusion principle. How do MacArthur's warblers show that it does not always hold? (3 marks)

Try it first, then show the solutionSolution

1. Two closely related species competing for the same resources cannot co-exist indefinitely; the competitively inferior one is eventually eliminated. (1)
2. This may hold only when resources are limiting; species may instead evolve ways to co-exist, such as resource partitioning. (1)
3. MacArthur found five closely related warbler species living on the same tree, avoiding competition by differences in their foraging behaviour. (1)

Answer: Inferior competitor eliminated; but warblers co-exist by partitioning resources through different foraging.

Parasitism, commensalism and mutualism

NCERT section 11.1.4 Population Interactions (parasitism, commensalism, mutualism)

Parasitism (+ −): free lodging and meals, so it has evolved in many groups.

• Many parasites are host-specific, and host and parasite co-evolve.
• Adaptations: loss of unnecessary sense organs, adhesive organs or suckers, loss of the digestive system, high reproductive capacity.
• Complex life cycles with intermediate hosts or vectors: the human liver fluke (a trematode) needs two intermediate hosts, a snail and a fish; the malarial parasite needs a mosquito vector.
• Most parasites harm the host: reduce survival, growth, reproduction and population density, and make it more vulnerable to predation.
• Ectoparasites feed on the outside: lice on humans, ticks on dogs, copepods on marine fish, and Cuscuta, a plant parasite on hedge plants that has lost its chlorophyll and leaves. (The female mosquito is not considered a parasite though it needs our blood for reproduction: it does not live on or in us.)
• Endoparasites live inside (liver, kidney, lungs, RBCs); they have more complex life cycles, simplified bodies and high reproductive potential.
• Brood parasitism: the parasitic bird lays eggs in the host's nest for the host to incubate; its eggs have evolved to resemble the host's in size and colour. Example: cuckoo (koel) and crow, in spring to summer.

Commensalism (+ 0):

• Orchid growing as an epiphyte on a mango branch.
• Barnacles on the back of a whale.
• Cattle egret and grazing cattle: cattle stir up insects that egrets catch.
• Clown fish among the stinging tentacles of a sea anemone: the fish gets protection; the anemone gets no apparent benefit.

Mutualism (+ +):

• Lichens: fungus + photosynthesising algae or cyanobacteria.
• Mycorrhizae: fungi help roots absorb nutrients; the plant gives carbohydrates.
• Plant–animal mutualism: plants pay pollinators with pollen and nectar, and seed dispersers with fruits; systems must guard against 'cheaters' (nectar thieves), so partners often co-evolve.
• Fig and wasp: a one-to-one relationship; a fig species is pollinated only by its partner wasp. The female wasp uses the fruit as an egg-laying site and its developing seeds to feed larvae, pollinating the fig while searching for egg-laying sites.
• Orchids: many floral patterns attract the right pollinators (bees, bumblebees). The Mediterranean orchid Ophrys uses 'sexual deceit': one petal resembles a female bee; males pseudocopulate with the flower and carry pollen to another flower. If the female bee's colour patterns change, the orchid must co-evolve to keep pollinated.

Amensalism (− 0): one species is harmed, the other unaffected (named in NCERT's table).

Worked example

Name the interaction and justify: (a) orchid on a mango branch, (b) cuckoo laying eggs in a crow's nest, (c) fig and fig wasp, (d) Cuscuta on a hedge plant. (2 marks)

Try it first, then show the solutionSolution

1. (a) Commensalism: the orchid gains a place to grow; the mango is unaffected. (b) Brood parasitism: the crow raises the cuckoo's young at its own cost. (1)
2. (c) Mutualism: the wasp pollinates the fig; the fig provides an egg-laying site and seeds for larvae. (d) Parasitism (ectoparasite): Cuscuta draws nutrition from the host. (1)

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

Organisms and Populations in real life

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

Topics in this chapter

• Population Growth: Exponential and Logistic
• Predation, Parasitism and Mutualism
• Competition and Competitive Exclusion

Organisms and Populations: key facts

Per capita rates

$\text{birth rate}=\frac{\text{births}}{N};\text{death rate}=\frac{\text{deaths}}{N}$

Lotus: 8/20 = 0.4; fruit flies: 4/40 = 0.1.

Change in population density

$N_{t+1}=N_t+[(B+I)-(D+E)]$

B births, I immigrants, D deaths, E emigrants.

Exponential growth

$\frac{dN}{dt}=rN;N_t=N_0{e}^{rt};r=b-d$

J-shaped curve; e = 2.71828.

Doubling time

$r=\frac{\ln 2}{T}\approx \frac{0.693}{T}$

T = doubling time.

Logistic growth

$\frac{dN}{dt}=rN\left(\frac{K-N}{K}\right)$

S-shaped curve; growth fastest at N = K/2; stops at N = K.

Interactions

$(+,+)\text{mutualism};(-,-)\text{competition};(+,-)\text{predation, parasitism};(+,0)\text{commensalism};(-,0)\text{amensalism}$

Predation, parasitism and commensalism involve close living.

Organisms and Populations: diagrams to practise

Practise each one on paper: draw it from the checklist, then tick off every label.

How the board asks Organisms and Populations

The question patterns to expect, and the method that works for each.

Tricks to remember Organisms and Populations

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

• B-I up, D-E down

  Births and Immigration add; Deaths and Emigration subtract: Nₜ₊₁ = Nₜ + (B + I) − (D + E).

  Use it when: Population change numericals.

• J for 'Jungle with no limits', S for 'Stops at K'

  Exponential = J-shaped, unlimited resources; logistic = S-shaped, levels off at carrying capacity K.

  Use it when: Comparing growth curves.

• Pyramid shapes: Triangle, Bell, Urn

  Broad base = expanding; bell = stable; urn (narrow base) = declining.

  Use it when: Age pyramid questions.

• Signs table: 'MC-PP-CA'

  Mutualism ++, Competition −−, Predation +−, Parasitism +−, Commensalism +0, Amensalism −0.

  Use it when: Interaction classification.

• Barnacles: 'Big Balanus Bullies'

  Larger Balanus excludes smaller Chthamalus in Connell's Scottish coast study.

  Use it when: Competition examples.

• Five warblers, one tree

  MacArthur: different foraging behaviour (resource partitioning) lets them co-exist.

  Use it when: Exceptions to competitive exclusion.

Organisms and Populations: board-level solved questions

Ordered from board level to HOTS. Cover the solution, write your own, then compare step by step.

Case-study practice (Section D style)

Read the situation, then answer every part. In the board paper this is a 4-mark question split into 1 + 2 + 1 marks.

Where the marks are

How a typical answer is split into step marks. Show every step: a right final answer alone rarely earns full marks.

Where students lose marks in Organisms and Populations

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

• ✗Mistake: Using total numbers instead of per capita rates

  Wrong working

  8 lotus plants were born, so the birth rate is 8.

  Right working

  Birth rate = 8/20 = 0.4 offspring per lotus per year.

  ✓Do this: Divide by the population size and give units (per individual per unit time).

  Usually costs: 1 mark.

• ✗Mistake: Writing the logistic equation without the (K − N)/K factor or swapping K and N

  ✓Do this: dN/dt = rN(K − N)/K; check that growth becomes zero when N = K.

  Usually costs: 1 mark.

• ✗Mistake: Calling the orchid on a mango tree a parasite

  ✓Do this: The orchid is an epiphyte using the branch for support; the mango is unaffected: commensalism.

  Usually costs: 1 mark.

• ✗Mistake: Saying competition happens only between closely related species and only when resources are limiting

  ✓Do this: Unrelated species can compete (flamingoes and fishes for zooplankton), and interference competition can occur even with abundant resources.

  Usually costs: 1 mark.

• ✗Mistake: Calling the female mosquito a parasite

  ✓Do this: NCERT notes the female mosquito is not considered a parasite even though it needs blood for reproduction (it does not live on or in the host).

  Usually costs: ½ mark.

• ✗Mistake: Answering questions on abiotic factors or adaptations from this chapter

  ✓Do this: Organism and its environment, abiotic factors, responses and adaptations are not in the 2026-27 syllabus; focus on populations.

  Usually costs: Time and marks spent on unasked content.

NCERT exercise guide: Organisms and Populations

What each exercise in the NCERT chapter tests, how to approach it, and a question of the same kind solved step by step. Questions here are written in our own words.

Organisms and Populations: previous year questions

• Asked in 7 of the 8 board papers we hold (2023 to 2026), on average 3.3 marks per paper; 12 questions in all, counting OR alternatives. In 4 of 4 sample papers.

Counted from the real papers on our previous year papers page, not estimated. Average marks are over every board paper we hold, with an OR pair counted once. Chapter tags are ours.

Solved board questions from Organisms and Populations

Real questions from recent papers. Try each one first, then open our step-by-step solution with marks per step.

Every Organisms and Populations question, by year

Practice questions on Organisms and Populations

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

1. Q1 · 1 mark · MCQBoard level

   Which of the following is a (−, 0) interaction?
   (a) Commensalism (b) Amensalism (c) Competition (d) Parasitism

   Hint

   One harmed, one unaffected.

   Answer

   (b) Amensalism

2. Q2 · 1 mark · Assertion-reasonBoard level

   Assertion (A): In a logistic growth curve, population growth slows down as density approaches K.
   Reason (R): The factor (K − N)/K approaches zero as N approaches K.
   (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

   Look at the equation.

   Answer

   (a)

3. Q3 · 1 mark · MCQHarder board level

   In a population of 200 individuals, 30 are born and 20 die in a year. The per capita birth rate is:
   (a) 0.10 (b) 0.15 (c) 0.05 (d) 0.25

   Hint

   Births ÷ N.

   Answer

   (b) 0.15 (30/200)

4. Q4 · 2 marks · Very short answerBoard level

   Define population and community. Give one example of each. (2 marks)

   Hint

   One species versus many.

   Answer

   Population: individuals of one species in a defined area, sharing or competing for resources and potentially interbreeding, e.g. lotus plants in a pond (1). Community: populations of different species living and interacting in an area, e.g. the plants, animals and microbes of a pond (1).

5. Q5 · 2 marks · Very short answerHarder board level

   Why is per cent cover or biomass a better measure than number for some populations? Give an NCERT example. (2 marks)

   Hint

   Banyan and Parthenium.

   Answer

   When individuals differ greatly in size, numbers misrepresent their importance (1). One huge banyan among 200 Parthenium plants would seem insignificant by count, though its canopy dominates; cover or biomass reflects its role (1).

6. Q6 · 2 marks · Very short answerBoard level

   Explain brood parasitism with an example, and the adaptation of the parasite's eggs. (2 marks)

   Hint

   Koel and crow.

   Answer

   The parasitic bird lays eggs in the host's nest and lets the host incubate them, e.g. the cuckoo (koel) laying in a crow's nest (1). Its eggs have evolved to resemble the host's eggs in size and colour, reducing the chance of being detected and thrown out (1).

7. Q7 · 3 marks · Short answerHarder board level

   List four adaptations of parasites and explain why the human liver fluke has a complex life cycle. (3 marks)

   Hint

   Lose, cling, reproduce.

   Answer

   Loss of unnecessary sense organs; adhesive organs or suckers; loss of the digestive system; high reproductive capacity (2). The liver fluke depends on two intermediate hosts, a snail and a fish, to reach and infect its primary host (1).

8. Q8 · 3 marks · Short answerHarder board level

   Give three pieces of evidence that competition occurs in nature. (3 marks)

   Hint

   Tortoise, barnacles, flamingoes.

   Answer

   The Abingdon tortoise of the Galapagos went extinct within a decade of goats being introduced, apparently because goats browsed more efficiently (1). Competitive release: Connell showed the superior barnacle Balanus excludes Chthamalus on Scottish rocky coasts, and Chthamalus expands when Balanus is removed (1). Unrelated flamingoes and fishes compete for zooplankton in South American lakes (1).

9. Q9 · 5 marks · Long answerHarder board level

   (a) Draw the logistic growth curve and label its phases. (2)
   (b) Write its equation and explain each term. (2)
   (c) Why is it more realistic than exponential growth? (1) (5 marks)

   Hint

   S-curve with K.

   Answer

   (a) S-shaped curve of N against time with lag phase, acceleration, deceleration and asymptote at the dashed K line (2). (b) dN/dt = rN(K − N)/K, where N is population density at time t, r the intrinsic rate of natural increase, and K the carrying capacity; (K − N)/K is the fraction of resources still available (2). (c) Resources for most populations are finite and become limiting, so growth must level off (1).

10. Q10 · 3 marks · Short answerHOTS / competency

    Suppose a lake has a population of 2000 fish with r = 0.1 per year and K = 10,000. A fishing company wants to harvest the maximum sustainable number each year. At about what population size should the fish be kept, and roughly how many can be harvested yearly at that size? (3 marks)

    Hint

    Logistic growth peaks at K/2.

    Answer

    Growth dN/dt = rN(K − N)/K is greatest at N = K/2 = 5000 (1). At N = 5000: dN/dt = 0.1 × 5000 × (5000/10,000) = 500 × 0.5 = 250 fish per year (1). Harvesting about 250 per year while holding the population near 5000 keeps it stable; check at N = 4000: 0.1 × 4000 × 0.6 = 240 and N = 6000: 0.1 × 6000 × 0.4 = 240, both lower (1).

11. Q11 · 2 marks · Very short answerHOTS / competency

    A newly formed island is colonised by a species of lizard. In the first few years, which process contributes most to population growth, and why? (2 marks)

    Hint

    Special conditions.

    Answer

    Immigration (1). When a new habitat is being colonised, arrivals from elsewhere may add more individuals than births, which matter more only once a breeding population is established (1).

12. Q12 · 2 marks · Very short answerHarder board level

    Explain with an example why predators are described as 'prudent'. (2 marks)

    Hint

    Over-exploitation.

    Answer

    If a predator were too efficient and over-exploited its prey, the prey could become extinct, and then the predator would also die out for lack of food (1). So natural predators do not wipe out their prey, keeping both populations going (e.g. predators regulating prey without eliminating them) (1).

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. 1.Name four attributes of a population not found in an individual.Show answer

   Birth rate, death rate, sex ratio and age distribution.

2. 2.What is the r value of the Norway rat and the flour beetle?Show answer

   0.015 and 0.12 respectively.

3. 3.What was India's r value in 1981?Show answer

   0.0205.

4. 4.What is the shape of an exponential growth curve?Show answer

   J-shaped.

5. 5.What is carrying capacity?Show answer

   The maximum population size a habitat's resources can support (K).

6. 6.What is Darwinian fitness?Show answer

   Reproductive fitness, a high r value.

7. 7.Which organisms breed only once in their lifetime?Show answer

   Pacific salmon fish and bamboo.

8. 8.What percentage of insects are phytophagous?Show answer

   Nearly 25%.

9. 9.Which poisonous chemical does Calotropis produce?Show answer

   Cardiac glycosides.

10. 10.Why did the Abingdon tortoise become extinct?Show answer

    Goats introduced to the Galapagos browsed more efficiently and out-competed it within a decade.

11. 11.Name the two intermediate hosts of the human liver fluke.Show answer

    A snail and a fish.

12. 12.What is 'sexual deceit' in Ophrys?Show answer

    One petal resembles a female bee, so male bees pseudocopulate with the flower and carry pollen between flowers.

Quick revision checklist

The night before the exam: can you do each of these without looking?

• ☐Can you calculate per capita birth and death rates?
• ☐Can you identify expanding, stable and declining age pyramids?
• ☐Can you choose the right measure of density for a banyan tree, fish in a lake and tigers?
• ☐Can you use Nₜ₊₁ = Nₜ + [(B + I) − (D + E)]?
• ☐Can you derive r from a doubling time and use Nₜ = N₀e^(rt)?
• ☐Can you draw J- and S-shaped curves and calculate logistic dN/dt?
• ☐Can you name the six interactions with signs and an example each?
• ☐Can you explain Gause's principle, competitive release, resource partitioning and plant defences?

Organisms and Populations: frequently asked questions