NCERT Solutions for Class 12 Biology Chapter 11: Organisms and Populations
These Class 12 Biology Chapter 11 solutions cover Organisms and Populations from the NCERT textbook (session 2026–27). The chapter introduces ecology at the population level — how individuals of a species live together as populations, the attributes a population has, how populations grow (exponential and logistic models), life-history variation, and the six kinds of interactions between species. Every end-of-chapter exercise question is reproduced verbatim below and answered in exam-ready style.
Class: 12Subject: BiologyChapter: 11Name: Organisms and PopulationsUnit: EcologySession: 2026–27
Ecology studies the interactions among organisms and between an organism and its physical (abiotic) environment, working at four levels of biological organisation — organisms, populations, communities and biomes. This chapter focuses on the population level. A population is a group of individuals of one species living in a defined geographical area, sharing or competing for similar resources and potentially interbreeding. Unlike a single individual, a population has measurable attributes — birth and death rates, sex ratio and age distribution (shown as age pyramids). Population size, or density (N), changes through natality, mortality, immigration and emigration. Growth follows an exponential (J-shaped) pattern when resources are unlimited and a logistic (S-shaped/sigmoid) pattern bounded by the habitat’s carrying capacity (K) when resources are limiting. The chapter ends with six interspecific interactions — mutualism, competition, predation, parasitism, commensalism and amensalism.
Key Concepts & Formulas
Population: a group of individuals of the same species living in a well-defined geographical area, sharing or competing for resources and potentially interbreeding.
Population density (N): the size of a population in a habitat at a given time; can be expressed as numbers, biomass or per cent cover.
Four basic processes changing density: natality (births) and immigration increase it; mortality (deaths) and emigration decrease it.
NCERT Solutions – Organisms and Populations (Exercises)
All ten end-of-chapter questions are reproduced exactly as in the NCERT textbook; the answers below are original and written in CBSE exam-ready style.
1. List the attributes that populations possess but not individuals.
ANSWERA population shows several attributes that a single organism does not:(i) Birth rate (natality) — the per-capita number of births added to the population in a given period.(ii) Death rate (mortality) — the per-capita number of deaths in the population in a given period.(iii) Sex ratio — the proportion of males to females (an individual is simply male or female, but a population has a ratio).(iv) Age distribution / age pyramid — the proportion of individuals in different age groups, whose shape shows whether the population is growing, stable or declining.(v) Population density (N) — the size of the population per unit area, measured as numbers, biomass or per cent cover.
2. If a population growing exponentially double in size in 3 years, what is the intrinsic rate of increase (r) of the population?
SOLUTIONFor exponential growth, Nt = N0 ert.If the population doubles, Nt = 2N0 and t = 3 years.So 2N0 = N0 e3r ⇒ 2 = e3r.Taking natural logarithm: ln 2 = 3r ⇒ 0.693 = 3r.r = 0.693 / 3 = 0.231 per year (intrinsic rate of natural increase).
3. Name important defence mechanisms in plants against herbivory.
ANSWERBecause plants cannot run from herbivores, they have evolved morphological and chemical defences:Morphological: thorns (as in Acacia and Cactus), spines, sharp-edged or hairy leaves that physically discourage grazing.Chemical: production and storage of toxic or deterrent substances that make the herbivore sick, inhibit feeding or digestion, disrupt reproduction or even kill it — for example the cardiac glycosides of Calotropis, and compounds such as nicotine, caffeine, quinine, strychnine and opium, which plants make as defences against grazers and browsers.
4. An orchid plant is growing on the branch of mango tree. How do you describe this interaction between the orchid and the mango tree?
ANSWERThis is an example of commensalism (+ 0 interaction). The orchid grows as an epiphyte on the mango branch and is benefited — it gets support, space and better access to sunlight. The mango tree is neither harmed nor benefited. Since one partner gains and the other is unaffected, the relationship is commensalism.
5. What is the ecological principle behind the biological control method of managing with pest insects?
ANSWERThe principle is predation — the ability of a predator to regulate (keep under control) the population of its prey. In biological control, a natural predator (or parasite/pathogen) of the pest is introduced so that it preys upon the pest and keeps its numbers below the level at which it causes damage.A classic illustration is the control of the invasive prickly-pear cactus in Australia by a cactus-feeding moth (Cactoblastis) brought from the cactus’s natural habitat. This avoids harmful chemical pesticides.
6. Define population and community.
ANSWERPopulation: a group of individuals of the same species living in a well-defined geographical area at a given time, sharing or competing for similar resources and potentially interbreeding (e.g. all the lotus plants in a pond).Community: an assemblage of populations of different species living together and interacting in the same habitat or area (e.g. all the plants, animals and microbes of a pond together form a community).
7. Define the following terms and give one example for each:
(a) Commensalism(b) Parasitism(c) Camouflage(d) Mutualism(e) Interspecific competition
ANSWER(a) Commensalism (+ 0): an interaction in which one species is benefited and the other is neither harmed nor benefited. Example: the cattle egret foraging beside grazing cattle (the egret gains insects; the cattle are unaffected). Also: barnacles on a whale’s back.(b) Parasitism (+ −): an interaction in which one organism (the parasite) lives on or in another (the host), obtains food and shelter from it, and harms it. Example:Cuscuta (a parasitic plant) growing on a host plant; ticks on dogs.(c) Camouflage: a defence in which prey are cryptically coloured or shaped to blend with their surroundings so that predators cannot detect them easily. Example: many insects and frogs that match the colour of leaves or bark.(d) Mutualism (+ +): an interaction that benefits both interacting species. Example: lichen, an association of a fungus and a photosynthesising alga/cyanobacterium; also mycorrhizae and the fig–wasp relationship.(e) Interspecific competition (− −): competition between individuals of two different species for the same limiting resource, lowering the fitness of at least one. Example: visiting flamingoes and resident fishes competing for zooplankton in shallow South-American lakes.
8. With the help of suitable diagram describe the logistic population growth curve.
ANSWERIn nature no population has unlimited resources, so growth eventually becomes limited. When a population grows in a habitat with limited resources, it follows logistic (Verhulst–Pearl) growth, described by:
dN/dt = rN × (K − N)/K
DIAGRAM (described)When N is plotted against time (t), the curve is S-shaped (sigmoid) and passes through these phases:(i) Lag phase: growth is slow at first as the small population establishes.(ii) Acceleration (log/exponential) phase: numbers rise steeply as resources are still ample.(iii) Deceleration phase: growth slows as resources become limiting and competition increases.(iv) Asymptote (stationary phase): the population density levels off at the carrying capacity (K) — the maximum number the habitat can support — where dN/dt approaches zero.The sigmoid curve therefore rises gently, then steeply, then flattens along a horizontal line at K. Because resources for most populations are finite, the logistic model is considered the more realistic one. (In the exam, sketch the S-shaped curve with the X-axis = time, Y-axis = population density N, and a dashed horizontal line marked K.)
9. Select the statement which explains best parasitism.
(a) One organism is benefited.(b) Both the organisms are benefited.(c) One organism is benefited, other is not affected.(d) One organism is benefited, other is affected.
ANSWER(d) One organism is benefited, other is affected.In parasitism the parasite is benefited (it gets food and shelter) while the host is harmed (its survival, growth and reproduction are reduced). Option (a) is incomplete, (b) describes mutualism and (c) describes commensalism.
10. List any three important characteristics of a population and explain.
ANSWER(i) Population density (N): the number (or biomass/per cent cover) of individuals per unit area or volume at a given time. It indicates the population’s status in the habitat and changes with births, deaths, immigration and emigration.(ii) Natality and mortality (birth and death rates): natality is the per-capita number of births added in a period; mortality is the per-capita number of deaths. Together they are the chief factors that decide whether a population grows or declines.(iii) Age distribution (age pyramid): the proportion of individuals in different age groups. Plotted as an age pyramid, its shape reveals whether the population is growing (broad base), stable or declining (narrow base). (Sex ratio is another valid characteristic.)
Extra Practice Questions
Short Answer Type Questions
Q1. Distinguish between exponential and logistic growth.
ANSWERExponential growth occurs when resources are unlimited; it gives a J-shaped curve and is described by dN/dt = rN. Logistic growth occurs when resources are limited; it gives an S-shaped (sigmoid) curve bounded by the carrying capacity K and is described by dN/dt = rN(K−N)/K. Logistic growth is the more realistic model in nature.
Q2. What is carrying capacity (K)?
ANSWERCarrying capacity is the maximum population size that a particular habitat can support with its available resources. Beyond K no further growth is possible, so in logistic growth the population density levels off (asymptote) at K.
Q3. Why is the female mosquito not considered a parasite even though it needs blood?
ANSWERThe female mosquito only takes a brief blood meal for egg development; it does not live on or in the host, does not depend on a single host for its lodging and survival, and does not establish a continuous, harmful association. Hence it is not a true parasite.
Q4. State Gause’s Competitive Exclusion Principle.
ANSWERIt states that two closely related species competing for the same limiting resources cannot co-exist indefinitely, and the competitively inferior species is eventually eliminated. This holds when resources are limiting; otherwise species may evolve mechanisms such as resource partitioning to co-exist.
Q5. What is resource partitioning? Give an example.
ANSWERResource partitioning is a mechanism by which competing species avoid competition by using the same resource at different times or in different ways. MacArthur showed that five closely related warbler species on the same tree co-existed by foraging in different parts of the tree at different times.
Long Answer Type Questions
Q1. Describe predation and explain its ecological roles.
ANSWERPredation is a (+ −) interaction in which one organism (predator) kills and feeds on another (prey); herbivores feeding on plants are, in the broad ecological sense, also predators. Predation transfers the energy fixed by plants to higher trophic levels, acting as a conduit for energy flow. Predators keep prey populations under control — without them, prey could reach densities that destabilise the ecosystem (e.g. invasive prickly-pear cactus in Australia, controlled by an introduced moth). Predators also maintain species diversity by reducing the intensity of competition among prey species; removing the starfish Pisaster from an intertidal area led to the extinction of more than ten invertebrate species through competition. However, ‘prudent’ predators avoid over-exploiting prey, since wiping out the prey would also doom the predator. Prey, in turn, evolve defences such as camouflage, distastefulness and toxicity (e.g. the Monarch butterfly).
Q2. Explain the various types of population interactions with examples.
ANSWERMutualism (+ +): both species benefit, e.g. lichens (fungus + alga), mycorrhizae, and the fig–wasp partnership. Competition (− −): both species suffer when fitness is lowered by another’s presence, e.g. flamingoes and fishes competing for zooplankton. Predation (+ −): predator benefits, prey is harmed, e.g. tiger and deer. Parasitism (+ −): parasite benefits, host is harmed, e.g. Cuscuta on a host plant or the liver fluke. Commensalism (+ 0): one benefits, the other is unaffected, e.g. orchid on a mango branch, cattle egret with grazing cattle. Amensalism (− 0): one species is harmed while the other is unaffected. These interactions may be beneficial, detrimental or neutral and shape the structure of biological communities.
Q3. What are the four basic processes that change population density? Derive the relation for density at time t+1.
ANSWERPopulation density fluctuates due to four processes: natality (B) and immigration (I), which add individuals, and mortality (D) and emigration (E), which remove them. If Nt is the density at time t, then the density at time t+1 is Nt+1 = Nt + [(B + I) − (D + E)]. The population grows when (B + I) exceeds (D + E) and declines when the reverse is true. Under normal conditions births and deaths are the most important factors; immigration and emigration become significant only in special situations, such as when a new habitat is being colonised.
MCQs & Answer Key
1. Which of the following is an attribute of a population but not of an individual?
(a) birth (b) death (c) sex ratio (d) age
2. The integral form of the exponential growth equation is:
(a) Nt = N0 ert (b) dN/dt = rN (c) dN/dt = rN(K−N)/K (d) Nt+1 = Nt + B
3. The logistic growth curve is:
(a) J-shaped (b) S-shaped (sigmoid) (c) a straight line (d) U-shaped
4. The maximum population size a habitat can support is its:
(a) biotic potential (b) intrinsic rate r (c) carrying capacity (K) (d) density N
For each Assertion–Reason question, choose: (A) Both true and the Reason correctly explains the Assertion; (B) Both true but the Reason is not the correct explanation; (C) Assertion true, Reason false; (D) Assertion false, Reason true.
A-R 1. Assertion: A population has a sex ratio but an individual does not.
Reason: An individual is either male or female, while a population has a proportion of males to females.
A-R 2. Assertion: The logistic growth model is considered more realistic than the exponential model.
Reason: Resources for most populations are finite and become limiting sooner or later.
A-R 3. Assertion: In commensalism one species benefits while the other is harmed.
Reason: In an orchid–mango interaction the mango tree is harmed by the orchid.
A-R 4. Assertion: Predators in nature tend to be ‘prudent’.
Reason: If a predator over-exploits its prey to extinction, the predator too will become extinct for lack of food.
A-R 5. Assertion: Plants have evolved morphological and chemical defences against herbivores.
Reason: Unlike animals, plants cannot run away from their predators.
Answer key: 1-(A), 2-(A), 3-(D), 4-(A), 5-(A).
Common Mistakes to Avoid
Watch out for these
Confusing population (one species) with community (many species together).
Mixing up commensalism (+ 0) with mutualism (+ +) — in commensalism only one partner gains.
Forgetting to take natural log in r-calculations — doubling time uses ln 2 = 0.693, not log 2.
Drawing the logistic curve as J-shaped — it is S-shaped (sigmoid) and flattens at K.
Calling the female mosquito a parasite — it only takes a brief blood meal and is not a true parasite.
Writing parasitism as “one benefits, other unaffected” — in parasitism the host is harmed.
How to score full marks in this chapter
Memorise the six interactions with their (+/−/0) signs and one example each — these are guaranteed marks. Always write the growth equations with correct symbols (dN/dt = rN for exponential; dN/dt = rN(K−N)/K for logistic) and label the four phases of the sigmoid curve. For numericals, use Nt = N0ert and remember ln 2 = 0.693. When asked to “describe with a diagram”, sketch the curve with axes labelled and K marked, even in a text answer state what the diagram shows.
Frequently Asked Questions
What is Class 12 Biology Chapter 11 Organisms and Populations about?
Chapter 11 explores ecology at the population level — what a population is, the attributes it possesses (birth and death rates, sex ratio, age distribution), how populations grow through exponential and logistic models bounded by carrying capacity, life-history variation, and the six interspecific interactions (mutualism, competition, predation, parasitism, commensalism, amensalism).
What is the difference between exponential and logistic growth?
Exponential growth happens when resources are unlimited, giving a J-shaped curve (dN/dt = rN). Logistic growth happens when resources are limited, giving an S-shaped curve that levels off at the carrying capacity K (dN/dt = rN(K−N)/K). Logistic growth is the more realistic model in nature.
How do you calculate the intrinsic rate of increase from doubling time?
Use Nt = N0ert. If the population doubles in time t, then 2 = ert, so r = ln 2 / t = 0.693 / t. For example, doubling in 3 years gives r = 0.693 / 3 = 0.231 per year.
Are these Class 12 Biology Chapter 11 solutions free?
Yes. All ClearStudy NCERT Solutions for Class 12 Biology are free and follow the official NCERT textbook for session 2026-27.
