Home/Biology/Chapter 11 — Organisms and Populations
Class 12 Biology

Organisms and Populations

Population attributes, age pyramids, exponential and logistic growth, life history variation and population interactions — with SVG diagrams, a growth-curve simulator, MCQs, A-R and PYQs.

4NCERT Sub-sections
12MCQs
6Assertion–Reason
5PYQs
Section 01

Population Attributes

11.0Ecology and Levels of Organisation

Ramdeo Misra (1908–1998) — 'Father of Ecology in India'; BHU, Varanasi; studied tropical communities, succession, productivity and nutrient cycling; his efforts led to the National Committee for Environmental Planning and Coordination (1972) and later the Ministry of Environment and Forests (1984).
  • Ecology studies interactions among organisms and between organisms and their physical (abiotic) environment. It is concerned with four levels: organisms, populations, communities and biomes.
  • 'How-type' questions seek the mechanism; 'why-type' questions seek the significance (e.g. how a bird sings vs why it sings).

11.1.1What is a Population?

Population

A group of individuals of a species living in a well-defined geographical area, sharing or competing for similar resources and potentially interbreeding (even asexually reproducing groups count). Examples: cormorants in a wetland, teak trees in a forest, bacteria in a culture plate, lotus plants in a pond.

Natural selection operates at the population level, so population ecology links ecology with population genetics and evolution. A population has attributes that an individual lacks:

  • Birth rate and death rate (per capita). Lotus: 20 plants + 8 new → birth rate = 8/20 = 0.4 offspring per lotus per year. Fruit flies: 4 of 40 die in a week → death rate = 4/40 = 0.1 per fly per week.
  • Sex ratio — e.g. 60% females : 40% males.
  • Age distribution — plotted as an age pyramid; its shape shows whether the population is growing, stable or declining.
  • Population size (density, N) — can be numbers, per cent cover or biomass, or a relative density (fish per trap); tiger census uses pug marks and faecal pellets (indirect estimates).
V1 · Age pyramids — growing, stable and declining populations Expanding (growing) broad base — many young ♂ Male Female ♀ Stable similar numbers across ages ♂ Male Female ♀ Declining narrow base — few young ♂ Male Female ♀ Pre-reproductive Reproductive Post-reproductive
Remember: Siberian cranes at Bharatpur may be <10 in a year; Chlamydomonas in a pond may be millions. A single huge banyan vs 200 Parthenium plants — here per cent cover/biomass is a better measure than numbers.
Section 02

Population Growth

11.1.2Four Basic Processes

  • Natality (B) — births added; Mortality (D) — deaths; Immigration (I) — individuals entering from elsewhere; Emigration (E) — individuals leaving. Natality + immigration ↑ density; mortality + emigration ↓ density.
  • Density at time t + 1: Nt+1 = Nt + [(B + I) − (D + E)]. Normally births and deaths matter most; immigration matters when a new habitat is being colonised.
V2 · What changes population density Population density N t Natality (B) — births Immigration (I) Mortality (D) — deaths Emigration (E) N t+1 = N t + [ (B + I) − (D + E) ]

11.1.2aExponential Growth

  • With unlimited resources, growth is exponential (geometric). With per-capita birth rate b and death rate d: dN/dt = (b − d) × N = rN, where r = intrinsic rate of natural increase.
  • Integral form: Nt = N0 ert (e = 2.71828). Plot of N vs t = J-shaped curve.
  • Values of r: Norway rat 0.015, flour beetle 0.12, human population of India (1981) 0.0205.
  • Chess-board anecdote: doubling wheat grains on each of 64 squares exceeds a kingdom's total wheat — as would one Paramecium doubling daily for 64 days with unlimited food and space.

11.1.2bLogistic Growth

  • In nature resources are limited → competition. A habitat has a maximum carrying capacity (K) for a species.
  • Growth shows a lag phase, then acceleration and deceleration, finally an asymptote at K → sigmoid (S-shaped) curve: Verhulst–Pearl logistic growth.
  • dN/dt = rN [(K − N)/K]. It is considered more realistic since resources are finite.
V3 · Exponential (J) and logistic (S) growth K (carrying capacity) a · Exponential (J-shaped) dN/dt = rN · resources unlimited b · Logistic (S-shaped) dN/dt = rN (K−N)/K Lag phase Acceleration Deceleration Asymptote Time (t) → Population density (N)
Try it · growth-curve simulator

Change the values and compare exponential and logistic growth.

Section 03

Life History Variation

11.1.3Reproductive Strategies

Populations evolve to maximise reproductive (Darwinian) fitness — a high r — in their habitat. Life-history traits evolve under constraints of abiotic and biotic components of the habitat.

Strategy Examples
Breed only once in a lifetime Pacific salmon fish, bamboo
Breed many times Most birds and mammals
Large number of small offspring Oysters, pelagic fishes
Small number of large offspring Birds, mammals
Section 04

Population Interactions

11.1.4Interspecific Interactions

No habitat has just one species — even a plant needs soil microbes to recycle nutrients and animals for pollination. Interactions between two species may be beneficial (+), detrimental (−) or neutral (0).

V4 · Outcomes of interspecific interactions (Species A / Species B) Interaction Species A Species B What happens / example Mutualism + + Lichens · fig–wasp · mycorrhiza Competition − − Flamingoes and fishes · Balanus vs Chthamalus Predation + − Predator–prey: Pisaster, tiger–deer Parasitism + − Cuscuta, liver fluke, cuckoo (brood parasite) Commensalism + 0 Orchid on mango · cattle egret · clown fish Amensalism − 0 One species harmed, the other unaffected
  • Predation, parasitism and commensalism — the interacting species live closely together. In mutualism both benefit; in competition both lose.
Section 04 (contd.)

Predation

(i)Predation and Defences

  • Predation transfers energy fixed by plants to higher trophic levels ('conduits'). A sparrow eating seed is a predator too; herbivores are broadly similar.
  • Roles: keep prey populations under control; unchecked exotic species become invasive — the prickly pear cactus in Australia (early 1920s) spread over millions of hectares until a cactus-feeding moth was introduced. Biological control of pests relies on this.
  • Predators maintain species diversity by reducing competition among prey: when the starfish Pisaster was removed from a rocky intertidal area of the American Pacific coast, over 10 invertebrate species became extinct within a year.
  • An over-efficient predator may wipe out its prey and itself — so predators are 'prudent'.
  • Prey defences: camouflage (cryptic colouration of some insects and frogs); poisonous/distasteful species — Monarch butterfly acquires a chemical as a caterpillar from a poisonous weed.
  • Plant defences (~25% of insects are phytophagous): thorns (Acacia, cactus); chemicals — Calotropis makes poisonous cardiac glycosides; nicotine, caffeine, quinine, strychnine, opium are plant defences against grazers.
Section 04 (contd.)

Competition

(ii)Competition

Definition

A process in which the fitness of one species (its r) is significantly lower in the presence of another species.

  • Not only closely related species compete: flamingoes and resident fishes in South American lakes compete for zooplankton. Resources need not be limiting — in interference competition one species' feeding efficiency is reduced by the other's presence.
  • Gause's Competitive Exclusion Principle: two closely related species competing for the same limiting resources cannot co-exist indefinitely; the inferior is eliminated.
  • Evidence: Abingdon tortoise (Galapagos) became extinct within a decade after goats were introduced. Competitive release: a restricted species expands its range when the superior competitor is removed — Connell's experiment on Scottish coasts: larger Balanus excludes smaller Chthamalus.
  • Resource partitioning promotes co-existence: MacArthur — five closely related warblers on the same tree avoid competition through behavioural differences in foraging.
V5 · Competitive release (Connell's barnacle experiment) remove Balanus Balanus present (superior competitor) Competitive release large Balanus dominates; small Chthamalus restricted Chthamalus expands its range dramatically
Section 04 (contd.)

Parasitism and Commensalism

(iii)Parasitism

  • Many parasites are host-specific and co-evolve with the host. Adaptations: loss of unnecessary sense organs, adhesive organs/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.
  • Ectoparasites feed on the external surface: lice on humans, ticks on dogs, copepods on marine fish; Cuscuta (parasitic plant that lost chlorophyll and leaves). Endoparasites live inside the host (liver, kidney, lungs, RBCs).
  • Brood parasitism: the parasitic bird lays eggs in the host's nest; the eggs resemble the host's eggs (e.g. cuckoo/koel and crow).
  • The female mosquito is not usually considered a parasite though it needs blood for reproduction.

(iv)Commensalism

  • One species benefits; the other is neither harmed nor benefited. Examples: orchid (epiphyte) on a mango branch; barnacles on a whale's back; cattle egret and grazing cattle (cattle flush out insects); clown fish and sea anemone (fish gets protection among stinging tentacles).
Section 04 (contd.)

Mutualism

(v)Mutualism

  • Lichens: fungus + photosynthesising algae or cyanobacteria. Mycorrhizae: fungi + roots of higher plants — fungi absorb nutrients; plant gives carbohydrates.
  • Plant–animal mutualism: pollen, nectar (rewards for pollinators), juicy fruits (for seed dispersers). The system must be safeguarded against 'cheaters'; leads to co-evolution.
  • Fig–wasp: each fig species is pollinated only by its partner wasp; the wasp lays eggs in the fruit and the larvae feed on developing seeds, in return for pollination.
  • Ophrys orchid (Mediterranean): a petal resembles the female bee (sexual deceit); male bee 'pseudocopulates' and carries pollen. If the female bee's pattern changes, the orchid must co-evolve to keep the resemblance.
V6 · Fig and wasp — an obligate mutualism Female wasp Fig inflorescence / fruit Pollinates the fig while searching for egg-laying sites Offers developing seeds as food for wasp larvae One fig species ↔ one wasp species → tight CO-EVOLUTION
Practice

📝 Solved Examples with Step Marking

📋 2 Marks

Q1. If a population growing exponentially doubles in 3 years, what is its intrinsic rate of increase (r)?

Use Nt = N₀ert 1 Mark

Nt = 2N₀ when t = 3 → 2 = e3r → 3r = ln 2.

Calculate 1 Mark

r = ln 2 ÷ 3 = 0.693 ÷ 3 ≈ 0.23 per year.

📋 3 Marks

Q2. A pond has 20 lotus plants. During the year 8 new plants are added and 2 die. Find the birth rate, death rate and new population size (no migration).

Birth rate 1 Mark

8 ÷ 20 = 0.4 per lotus per year.

Death rate 1 Mark

2 ÷ 20 = 0.1 per lotus per year.

New size 1 Mark

Nt+1 = 20 + [(8 + 0) − (2 + 0)] = 26.

📋 3 Marks

Q3. Name the interaction in each: (a) orchid on a mango branch, (b) cuckoo laying eggs in a crow's nest, (c) clown fish and sea anemone.

(a) 1 Mark

Commensalism (+, 0) — orchid benefits; mango neither harmed nor benefited.

(b) 1 Mark

Brood parasitism (a type of parasitism, +, −).

(c) 1 Mark

Commensalism — fish gets protection; anemone derives no apparent benefit.

Test Yourself

✍️ MCQ Practice

Score: 0 / 12
New Pattern

🎯 Assertion–Reason

(A) Both true, R explains A. (B) Both true, R does NOT explain A. (C) A true, R false. (D) A false, R true.

Score: 0 / 6
Exam Focus

📖 Important Board Questions

NCERT

Q. List the attributes that populations possess but individuals do not.

Answer 2 Marks

Birth rate, death rate, sex ratio, age distribution (age pyramid) and population size/density — properties of the group, not of an individual.

NCERT

Q. With the help of a diagram describe the logistic population growth curve.

Answer 3 Marks

With limited resources growth shows a lag phase, acceleration, deceleration and an asymptote at carrying capacity K — a sigmoid curve (Verhulst–Pearl): dN/dt = rN(K−N)/K. (Draw V3.)

NCERT

Q. Name important defence mechanisms in plants against herbivory.

Answer 2 Marks

Morphological: thorns (Acacia, cactus). Chemical: poisonous or distasteful substances — cardiac glycosides in Calotropis; nicotine, caffeine, quinine, strychnine, opium.

NCERT

Q. Define (a) commensalism, (b) parasitism, (c) camouflage, (d) mutualism, (e) interspecific competition — with one example each.

Answer 5 Marks

(a) +/0 — orchid on mango; (b) +/− — Cuscuta on host; (c) body colour/pattern blending with surroundings — cryptic frogs and insects; (d) +/+ — lichens, fig–wasp; (e) fitness of one species lowered by another — flamingoes and fishes for zooplankton.

NCERT

Q. What is the ecological principle behind biological control of pest insects?

Answer 2 Marks

Predation — predators regulate prey populations; introducing a natural predator/parasite of the pest keeps it under control (e.g. the moth controlling prickly pear in Australia).