Organisms and Populations – Class 12 Biology Chapter 11 Notes & NCERT PDF

Class 12BiologyChapter 11NCERT book: Biology - Class 12

Notes and a simple summary of Chapter 11 of the NCERT Class 12 Biology book, with the official chapter PDF, flashcards, an MCQ quiz and an AI tutor for your doubts.

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Chapter 11: Organisms and Populations

Introduction

This chapter opens Unit X, Ecology, which studies interactions among organisms and between organisms and their physical environment. The unit introduction honours Ramdeo Misra (1908–1998), the Father of Ecology in India, who earned his Ph.D. in ecology from Leeds (1937) and built ecology teaching and research at Banaras Hindu University; his efforts led to the National Committee for Environmental Planning and Coordination (1972) and, in turn, the Ministry of Environment and Forests (1984). In this chapter the focus is the population — its attributes, growth models, life-history strategies and the interactions between species. Expect numericals (birth rate, r), graph questions (J and S curves) and "name the interaction" questions in the board exam.

Key Concepts

11.1 Populations

A population is a group of individuals of a species living in a well-defined geographical area, sharing or competing for similar resources and potentially interbreeding. Selection operates on individuals, but evolution is seen at the population level.

11.1.1 Population Attributes. A population has attributes an individual cannot have:

Birth rate — per capita births: if 8 new lotus plants appear in a pond of 20 during a year, birth rate = 8/20 = 0.4 offspring per lotus per year.

Death rate — 4 of 40 fruit flies dying in a week gives 4/40 = 0.1 individuals per fruit fly per week.

Sex ratio — e.g. 60% females, 40% males.

Age distribution — plotted as age pyramids (pre-reproductive, reproductive, post-reproductive). The shape shows the status: expanding (triangular, broad base), stable (bell-shaped) or declining (urn-shaped, narrow base).

Population density (N) — not always counted as numbers. In an area with 200 Parthenium plants and one huge banyan tree, per cent cover or biomass is a more meaningful measure. Relative density is often enough: fish caught per trap; pug marks and faecal pellets for tigers.

11.1.2 Population Growth. Density changes by four processes: natality (B) and immigration (I) increase it; mortality (D) and emigration (E) decrease it:

N(t+1) = N(t) + [(B + I) − (D + E)].

Growth models.

(i) Exponential growth — when resources are unlimited: dN/dt = (b − d) × N; with r = b − d, dN/dt = rN. Here r is the intrinsic rate of natural increase. Examples: Norway rat r = 0.015, flour beetle r = 0.12; for India in 1981, r = 0.0205. The integrated form is N(t) = N(0) e^(rt). The plot is a J-shaped curve. The chessboard-and-wheat story and the growth of Paramecium show how fast this can be.

(ii) Logistic growth — resources are finite, so there is a carrying capacity (K) beyond which growth stops. The growth has a lag phase, acceleration, deceleration and an asymptote at K — the sigmoid (S-shaped) curve, called Verhulst–Pearl logistic growth: dN/dt = rN (K − N)/K. It is considered more realistic.

11.1.3 Life History Variation. Populations evolve to maximise reproductive fitness (Darwinian fitness, high r). Some organisms breed only once (Pacific salmon, bamboo); others breed many times (most birds and mammals). Some produce many small offspring (oysters, pelagic fishes); others produce a few large ones (birds, mammals).

11.1.4 Population Interactions (Table 11.1).

Mutualism (+/+), Competition (−/−), Predation (+/−), Parasitism (+/−), Commensalism (+/0), Amensalism (−/0).

Predation transfers energy across trophic levels and keeps prey in check. The prickly pear cactus in Australia spread uncontrollably until a cactus-feeding moth was introduced. When the starfish Pisaster was removed from an American intertidal community, more than 10 species of invertebrates became extinct within a year through interspecific competition. Prey defences: camouflage (cryptic colouring in frogs and insects), distastefulness — the Monarch butterfly acquires a chemical by feeding on a poisonous weed as a caterpillar. Nearly 25% of insects are phytophagous. Plants defend with thorns (Acacia, cactus) and chemicals — Calotropis makes highly poisonous cardiac glycosides; nicotine, caffeine, quinine, strychnine and opium are also defences.

Competition can occur between unrelated species (flamingoes and resident fishes compete for zooplankton in South American lakes) and even without shared resources — interference competition. The Abingdon tortoise of the Galapagos became extinct within a decade of goats being introduced. Competitive release: a species expands its range when a competitor is removed. Gause's competitive exclusion principle: two closely related species competing for the same resources cannot coexist indefinitely; the inferior one is eliminated. Connell's experiments on the rocky coast of Scotland: the larger barnacle *Balanus* excludes the smaller *Chathamalus* from the intertidal zone. Resource partitioning: MacArthur's five closely related warbler species coexist by foraging in different parts of the same tree.

Parasitism: parasites are host-specific and co-evolve with hosts; they lose unnecessary organs, have adhesive organs and high reproductive capacity. The human liver fluke needs two intermediate hosts (a snail and a fish). The malarial parasite needs a vector (mosquito). Ectoparasites: lice on humans, ticks on dogs, copepods on marine fish, and *Cuscuta* (a parasitic plant with no chlorophyll) on hedge plants. The female mosquito is not considered a parasite though it needs our blood. Endoparasites live inside the host and have simple life forms. Brood parasitism: the koel lays eggs in the crow's nest; its eggs resemble the host's.

Commensalism: an orchid growing as an epiphyte on a mango branch; barnacles on a whale; cattle egret with grazing cattle; clown fish living among the stinging tentacles of a sea anemone.

Mutualism: lichens (fungus + photosynthesising alga/cyanobacterium); mycorrhizae (fungi + roots of higher plants); fig and wasp — every fig species has a one-to-one pollinating wasp species; the Mediterranean orchid *Ophrys* uses sexual deceit — a petal resembles a female bee, so the male bee pseudocopulates and transfers pollen.

Amensalism: one species is harmed while the other is unaffected.

Diagrams to Know

Figure 11.2 — Age pyramids: three shapes — broad-base triangle (expanding), bell (stable), urn (declining).

Figure 11.3 — Growth curves: J-curve (exponential, a) rising without limit; S-curve (logistic, b) levelling at K. Label axes: time and population density (N).

Figure 11.4 — Mutual relationship: the fig flower pollinated by its specific wasp.

Figure 11.5 — Sexual deceit: Ophrys petal resembling a female bee.

Key-Facts Sheet

Birth rate example 8/20 = 0.4; death rate example 4/40 = 0.1.

Exponential: dN/dt = rN; N(t) = N(0)e^(rt); J-curve.

Logistic: dN/dt = rN(K − N)/K; S-curve; K = carrying capacity.

r: Norway rat 0.015; flour beetle 0.12; India (1981) 0.0205.

Breed once: Pacific salmon, bamboo.

Liver fluke: 2 intermediate hosts — snail, fish.

Balanus vs Chathamalus (Connell, Scotland); 5 warblers (MacArthur).

Board Exam Focus

NCERT Exercise 2 style numerical: if a population doubles in 3 years, r = ln 2 / 3 = 0.231 per year (about 0.23).

Draw and compare exponential and logistic curves; write both equations.

List population attributes not shown by individuals.

Explain competitive exclusion and resource partitioning with examples.

Distinguish ectoparasite, endoparasite and brood parasite with examples.

Give examples of commensalism and mutualism; explain sexual deceit in Ophrys.

Plant defences against herbivores; anti-predator adaptations of prey.

Key Terms

Population — समष्टि

Natality — जन्म दर

Mortality — मृत्यु दर

Immigration — आप्रवासन

Emigration — उत्प्रवासन

Age pyramid — आयु पिरामिड

Carrying capacity — पोषण क्षमता

Intrinsic rate of natural increase — प्राकृतिक वृद्धि की स्वाभाविक दर (r)

Mutualism — सहोपकारिता

Competition — स्पर्धा

Predation — परभक्षण

Parasitism — परजीविता

Commensalism — सहभोजिता

Amensalism — अंतरजातीय परजीविता / एकपक्षीय हानि

Brood parasitism — नीड़ परजीविता

Common Mistakes

Writing birth rate as a number of births instead of a per capita rate.

Confusing the curves: exponential is J-shaped, logistic is S-shaped (sigmoid).

Forgetting that immigration increases and emigration decreases density.

Calling the female mosquito a parasite — NCERT says it is not.

Labelling the orchid on mango as parasitism — it is commensalism.

Swapping Connell's barnacles: Balanus (larger) excludes Chathamalus.

💡 Key Learning Points

✓A population has attributes such as birth rate, death rate, sex ratio, age distribution and density that individuals do not have.
✓Density changes by natality and immigration (increase) and mortality and emigration (decrease): N(t+1) = N(t) + [(B + I) − (D + E)].
✓Exponential growth (dN/dt = rN) gives a J-curve; logistic growth (dN/dt = rN(K − N)/K) gives a sigmoid curve limited by carrying capacity K.
✓Species interact through mutualism (+/+), competition (−/−), predation and parasitism (+/−), commensalism (+/0) and amensalism (−/0).
✓Classic examples: Gause's competitive exclusion, Connell's barnacles, MacArthur's warblers, liver fluke, koel–crow brood parasitism, fig–wasp and Ophrys sexual deceit.

👨‍🏫 Teaching Tips

→Begin with a quick story of Ramdeo Misra to connect Indian ecology to the chapter.
→Work the lotus (8/20) and fruit fly (4/40) examples on the board, then give a fresh per capita rate problem.
→Let students plot both growth curves on graph paper from a small table of values to see J versus S shapes.
→Use the +/−/0 sign code as a grid students fill for each interaction.
→Show pictures of the koel and crow eggs, clown fish and anemone, and Ophrys to make examples vivid.
→Solve the doubling-time numerical (r = ln 2 / 3) step by step using N(t) = N(0)e^(rt).
→Ask 'what happens if...?' questions — remove Pisaster, introduce goats, remove Balanus — to build reasoning.
→Close with a matching exercise of 12 examples to their interaction type.

📋 Assessment Questions

Can the student calculate birth rate, death rate and r from given data?
Can the student draw, label and explain J and S growth curves with equations?
Can the student interpret expanding, stable and declining age pyramids?
Can the student classify an example into the correct population interaction and justify it?
Can the student explain competitive exclusion, resource partitioning and plant defences with NCERT examples?

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