Chapter 11 — Organisms and Populations

NCERT Class 12 Biology Chapter 11, Organisms and Populations, covers ecology at the population level — including population attributes, growth models (exponential and logistic), life history variation, and interspecific interactions such as predation, competition, parasitism, commensalism, and mutualism.

Chapter 11 of NCERT Class 12 Biology explores ecology at the population level. A population is a group of individuals of the same species sharing resources in a defined geographical area. Populations have attributes individuals lack — birth rates, death rates, sex ratio, and age distribution. Population growth follows exponential patterns under unlimited resources (J-shaped curve) and logistic patterns under limited resources (S-shaped sigmoid curve), bounded by carrying capacity (K). The chapter also covers interspecific interactions: mutualism, competition, predation, parasitism, commensalism, and amensalism, explaining how these shape community structure and evolution.

1. 01Population attributes include birth rate, death rate, sex ratio, and age distribution — attributes that individual organisms do not possess.
2. 02Population growth is exponential (dN/dt = rN) when resources are unlimited, producing a J-shaped curve; it becomes logistic (Verhulst-Pearl model) when resources are limiting, producing an S-shaped sigmoid curve bounded by carrying capacity (K).
3. 03The intrinsic rate of natural increase (r) measures a population's inherent growth potential; for Norway rat r = 0.015, flour beetle r = 0.12, and India's human population in 1981 had r = 0.0205.
4. 04Interspecific interactions are classified by outcome: mutualism (+/+), competition (−/−), predation (+/−), parasitism (+/−), commensalism (+/0), and amensalism (−/0).
5. 05Gause's Competitive Exclusion Principle states that two closely related species competing for the same limiting resource cannot coexist indefinitely; the inferior competitor is eventually eliminated.
6. 06Plants have evolved morphological defences (thorns in Acacia, Cactus) and chemical defences (cardiac glycosides in Calotropis, nicotine, quinine) against herbivory.