Chapter 11 — Organisms and Populations

Chapter 11 of the Class 12 Biology NCERT textbook, "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.

• Populations have emergent attributes — A population is a group of one species sharing a habitat, and unlike a single organism it has properties such as birth and death rates, sex ratio, and age distribution that describe the group as a whole.
• How populations grow — With unlimited resources growth is exponential, giving a J-shaped curve, but limited resources impose logistic growth toward a carrying capacity, producing the S-shaped curve that models real populations.
• Interspecific interactions — Species affect each other through mutualism, competition, predation, parasitism, commensalism, and amensalism, and these relationships classified by who benefits or is harmed shape communities and drive evolution.
• Competition and defence adaptations — Gause's exclusion principle explains why two species cannot indefinitely share the same limiting resource, while plants evolve thorns and toxic chemicals as defences against herbivores, illustrating adaptation through interaction.

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.