Organisms and Populations Class 12 Notes जीव और समष्टियाँ — कक्षा 12 जीव विज्ञान अध्याय 11

Revise these first (prerequisites)

• Meaning of species (Class 9–10)
• Reading the graph: x-axis time, y-axis number
• The idea of predator/prey in a food chain (Class 10)

1. What is a population?

   A group of organisms of the same species in one place — like all the fish in a pond or all the chital in a forest — is called a population (समष्टि). They share resources among themselves and breed among themselves.

2. The 'details' of a population

   An individual is born and dies, but a population has a birth rate, death rate, sex ratio and age distribution. Just as a school 'class' has an average mark, not any single student.

3. How does a population increase or decrease?

   Four factors change population size: births (natality) and arrivals from outside (immigration) increase it; deaths (mortality) and departures (emigration) decrease it. The calculation is simple: those coming in minus those going out.

4. J-curve and S-curve

   If food and space are unlimited, a population grows rapidly — the graph is J-shaped (exponential). In reality, resources are limited, so growth slows and levels off at a limit — the graph is S-shaped (logistic). This limit is the carrying capacity (K).

5. Relationships among organisms

   Different species interact with each other: a lion eats a deer (predation), two species fight for the same food (competition), one lives on the body of another (parasitism), and sometimes both benefit (mutualism). Just as society has friends, enemies and neighbours — so does nature.

6. Remember using +, −, 0

   Write each interaction with its signs: predation/parasitism (+, −), competition (−, −), mutualism (+, +), commensalism (+, 0), amensalism (−, 0). Memorise this one table and the whole topic becomes easy.

Quick check ✔

Basics clear? Now move on to Learn in Detail 👆

As per NCERT 2026-27 — every point you need for the board exam.

1. Population

   Individuals of the same species in an area that share/compete for resources and can interbreed. Attributes belong to the population, not to the individual.

2. Population attributes

   Birth rate (natality) and death rate (mortality) are expressed per capita. Example: a pond has 20 lotus plants + 8 new ones = 8/20 = 0.4 offspring per lotus per year. Sex ratio (e.g., 60% female, 40% male).

3. Age pyramids

   Pre-reproductive, reproductive, post-reproductive. Shapes: expanding (broad triangular base), stable, declining. Age pyramids are drawn for human populations.

4. Population density

   Not always numbers — sometimes percent cover or biomass is better. Tiger census: pugmarks and fecal pellets.

5. Population growth

   Density increases due to natality (B) and immigration (I); decreases due to mortality (D) and emigration (E). N_t+1 = N_t + [(B + I) − (D + E)].

6. Exponential growth

   Resources unlimited → J-shaped curve. dN/dt = rN; N_t = N₀e^rt. r = intrinsic rate of natural increase (Norway rat 0.015, flour beetle 0.12; India in 1981: 0.0205).

7. Logistic growth

   Resources limited → S-shaped (sigmoid) curve: lag phase → acceleration → deceleration → asymptote (K = carrying capacity). Verhulst–Pearl: dN/dt = rN(K − N)/K. More realistic.

8. Population interactions

   Mutualism (+,+), competition (−,−), predation (+,−), parasitism (+,−), commensalism (+,0), amensalism (−,0).

9. Predation

   Energy transfer, controlling prey populations, maintaining species diversity. Prickly pear cactus in Australia — controlled by introducing a moth (predator). Prey defences: camouflage, Monarch butterfly (poisonous — its caterpillar feeds on milkweed), cardiac glycosides of Calotropis.

10. Competition

    Gause's competitive exclusion principle: when two closely related species compete for the same resource, the inferior one is eliminated. Galapagos goats and Abingdon tortoise. Interference competition is also possible. Resource partitioning — MacArthur's warblers.

11. Parasitism

    Host-specific; adaptations of parasites: loss of sense organs, presence of adhesive organs/suckers, loss of digestive system, high reproductive capacity. Ectoparasites (lice, Cuscuta), endoparasites (liver fluke — 2 intermediate hosts: snail, fish). Brood parasitism: the koel lays its eggs in the crow's nest.

12. Commensalism

    Orchid on a mango branch (epiphyte), barnacles on the back of a whale, cattle egret and grazing cattle, sea anemone and clown fish.

13. Mutualism

    Lichen (fungus + algae/cyanobacteria), mycorrhiza (fungus + plant roots). Fig tree and wasp; Mediterranean orchid Ophrys — petal resembles a female bee (sexual deceit) → pollination through pseudocopulation.

The concept in depth — the why and the how. Everything stays within the NCERT syllabus.

1. Why is the logistic model more realistic?

   No population has unlimited resources, so exponential growth cannot continue forever. Logistic (Verhulst–Pearl) equation: dN/dt = rN[(K − N)/K]. When N is small, (K−N)/K ≈ 1 and growth is fast; when N approaches K, this term tends towards 0 and growth stops. The graph shows lag phase → acceleration → deceleration → asymptote (K).

2. How density is measured — counting every individual is not always necessary

   Large organisms present in small numbers are easy to count, but not bacteria or grass. For plants, percent cover or biomass is used; for fish, the number caught per trap (relative density); and for animals like tigers, an indirect estimate from pugmarks and faecal pellets. Often, relative density alone is sufficient.

3. Ecological role of predators

   Predators keep prey populations under control and help maintain species diversity by reducing the intensity of competition among competing prey species. The spread of prickly pear cactus introduced into Australia was checked only when a cactus-feeding predator (a moth) from its native area was introduced. Prey defences: camouflage in frogs, the Monarch butterfly being distasteful to predators (due to feeding on a poisonous weed in the caterpillar stage), cardiac glycosides of Calotropis.

4. Competition — Gause and Connell

   Gause's competitive exclusion principle: two closely related species competing for the same resources cannot coexist for long; the weaker competitor is eliminated. Connell showed on the rocky sea coast of Scotland that the larger barnacle Balanus excludes the smaller Chthamalus from the intertidal area. In the Galapagos, the Abingdon tortoise became extinct within 10 years of goats being introduced. But species can also coexist through resource partitioning (different feeding times/patterns) — MacArthur's warblers.

5. Co-evolution in mutualism

   Each fig species is pollinated only by its own partner wasp species; the wasp lays eggs inside the fig, and the fig's seeds serve as food for the larvae. The Mediterranean orchid Ophrys has a petal that resembles a female bee, attracting male bees for 'pseudocopulation' and thus getting pollinated — if the bee's appearance changes, the orchid must change along with it (co-evolution). Lichens (fungus + algae/cyanobacteria) and mycorrhizae (fungus + plant roots) are also examples of mutualism.

Common confusions — cleared up

1. Commensalism vs Mutualism

   In commensalism one benefits while the other is neither benefited nor harmed (+, 0) — e.g. orchid on mango, cattle egret and cattle. In mutualism both benefit (+, +).

2. Parasitism vs Predation

   A predator kills and eats its prey; a parasite lives on its host and usually does not kill it immediately, because the host's survival is essential for the parasite.

3. r vs K

   r = intrinsic rate of natural increase (how fast the population can grow). K = carrying capacity (the maximum number the environment can support).

4. Brood parasitism

   The cuckoo lays its eggs in the crow's nest and the crow incubates them — this is brood parasitism. The cuckoo's eggs resemble the crow's eggs in colour and size.

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