Organisms and Populations

This is the eleventh chapter of NCERT's Class 12 Biology textbook (pages 190–204 of the 2026-27 reprint, after the Unit X opener and the Ramdeo Misra box on pages 188–189). It opens Unit X, Ecology, and has one main section, 11.1 Populations, in four parts — population attributes (11.1.1), population growth with growth models (11.1.2), life history variation (11.1.3) and population interactions (11.1.4: predation, competition, parasitism, commensalism, mutualism) — with Figures 11.1–11.5, Table 11.1, a summary and 10 exercises. This page uses ten concept sections; the life-history one is short because the book's is.

• Neighbouring chapters: Ecosystem (Chapter 12) takes the next step — food chains, productivity, energy flow and succession; Biodiversity and Conservation (Chapter 13) uses the ideas of invasive species, co-extinction (the fig and its wasp) and keystone predators. Evolution (Chapter 6) supplies natural selection and the struggle for existence; Sexual Reproduction in Flowering Plants (Chapter 1) covers pollinators and the outbreeding devices behind them; Microbes in Human Welfare (Chapter 8) covers biocontrol agents, mycorrhiza and biofertilisers; Human Health and Disease (Chapter 7) has the malaria parasite and its mosquito vector.
• Prelims — environment and ecology: lichens (1984, 2014), mycorrhiza (1990, 1999), epiphytes (2001), keystone species (2000), camouflage (1979), which is not a parasite (1984), biological control (1991, 2008), birds feeding beside cattle (2014), symbiosis across groups (2021) and the fig and its co-evolved wasp (2024).
• GS3 — Environment: carrying capacity and sustainable development (2019); invasive species and biological control. GS1 touches it through population and its growth (age pyramids, growth rates, the census).

• Ecology studies interactions among organisms and with the abiotic environment, at four levels — organisms, populations, communities, biomes. A population: individuals of one species in a defined area, sharing or competing for resources, potentially interbreeding (asexual groups count too). Natural selection works at the population level.
• Attributes only a population has: birth rate and death rate (per capita: 8 new lotus on 20 = 0.4; 4 of 40 flies die = 0.1 per week), sex ratio, age distribution → age pyramid (expanding, stable, declining).
• Density (N): numbers, or per cent cover / biomass (one banyan vs 200 Parthenium), or relative (fish per trap), often indirect (tiger census by pug marks and faecal pellets). N_t+1 = N_t + [(B + I) − (D + E)].
• Exponential: dN/dt = rN, N_t = N₀e^rt, J-shaped; r = b − d, the intrinsic rate of natural increase (Norway rat 0.015, flour beetle 0.12, India 1981 0.0205). Logistic (Verhulst-Pearl): dN/dt = rN(K − N)/K, sigmoid, lag → acceleration → deceleration → asymptote at carrying capacity K; more realistic.
• Interactions: mutualism +/+, competition −/−, predation and parasitism +/−, commensalism +/0, amensalism −/0. Predators transfer energy, control prey (prickly pear and a moth; biological control), keep diversity (Pisaster). Plant defences: thorns, chemicals (Calotropis's cardiac glycosides). Monarch butterfly is distasteful.
• Competition: fitness (r) lowered by another species; Gause's competitive exclusion; Abingdon tortoise and goats; competitive release (Connell's barnacles); resource partitioning (MacArthur's 5 warblers). Parasites: co-evolve, ecto (lice, ticks, Cuscuta) and endo; liver fluke (snail, fish); brood parasitism (koel–crow). Commensalism: orchid on mango, barnacles on whale, cattle egret, clown fish. Mutualism: lichens, mycorrhizae, fig–wasp, Ophrys sexual deceit.

Ecology studies the interactions among organisms and between an organism and its physical (abiotic) environment, at four levels — organisms, populations, communities and biomes; this chapter works at the population level. In nature individuals rarely live alone: most live in groups in a well-defined area, share or compete for similar resources and potentially interbreed — a population. A group produced by asexual reproduction is also treated as a population. Examples: the cormorants of a wetland, rats in an abandoned house, teak trees in a forest tract, bacteria on a culture plate, lotus plants in a pond. Although the individual copes with a changed environment, natural selection operates at the population level — which is why population ecology links ecology to population genetics and evolution.

Birth and death rates. An individual has births and deaths; a population has rates, expressed per capita. If a pond had 20 lotus plants last year and 8 new ones were added (28 now), the birth rate is 8/20 = 0.4 offspring per lotus per year. If 4 of 40 laboratory fruit flies die in a week, the death rate is 4/40 = 0.1 individuals per fruit fly per week. Sex ratio: an individual is male or female; a population has a ratio (e.g. 60 per cent females, 40 per cent males). Age distribution: the per cent of individuals in each age group, plotted as an age pyramid; for humans the pyramid shows males and females side by side, and its shape shows whether the population is growing, stable or declining.

The size of a population tells us its status in the habitat; any process we study — the outcome of competition, the impact of a predator, the effect of a pesticide — is judged by the change in population size. Sizes range from fewer than 10 (Siberian cranes at Bharatpur wetlands in any year) to millions (Chlamydomonas in a pond). Population size is technically population density (N), and need not be a number of individuals.

Population size is not static; it changes with food, predation pressure, weather and more, and these changes show whether a population is flourishing or declining. Density changes through four basic processes — two add, two remove: natality (births added during a period), mortality (deaths during the period), immigration (individuals of the same species coming in from elsewhere) and emigration (individuals leaving). If N is the density at time t, then N_t+1 = N_t + [(B + I) − (D + E)]. Density rises if B + I exceeds D + E. Normally births and deaths matter most; immigration and emigration matter only in special conditions — for instance, when a new habitat is being colonised, immigration may contribute more than births.

Exponential growth. When resources (food and space) are unlimited, each species can realise fully its innate potential to grow in number, as Darwin observed. If b and d are the per capita birth and death rates, then dN/dt = (b − d) × N; letting r = b − d, dN/dt = rN. r is the intrinsic rate of natural increase, an important parameter for assessing the impact of any biotic or abiotic factor on growth. NCERT's values: Norway rat 0.015, flour beetle 0.12, India's human population in 1981, 0.0205. Plotting N against time gives a J-shaped curve; the integral form is N_t = N₀e^rt (e = 2.71828, the base of natural logarithms). Any exponentially growing species can reach enormous densities quickly: Darwin showed even the slow-breeding elephant would, without checks.

NCERT's anecdote: a minister who wins a game of chess asks the king for one wheat grain on the first square, two on the second, four on the third, doubling to the 64th square; by half the board the king realises all the wheat of his kingdom would not suffice. The arithmetic: the first 32 squares hold 2³² − 1 ≈ 4.3 billion grains, and the whole board 2⁶⁴ − 1 ≈ 1.8 × 10¹⁹. A single Paramecium dividing once a day would likewise reach 2⁶⁴ ≈ 1.8 × 10¹⁹ cells in 64 days, if food and space were unlimited (all asserted in the source file).

Logistic growth. No population has unlimited resources; individuals compete, and the ‘fittest’ survive and reproduce. A habitat can support a maximum number — nature's carrying capacity (K). A population growing with limited resources shows a lag phase, then acceleration and deceleration, and finally an asymptote as density reaches K: a sigmoid (S-shaped) curve, called Verhulst-Pearl logistic growth: dN/dt = rN (K − N)/K. Since resources for most animal populations are finite and become limiting sooner or later, the logistic model is considered more realistic.

The table records positions, not a verdict.

Populations evolve to maximise their reproductive fitness, also called Darwinian fitness (a high r), in their habitat; under a given set of selection pressures organisms evolve towards the most efficient reproductive strategy. Some breed only once in a lifetime (Pacific salmon, bamboo); others many times (most birds and mammals). Some produce many small offspring (oysters, pelagic fishes); others few large offspring (birds, mammals). Ecologists suggest life-history traits evolved in relation to the constraints of the abiotic and biotic components of the habitat; this is an active research area. NCERT gives no figure here.

No natural habitat holds just one species; even a plant, which makes its own food, needs soil microbes to return nutrients and often an animal to pollinate it. Animals, plants and microbes interact to form a biological community. Interspecific interactions arise between populations of two different species and can be beneficial (+), detrimental (−) or neutral (0) for each. Both benefit in mutualism and both lose in competition; in parasitism and predation one benefits (parasite, predator) and the other is harmed (host, prey); in commensalism one benefits and the other is unaffected; in amensalism one is harmed and the other unaffected. Predation, parasitism and commensalism share one feature — the species live closely together.

Predation is nature's way of transferring to higher trophic levels the energy fixed by plants. Tiger and deer come to mind, but a sparrow eating a seed is also a predator, and herbivores are, in a broad ecological context, not very different from predators. Predators also keep prey populations under control; without them prey could reach very high densities and destabilise the ecosystem. Exotic species introduced into a new area often become invasive because the invaded land lacks their natural predators: the prickly pear cactus introduced into Australia (NCERT: early 1920s) spread over millions of hectares of rangeland until a cactus-feeding moth from its natural habitat was introduced. Biological control of agricultural pests rests on this ability of a predator to regulate its prey. Predators also maintain species diversity by reducing competition among prey: in rocky intertidal communities of the American Pacific coast, removing the starfish Pisaster from an enclosed area led to more than 10 invertebrate species becoming extinct within a year, through interspecific competition.

A predator that overexploits its prey may drive it extinct and then starve itself — so predators in nature are ‘prudent’. Prey have evolved defences: cryptic colouring (camouflage) in some insects and frogs; being poisonous; the Monarch butterfly is highly distasteful to birds because of a chemical it acquires as a caterpillar by feeding on a poisonous weed. For plants, herbivores are the predators; nearly 25 per cent of insects are phytophagous, and plants cannot run away. Their defences: thorns (Acacia, cactus) — the commonest morphological defence — and chemicals that sicken the herbivore, inhibit feeding or digestion, disrupt reproduction or kill it. Calotropis makes highly poisonous cardiac glycosides, so cattle and goats never browse it; nicotine, caffeine, quinine, strychnine and opium are produced by plants as defences against grazers and browsers.

Darwin considered interspecific competition a potent force in evolution. It is often thought to occur only between closely related species competing for limiting resources, but neither is necessary. Unrelated species compete: in some shallow South American lakes, visiting flamingoes and resident fishes compete for zooplankton. And resources need not be limiting: in interference competition one species' feeding efficiency falls because of the other's interfering presence even when food and space are abundant. So competition is best defined as a process in which the fitness of one species (its r) is significantly lower in the presence of another.

In laboratory experiments, as Gause and others showed, when resources are limited the competitively superior species eventually eliminates the other; evidence for such competitive exclusion in nature is less conclusive, though strong circumstantial cases exist: the Abingdon tortoise of the Galapagos went extinct within a decade of goats being introduced, apparently because the goats browsed more efficiently. Competitive release: a species confined to a small area by a superior competitor expands dramatically when that competitor is removed — Connell's field experiments on the rocky coast of Scotland showed the larger Balanus barnacle dominating the intertidal zone and excluding the smaller Chthamalus. Herbivores and plants seem more adversely affected by competition than carnivores. Gause's Competitive Exclusion Principle: two closely related species competing for the same resources cannot co-exist indefinitely, and the inferior one is eliminated — perhaps true if resources are limiting, but more recent studies show species may evolve mechanisms for co-existence, such as resource partitioning: different feeding times or foraging patterns. MacArthur showed five closely related warbler species living on the same tree co-existing through behavioural differences in foraging.

Parasitism ensures free lodging and meals, so it has evolved in many groups, from plants to higher vertebrates. Many parasites are host-specific, so host and parasite co-evolve: if the host evolves ways to reject or resist the parasite, the parasite must evolve ways to counter them. Parasites' special adaptations: loss of unnecessary sense organs, adhesive organs or suckers, loss of digestive system and high reproductive capacity. Life cycles are often complex, with one or two intermediate hosts or vectors: the human liver fluke (a trematode) needs two intermediate hosts, a snail and a fish; the malarial parasite needs a vector, the mosquito. Most parasites harm the host — lowering its survival, growth, reproduction and population density, or weakening it so it falls to predators.

Ectoparasites feed on the host's external surface: lice on humans, ticks on dogs, ectoparasitic copepods on many marine fish, and Cuscuta, a parasitic plant on hedge plants that has lost its chlorophyll and leaves and takes its nutrition from the host. The female mosquito is not considered a parasite, although it needs our blood to reproduce. Endoparasites live inside the host (liver, kidney, lungs, red blood cells); their life cycles are more complex because of extreme specialisation, with simplified morphology and anatomy but high reproductive potential. Brood parasitism in birds: the parasitic bird lays its eggs in a host's nest and lets the host incubate them; its eggs have evolved to resemble the host's in size and colour so they are not ejected — watch the cuckoo (koel) and the crow in the breeding season (spring to summer).

Commensalism: one species benefits, the other is neither harmed nor benefited. An orchid growing as an epiphyte on a mango branch and barnacles on a whale's back benefit, while the mango tree and the whale get no apparent benefit. The cattle egret forages beside grazing cattle, which flush out insects from the vegetation that the egret would otherwise find hard to catch. Clown fish living among the stinging tentacles of a sea anemone get protection from predators; the anemone appears to gain nothing.

Mutualism: both benefit. Lichens are an intimate association between a fungus and photosynthesising algae or cyanobacteria; mycorrhizae are associations between fungi and the roots of higher plants — the fungi help absorb essential nutrients from the soil, and the plant gives the fungi energy-yielding carbohydrates. The most spectacular examples are plant–animal relationships. Plants pay ‘fees’ — pollen and nectar for pollinators, juicy fruits for seed dispersers — and the system must be guarded against ‘cheaters’ such as nectar thieves; hence plant–animal interactions often involve co-evolution. Many fig species have a tight one-to-one relationship with a pollinator wasp species: a fig can be pollinated only by its partner wasp. The female wasp uses the fruit as an oviposition site and the developing seeds to nourish her larvae; while searching for egg-laying sites she pollinates the fig inflorescence, and in return the fig gives up some developing seeds. Orchids show great floral diversity to attract the right pollinator (bees and bumblebees). Not all offer rewards: the Mediterranean orchid Ophrys uses sexual deceit — one petal resembles the female bee in size, colour and markings; the male bee ‘pseudocopulates’ with it, is dusted with pollen, and transfers it to the next flower. If the female bee's colour pattern changed even slightly, the orchid would have to co-evolve to keep pollinating.

The chapter poses several questions and activities in its running text.

Numbers

One-liners

• Natural selection operates at the population level.
• Birth and death rates are per capita rates.
• An age pyramid with a wide base indicates a growing population.
• Population density can be measured as numbers, per cent cover or biomass.
• Tiger censuses have used pug marks and faecal pellets.
• Natality and immigration increase population density.
• Mortality and emigration decrease population density.
• r equals the per capita birth rate minus the per capita death rate.
• Exponential growth gives a J-shaped curve.
• Logistic growth gives a sigmoid curve levelling at K.
• Carrying capacity is the maximum population a habitat can support.
• Pacific salmon and bamboo breed only once in a lifetime.
• Predation transfers energy to higher trophic levels.
• Biological control relies on predators regulating prey.
• Calotropis produces cardiac glycosides.
• Gause stated the Competitive Exclusion Principle.
• Connell showed competitive release in barnacles.
• MacArthur's warblers co-exist by resource partitioning.
• The liver fluke needs a snail and a fish as intermediate hosts.
• The koel is a brood parasite of the crow.
• An orchid on a mango tree is a commensal.
• Lichens are fungi living with algae or cyanobacteria.
• Each fig species has its own partner pollinator wasp.
• Ophrys gets pollinated by sexual deceit.

Traps to avoid

• Only populations, not individuals, have birth rates and sex ratios.
• A declining pyramid has a base narrower than its middle.
• NCERT's population density means population size.
• J-shaped is exponential; S-shaped is logistic.
• Logistic growth is fastest at K/2, not at K.
• Exercise 2's r is ln 2 divided by 3, about 0.231.
• Herbivores are predators of plants in the ecological sense.
• Competition can occur without limiting resources.
• Competitive release supports the existence of competition.
• The female mosquito is a vector, not a parasite.
• Cuscuta is a parasite; an epiphytic orchid is a commensal.
• Brood parasitism is parasitism, not commensalism.
• An epiphyte depends on its host only for support.
• Predation and parasitism share the same signs (+, −).

Prelims — what UPSC asks

• Interactions and partners: lichens are algae and fungi (1984, 2014); mycorrhiza is fungi with roots of higher plants (1990, 1999); epiphytes depend on hosts for mechanical support (2001); birds feeding beside cattle — the Common Myna (2014); cnidarians, fungi and protozoa all form symbioses (2021); the fig and its co-evolved, only pollinator (2024).
• Predators, parasites, control: camouflage (1979); the housefly is not a parasite (1984); wasps and birds for biological control (1991); Gambusia against mosquitoes (2008); keystone species (2000).
• Key notes: 1984 (not a parasite) keys the housefly — sound; by NCERT's mosquito reasoning a bed bug, which visits only to feed, is a borderline parasite, but the housefly is clearly not one. 1991 keys wasps and birds — sound; spiders also eat insects, but wasps (parasitoids) and birds are the standard biocontrol agents. 1979 asks what stripes indicate and keys camouflage — the intended answer; stripes can also be warning or social signals, so treat it as a dated question.
• Likely next: per capita rates and their arithmetic, the four processes, J vs S curves and K, the meaning of r, life-history examples (salmon, bamboo), the sign table, the Monarch butterfly, Calotropis, competitive release, resource partitioning, the liver fluke's two hosts, brood parasitism, Ophrys, invasive species and biocontrol, keystone predators.

Mains — where it appears and how to answer

• GS3 2019 (carrying capacity and sustainable development): define K from the logistic model (dN/dt = rN(K − N)/K, the S-curve's asymptote), then translate it to a region — water, land, waste-absorbing capacity, tourism and urban load — and to planning tools (carrying-capacity studies for hill towns, environmental impact assessment). Note that human carrying capacity changes with technology and consumption, and present differing views as attributed positions.
• GS1 population questions: use the age pyramid and per capita rates to explain growth, the demographic dividend and ageing; use r = b − d and doubling time (ln 2 ÷ r) for quick quantitative points; keep the policy debate as attributed positions.

Tap an option. NCERT's own exercise questions come first; the rest test the figures and common confusions.

Open any question to attempt it in the Question bank — your attempt is saved to your progress.