NCERT Solutions for Class 12 Biology Chapter 11: Organisms and Populations

An individual organism can only be born once and die once, and is simply either male or female — it cannot have a ‘rate’ of anything. A population, being a group of individuals of a species sharing a defined geographical area, has emergent, group-level attributes that no single individual can possess:

1. Birth rate (natality rate). While an individual simply has births, a population has a per capita birth rate — e.g., if 20 lotus plants in a pond produce 8 new plants in a year, the birth rate is 8/20 = 0.4 offspring per lotus per year.
2. Death rate (mortality rate). Similarly, a population has a per capita death rate — e.g., if 4 of 40 fruitflies in a lab population die in a week, the death rate is 4/40 = 0.1 individual per fruitfly per week.
3. Sex ratio. An individual is simply male or female, but a population has a sex ratio — e.g., ‘60% of the population are females and 40% males’.
4. Age distribution. A population at any time is composed of individuals of different ages; plotting the percentage of individuals in each age group gives an age pyramid, whose shape reveals whether the population is growing, stable, or declining.
5. Population density/size. Only a population (not an individual) has a size or density (N), which may be measured as total number, biomass, or per cent cover depending on the species.

Conclusion: populations possess emergent, group-level attributes — birth rate, death rate, sex ratio, age distribution and population density — that are meaningless for a single individual organism, which can only be born, die, and be one sex at a time.

For exponential growth, NCERT gives the integral form of the growth equation as N_t = N₀e^rt, where N₀ is the population density at time zero, N_t is the density after time t, and r is the intrinsic rate of natural increase.

1. Set up the doubling condition. The population doubles in 3 years, so N_t = 2N₀ when t = 3.
2. Substitute into the equation: 2N₀ = N₀e^r(3)
3. Cancel N₀ from both sides: 2 = e^3r
4. Take the natural logarithm of both sides: ln(2) = 3r
5. Solve for r: r = ln(2) / 3 = 0.693 / 3 ≈ 0.231 per year

Conclusion: using N_t = N₀e^rt with the doubling condition N_t = 2N₀ at t = 3 years gives an intrinsic rate of natural increase r = ln(2)/3 ≈ 0.231 per year.

Nearly 25 per cent of all insects are phytophagous (feed on plants), and the problem is especially severe for plants because, unlike animals, they cannot run away from their predators (herbivores). Plants have therefore evolved an astonishing variety of morphological and chemical defences:

Conclusion: because plants cannot flee their herbivores, they defend themselves morphologically with structures like thorns (Acacia, cactus) and chemically with toxic compounds like the cardiac glycosides of Calotropis and commercially significant defence chemicals such as nicotine, caffeine, quinine, strychnine and opium.

This interaction is commensalism — the interspecific interaction in which one species benefits while the other is neither harmed nor benefited (a ‘+, 0’ outcome).

1. The orchid, growing as an epiphyte on the mango branch, benefits — it gains a physical growing surface, better access to sunlight, and improved air circulation, all useful for a plant that is not rooted in soil.
2. The mango tree derives no apparent benefit from hosting the orchid, but it is also not harmed by it — the orchid only uses the branch as physical support and does not draw nutrients from the mango tree’s tissues (unlike a true parasite).

Conclusion: the orchid–mango relationship is commensalism, since the epiphytic orchid gains physical support and better light access from growing on the mango branch while the mango tree itself is neither helped nor harmed by the association.

The ecological principle behind biological pest control is predation — specifically, that predators keep prey (pest) populations under control. By deliberately introducing a pest’s natural predator into an area, the pest population can be regulated without resorting to toxic chemical insecticides.

1. Why pests become problematic in new areas. When an exotic species is introduced into a new geographical area, it often becomes invasive and spreads rapidly, specifically because the invaded land lacks its natural predators — nothing is present to keep its population density in check.
2. The classic example — prickly pear cactus in Australia. The prickly pear cactus, introduced into Australia in the early 1920s, spread explosively into millions of hectares of rangeland. It was finally brought under control only after a cactus-feeding predator (a moth) from its natural habitat was deliberately introduced into the country.
3. The underlying principle applied to agriculture. Biological control methods in agricultural pest management are based directly on this same ability of a predator to regulate a prey (pest) population — by reintroducing or encouraging the pest’s natural enemy, its numbers can be brought down to manageable levels, just as happens naturally in an undisturbed ecosystem.

Conclusion: biological pest control works on the ecological principle that predators regulate prey populations, so deliberately introducing a pest’s natural predator — as was done with a cactus-feeding moth to control invasive prickly pear cactus in Australia — can bring an unchecked pest population back under control.

Population: A population is a group of individuals of a particular species, living in a well-defined geographical area, at a given time, that share or compete for similar resources and are potentially interbreeding (though groups formed through asexual reproduction are also considered populations for ecological purposes). All the cormorants in a wetland, rats in an abandoned dwelling, teakwood trees in a forest tract, bacteria in a culture plate, and lotus plants in a pond are all examples of populations.

Community: Ecology is concerned with four levels of biological organisation — organisms, populations, communities and biomes. A community is the next level up from a population: it is an assemblage of populations of different species living and interacting with one another within the same defined geographical area/habitat. A community is thus built from multiple interacting populations — linked through interactions such as predation, competition, parasitism, commensalism and mutualism — just as a population is built from multiple interacting individuals.

Conclusion: a population is a group of interbreeding individuals of a single species sharing a defined area and its resources, while a community is the broader assemblage of many such populations of different species interacting together in that same area.

Conclusion: these five terms capture the full spectrum of how species interact — from one-sided benefit without harm (commensalism) or with harm (parasitism), through defensive blending-in (camouflage), to mutual benefit (mutualism) and mutual disadvantage (interspecific competition) — each illustrated by a specific, well-documented natural example.

No population of any species in nature has unlimited resources at its disposal, so true exponential growth cannot continue indefinitely. The logistic growth model — also called the Verhulst-Pearl Logistic Growth — describes how a population grows realistically in a habitat with limited resources.

1. The governing equation. Logistic growth is described by dN/dt = rN(K–N)/K, where N is the population density at time t, r is the intrinsic rate of natural increase, and K is the carrying capacity — the maximum population size a given habitat can sustain.
2. The four phases of growth. A population growing logistically shows, in sequence: a lag phase (slow initial growth), a phase of acceleration (growth speeds up as the population establishes itself), a phase of deceleration (growth slows as resources become limiting and competition increases), and finally an asymptote, where the population density levels off at the carrying capacity K and growth effectively stops.
3. The resulting curve. Plotting population density (N) against time (t) produces a sigmoid (S-shaped) curve — in clear contrast to the unchecked, J-shaped exponential curve (dN/dt = rN) that would result if resources were unlimited.
4. Why it is considered realistic. Since resources for growth are finite and become limiting sooner or later for virtually every real population, the logistic model — which explicitly builds in a carrying capacity ceiling — is considered the more realistic representation of how populations actually grow in nature, compared to the exponential model.

Conclusion: the logistic growth curve is the realistic, S-shaped (sigmoid) pattern — lag, acceleration, deceleration, then an asymptote — that a population follows as it grows toward the carrying capacity K of its finite-resource habitat, governed by the equation dN/dt = rN(K–N)/K.

Correct option: (d) One organism is benefited, other is affected.

In parasitism, the parasite derives nourishment and benefit from its host, while the host is harmed in the process — its survival, growth or reproduction may be reduced, and it may even be rendered more vulnerable to predation by being made physically weaker. This matches exactly the ‘+, –’ outcome NCERT assigns to parasitism in its table of population interactions (Table 11.1), where the parasite is ‘+’ (benefited) and the host is ‘–’ (harmed/‘affected’).

Conclusion: the correct answer is (d) — parasitism is best defined as an interaction in which one organism (the parasite) benefits while the other (the host) is actively harmed, distinguishing it from commensalism (other unaffected) and mutualism (both benefit).

A population, as a group-level entity, has several defining characteristics that an individual organism does not possess. Three important ones are:

1. Birth rate and death rate. A population is characterised by per capita birth and death rates rather than simple counts of births/deaths. For example, a lotus population of 20 that gains 8 new plants in a year has a birth rate of 8/20 = 0.4 offspring per lotus per year; a fruitfly population of 40 that loses 4 individuals in a week has a death rate of 4/40 = 0.1 individual per fruitfly per week. These rates are the primary drivers of changes in population density.
2. Sex ratio. Unlike an individual, who is simply male or female, a population has a sex ratio — the proportion of males to females — which can be expressed as a percentage (e.g., 60% female, 40% male). Sex ratio influences the reproductive potential and future growth rate of the population.
3. Age distribution (age pyramid). A population at any time consists of individuals of different ages. Plotting the percentage of individuals in each age group produces an age pyramid; the shape of this pyramid indicates whether the population is growing, stable, or declining, making it a powerful tool for predicting a population’s future trajectory.

Conclusion: birth and death rates, sex ratio, and age distribution (summarised in the age pyramid) are three defining characteristics that apply only to a population as a whole, and together they reveal whether a population is thriving, stable, or heading toward decline.