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NCERT Solutions · Class 12 Biology Organisms and Populations

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Exercises 11.1–11.10

  1. Exercise 11.1

    List the attributes that populations possess but not individuals.

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    A population has birth rates and death rates, a sex ratio, an age distribution and a population density — an individual organism has none of these.
    Birth rate (natality) and death rate (mortality): an individual may have a birth and a death, but only a population has rates, expressed per capita. If $\displaystyle 20$ lotus plants in a pond add $\displaystyle 8$ new plants, the birth rate is $\displaystyle 8$/$\displaystyle 20$ = $\displaystyle 0.4$ offspring per lotus per year; if $\displaystyle 4$ of $\displaystyle 40$ fruitflies die in a week, the death rate is $\displaystyle 4$/$\displaystyle 40$ = $\displaystyle 0.1$ individuals per fruitfly per week.
    Sex ratio: an individual is either male or female; a population has a ratio, e.g. $\displaystyle 60$ per cent females and $\displaystyle 40$ per cent males.
    Age distribution: a population at any time is made of individuals of different ages; plotted as an age pyramid, whose shape shows whether the population is growing, stable or declining. An individual has only one age.
    Population density (N): the size of the population, measured as total number, per cent cover or biomass — meaningless for a single individual.

  2. Exercise 11.2

    If a population growing exponentially double in size in $\displaystyle 3$ years, what is the intrinsic rate of increase (r) of the population?

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    \(\displaystyle r = 0.231\) per year.
    The integral form of the exponential growth equation is \(\displaystyle N_t = N_0 e^{rt}\).
    Doubling in $\displaystyle 3$ years means \(\displaystyle N_t = 2N_0\) and \(\displaystyle t = 3\), so \(\displaystyle 2N_0 = N_0 e^{3r}\).
    Therefore \(\displaystyle e^{3r} = 2\), giving \(\displaystyle 3r = \ln 2 = 0.693\).
    \(\displaystyle r = 0.693/3 = 0.231\) per year — the intrinsic rate of natural increase.

  3. Exercise 11.3

    Name important defence mechanisms in plants against herbivory.

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    Plants defend themselves against herbivory by morphological and chemical means, because unlike animals they cannot run away from their predators.
    Thorns: the most common morphological defence, as in Acacia and Cactus.
    Chemical defences: many plants produce and store chemicals that make the herbivore sick when eaten, inhibit its feeding or digestion, disrupt its reproduction, or even kill it.
    Calotropis produces highly poisonous cardiac glycosides — which is why cattle and goats are never seen browsing on it.
    Nicotine, caffeine, quinine, strychnine and opium — substances we extract commercially — are actually produced by plants as defences against grazers and browsers.

  4. Exercise 11.4

    An orchid plant is growing on the branch of mango tree. How do you describe this interaction between the orchid and the mango tree?

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    This is commensalism (+ / $\displaystyle 0$).
    The orchid grows as an epiphyte on the mango branch and is benefited — it gains support and a place in the light.
    The mango tree is neither harmed nor benefited; it derives no apparent benefit from hosting the orchid.
    The chapter groups this with barnacles growing on the back of a whale, another case where one partner gains and the other is unaffected.

  5. Exercise 11.5

    What is the ecological principle behind the biological control method of managing with pest insects?

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    The principle is predation — a predator's ability to regulate the population of its prey.
    Predators keep prey populations under control; but for predators, prey species could reach very high population densities and cause ecosystem instability.
    Exotic species become invasive in a new area precisely because the invaded land does not have their natural predators, so introducing that natural predator restores the check.
    Example from the chapter: the prickly pear cactus introduced into Australia in the early 1920s spread over millions of hectares of rangeland, and was brought under control only after a cactus-feeding predator (a moth) from its natural habitat was introduced.
    Biological control of agricultural pests works the same way — the pest's natural predator is used in place of chemical pesticide.

  6. Exercise 11.6

    Define population and community.

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    Population: a group of individuals of the same species living in a well-defined geographical area, sharing or competing for similar resources and potentially interbreeding.
    For ecological purposes a group of individuals produced even by asexual reproduction is also counted as a population — e.g. all the cormorants in a wetland, rats in an abandoned dwelling, teakwood trees in a forest tract, bacteria in a culture plate, lotus plants in a pond.
    Community: the assemblage formed when populations of different species — animals, plants and microbes — live together in a habitat and interact with one another in various ways.
    No species can live in isolation, so even a minimal community carries many interactive linkages, though not all of them are readily apparent. Community is one of the four levels of organisation ecology deals with: organisms, populations, communities and biomes.

  7. Exercise 11.7

    Define the following terms and give one example for each:
    (a)
    Commensalism
    (b)
    Parasitism
    (c)
    Camouflage
    (d)
    Mutualism
    (e)
    Interspecific competition

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    NCERT publishes no answers for this textbook, so there is nothing in the book to check this working against. It has also not yet been read through by hand.

    (a) Commensalism
    An interaction in which one species is benefited and the other is neither harmed nor benefited (+ / $\displaystyle 0$).
    Example: the cattle egret foraging beside grazing cattle — the moving cattle stir up and flush out insects for the egret, while the cattle gain nothing.
    (b) Parasitism
    An interaction in which one species, the parasite, benefits by getting free lodging and meals from the other, the host, which is harmed (+ / –). Parasites may reduce the host's survival, growth and reproduction and lower its population density.
    Example: *Cuscuta* growing on hedge plants — it has lost its chlorophyll and leaves and derives its nutrition from the host plant. (Lice on humans and ticks on dogs are ectoparasitic examples.)
    (c) Camouflage
    Cryptic colouration by which a prey species blends with its surroundings so that the predator cannot detect it easily; it is a prey defence that lessens the impact of predation.
    Example: some species of insects and frogs are cryptically coloured to avoid being spotted by predators.
    (d) Mutualism
    An interaction that confers benefits on both the interacting species (+ / +).
    Example: lichens, an intimate association between a fungus and photosynthesising algae or cyanobacteria. (Mycorrhizae, and the fig tree with its partner wasp, are equally good examples.)
    (e) Interspecific competition
    A process in which the fitness of one species — measured as its intrinsic rate of increase \(\displaystyle r\) — is significantly lowered in the presence of another species (– / –). It can occur even between unrelated species, and even when resources are not limiting, as in interference competition.
    Example: in some shallow South American lakes, visiting flamingoes and resident fishes compete for their common food, the zooplankton.

  8. Exercise 11.8

    With the help of suitable diagram describe the logistic population growth curve.

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    ncert_XII_Biology_fig_11-8
    The logistic curve is sigmoid (S-shaped), because no population in nature has unlimited resources — a habitat can support only a maximum number, its carrying capacity (K).
    Limited resources lead to competition between individuals, and eventually the 'fittest' individual survives and reproduces.
    The growth passes through four stages: an initial lag phase, then a phase of acceleration, then a phase of deceleration, and finally an asymptote, when the population density reaches K and levels off.
    This is called Verhulst–Pearl logistic growth and is described by \(\displaystyle \frac{dN}{dt} = rN\left(\frac{K-N}{K}\right) \), where N is population density at time t, r is the intrinsic rate of natural increase and K is the carrying capacity.
    Because resources for most animal populations are finite and become limiting sooner or later, the logistic model is considered the more realistic one.
    What your diagram must show
    Draw the x-axis as Time (t) and the y-axis as Population density (N).
    Draw a horizontal broken line across the upper part of the graph and label it K (carrying capacity).
    Draw the logistic curve starting low and almost flat near the origin (lag phase), then rising steeply (acceleration), then bending over (deceleration), and finally running flat just below and along the broken line without crossing it (asymptote). Label this curve b — logistic growth.
    For contrast, draw a second curve from the same starting point that rises and keeps rising ever more steeply off the top of the graph, J-shaped, and label it a — exponential growth.
    Mark the four phases along the logistic curve: lag, acceleration, deceleration, asymptote.

  9. Exercise 11.9

    Select the statement which explains best parasitism.
    (a)
    One organism is benefited.
    (b)
    Both the organisms are benefited.
    (c)
    One organism is benefited, other is not affected.
    (d)
    One organism is benefited, other is affected.

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    (d) One organism is benefited, other is affected.
    In parasitism the parasite is benefited — it gets free lodging and meals from the host's body.
    The host is harmed: majority of parasites reduce the survival, growth and reproduction of the host, lower its population density, and may render it more vulnerable to predation by making it physically weak.
    So the interaction is scored (+ / –), the same sign pattern as predation.
    Why the others are wrong: (a) is incomplete because it ignores the harm to the host; (b) describes mutualism; (c) describes commensalism.

  10. Exercise 11.10

    List any three important characteristics of a population and explain.

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    Three important characteristics of a population are its birth and death rates, its sex ratio and its age distribution.
    (i) Birth rate (natality) and death rate (mortality)
    These are per capita rates, not counts — the change in numbers with respect to the members of the population.
    If $\displaystyle 20$ lotus plants in a pond add $\displaystyle 8$ new plants, the birth rate is $\displaystyle 8$/$\displaystyle 20$ = $\displaystyle 0.4$ offspring per lotus per year; if $\displaystyle 4$ of $\displaystyle 40$ fruitflies die in a week, the death rate is $\displaystyle 4$/$\displaystyle 40$ = $\displaystyle 0.1$ individuals per fruitfly per week.
    (ii) Sex ratio
    An individual is either male or female, but the population as a whole has a ratio of the two — for example, $\displaystyle 60$ per cent females and $\displaystyle 40$ per cent males.
    (iii) Age distribution
    A population at any given time is composed of individuals of different ages; the per cent of individuals in each age group is plotted as an age pyramid.
    The shape of the pyramid reflects the growth status of the population — growing, stable or declining.
    (A fourth commonly cited attribute is population density (N), the size of the population, which may be expressed as total number, per cent cover or biomass depending on the species.)