Q4. Name important defence mechanisms in plants against herbivory. (NCERT Exercise 3, page 204.)
Morphological defences, of which thorns, as in Acacia and cactus, are the most common; and chemical defences, substances that make the herbivore sick, inhibit feeding or digestion, disrupt its reproduction or kill it, such as the highly poisonous cardiac glycosides of Calotropis. Nicotine, caffeine, quinine, strychnine and opium are also plant defences against grazers and browsers (NCERT page 198).
Q5. What is the ecological principle behind the biological control method of managing pest insects? (NCERT Exercise 5, page 204.)
Predation: predators regulate prey populations. Biological control methods in agricultural pest control are based on the ability of the predator to regulate the prey population, as the moth from its natural habitat controlled the invasive prickly pear in Australia (NCERT pages 197–198).
Q6. Define population and community. (NCERT Exercise 6, page 204.)
A population is a group of individuals of a species living in a well-defined geographical area, sharing or competing for similar resources and potentially interbreeding (NCERT page 191). A community is formed by the populations of different species, animals, plants and microbes, living in an area and interacting in various ways (reasoned from NCERT page 196: they “interact in various ways to form a biological community”).
Q7. Define the following terms and give one example of each: (a) commensalism; (b) parasitism; (c) camouflage; (d) mutualism; (e) interspecific competition. (NCERT Exercise 7, page 204.)
(a) Commensalism: one species benefits, the other is neither harmed nor benefited; the cattle egret and grazing cattle. (b) Parasitism: one species, the parasite, benefits at the expense of the other, the host; Cuscuta on hedge plants. (c) Camouflage: cryptic colouring that keeps prey from being detected easily by predators; some insects and frogs. (d) Mutualism: both species benefit; the fig and its pollinator wasp. (e) Interspecific competition: the fitness of one species is significantly lower in the presence of another; flamingoes and fishes competing for zooplankton in South American lakes (NCERT pages 197–202).
Q8. With the help of a suitable diagram, describe the logistic population growth curve. (NCERT Exercise 8, page 204.)
In a habitat with limited resources, a population shows a lag phase, then acceleration, then deceleration, and finally an asymptote at the carrying capacity K; plotting N against time gives a sigmoid (S-shaped) curve, Verhulst–Pearl logistic growth, described by dN/dt = rN(K − N)/K (NCERT page 195). The diagram is the logistic curve in the growth-curve figure on this page.
Q9. List any three important characteristics of a population and explain them. (NCERT Exercise 10, page 204.)
Any three of: (1) birth rate, the per capita number of births in a period, for example 8 new lotus plants among 20 gives 0.4 per lotus per year; (2) death rate, the per capita number of deaths in a period; (3) sex ratio, the proportions of females and males, such as 60 and 40 per cent; (4) age distribution, plotted as an age pyramid whose shape shows whether the population is growing, stable or declining (NCERT page 191).
Q10. In NCERT’s chess-board story, how many grains lie on the first 32 squares, half the board? And how many Paramecium would one individual give after 64 days of doubling every day?
The first 32 squares hold 1 + 2 + 4 + … + 231 = 232 − 1 = 4 294 967 295 grains. After 64 doublings, one Paramecium becomes 264 ≈ 1.8 × 1019 individuals (our arithmetic; NCERT pages 194–195).