Q4. Which one of the following has the largest population in a food chain? (a) producers; (b) primary consumers; (c) secondary consumers; (d) decomposers. (NCERT Exercise 2, page 215.)
(d) Decomposers. They are mainly bacteria and fungi (NCERT page 210), mostly microscopic organisms present in very large numbers wherever there is dead matter, so counted as individuals they outnumber every other group (our reasoning). Among the levels of a grazing food chain, which is what NCERT’s pyramids show, the producers are the largest: about 5.8 million plants against 3 top carnivores in NCERT Figure 12.4 (a) (page 212); decomposers are given no place in those pyramids (page 214).
Q5. What is the percentage of photosynthetically active radiation (PAR) in the incident solar radiation? (a) 100%; (b) 50%; (c) 1–5%; (d) 2–10%. (NCERT Exercise 5, page 215.)
(b) 50%, the nearest option: NCERT says PAR is less than 50 per cent of the incident solar radiation (page 209). Option (d), 2–10 per cent, is the share of the PAR that plants capture, a different quantity; option (c), 1–5 per cent, is roughly what that capture comes to as a share of all the sunlight (our arithmetic).
Q6. Distinguish between a food chain and a food web. (NCERT Exercise 6(d), page 215.)
A food chain is a single sequence in which an animal feeds on a plant or on another animal and is in turn food for another, as in grass → goat → man. A food web is the network formed when food chains interconnect: detritus food chain organisms are prey to grazing food chain animals, and omnivores such as cockroaches and crows feed at more than one level (NCERT pages 209–210).
Q7. Assign each to its trophic level, as in NCERT Figure 12.2: cow, wolf, phytoplankton, lion, grasshopper, trees.
First (producers): phytoplankton, trees. Second (herbivores): cow, grasshopper. Third (carnivores): wolf. Fourth (top carnivores): lion (NCERT page 210). A trophic level is a functional level, though: a lion eating a herbivore is feeding at the third level (reasoned from NCERT page 213).
Q8. Under the 10 per cent law, the secondary carnivores of a chain receive 45 J. What NPP did the producers supply over the same area and interval, and how much reached the herbivores?
Secondary carnivores are at the fourth level, three transfers above the producers: 45 × 10 × 10 × 10 = 45 000 J at the producers, and 45 000 × 0.1 = 4500 J at the herbivores (NCERT page 211).
Q9. What are saprotrophs, and how do they feed?
Saprotrophs (sapro: to decompose) are the decomposers, heterotrophs that are mainly fungi and bacteria. They secrete digestive enzymes that break down dead and waste materials into simple, inorganic materials, which they then absorb (NCERT page 210).
Q10. Why must every ecosystem receive a constant supply of energy?
Energy flows one way and is lost as heat at each step, and by the second law of thermodynamics ecosystems need a constant supply of energy to synthesise the molecules they require, to counteract the universal tendency toward increasing disorderliness (NCERT page 209).