Q4. Fill in the blank: in aquatic ecosystems, the limiting factor for the productivity is ______. (NCERT Exercise 1(c), page 215.)
Light: aquatic producers can photosynthesise only in the sunlit surface layers (our reasoning). Beyond the NCERT text, NASA Earth Observatory (public domain) adds that over much of the open ocean nutrients, such as nitrate and iron, are also scarce there and limit phytoplankton growth. For this blank write light. For NCERT’s in-text question on the low productivity of the ocean (page 207), give both reasons, as the section on land and oceans does: light only near the surface, and few nutrients in that sunlit layer.
Q5. Secondary producers are: (a) herbivores; (b) producers; (c) carnivores; (d) none of the above. (NCERT Exercise 4, page 215.)
(a) Herbivores. NCERT defines secondary productivity as the rate of formation of new organic matter by consumers (page 207), and herbivores are the first consumers to turn the producers’ NPP into new organic matter. The question is loosely worded. Carnivores, option (c), also form new organic matter, and option (d) has a case too: NCERT speaks of secondary productivity, not secondary producers, and calls the green plants (in water, phytoplankton, algae and higher plants) the producers (page 209), so in that strict sense no consumer is a producer. Producers, option (b), are the primary producers. In a test, mark (a), which follows NCERT’s definition of secondary productivity.
Q6. Beyond the NCERT text: taking NASA’s mean radius of the Earth, 6371 km (public domain), the Earth’s surface is 4πR ≈ 5.10 × 10 m. Using NCERT’s figures, about how many grams of dry organic matter does an average square metre of land, and of ocean, produce each year?
Land: 30 per cent of the surface ≈ 1.53 × 10 m, producing 115 billion tons = 1.15 × 10 g, so about (1.15 × 10) ÷ (1.53 × 10) ≈ 750 g m yr. Ocean: 70 per cent ≈ 3.57 × 10 m, producing 55 billion tons = 5.5 × 10 g, so about 154 g m yr. The ratio, 750 ÷ 154 ≈ 4.9, matches Example 2 (our arithmetic, NCERT page 207, with tons taken as metric tonnes of 1000 kg).
Q7. What percentage of the biosphere’s annual NPP comes from the oceans, according to NCERT’s figures?
55 ÷ 170 × 100 ≈ 32 per cent (32.4), from about 70 per cent of the surface (our arithmetic from NCERT page 207).
Q8. Why is productivity expressed per unit area per year, as g m yr or (kcal m) yr?
Because it is a rate: dividing by area and by time puts ecosystems of different sizes, measured over different periods, on one scale, so that the productivity of different ecosystems can be compared (NCERT page 207).
Q9. Which organisms can use the net primary productivity of an ecosystem, and why is NPP smaller than GPP?
The heterotrophs, herbivores and decomposers, use the NPP. It is smaller than GPP because a considerable amount of GPP is used by the plants themselves in respiration (R): NPP = GPP − R (NCERT page 207).