Consider the following statements regarding the productivity of an ecosystem:

Question

Consider the following statements regarding the productivity of an ecosystem:

i. Gross primary productivity is the rate of formation of organic matter by consumers.

ii. Net primary productivity is the variable biomes for the consumption to heterotrophs.

The correct answer/s is/are:

1. A. i only
2. B. ii only (Correct answer)
3. C. Both i and ii
4. D. Neither i nor ii

Correct Answer

Option B — ii only

Detailed Solution & Explanation

The correct answer is ii only.

Key Points

• Statement i is incorrect. Gross Primary Productivity (GPP) is the rate of production of organic matter by producers (autotrophs) during photosynthesis — not by consumers. Consumers do not fix energy; they are secondary producers of biomass at best.
• Statement ii is correct. Net Primary Productivity (NPP) is the biomass that remains after the producers' own respiratory losses, and it is this that is available to heterotrophs — the herbivores and decomposers of the ecosystem.
• The relationship is simply NPP = GPP − R, where R is respiration by the producers.

Additional Information

• Productivity is a rate, not a stock. It is expressed as grams of biomass, or kilocalories, per square metre per year (g m⁻² yr⁻¹). Standing crop biomass is the stock — the amount present at a given moment — and the two must not be confused. Oceanic phytoplankton have a low standing crop but a very high turnover rate.
• Secondary productivity is the rate at which consumers assimilate energy and build new organic matter — the same idea applied one trophic level up.
• The 10% law of Lindeman: only about 10% of the energy at one trophic level is transferred to the next; the rest is lost as heat in respiration and in unassimilated matter. This is why food chains rarely exceed four or five trophic levels, and why an inverted pyramid of energy is impossible.
• Global figures: the annual net primary productivity of the whole biosphere is roughly 170 billion tonnes of dry organic matter, of which the oceans contribute only about 55 billion tonnes despite covering 71% of the surface — the oceans are, in productivity terms, largely a desert with fertile margins.
• The most productive ecosystems are tropical rainforests, coral reefs, estuaries and swamps; the least productive are open oceans, deserts and tundra.
• Ecological pyramids: the pyramid of energy is always upright; the pyramid of numbers can be inverted, as in a single tree supporting many insects; the pyramid of biomass can be inverted in aquatic systems, where a small phytoplankton biomass supports a larger zooplankton biomass at any instant.