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NCERT Exemplar · Q3

Q.Productivity is the rate of production of biomass expressed in terms of:
i. (kcal m^-3) yr^-1
ii. g^-2 yr^-1
iii. g^-1 yr^-1
iv. (kcal m^-2) yr^-1

(a) ii
(b) iii
(c) ii and iv
(d) i and iii
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✓ Free question

Productivity is measured as biomass produced per unit area per unit time, so the correct units are those that include area (m⁻²) and time (yr⁻¹), making options ii and iv the valid ones.

When we talk about productivity in ecology, we are referring to the rate at which biomass — the organic matter produced by plants through photosynthesis — is generated in an ecosystem. This is a fundamental concept in the study of energy flow and nutrient cycling. The NCERT textbook defines productivity as the rate of biomass production, and it expresses this rate in terms of weight (grams) or energy (kilocalories) per unit area per unit time.

The key here is the phrase "per unit area." Productivity is always measured over a specific area of land or water, because we need to compare how productive different ecosystems are. For example, a tropical rainforest produces far more biomass per square metre than a desert. So the units must include an area component, typically square metres (m⁻²).

Now look at the options given:

  • i. (kcal m⁻³) yr⁻¹ — This uses cubic metres (m⁻³), which is a unit of volume, not area. This would be appropriate for measuring productivity in a three‑dimensional space like a water column, but the standard ecological definition for terrestrial ecosystems uses area. The NCERT, however, does mention that for aquatic ecosystems, productivity can be expressed per unit volume, but the question is about the general expression of productivity. In the standard NCERT treatment, productivity is given as weight or energy per unit area per year. So option i is not the typical expression.

  • ii. g⁻² yr⁻¹ — This is grams per square metre per year. It correctly includes area (m⁻²) and time (yr⁻¹). This is a standard unit.

  • iii. g⁻¹ yr⁻¹ — This is grams per gram per year, which is a ratio (like relative growth rate), not a rate of biomass production per unit area. This is not a measure of productivity in the sense used in ecosystem ecology.

  • iv. (kcal m⁻²) yr⁻¹ — This is kilocalories per square metre per year. It also correctly includes area and time, and is the energy‑based equivalent of option ii.

Note

The NCERT textbook explicitly states that productivity is expressed in terms of weight (g⁻² yr⁻¹) or energy (kcal m⁻² yr⁻¹). It does not use volume‑based units for the general definition.

Therefore, the correct choices are ii and iv.

Important

Always remember: productivity is a rate per unit area per unit time. If the unit lacks an area component (like m⁻²), it is not a measure of productivity in the standard ecological sense.

✓Final answer

In short, productivity is expressed as biomass per unit area per year, so the correct options are ii (g⁻² yr⁻¹) and iv (kcal m⁻² yr⁻¹), which corresponds to answer (C).

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