NEET 2024 — Biology solution

NEET2024Biology

In an ecosystem if the Net Primary Productivity (NPP) of first trophic level is $100 x$ (kcal m $^{- 2}$ ) yr $^{- 1}$ , what would be the GPP (Gross Primary Productivity) of the third trophic level of the same ecosystem?

A
$10 x$ (kcal m $^{- 2}$ ) yr $^{- 1}$
B
$\frac{100 x}{3 x}$ (kcal m $^{- 2}$ ) yr $^{- 1}$
C
$\frac{x}{10}$ (kcal m $^{- 2}$ ) yr $^{- 1}$
D
$x$ (kcal m $^{- 2}$ ) yr $^{- 1}$

Solution

NPP represents the biomass available to the next trophic level. Therefore, the NPP of producers ( $100 x$ ) is available to the primary consumers (second trophic level).

According to the 10% law, only 10% of this energy is transferred to the next trophic level.

Therefore, the energy available at the third trophic level is:

$\frac{10}{100} \times 100 x = 10 x$

Hence,

$10 x$

Note: Strictly speaking, GPP (Gross Primary Productivity) is defined only for producers (the first trophic level). Consumers do not have a GPP. Therefore, asking for the "GPP of the third trophic level" is conceptually incorrect.

However, in many entrance-exam questions, the term "productivity" is often used loosely and the examiner usually intends to ask for the energy available at the third trophic level. In that interpretation, the expected answer is:

$10 x$

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