In a terrestrial ecosystem, the Gross Primary Productivity (GPP) of...

The Gross Primary Productivity (GPP) of producers is $20,000 \text{ kcal m}^{-2} \text{ yr}^{-1}$.

The energy utilized for respiration (R) by producers is $20\%$ of GPP.

$R = \dfrac{20}{100} \times 20,000 = 4,000 \text{ kcal m}^{-2} \text{ yr}^{-1}$

The Net Primary Productivity (NPP) is the energy available to the primary consumers (herbivores).

$\text{NPP} = \text{GPP} - R$

$\text{NPP} = 20,000 - 4,000 = 16,000 \text{ kcal m}^{-2} \text{ yr}^{-1}$

According to Lindeman's $10\%$ law of energy transfer, only $10\%$ of the energy is passed on to the next trophic level.

Energy available to primary consumers (herbivores) = $16,000 \text{ kcal m}^{-2} \text{ yr}^{-1}$

Energy available to secondary consumers (primary carnivores) = $10\%$ of $16,000 = 1,600 \text{ kcal m}^{-2} \text{ yr}^{-1}$

Energy available to tertiary consumers (secondary carnivores) = $10\%$ of $1,600 = 160 \text{ kcal m}^{-2} \text{ yr}^{-1}$

Answer: $160 \text{ kcal m}^{-2} \text{ yr}^{-1}$