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Difficulty: Very hardEnergy Flow, Food Chains, and Ecological Pyramids

In a humid tropical forest ecosystem, the producers capture solar energy resulting in a Gross Primary Productivity (GPP) of 40000 kJ/m2/yr40{}000\text{ kJ/m}^2/\text{yr}. Autotrophic respiration accounts for 55%55\% of this captured energy. Primary consumers assimilate 10%10\% of the Net Primary Productivity (NPP) available to them, while spending 60%60\% of their assimilated energy on cellular respiration. What is the total energy available for secondary consumers at the third trophic level?

  1. 720 kJ/m2/yr720\text{ kJ/m}^2/\text{yr}Answer
  2. B
    1800 kJ/m2/yr1{}800\text{ kJ/m}^2/\text{yr}
  3. C
    1600 kJ/m2/yr1{}600\text{ kJ/m}^2/\text{yr}
  4. D
    −720 kJ/m2/yr-720\text{ kJ/m}^2/\text{yr}, representing an inverted energy flow pyramid

Answer

The total energy available for secondary consumers is 720 kJ/m2/yr720\text{ kJ/m}^2/\text{yr}.
The correct calculation yields 720 kJ/m2/yr720\text{ kJ/m}^2/\text{yr} by systematically subtracting autotrophic metabolic costs (55%55\% of GPP), taking the 10%10\% ecological assimilation rate into primary consumers, and subtracting herbivore metabolic maintenance (60%60\% of assimilated energy).

Step-by-Step Solution

1
Calculate Net Primary Productivity (NPP) of the producers.
NPP=GPP−Rproducers=40000−(0.55×40000)=18000 kJ/m2/yr\text{NPP} = \text{GPP} - R_{\text{producers}} = 40{}000 - (0.55 \times 40{}000) = 18{}000\text{ kJ/m}^2/\text{yr}.
Plant respiration consumes 55%55\% of GPP, leaving 45%45\% as biomass available to herbivores.
2
Determine energy assimilated by primary consumers.
Assimilated Energy=0.10×18000=1800 kJ/m2/yr\text{Assimilated Energy} = 0.10 \times 18{}000 = 1{}800\text{ kJ/m}^2/\text{yr}.
Primary consumers transfer 10%10\% of the available plant biomass (NPP) into their tissue assimilation pathway.
3
Deduct respiratory losses of primary consumers to find net secondary productivity available for the third trophic level.
Available Energy=1800×(1−0.60)=720 kJ/m2/yr\text{Available Energy} = 1{}800 \times (1 - 0.60) = 720\text{ kJ/m}^2/\text{yr}.
Herbivores expend 60%60\% of their assimilated energy on metabolic processes, leaving 40%40\% incorporated into new biomass accessible to secondary consumers.

Key Concept

Energy Transfer Efficiency and Productivity Calculations Across Trophic Levels
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Topics

Energy Flow, Food Chains, and Ecological Pyramids

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