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

In a savanna ecosystem, primary producers have a gross primary productivity (GPP) of 20000 kcal m−2 yr−120{}000\text{ kcal m}^{-2}\text{ yr}^{-1}, but consume 50%50\% of this energy through autotrophic cellular respiration (RAR_A). Assuming a constant ecological efficiency of 10%10\% for energy transfer between consecutive trophic levels, what is the total amount of energy per square meter per year available to tertiary consumers?

  1. 10 kcal m−2 yr−110\text{ kcal m}^{-2}\text{ yr}^{-1}Answer
  2. B
    20 kcal m−2 yr−120\text{ kcal m}^{-2}\text{ yr}^{-1}
  3. C
    100 kcal m−2 yr−1100\text{ kcal m}^{-2}\text{ yr}^{-1}
  4. D
    1000 kcal m−2 yr−11{}000\text{ kcal m}^{-2}\text{ yr}^{-1}

Answer

The total energy available to tertiary consumers is 10 kcal m−2 yr−110\text{ kcal m}^{-2}\text{ yr}^{-1}.
To find the energy available to tertiary consumers, first calculate Net Primary Productivity: NPP=GPP−RA=20000−10000=10000 kcal m−2 yr−1\text{NPP} = \text{GPP} - R_A = 20{}000 - 10{}000 = 10{}000\text{ kcal m}^{-2}\text{ yr}^{-1}. Then apply the 10%10\% transfer efficiency across three consumer steps: primary consumers receive 1000 kcal m−2 yr−11{}000\text{ kcal m}^{-2}\text{ yr}^{-1}, secondary consumers receive 100 kcal m−2 yr−1100\text{ kcal m}^{-2}\text{ yr}^{-1}, and tertiary consumers receive 10 kcal m−2 yr−110\text{ kcal m}^{-2}\text{ yr}^{-1}.

Step-by-Step Solution

1
Calculate Net Primary Productivity (NPP) of the primary producers
NPP=GPP−RA=20000−(0.50×20000)=10000 kcal m−2 yr−1\text{NPP} = \text{GPP} - R_A = 20{}000 - (0.50 \times 20{}000) = 10{}000\text{ kcal m}^{-2}\text{ yr}^{-1}
Only energy stored as organic biomass after metabolic respiration is available to herbivores.
2
Calculate energy transferred to primary consumers (trophic level 2)
Energy2=10% of 10000=1000 kcal m−2 yr−1\text{Energy}_2 = 10\% \text{ of } 10{}000 = 1{}000\text{ kcal m}^{-2}\text{ yr}^{-1}
According to the 10%10\% law of energy transfer, only one-tenth of available energy passes to primary consumers.
3
Calculate energy transferred to secondary consumers (trophic level 3)
Energy3=10% of 1000=100 kcal m−2 yr−1\text{Energy}_3 = 10\% \text{ of } 1{}000 = 100\text{ kcal m}^{-2}\text{ yr}^{-1}
Apply the 10%10\% transfer efficiency from primary consumers to secondary consumers.
4
Calculate energy transferred to tertiary consumers (trophic level 4)
Energy4=10% of 100=10 kcal m−2 yr−1\text{Energy}_4 = 10\% \text{ of } 100 = 10\text{ kcal m}^{-2}\text{ yr}^{-1}
Apply the 10%10\% transfer efficiency from secondary consumers to tertiary consumers.

Key Concept

Calculation of net primary productivity and progressive thermodynamic energy attenuation across trophic levels.
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Topics

Energy Flow, Food Chains, and Ecological Pyramids

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