Why would net primary productivity be more useful than gross primary productivity for predicting the number of species and population sizes of heterotrophic organisms found in an ecosystem?

Question

a. GPP can vary as the amount of sunlight varies, but because the amount of energy used by plants in cellular respiration is constant, NPP will therefore always be a constant.

b. The magnitude of GPP is dependent on the amount of sunlight captured by plants while the magnitude of NPP is independent of the amount of sunlight.

c. NPP is the “excess” amount of photosynthetic output available after plants have used their photosynthate to maintain themselves and therefore reflects the total amount of energy available to heterotrophic organisms.

d. NPP will always be a smaller number than GPP and will therefore be a conservative measure of how many heterotrophic organisms can be expected to be supported in an ecosystem.

Answer 1

Hello Danielle S.

It is important to keep in mind that Net Primary Productivity is essentially Gross Primary Productivity - Metabolism / Maintenance Cost, or (NPP = GPP - M). The multiple choice answer ought to reflect this answer. The best answer appears to be C because NPP is indeed the "excess" amount of photosynthetic output after some photosynthate is used for maintenance. Excess photosynthetic output may be anabolized sugars stored as starch, which heterotrophic organisms can consume.

The answer is not A because the amount of energy used by plants is not constant. The amount of energy produced depends on a number of factors, including sunlight, CO2 availability, sugar sources, and sugar or metabolic sinks.

The answer is not B because NPP is not independent of sunlight. Since GPP is dependent on amount of sunlight, then NPP is well. See above formula.

The answer is not D because although NPP will be a smaller number than GPP and will thus be a more conservative estimation of heterotroph populations, it does not explain why NPP would be more accurate, which answer C does.

Thank you for asking. Thoughts, comments, questions, concerns, criticism? Feel free to respond.