A forest has a GPP of 2,000 g C/m²/year. Autotrophic respiration consumes 800 g C/m²/year. A researcher asks: how much carbon is actually available to herbivores and decomposers? Which answer is correct?
A2,000 g C/m²/year — all photosynthetically fixed carbon is available to the food web
B1,200 g C/m²/year — NPP = GPP minus autotrophic respiration
C800 g C/m²/year — respiration represents the net energy gain of the ecosystem
D2,800 g C/m²/year — GPP and respiration together represent total carbon turnover
NPP = GPP − Ra = 2,000 − 800 = 1,200 g C/m²/year. Only this remainder — the carbon fixed beyond what producers use for their own metabolism — accumulates as biomass available to consumers. The most common error is treating GPP (total photosynthesis) as the energy available to the food web, ignoring that producers must burn a large fraction of their own production just to stay alive and grow.
Question 2 Multiple Choice
Two ecosystems have equal GPP. Ecosystem A's NPP is 90% of its GPP. Ecosystem B's NPP is only 35% of its GPP. Which statement best describes the consequence for animal communities?
ABoth ecosystems support equal animal biomass because their GPP is identical
BEcosystem B supports more animals because higher autotrophic respiration indicates more metabolic activity
CEcosystem A can support substantially more consumer biomass because a larger fraction of fixed carbon becomes available as biomass — NPP, not GPP, is the constraint on food webs
DGPP is the only ecologically relevant productivity measure; NPP/GPP ratio is a technical detail
Consumer biomass is ultimately constrained by NPP — the energy that actually enters the food web as biomass. Ecosystem A has NPP ≈ 0.90 × GPP while Ecosystem B has NPP ≈ 0.35 × GPP. Even with identical GPP, Ecosystem A has roughly 2.6× more energy available to herbivores and decomposers. Comparing GPP values without accounting for autotrophic respiration gives a misleading picture of how much the ecosystem can actually support.
Question 3 True / False
NPP, not GPP, determines the maximum biomass available to herbivores and all higher trophic levels.
TTrue
FFalse
Answer: True
GPP includes energy that producers use for their own cellular respiration (Ra) — energy that is converted to heat and never enters the food web as biomass. Only NPP (GPP − Ra) accumulates as plant tissue — leaves, wood, seeds, roots — that consumers can eat or decomposers can break down. NPP is the fundamental energy budget constraint on everything above the producer level in a food web.
Question 4 True / False
A region with high GPP necessarily supports more consumer biomass than a region with lower GPP.
TTrue
FFalse
Answer: False
What matters is NPP (GPP − Ra), not GPP alone. A high-GPP ecosystem with very high autotrophic respiration rates may have lower NPP than a lower-GPP ecosystem whose producers are more efficient at converting fixed carbon to biomass. Consumer biomass is constrained by NPP. Two ecosystems with the same GPP can support dramatically different food webs depending on how much of that GPP the producers themselves consume. Comparing GPP without knowing Ra leads to incorrect predictions.
Question 5 Short Answer
Why do ecologists use NPP rather than GPP as the key measure of ecosystem productivity when studying food webs and energy flow?
Think about your answer, then reveal below.
Model answer: Because GPP includes the energy producers use for their own metabolism (autotrophic respiration, Ra), which is never available to consumers. NPP = GPP − Ra is what actually accumulates as biomass — the plant tissue that herbivores eat and decomposers process. NPP is the energy 'available to the rest of the food web,' so it is the true constraint on how many organisms the ecosystem can support at higher trophic levels. GPP tells you how much sunlight the ecosystem captures; NPP tells you how much of that capture actually enters the food web.
The factory analogy from the explainer captures this: GPP is total production-line output, Ra is the energy cost of running the factory (workers' meals, machinery, lighting), and NPP is the finished goods available for consumers. You cannot feed consumers with the energy used to run the factory.