Question

In a stable ecosystem, the relationship between Gross Primary Productivity (GPP) and community respiration (R) is:

• A. GPP $<$ R
• B. GPP = R
• C. GPP $>$ R always
• D. GPP independent of R

Hint:

The P/R ratio (Production to Respiration) is a reliable indicator of ecosystem maturity.
In a stable climax community, the P/R ratio is always equal to $1$.
If P/R $>$ $1$, it is an autotrophic or successional system.

Answer 1

The Correct Option is B

A useful way to settle this is to ask what would actually happen to the ecosystem over time under each proposed relationship between Gross Primary Productivity (\(GPP\)) and total community respiration (\(R\)), then see which outcome matches what we mean by a stable system.

1. \(GPP \lt R\): if the community keeps burning more energy than producers replace, the standing biomass would shrink year after year until the ecosystem thinned out or collapsed. That is the signature of a declining system, not a stable one.
2. \(GPP = R\): if fixation and consumption stay equal, biomass neither piles up nor drains away, so the ecosystem simply persists in its current form indefinitely. That outcome, an unchanging biomass level over time, is precisely what stability means.
3. \(GPP \gt R\) always: if production permanently outran respiration, biomass would keep piling up without end. In practice this only happens temporarily while a young ecosystem is building itself up; it cannot continue forever because space, nutrients, and light are limited, so this cannot be the steady, long-term condition of a mature, stable system.
4. GPP independent of R: if the two were truly unrelated, respiration would not rise or fall with how much the producers were fixing, which contradicts the direct dependence of consumer and decomposer activity on the organic matter that producers supply.

Only equality between energy capture and energy expenditure leaves the ecosystem unchanged over time, which is the defining feature of stability, so the correct answer is GPP = R.