Here is the puzzle at the heart of this page. A young ecosystem captures very little energy, yet it piles on biomass fast. A mature rainforest captures enormous amounts of energy, yet its biomass barely grows at all. Once you see why, this whole topic falls into place.
📘 What you need to know
Gross productivity (GP) is the total energy captured by producers in photosynthesis.
Net productivity (NP) is what is left after the producers respire. It is the energy available for growth and for everything that eats them.
NP = GP − R. Both are rates: energy per unit area per unit time.
Early succession: GP is low (few producers, poor conditions) but NP as a percentage of GP is high, so biomass builds quickly.
Mid succession: GP rises as more and bigger producers arrive. NP still rises but starts to slow, then peaks.
Late succession: GP is high but respiration is nearly as high, so NP falls close to zero. Biomass stops accumulating and simply cycles.
At the climax the productivity to respiration ratio is about 1 — the system uses energy as fast as it makes it.
The equation everything hangs on
NP = GP − R • units: kJ m−2 yr−1
Early stages: small harvest, big share kept
On bare rock or new sand there is not much photosynthesis going on, so gross productivity is low. Three reasons:
Harsh conditions — bare rock, no soil, no water held.
Few nutrients — nothing for producers to build tissue with.
Low density of producers — only mosses and lichens, so very little leaf area catching light.
But look at what happens to the energy they do capture. There are almost no consumers, and the plants themselves are small with very little non-photosynthetic tissue to keep alive. Total respiration is tiny. So a large share of the captured energy is left over as net productivity and goes straight into new growth.
That is why a pioneer community, which produces very little in absolute terms, still gains biomass quickly compared with the amount it captures.
Mid stages: the peak
As soil forms, bigger plants can establish. More producers with more leaves means GP rises, and a mixture of plant heights and shapes catches light more efficiently.
Net productivity keeps rising too, but more slowly, and eventually it peaks. Two things pull it down:
Respiration rises. There are more organisms in total, including consumers and decomposers, and the plants themselves now have trunks, branches and roots that respire but do not photosynthesise.
Competition rises. Plants shade each other and compete for water and nutrients, so growth slows down.
Late stages: high production, almost no gain
At the climax, GP is at its highest — a mature forest is one of the most productive systems on Earth. But almost all of that energy is used up in respiration by the producers and by the huge community of consumers and decomposers living off them.
So NP drops towards zero. Biomass is no longer piling up; it is being recycled. A tree falls, decomposers break it down, the nutrients go back into the soil and a new tree grows in the gap. The total stays roughly constant.
The blue line is not a separate measurement. It is simply the distance between the green and red lines, which is why it can rise while both of the others are still rising.
The same idea as a share of the energy
Low GP with a big share kept beats high GP with almost nothing left over — if what you care about is how fast biomass accumulates.
Pioneer against climax, side by side
Measure
Pioneer community
Climax community
Gross productivity (GP)
Low
High
Total respiration (R)
Very low
Very high
Net productivity (NP)
Low in absolute terms, but rising fast
Close to zero
NP as a share of GP
High
Low
GP to R ratio
Well above 1
About 1
Biomass
Low, but increasing quickly
High and roughly constant
Energy flow
Simple and linear
Complex and cyclic
Nutrient cycling
Leaky, relies on outside inputs
Tight, nutrients recycled internally
If you only remember one sentence from this page, make it this: gross productivity rises all the way through succession, but net productivity rises and then falls. The two behave completely differently, and mixing them up is the single most expensive mistake in this topic.
Worked examples
WORKED EXAMPLE 1
In a young woodland, producers capture 12 000 kJ m−2 yr−1 and respire 4 200 kJ m−2 yr−1. Calculate NP and NP as a percentage of GP.
Step 1: use the equationNP = GP − R = 12 000 − 4 200NP = 7 800 kJ m−2 yr−1Step 2: turn it into a percentage7 800 / 12 000 × 100 = 65%NP is 65% of GPKeep the units on the final answer. Dropping them loses a mark on data questions.
WORKED EXAMPLE 2
A mature forest nearby has GP = 45 000 and R = 44 100 kJ m−2 yr−1. Calculate NP as a percentage of GP and explain what this tells you about the forest.
Step 1: find NPNP = 45 000 − 44 100 = 900 kJ m−2 yr−1Step 2: as a percentage900 / 45 000 × 100 = 2%NP is 2% of GPStep 3: what it means
Almost all the energy captured is used in respiration, so hardly any biomass is added each year. The forest is at or near its climax and is in a steady state.
Notice GP here is nearly four times the young woodland, yet NP is far lower. That is the whole point of this topic.
WORKED EXAMPLE 3
Explain why total biomass stops increasing in a climax community even though gross productivity is high. (3 marks)
Mark 1: what respiration is doing
The community now contains many producers, consumers and decomposers, so total respiration is very high.
Mark 2: the consequence for NP
Energy is being used up almost as fast as it is captured, so NP is close to zero.
Mark 3: what that means for biomass
With no surplus energy, new growth only replaces what dies, so biomass cycles between organisms instead of accumulating.
Production and respiration are balanced, so biomass stays constant
💡 Exam tip
Write out NP = GP − R before you calculate anything. It earns method marks even if the arithmetic slips.
Read carefully whether the question wants NP in absolute terms or as a share of GP. They change in opposite directions early on.
Say “productivity is a rate” if asked to compare it with biomass. Rate versus store is a classic one-mark distinction.
When describing a graph, give values and turning points: where NP peaks, where the lines converge.
Units are energy per area per time. Copy them from the question and keep them.
Link back to succession: rising GP is because there are more and bigger producers; rising R is because there are more organisms in total.
⚠ Common mix-up
NP is not biomass. NP is the rate at which energy becomes available for growth; biomass is the store that has built up.
Saying productivity falls at the climax. Gross productivity is at its highest there. It is net productivity that falls.
Thinking low NP means an unhealthy system. Near-zero NP is exactly what a mature, balanced ecosystem looks like.
Forgetting consumers and decomposers when explaining why respiration climbs.
Mixing up percentage and absolute values when comparing two stages.
Assuming the peak of NP is at the climax. It happens in the middle stages, well before the community settles.
Up next: r and K Reproductive Strategies — why the species that thrive in pioneer communities breed in a completely different way from the ones that dominate at the climax.
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