Reading Energy and Matter Diagrams Quantitatively

You’ll be able to

Apply the 10% rule across trophic levels and compute available energy
Distinguish gross from net primary productivity and use the relationship
Explain why matter cycles while energy flows one way

The one asymmetry the whole course rests on

Energy flows through an ecosystem; matter cycles within it. Energy enters as sunlight, moves up trophic levels losing most of itself as heat at each transfer, and leaves the system permanently — it cannot be reused. Matter has nowhere to go: the same carbon, nitrogen and phosphorus atoms are used, released and taken up again indefinitely. Almost every Unit 1 free-response question turns on keeping these straight, and the giveaway phrasing is directional: a question about energy asks how much reaches a level, while a question about matter asks where an atom goes next.

The 10% rule, applied properly

Roughly 10% of the energy at one trophic level becomes biomass at the next; the other 90% is lost, mostly as heat from cellular respiration, with some never consumed and some passed as waste. Two consequences the exam tests. First, the arithmetic compounds: a producer level with 10,000 kcal supports about 1,000 at primary consumers, 100 at secondary and 10 at tertiary — which is why food chains rarely exceed four or five levels and why top predators need enormous ranges. Second, it explains a policy result: eating lower on the food chain feeds more people from the same land, because each level discarded recovers a factor of ten. Note that 10% is a rough average, and real transfer efficiencies range from about 1% to 20%.

GPP, NPP, and respiration

Three quantities that students routinely conflate. Gross primary productivity (GPP) is the total energy producers capture by photosynthesis. Respiration (R) is what they burn to stay alive. Net primary productivity (NPP) = GPP − R is what is left over and stored as biomass — and it is the only part available to consumers, which is why NPP rather than GPP is the number that matters for the rest of the food web. Units are energy per area per time, typically kcal/m²/year or grams of carbon per square meter per year. Highest NPP occurs in tropical rainforests, estuaries, swamps and coral reefs; lowest in deserts, tundra and the open ocean — although the open ocean's enormous area makes its total contribution large despite low productivity per square meter, which is a distinction worth stating precisely.

Productivity and trophic transfer

NPP = GPP − R energy at next level ≈ 0.10 × energy at current level

NPP is what consumers can eat. The 10% figure is an average; real efficiencies run from about 1% to 20%.

Worked example

A grassland has a GPP of 12,000 kcal/m²/year and plant respiration of 5,000 kcal/m²/year. Find NPP, and estimate the energy reaching secondary consumers.

1.NPP = GPP − R = 12,000 − 5,000 = 7,000 kcal/m²/year. This is the energy stored as plant biomass and available to herbivores.
2.Primary consumers receive about 10% of NPP: 0.10 × 7,000 = 700 kcal/m²/year.
3.Secondary consumers receive about 10% of that: 0.10 × 700 = 70 kcal/m²/year.
4.Check the compounding: two transfers means 7,000 × 0.01 = 70, which confirms the arithmetic.

Answer: NPP = 7,000 kcal/m²/year, and secondary consumers receive roughly 70 kcal/m²/year. Note that the 5,000 kcal of respiration never enters the food web at all — it leaves as heat.

On the exam

Start every trophic calculation from NPP, not GPP. Using GPP inflates the answer by the amount of plant respiration and is the most common error on this question type. If the prompt gives you GPP and respiration, the subtraction is the first thing the rubric looks for.

Checkpoint

An ecosystem has GPP of 8,000 kcal/m²/year and NPP of 3,000 kcal/m²/year. Plant respiration is

Checkpoint

Which statement correctly contrasts energy and matter in an ecosystem?

Answer the 2 checkpoints as you read.