How much energy do producers capture, and what determines the productivity of an ecosystem?
Topic 1.8 Primary Productivity: define gross and net primary productivity, explain the factors that control them, and calculate net primary productivity from data.
A focused answer to APES Topic 1.8, covering gross and net primary productivity, respiration, the GPP-NPP relationship, limiting factors, productivity across biomes, and ecological efficiency, with a full worked NPP calculation.
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What this topic is asking
The College Board (Topic 1.8) wants you to distinguish gross primary productivity (GPP) from net primary productivity (NPP), to apply the relationship , and to explain what controls productivity. This is a quantitative topic: you should be able to calculate NPP from data and compare productivity across ecosystems.
Gross and net primary productivity
The key relationship, which the exam expects you to use confidently, is:
Productivity is usually expressed as energy per area per time (for example kcal/m^2/year) or as biomass per area per time (g/m^2/year).
Why NPP matters
What limits productivity
Productivity is controlled by the resources producers need:
- Sunlight: the energy source for photosynthesis; limited in deep water, dense shade and at high latitudes.
- Water: essential for photosynthesis; scarce in deserts.
- Temperature: enzymes and growth slow in the cold; tundra and boreal systems are limited by it.
- Nutrients: nitrogen and phosphorus are common limiting nutrients; productivity rises where they are abundant.
Because of these factors, productivity varies enormously between biomes.
Productivity across ecosystems
The most productive ecosystems combine warmth, water, light and nutrients: tropical rainforests, estuaries, coral reefs and wetlands have high NPP. The least productive are deserts (limited by water), tundra (limited by temperature) and the open ocean (limited by nutrients), which despite its vast area has low NPP per unit area. This pattern ties Topic 1.8 directly back to the biomes of Topics 1.2 and 1.3: the same abiotic factors that define a biome also set its productivity. Productivity also connects forward to the energy-flow topics, because NPP is the starting amount of energy that then passes, with heavy losses, up the trophic levels. When you are asked why a particular ecosystem supports so much (or so little) life, the answer almost always traces back to which resource is limiting its primary productivity.
Try this
Q1. Define gross primary productivity. [1 point]
- Cue. The total rate at which producers capture and store energy by photosynthesis, before any is used in respiration.
Q2. Calculate the NPP of an ecosystem with a GPP of 10,000 kcal/m^2/year where producers respire 4,500 kcal/m^2/year. [1 point]
- Cue. kcal/m^2/year.
Exam-style practice questions
Practice questions written in the style of College Board exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
AP 2021 (style)4 marksSection II (FRQ, quantitative). In a grassland, producers fix energy by photosynthesis at a gross primary productivity of 20,000 kcal per square meter per year. The producers use 12,000 kcal per square meter per year in cellular respiration. (a) Define net primary productivity. (b) Calculate the net primary productivity of this grassland. (c) Calculate the percentage of gross primary productivity that is available to consumers. (d) Identify two factors that could limit primary productivity in this ecosystem.Show worked answer →
A 4-point quantitative FRQ on primary productivity.
(a) Define (1 point): net primary productivity (NPP) is the energy fixed by producers that remains after their own respiration, that is, the energy available to the next trophic level.
(b) Calculate (1 point): kcal/m^2/year.
(c) Calculate (1 point): percentage available = .
(d) Identify (1 point): any two of sunlight, water, temperature, or nutrient (such as nitrogen or phosphorus) availability.
Markers reward a correct NPP definition, the subtraction GPP minus respiration, the percentage calculation, and two valid limiting factors.
AP 2019 (style)1 marksSection I (multiple choice). Which equation correctly relates the productivity terms? (A) NPP = GPP + respiration (B) GPP = NPP - respiration (C) NPP = GPP - respiration (D) NPP = respiration - GPP. Justify your choice.Show worked answer →
A 1-point MCQ on the productivity relationship. The answer is (C).
Net primary productivity equals gross primary productivity minus the energy producers use in respiration: NPP = GPP - respiration. Option (A) adds respiration incorrectly, (B) rearranges it wrongly, and (D) reverses it. The trap is forgetting that respiration is subtracted because producers consume some of the energy they fix.
Related dot points
- Topic 1.9 Trophic Levels: describe the trophic levels of an ecosystem and explain the roles of producers, consumers and decomposers in transferring energy and matter.
A focused answer to APES Topic 1.9, covering producers, primary, secondary and tertiary consumers, decomposers and detritivores, autotrophs and heterotrophs, and how energy and matter move through trophic levels, with a worked classification question.
- Topic 1.10 Energy Flow and the 10% Rule: explain how energy is lost between trophic levels, apply the 10% rule, and calculate energy transfer and ecological efficiency.
A focused answer to APES Topic 1.10, covering the one-way flow of energy, the 10% rule, why energy is lost as heat and through respiration, ecological efficiency, and energy pyramids, with full worked multi-level energy calculations.
- Topic 1.11 Food Chains and Food Webs: describe how food chains and food webs represent the flow of energy and matter, and predict the effects of changes to a food web.
A focused answer to APES Topic 1.11, covering food chains and food webs, how energy and matter flow through them, keystone species, trophic cascades, and predicting the effects of removing a species, with a worked food-web disruption question.
- Topic 1.4 The Carbon Cycle: describe the major reservoirs and fluxes of the carbon cycle and explain how natural processes and human activities move carbon between them.
A focused answer to APES Topic 1.4, covering carbon reservoirs and fluxes, photosynthesis and respiration, decomposition, combustion, the ocean as a carbon sink, and how fossil fuel burning alters the cycle, with a worked carbon-flux calculation.
- Topic 1.2 Terrestrial Biomes: describe the global distribution of the major terrestrial biomes and explain how temperature and precipitation determine the type of biome found in a region.
A focused answer to APES Topic 1.2, covering the major terrestrial biomes, how temperature and precipitation define them, latitude and altitude patterns, and biome shifts under a changing climate, with a worked climograph question.
Sources & how we know this
- AP Environmental Science Course and Exam Description — College Board (2020)