How does energy flow through an ecosystem, and why is it lost between levels?
Topic 8.2 Energy Flow Through Ecosystems: explain how energy flows through trophic levels and why energy is lost between levels.
A focused answer to AP Biology Topic 8.2, covering trophic levels, food chains and webs, the 10 percent rule, energy pyramids, productivity, and why energy decreases up the chain, with a worked energy-transfer calculation.
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What this topic is asking
The College Board (Topic 8.2) wants you to explain how energy flows through an ecosystem from producers through trophic levels, why energy decreases at each level (the ~10% rule), and how this shapes food chains, food webs and energy pyramids.
Trophic levels and food chains
Why energy decreases: the 10% rule
Productivity
Net primary productivity is the key figure for the rest of the food web, because it is the energy actually available to consumers. It varies hugely between ecosystems: tropical rainforests and coral reefs are highly productive, while deserts and open ocean are low, which is why they support very different amounts of consumer biomass.
It is important to separate two things that move through an ecosystem. Energy flows one way (sunlight, to chemical energy in producers, to consumers, and out as heat) and must be constantly resupplied by the sun. Matter (carbon, nitrogen, water) cycles, being used and reused as decomposers return it to the environment. A common exam task is to read an energy-flow diagram and distinguish the one-way flow of energy from the cycling of matter.
Try this
Q1. State roughly what fraction of energy passes from one trophic level to the next. [1 point]
- Cue. About 10% (the other ~90% is lost, mostly as heat from respiration).
Q2. Explain why food chains rarely have more than four or five trophic levels. [2 points]
- Cue. Because only ~10% of energy passes up each level, there is too little energy left after several transfers to support another level of consumers.
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 2020 (style)4 marksSection II (long FRQ excerpt, data). Producers in an ecosystem capture 10000 kJ of energy. (a) Using the 10% rule, calculate the energy available to the primary consumers and to the secondary consumers. (b) Explain why energy decreases at each higher trophic level.Show worked answer →
A 4-point calculate-and-explain FRQ on energy flow.
(a) Calculate (2 points): (1 point) primary consumers receive about kJ; (1 point) secondary consumers receive about kJ.
(b) Explain (2 points): (1 point) at each level, most energy is lost as heat through cellular respiration, and some is used for movement, growth and not all of an organism is eaten or digested; (1 point) so only about 10% of the energy at one level is stored as biomass available to the next, which is why energy decreases up the chain.
Markers reward the correct 10% calculations and explaining the loss as respiration heat plus uneaten or undigested material.
AP 2017 (style)1 marksSection I (multiple choice). In an energy pyramid, the amount of available energy is greatest at the level of the: (A) producers. (B) primary consumers. (C) secondary consumers. (D) top predators.Show worked answer →
The correct answer is (A).
Producers form the base of the energy pyramid and contain the most available energy. Energy decreases at each higher trophic level (about 90% is lost between levels), so producers always have the most and top predators the least.
Related dot points
- Topic 8.3 Population Ecology: explain exponential and logistic growth, carrying capacity, and the factors that regulate population size.
A focused answer to AP Biology Topic 8.3, covering exponential and logistic growth, carrying capacity, growth rate calculations, and the factors that shape population size, with a worked growth-rate calculation.
- Topic 8.5 Community Ecology: explain the types of interactions between species in a community and their effects on the species involved.
A focused answer to AP Biology Topic 8.5, covering competition, predation, the niche, symbiosis (mutualism, commensalism, parasitism), keystone species and trophic relationships, with a worked interaction example.
- Topic 3.5 Photosynthesis: explain how the light-dependent reactions and the Calvin cycle capture light energy and use it to fix carbon dioxide into sugar.
A focused answer to AP Biology Topic 3.5, covering the light-dependent reactions, the electron transport chain, chemiosmosis, the Calvin cycle, and how light energy is converted to the chemical energy of sugars.
- Topic 3.6 Cellular Respiration: explain how glycolysis, the Krebs cycle and oxidative phosphorylation release energy from glucose to make ATP, and how fermentation allows ATP production without oxygen.
A focused answer to AP Biology Topic 3.6, covering glycolysis, the Krebs cycle, oxidative phosphorylation, chemiosmosis, the role of oxygen, and fermentation, with the link back to photosynthesis.
- Topic 8.6 Biodiversity: explain how biodiversity contributes to ecosystem stability and resilience.
A focused answer to AP Biology Topic 8.6, covering species and genetic diversity, how diversity supports ecosystem stability and resilience, the effects of low diversity, and a worked example using a diversity comparison.
Sources & how we know this
- AP Biology Course and Exam Description — College Board (2020)