Trophic efficiency is the percentage of energy that transfers from one trophic level to the next in a food chain. The 10% rule (CED EK ENG-1.C.1) says only about 10% gets passed on, while roughly 90% is lost as heat and through respiration.
Trophic efficiency is how much of the energy at one feeding level actually makes it to the next one up. The short answer from the CED: about 10%. That's the 10% rule (EK ENG-1.C.1). If producers hold 100,000 units of energy, the herbivores that eat them get roughly 10,000, the carnivores that eat those herbivores get about 1,000, and so on up the chain.
Where does the missing 90% go? It doesn't vanish. Organisms burn most of their energy just staying alive through cellular respiration, and that energy leaves the system as heat. This connects straight to the laws of thermodynamics (EK ENG-1.C.2): energy can't be created or destroyed, but every transfer is inefficient and loses usable energy to heat. So energy flows through an ecosystem one way and exits as heat. It doesn't cycle back like nutrients do.
This lives in Unit 1 (The Living World: Ecosystems), Topic 1.10, and supports learning objective AP Enviro 1.10.A, which asks you to determine how energy decreases as it flows through an ecosystem. It's the backbone of energy pyramids and the reason food chains rarely have more than four or five links. Once you only have about 0.01% of the original energy left, there isn't enough to support another level of predators. The 10% rule is also the math engine behind a ton of MCQ calculation questions, so getting comfortable multiplying and dividing by 10 here pays off all year.
Keep studying AP® Environmental Science Unit 1
Cellular Respiration (Unit 1)
Respiration is the main reason trophic efficiency is so low. Organisms burn most of the energy they take in to fuel their own life processes, and that energy escapes as heat instead of being stored for the next level to eat.
Laws of Thermodynamics (Unit 1)
The 10% rule is thermodynamics applied to ecosystems. The first law says energy is conserved, and the second law says every transfer loses usable energy as heat, which is exactly why 90% disappears at each step.
Food Webs and Trophic Levels (Unit 1)
Trophic efficiency is what limits how long a food chain can be. Energy drains by about 90% at each level, so top predators are rare because there's barely any energy left to support them.
Expect this mostly as quantitative MCQs. A stem gives you the energy at the bottom (say, producers capture 1.2% of 100,000 kJ/m²/year), then asks how much reaches herbivores, carnivores, or the top predator. You multiply by 0.10 for each level you go up. One classic version gives a marine pyramid (phytoplankton 100,000 → zooplankton 10,000 → small fish 1,000 → tuna 100 → sharks 10) and asks you to track the 10x drop. Application questions also test the implication: a rancher raising cattle instead of growing crops for people loses about 90% of the energy by adding a trophic level, which is why eating lower on the food chain feeds more people per unit of land. On FRQs, you'd use it to explain why energy decreases up a chain or why a system can only support so many top predators.
Energy flows one direction and exits as heat, so it must be constantly resupplied by the sun. Nutrients (like carbon and nitrogen) cycle and get reused. Trophic efficiency describes energy, not nutrients, so don't apply the 10% rule to matter cycles.
Trophic efficiency is the percentage of energy passed from one trophic level to the next, and the 10% rule says it's about 10%.
Roughly 90% of energy is lost at each level, mostly as heat from cellular respiration.
Energy loss is explained by the laws of thermodynamics, so the 10% rule isn't a coincidence, it's physics.
Because so much energy drains away, food chains are short and top predators are rare.
On the exam, multiply by 0.10 for each trophic level you move up to find available energy.
Eating lower on the food chain feeds more people because you skip the 90% loss of adding another level.
It's the percentage of energy that transfers from one trophic level to the next, which the 10% rule estimates at about 10% (EK ENG-1.C.1). The other roughly 90% is lost as heat and through respiration.
No. The 10% rule is an approximation, not a fixed law. Real ecosystems vary, but 10% is the number to use for AP calculations unless a problem gives you a different efficiency.
Most of it is used by organisms for their own life processes through cellular respiration and then leaves the ecosystem as heat. By the second law of thermodynamics, every energy transfer is inefficient.
Trophic efficiency tracks energy, which flows one way and exits as heat, so the sun has to keep refueling it. Nutrient cycling tracks matter like carbon and nitrogen, which gets reused over and over. Don't apply the 10% rule to nutrients.
Because energy drops about 90% at each trophic level. By the time you reach the top, there's so little energy left that it can only support a tiny number of large predators like sharks or raptors.
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