AP® Biology · Unit 8 · Topic 8.2
Trophic Levels and Energy Flow
Only about 10 percent of the energy stored at one trophic level makes it into the bodies of the organisms at the next level up, because most is lost as heat. That is why food chains are short and top predators are rare.
Producers capture energy from sunlight through photosynthesis and build it into sugars and tissue. When a primary consumer eats a producer, it does not keep all that energy. Most of it is spent on staying alive: moving, breathing, maintaining body temperature, repairing cells. All of that work releases heat, and heat energy escapes the ecosystem instead of passing to the next consumer. Only the energy actually stored as new growth or offspring is available to whatever eats that organism next. On average, about 10 percent of the energy at one trophic level ends up stored in the level above it. The other 90 percent is lost as heat, used in metabolism, or left in waste and uneaten parts.
This steady loss is why an energy pyramid is always wide at the base and narrow at the top. Producers sit on the bottom with the most energy, primary consumers above them with roughly a tenth as much, then secondary and tertiary consumers each losing another 90 percent. Because the losses compound, only a few trophic levels can exist before there is not enough energy left to support another one. That is the real reason a savanna supports tons of grass but only a handful of lions: the lions sit three or four transfers removed from the sun.
This one-way flow is very different from how carbon, nitrogen, and other matter behave in an ecosystem. Matter is recycled: carbon atoms move from air to plant to animal to decomposer and back to air, used again and again. Energy cannot do this. It arrives as sunlight, moves through the food chain once, and leaves permanently as heat, which is why ecosystems always need a constant new input of sunlight to keep functioning.
Key terms
- Trophic level
- A feeding position in a food chain, such as producer, primary consumer, or secondary consumer.
- Energy pyramid
- A diagram showing the energy available at each trophic level, narrowing sharply from producers to top consumers.
- Ten percent rule
- The general pattern that only about 10 percent of energy at one trophic level transfers to the next level up.
- Primary producer
- An organism, usually a plant or alga, that captures energy from sunlight to build organic matter.
- Gross versus net production
- Gross production is total energy captured by producers; net production is what remains after their own metabolism uses some up.
Watch it explained
- How it worksHow does roughly 90 percent of the energy vanish at every step up a food chain?
- How it worksWalk me through building an energy pyramid from producers up to top carnivores.
- Worked exampleFind the energy reaching a tertiary consumer when producers capture 20000 kilojoules.
- Common mistakeWhy doesn't energy cycle through an ecosystem the way carbon and nitrogen do?
- Going furtherWhat happens to the number of possible trophic levels when producers are scarce?
Check yourself
End of topic.