You met energy pyramids in Energy Pyramids. Now that you know cellular respiration, you can see where the "lost" energy goes.
Key idea
Energy enters an ecosystem as sunlight captured by producers, and flows up through trophic levels: primary consumers, secondary consumers and so on. On average only about 10% of the energy at one level becomes the bodies of the next. The rest is used for living and released as heat.
Why it works
An animal eats food, but it does not turn all of it into new body. Some is not digested and leaves as waste. Most of what is absorbed is burned in cellular respiration to power moving, breathing and keeping warm, and that energy ends up as heat. Only what is stored in new tissue is available to the next eater. Energy is never destroyed (that is the law of conservation of energy); it just leaves the food chain as heat that no organism can use.
Worked example: Up the pyramid
Producers in a meadow capture 50,000 kcal per square meter in a year. About how much energy reaches the secondary consumers?
- Primary consumers: 10% of 50,000 = 5,000 kcal.
- Secondary consumers: 10% of 5,000 = 500 kcal.
Answer: About 500 kcal per square meter per year.
Worked example: Measuring efficiency
Herbivores in a pond store 1,200 kcal; the fish that eat them store 90 kcal. What is the transfer efficiency?
- Efficiency = energy at the higher level ÷ energy at the lower level.
- 90 ÷ 1,200 = 0.075.
Answer: 7.5%, close to the usual rough 10%.
Because so much energy is lost at each step, food chains rarely have more than four or five levels, and top predators are few. Biomass (the mass of living things) and numbers usually shrink up the pyramid too, though not always: one large tree can feed thousands of insects, so a pyramid of numbers can be upside down.
Common mistake
Two errors are common. One is treating the 10% rule as exact; it is a rough average, and real transfers range from about 5% to 20%. The other is thinking the lost energy is destroyed. It is not: it is used in respiration and given off as heat, which flows out of the ecosystem.