Energy flows one way and ends as heat; atoms cycle. Each transfer keeps roughly a tenth.
Energy moves through an ecosystem one way: sunlight → producers → consumers → heat. At each step most of it never reaches the next level, which is why food chains are short and the top of the pyramid is small.
Key terms
- Trophic level — a feeding step. Producers come first, then primary consumers (herbivores), then secondary consumers that eat them, and so on.
- Net primary productivity (NPP) — what producers capture, minus what they burn in their own respiration. Every consumer and decomposer lives on it.
- Trophic efficiency — the percent of one level's stored energy that ends up stored in the next level. About 10 percent on average.
- Endotherm vs ectotherm — endotherms burn much of their food to stay warm; ectotherms use outside heat, so more food becomes tissue.
- Decomposers — fungi and bacteria that break down dead matter and wastes. They recycle nutrients, but their respiration releases the energy as heat.
How it works
- Producers capture energy from sunlight, or from inorganic chemicals by chemosynthesis, and store it in sugars. What they don't burn becomes new biomass.
- Consumers never get all of it. Much of each level is never eaten, and some of what is eaten passes out as feces. Decomposers get both.
- Of the energy a consumer absorbs, most is burned in cellular respiration to power movement, upkeep and, in endotherms, body heat. That energy leaves as heat for good.
- Only energy built into new tissue (growth and offspring) can feed the next level. On average, that's about 10 percent of the energy stored in the level below.
- Repeat at every link. After three or four transfers, too little energy is left to support another level, so food chains stay short.
Why it matters
Expect numbers. You'll get a pyramid or a table and be asked how much reaches a level, which step was most efficient, or what a drop in producers does to everything above. This topic also sets up a classic contrast: atoms like carbon cycle, but energy flows one way and ends as heat. Every ecosystem needs a constant energy input, usually sunlight.
Exam traps
| Trap | Actually |
|---|
| More NPP per square meter means a bigger share of Earth's total production. | NPP is a rate per area. Total = rate × area, so the huge open ocean outproduces tropical rainforests despite its low rate. |
| Secondary consumers get about 10 percent of the producers' energy. | Count transfers, not levels: producers → primary → secondary is two transfers. Multiply by 0.1 twice: about 1 percent. |
| The step that loses the fewest kilojoules is the most efficient. | Efficiency is a ratio. Divide each level by the level below, then compare percentages. Raw numbers shrink at every step anyway. |
| Decomposers recycle energy back to the producers. | They recycle nutrients. The energy in dead matter leaves as heat from their respiration, so producers must capture new energy. |
Worked example
In a prairie, plants store 20,000 kJ per m² per year as new biomass. Mice eat the plants and store 400 kJ per m² per year. Snakes eat the mice and store 60 kJ per m² per year. Which transfer is more efficient, and what best explains the difference?
- Efficiency is a ratio, so divide each level by the one below. Plants → mice: 400 ÷ 20,000 = 2 percent.
- Mice → snakes: 60 ÷ 400 = 15 percent. The second transfer is more than seven times as efficient.
- Ask where the energy went. Mice are small endotherms, and small animals burn more energy per gram than large ones. Snakes are ectotherms, so more of what they eat becomes tissue.
- Plant tissue is also harder to digest than meat, so mice lose more of their food as feces.
Mice → snakes, at 15 percent versus 2 percent. Snakes are ectotherms, so little of their food goes to making body heat.
An ectotherm turns a bigger share of what it eats into new biomass, and meat is easier to digest. The near miss is plants → mice, because it moves the most kilojoules. Energy pyramids always shrink as you go up, so raw amounts always favor the lower step. Only the percentage measures efficiency.