Energy flow and food webs

Energy enters most ecosystems as sunlight, captured by producers in photosynthesis. Primary consumers eat producers, secondary consumers eat primary consumers, and so on. Each feeding level is a trophic level. Decomposers break down dead matter from every level and return nutrients to the soil, so matter is recycled while energy flows through.

Only about 10% of the energy at one trophic level passes to the next. The rest is used in respiration for movement and warmth, or lost in waste and parts that are not eaten. That is why food chains rarely have more than four or five links, and why a pyramid of biomass gets narrower at the top.