Energy Flow Through Ecosystems: AP Biology Guide

Energy enters ecosystems when producers capture sunlight or chemical energy and store it in organic molecules. That energy then passes to consumers and decomposers, but much is lost as heat at each trophic transfer, so less usable energy remains at higher levels.

A producer is an autotroph that builds organic molecules from inorganic sources through photosynthesis or chemosynthesis. Producers form the base of most food webs because they are the primary entry point for energy into the ecosystem.

A consumer is a heterotroph that obtains energy by eating plants, animals, or other organisms. Consumers cannot make their own food from sunlight and must rely on energy stored in the tissues of other living things.

Decomposers break down dead organisms and waste products into simpler inorganic nutrients. They return those nutrients to soil and water so producers can use them again, but they release stored energy as heat rather than recycling it.

Trophic levels are the feeding positions in an ecosystem, such as producers, primary consumers, and higher-level consumers. Each level receives less usable energy than the level below it because organisms use most incoming energy for life processes and lose the rest as heat.

The 10% rule is a simplified model estimating that roughly 10% of energy at one trophic level becomes biomass at the next. Real ecosystems vary, but the model helps you predict why energy pyramids narrow sharply toward the top.

Organisms spend much of their energy on cellular respiration, movement, maintenance, and growth rather than passing it to the next consumer. Each transfer is inefficient, so a large fraction always leaves the system as heat.

An energy pyramid diagrams how much energy is available at each trophic level in an ecosystem. The base is always widest because producers capture the most energy, and each higher band is smaller because heat loss limits what can move upward.

A food chain traces one linear path of energy from a producer through a few consumers. A food web shows the many overlapping feeding links in a real community, which better reflects how species actually interact.

No—decomposers break down dead matter and return nutrients to the environment, but they do not send energy back up the food web. During decomposition, stored chemical energy is released as heat and is no longer available to support new biomass at higher trophic levels.

Energy flows in one direction through trophic levels and eventually exits ecosystems mainly as heat. Matter, by contrast, cycles repeatedly between living organisms, decomposers, and abiotic reservoirs such as soil, water, and air.

Start by naming the energy source, then follow food-web arrows from producers through each consumer level named in the prompt. Explain where energy is lost as heat and how that lower availability limits biomass, population size, or pyramid shape at higher trophic levels.

Only a small fraction of energy transfers between trophic levels, so little usable energy remains after a few steps. Most ecosystems support only three to five trophic levels because heat loss makes longer chains energetically unsustainable.

An energy pyramid measures usable energy at each trophic level and always narrows toward the top. A biomass pyramid shows total living mass at each level; it is usually upright, but fast-growing producers in aquatic systems can sometimes create an inverted biomass pyramid even when energy still decreases upward.