Trace the one-way flow of energy through trophic levels in food chains and food webs, using energy pyramids and the ten percent rule (Ohio's Learning Standards for Science, Biology, B.DI.2).

What this topic is asking

Ohio standard B.DI.2 (Ecosystems) expects students to trace energy flow through an ecosystem: who makes energy available, who passes it on, and why so much is lost. The Ohio Biology EOC turns this into items where you read a food chain or web, apply the ten percent rule, or predict knock-on effects of a change. The crosscutting idea is energy and matter: energy flows one way through an ecosystem (unlike matter, which cycles). This connects to the cellular processes of photosynthesis (which captures the energy) and cellular respiration (which releases it).

Producers, consumers, decomposers

Every ecosystem's energy starts with organisms that can capture it, and is passed along by those that eat.

• Producers (autotrophs). Organisms that make their own food, almost always by photosynthesis using sunlight (plants, algae, some bacteria). They are the base of the food chain and the entry point for energy into the ecosystem.
• Consumers (heterotrophs). Organisms that get energy by eating other organisms. Primary consumers are herbivores (eat producers); secondary consumers eat primary consumers; tertiary consumers eat secondary consumers. Carnivores eat animals, omnivores eat both.
• Decomposers. Bacteria and fungi that break down dead organisms and waste, returning nutrients to the soil and water. They recycle matter so producers can reuse it.

Trophic levels, food chains, and food webs

A trophic level is a feeding position in the chain: producers are the first trophic level, primary consumers the second, and so on. A food chain shows a single straight path of energy: for example, grass to grasshopper to frog to snake, with arrows pointing in the direction energy flows (from the eaten to the eater).

Real ecosystems are more complex than a single chain, because most organisms eat (and are eaten by) several others. A food web links many food chains into a network, showing all the feeding relationships in a community. The advantage of a web is that it reveals knock-on effects: a change in one organism (a disease in the producers, the removal of a predator) ripples through the connected species.

The ten percent rule

Energy transfer between trophic levels is inefficient. On average, only about 10% of the energy stored at one trophic level is passed on to the next; the other roughly 90% is lost. Most of the loss is heat released during cellular respiration and other life processes (movement, maintaining body temperature), and some energy is never consumed or leaves as undigested waste.

You can apply this as repeated multiplication by $0.1$ along the chain:

So if producers store $10{,}000$ units of energy, a primary consumer gets about $1{,}000$, a secondary consumer about $100$, and a tertiary consumer about $10$.

Energy pyramids and the one-way flow

An energy pyramid is a diagram showing the energy available at each trophic level. It is widest at the bottom (producers, the most energy) and narrows at each level up, because about 90% is lost at every step. The same shape usually appears for biomass (total mass of living material) and numbers of organisms.

The deepest point is that energy flows in one direction: in as sunlight, captured by producers, passed up the levels, and lost as heat at every step. It does not cycle back. This contrasts sharply with the cycling of matter, where atoms are reused again and again. The standard summary to remember is "energy flows, matter cycles."

Try this

Q1. In a food chain, which direction do the arrows point and why? [1]

• Cue. From the organism being eaten to the organism that eats it, showing the direction energy flows.

Q2. Producers store 8,000 units of energy. Using the ten percent rule, calculate the energy available to a secondary consumer. [1]

• Cue. Primary consumer $\approx 800$, secondary consumer $\approx 80$ units.