2. Food Webs and Energy Flow

Energy captured by producers flows through an ecosystem via feeding relationships. Tracing this flow is a core GED® Science skill.

Key feeding levels (trophic levels):

• Producers (autotrophs) — make their own food via photosynthesis (grass, algae, phytoplankton).
• Primary consumers (herbivores) — eat producers (rabbits, deer, caterpillars).
• Secondary consumers — eat primary consumers (foxes, frogs, small fish).
• Tertiary consumers — eat secondary consumers (eagles, large sharks).
• Decomposers — break down all levels (bacteria, fungi, earthworms).

Food chain vs. food web: A food chain is a single straight path; a food web shows ALL the overlapping chains. Real ecosystems are webs, not chains.

The 10% Rule: At each trophic level, only about 10% of the energy is passed on to the next level. The other 90% is used for the organism's own life processes or lost as heat. This is why there are always far more plants than herbivores, and far more herbivores than carnivores.

⚠️ Common misconception: removing one species from a food web does NOT only affect the species that ate it — it triggers a cascade of changes up and down the web.

💡 Tip: If a question shows a food web and asks what happens when a species is removed, trace both the predators (lose food) and the prey (population may explode) of the lost species.