01Trophic levels
Energy enters via producers (autotrophs) → primary consumers (herbivores) → secondary → tertiary consumers. Decomposers recycle nutrients from all levels.
Only about 10% of energy passes to the next level; the rest is lost as heat (respiration) and unused material. That limits food chains to about 4–5 levels.
02Productivity and metabolism
Gross primary productivity (GPP) = total photosynthesis. Net primary productivity (NPP) = GPP − respiration — what's available to consumers.
Endotherms use metabolic heat to hold body temperature (high energy cost); ectotherms rely on the environment (less food needed). Smaller endotherms have higher metabolic rate per gram.
03Notebook box
10% rule: 10,000 kcal producers → 1,000 → 100 → 10.
Worked example: GPP = 20,000 kcal, plant respiration = 8,000. NPP? → 12,000 kcal.