9.2 Energy Flow and Food Webs
Notes
Energy enters most ecosystems as sunlight and flows in one direction through feeding levels.
- Producers (autotrophs) make food. Consumers (heterotrophs) eat other organisms: herbivores, carnivores, omnivores.
- Decomposers (bacteria and fungi) break down dead matter and return nutrients to the soil.
- A food chain shows one path of energy; a food web shows many connected chains. Arrows point in the direction energy flows (toward the eater).
- Trophic levels: producers, primary consumers, secondary consumers, tertiary consumers.
- 10% rule: only about 10% of the energy at one level passes to the next; the rest is lost mostly as heat. This is why food chains are short.
- Biomagnification: toxins like DDT become more concentrated at higher trophic levels.
Practice
In the food chain grass → grasshopper → frog → snake, which is the secondary consumer?
The frog eats the primary consumer (grasshopper).
If the producers in an ecosystem have 10,000 kcal of energy, about how much energy reaches the secondary consumers?
10% of 10,000 is 1,000 for primary consumers; 10% of 1,000 is 100 for secondary consumers.
In a food web diagram, what do the arrows show?
An arrow from grass to rabbit means the rabbit gets energy from the grass.
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