Module 2: Food Chains and Food Webs; Energy Transfer and Bioaccumulation

1. Producers (autotrophs) make their own food through photosynthesis or chemosynthesis. Consumers (heterotrophs) eat other organisms. Decomposers break down dead matter and return nutrients to the soil.
2. A food chain shows a single path of energy transfer (grass → rabbit → fox). A food web shows all the interconnected feeding relationships in an ecosystem — far more realistic.
3. Trophic levels number the feeding positions: Level 1 = producers, Level 2 = primary consumers (herbivores), Level 3 = secondary consumers, Level 4 = tertiary consumers.
4. The 10% rule: only about 10% of the energy stored at one trophic level is transferred to the next. The rest is lost as heat during metabolism.
5. An ecological pyramid of biomass shows that the total mass of organisms decreases at each higher trophic level, reflecting the 10% energy loss.
6. Bioaccumulation is the buildup of a toxic substance in a single organism over its lifetime. Biomagnification is the increase in concentration of that toxin at each successive trophic level.
7. Classic example: DDT was a pesticide sprayed on lakes in the 1950s–70s. Tiny amounts dissolved in water, but fish accumulated it in fat, and bald eagles — eating many fish — accumulated lethal concentrations. This led to near-extinction and the DDT ban.
8. Indicator species are organisms sensitive to environmental change; their population health signals ecosystem health before broader damage is visible.

Netiquette from the syllabus: address ideas, not people. Complete sentences, correct spelling. Personal matters go to email, never the board.

Ask at home: If top predators vanish from an ecosystem, what happens to the animals they used to eat, and then what happens to the plants?

En español: Si los depredadores tope desaparecen de un ecosistema, ¿qué pasa con los animales que comían, y luego qué pasa con las plantas?