19.2 Food chains and food webs

Food chains as sequences showing energy transfer between organisms — arrow points in the direction of energy flow, beginning with a producer. Producers (plants) make their own organic nutrients via photosynthesis; consumers (animals) get energy by feeding; decomposers (bacteria/fungi) break down dead or waste organic material. Trophic levels: producer (1) → primary consumer/herbivore (2) → secondary/carnivore (3) → tertiary (4) → quaternary (5). Food webs as networks of interconnected chains. Pyramids of numbers (count) vs pyramids of biomass (total dry mass) — biomass preferred because it's never inverted. Human impact through overharvesting and introducing foreign species disrupts food webs. HL extension: pyramids of energy (kJ/m²/yr), the ~90% loss per trophic level (respiration + heat + indigestible material), why food chains have fewer than 5 levels, and why eating crops is ~10× more energy-efficient than eating livestock fed on crops.