Food Webs and Trophic Levels
How energy and matter flow through ecosystems from producers to consumers and decomposers, with roughly 10% transfer between levels.
A food web describes who eats whom in an ecosystem, mapping the flow of energy and matter between organisms. Its backbone is the trophic chain of levels: producers, consumers, and decomposers. Producers — plants, algae, and some bacteria — capture light energy through photosynthesis and build organic matter from carbon dioxide and water. Primary consumers (herbivores) eat producers, secondary consumers eat herbivores, tertiary consumers eat them, and decomposers (bacteria, fungi, detritivores) break down dead organic matter from every level, recycling nutrients.
Energy flow between levels is inefficient. The transfer efficiency between trophic levels averages about 10% — the famous rule first quantified by Raymond Lindeman in 1942 — because organisms use most consumed energy for their own metabolism and lose the rest as heat. The consequence is pyramidal: a grassland needs a huge base of plant biomass to support a small number of top predators, and few ecosystems can support more than four or five trophic levels. It also means energy flows one way — it cannot be recycled, only matter is recycled — so ecosystems depend on a continuous input of sunlight.
Food chains are simplifications; real ecosystems are tangled webs, because most consumers eat multiple prey and are eaten by multiple predators. Omnivory, scavenging, and detritus-feeding blur the levels. This complexity confers stability: a web with many alternative pathways can absorb the loss of one species, whereas a simple chain can collapse. Keystone species — such as sea otters controlling sea urchin populations in kelp forests, or wolves regulating deer in Yellowstone — exert influence far beyond their biomass.
Trophic structure also shapes the fate of pollutants: fat-soluble toxins such as DDT and mercury biomagnify, becoming more concentrated at each successive level, so top predators accumulate the highest doses. Disturbances that remove top predators or producers can trigger trophic cascades through the entire web, which is why ecological succession and ecosystem management are studied in terms of these energy pathways.
Tags
ecology ecosystems energy flow trophic levels
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