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A food web maps the feeding relationships among species in a community, with energy flowing from producers (autotrophs) through primary, secondary, and tertiary consumers (heterotrophs) to decomposers. Each feeding level is a trophic level. Food chains are linear sequences within the web; real webs are highly interconnected, conferring stability through redundancy. Omnivores feed at multiple trophic levels, and detritivores/decomposers recycle nutrients from dead organic matter. Food web structure determines how perturbations (species loss, invasions) propagate through communities.
Draw food webs for well-studied systems (e.g., kelp forest, grassland, salt marsh) and identify trophic positions. Trace energy flow through the web and identify which links are most important to overall stability. Compare food chain length across ecosystems and discuss why it varies.
Every living thing needs energy, and in any ecosystem that energy enters through *producers* — organisms that fix energy from sunlight or chemical sources through photosynthesis or chemosynthesis. Plants, algae, and cyanobacteria are the classic producers. Every other organism in the ecosystem ultimately gets its energy by eating something that traced its energy back to a producer. This flow of energy through a series of who-eats-whom relationships defines the *food web*.
A *trophic level* is a feeding position in this energy hierarchy. Producers occupy level 1. *Primary consumers* (herbivores) eat producers and occupy level 2. *Secondary consumers* eat primary consumers (level 3), and *tertiary consumers* eat secondary consumers (level 4). In practice, most species do not sit neatly at a single level — an omnivore like a bear eats berries (level 2), fish (level 3 or 4), and insects (level 2 or 3), giving it a fractional trophic position. A *food web* represents all the feeding links in a community simultaneously, which is far more accurate than any single food chain.
A critical insight is how energy is *lost* at each trophic transfer. When a grasshopper eats grass, it does not absorb all the grass's energy — most is lost to heat, respiration, and indigestible material. On average, only about 10% of the energy at one trophic level is incorporated into the biomass of the next. This is called *ecological efficiency* or the 10% rule. Starting with 10,000 units of plant energy: grasshoppers capture ~1,000, frogs ~100, hawks ~10. This rapid energy loss is why food chains are short — a sixth trophic level would have almost no energy to sustain a population — and why the total biomass of top predators in an ecosystem is always much smaller than the biomass of producers.
*Decomposers* — bacteria, fungi, and detritivores like earthworms — are often overlooked but are arguably the most important component of the food web. They break down dead organic matter from every trophic level, releasing bound nutrients back into forms that producers can use again. Without decomposers, nutrients would accumulate in dead biomass and producers would be starved of the nitrogen, phosphorus, and other elements they need. In a forest, far more energy flows through the detrital (decomposer) pathway than through the grazing pathway we typically picture.
Food web *stability* comes from redundancy — the more species that can fill a given role, the more robust the web is to losing any one of them. When a keystone species is removed, the effects can cascade through the entire web: the prey of that predator explodes in number, overconsumes its own prey or food source, and the ripple continues. This is a *trophic cascade*. Real-world examples include the reintroduction of wolves to Yellowstone, which suppressed elk overgrazing and allowed riverside vegetation to recover — a change that reshaped the entire ecosystem.