A food chain traces a single path of energy through an ecosystem. Grass is eaten by a grasshopper. The grasshopper is eaten by a frog. The frog is eaten by a snake. The snake is eaten by a hawk. Each link is a trophic level. The chain shows who eats whom, and it usually has three to five links. Longer chains are rare because energy is lost at every transfer.

Food chains are simple models. Real ecosystems are more complex. Most organisms eat more than one thing and are eaten by more than one predator. A hawk might eat snakes, rabbits, and birds. A snake might eat frogs, mice, and insects. When you map all these connections, you get a food web. Food chains are useful for teaching and for tracing specific contaminants. Mercury, for example, concentrates as it moves up a chain, reaching dangerous levels in top predators like tuna and swordfish.

Trophic levels

• Producers: plants and algae.
• Primary consumers: herbivores.
• Secondary consumers: carnivores that eat herbivores.
• Tertiary consumers: carnivores that eat other carnivores.

Food chains are not fixed. Remove one link and the chain reorganizes. Add an invasive species and the chain may collapse. The simplicity is useful. The reality is messier.