A food chain is a linear sequence of organisms through which nutrients and energy pass as one organism eats another, beginning with producers and ending with top consumers. In contrast, a food web is a complex, interconnected network of multiple food chains within an ecosystem, showing how energy flows through many pathways rather than just one. Food webs reveal the interdependence of organisms and illustrate how disturbances to one species can ripple through the entire community.
Each organism in a food chain occupies a specific trophic level, a hierarchical position defined by the number of energy-transfer steps from the primary producers. Producers, also called autotrophs, are organisms such as plants and algae that synthesize their own food from inorganic substances using photosynthesis or chemosynthesis. Primary consumers are herbivores that feed directly on producers, secondary consumers are carnivores or omnivores that feed on primary consumers, and tertiary consumers are predators that feed on secondary consumers. Decomposers, such as bacteria and fungi, break down dead organic matter and recycle nutrients back into the ecosystem.
Energy transfer between trophic levels is inefficient. The 10% rule states that only about 10% of the energy at one trophic level is transferred to the next, with the rest lost mainly as heat. This explains why food chains rarely extend beyond four or five trophic levels. When changes occur at the top of a food chain, they can cause reciprocal effects at lower trophic levels, a phenomenon known as a trophic cascade. The presence or absence of apex predators, which have no natural predators of their own, often drives these cascading effects throughout the ecosystem.