Food chains illustrate the flow of energy through various trophic levels in an ecosystem, depicting how energy moves from one organism to another.

In a food chain, each organism corresponds to a distinct trophic level, which outlines the transfer of energy and nutrients throughout the ecosystem. The first trophic level consists of producers, primarily plants or algae, which synthesize their own food via photosynthesis. These producers serve as the foundation for the second trophic level, comprised of primary consumers, or herbivores, that feed on the producers.

The third trophic level includes secondary consumers, typically carnivores that prey on primary consumers. Following this, the fourth trophic level consists of tertiary consumers, which are often the apex predators in the ecosystem. Each trophic level relies on the one beneath it for energy, which explains why the population of organisms tends to decrease at each successive level.

Energy transfer occurs in a food chain when one organism consumes another. However, it is important to note that not all energy is effectively transferred. Approximately $10\%$ of the energy is passed along to the next trophic level, while the remainder is lost as heat. This phenomenon is commonly referred to as the 10% rule, which helps explain why food chains seldom extend beyond four or five trophic levels.

Furthermore, food chains illustrate the interrelationships among different species within an ecosystem. For instance, if a species at one trophic level is removed or experiences a change in population size, it can trigger significant effects on the other levels, a phenomenon known as a trophic cascade.

In conclusion, food chains provide a straightforward yet powerful representation of the various trophic levels within an ecosystem. They highlight the flow of energy and nutrients from producers to consumers and underscore the interdependence of diverse species.