In an ecosystem, a food chain describes:

In an ecosystem, a food chain illustrates a single pathway through which energy flows and is transferred among organisms. It simplifies the interactions that occur in an ecosystem by showing a linear sequence where one organism eats another, thus obtaining energy. For instance, a simple food chain might depict plants (producers) being consumed by herbivores (primary consumers), which are then eaten by predators (secondary consumers).

This model emphasizes the direct relationship of energy transfer from one organism to another in a straightforward and clear manner. Understanding food chains is crucial because they help illustrate the foundational aspect of energy movement and the dependencies present in ecosystems, aiding in grasping concepts such as energy loss and the roles of various organisms within a given habitat.

The other choices refer to broader concepts that encompass more complex interactions. A complex network of feeding relationships describes a food web, which includes multiple food chains interlinked together. Discussing the role of decomposers involves the intricate breakdown of organic matter, which is important but distinct from the straightforward path of energy transfer emphasized in a food chain. Lastly, competition among different species illustrates an ecological interaction that affects population dynamics but does not define the concept of a food chain.