Food chains are represented by a food relationship between producers, consumers and decomposers, in which energy flows between these individuals from autotrophic beings, which, through photosynthesis or chemosynthesis, are able to transform inorganic into organic matter, which is incorporated into the chain from the moment an individual feeds on these organisms.
Algae, plants, and some bacteria and protozoa represent the producers. Consumers feed on producers (primary consumers); or those who feed on them (secondary consumers). Some bacteria and fungi, the so-called decomposers, feed on the mortal remains and excreta of these components of the food chain, recycling organic matter.
Food chain example:
Algae > crustacean > fish > shark > fungi and bacteria
producer – primary consumer – secondary consumer – tertiary consumer – decomposers
From the example in question, we can see that some of these organisms can feed on both plant and animal tissues, such as certain species of crustaceans and fish. Thus, we cannot consider that the food relationships of a given environment occur in a linear way, as represented in food chains.
Considering this aspect, food webs more faithfully reflect the relationship between producers, consumers and decomposers in an environment, being therefore considered a set of food chains. Here, decomposers act on all trophic levels.