Answer:
Because they can function in any of the defined trophic levels
Step-by-step explanation:
A trophic level refers to the steps of a food chain in an ecosystem. Each trophic level consists of different organisms depending on what they feed on. The first trophic level of every food chain contains the PRODUCER, which starts the food chain by making its own food via photosynthesis. The second trophic level consists of organisms called PRIMARY CONSUMER, which are herbivores that feed on the plants. The third trophic level consists SECONDARY CONSUMER, which are carnivores that feed on the primary consumer. Another carnivore called TERTIARY CONSUMER that feeds on the secondary consumer occupies the fourth trophic level.
Detritivores and decomposers are set of organisms that feed on dead matter, helping in their breakdown. Omnivores are organisms that can feed on both plants and animals. Hence, due to this unique characteristics possessed by these set of organisms, they can fit into any of the trophic levels except the first. For example, omnivores can occupy the second, third and fourth trophic levels.
On the other hand, detritivores and decomposers can operate in any trophic level as long as there is a dead matter. Therefore, it is difficult to assign one particular trophic level to these organisms.