The two groups of organisms that arguably play the greatest role in biological decomposition are:
Fungi: Fungi, including molds and mushrooms, are key decomposers in ecosystems. They break down dead organic matter, such as dead plants and animals, by secreting enzymes that break down complex organic compounds into simpler substances. Fungi play a crucial role in the early stages of decomposition.
Bacteria: Bacteria are also essential decomposers. They break down organic matter into simpler compounds through various metabolic processes. Bacteria are particularly important in breaking down organic materials like cellulose, lignin, and proteins.
While all the organisms you listed can be involved in decomposition to some extent, plants, fish, and mammals typically have other primary ecological roles, such as producers, herbivores, or predators, and are not the primary decomposers in ecosystems.