If the role of the plant-producers and the animal-consumers is generally well known, that of the microorganic decomposers is much less so.
(the role of the plant-producers and the animal-consumers; is generally known; ) (that of the microorganic decomposers; is so; much less)
the role of the plant-producers and the animal-consumers that of the microorganic decomposers
is generally known is so
much less