Are essential to recycling life materials because without them matter would pile up rather than recycle.

The given blank can be filled with decomposers.

Explanation:

The species that are placed at the level of decomposer in a biome plays an important part for the life on Earth. The decomposers dissociate dead animal and plant matter, due to which the nutrients in them gets recycled back into the ecosystem so that it can get used again.  

Fungi, mainly in the forests are the prime decomposers. One of their essential activity is to assists in discharging phosphorus and nitrogen from the dead and decaying matter. Other species are known as detritivores, like some of the insects, earthworms, woodlice, and sea cucumber also dissociate the dead matter, and however, they need to engulf it first unlike fungi, which uses biological and chemical procedures.