The food chain is a sequence of organisms through which the food and its energy pass with each member becoming the food of the later member.
It begins with detritus or dead organic matter. It is made up of decomposers which are heterotrophic organisms mainly fungi and bacteria. Detritivores act over it. Therefore food energy present in detritus passed into them. Detritivores and decomposers are consumed by smaller carnivores which in turn become food for larger carnivores and so on.
A common detritus food chain with earthworm as detritivores is given below.
Detritus → Earthworm → Sparrow → Falcon.