All animals depend on plants (directly or indirectly) for their food needs. They are hence called consumers and also heterotrops. If they feed on the producers, the plants, they are called primary consumers, and if the animals eat other animals which in turn eat the plants (or their produce) they are called secondary consumers likewise, you could have tertiary consumers too. Obviously the primary consumers will be herbivores. Some common herbivores are insects, birds and mammals in terrestrial ecosystem and molluses in aquatic ecosystem. The consumers that feed on these herbivoes are carivores, or more correctly primary carnivores (though secondary consumers). Those animals that depend on the primary carrivores for food are labelled secondary carnivores. A simple graging food chair (GFC) is depicted below:

Grass.................Goat............................Man

(producer)  (primary Consumer)  (Secondary consumer)

The detritus food chain (DFC) begins with the dead organic matter. It is made up of decomposers which are heterotrophic organisms, mainly fungi and bacteria. They meet their energy and nutrient requirements by degrading dead organic matter or detritus these are also known as saprophytes.

In an aquatic ecosystem, GFC is the major conduct for energy flow. As against this, in a terrestrial ecosystem, a much larger fraction of energy flowers through the detritus food chair through the GFC. Detritus food chair may be connected with the grazing food chair at some levels. These natural interconnection of food chains make it a food web.