The food energy passes from one trophic level to another trophic level.mostly from the lower to higher trophic lévels. When the path of food energy is .linear the components resemble the .links. of a chain and it is called .food chain.. Generally a food chain ends with decomposers. The three major types of food chains in an ecosystem are Grazing Food Chain, Parasitic Food Chain and Detritus Food Chain.
i) Grazing food chain : It is also known as predatory food chain, it begins with the green plants (producers) and the second third and fourth trophic levels are occupied by the herbivores, primary carnivores and secondary carnivores respectively, In some food chaiņs these is yet another trophic level - the climax.carnivbres. The number of trophic levels in food chains varies form 3 to 5 generally. Some examples from grazing food chain (GFC) are given below.
ii) Parasitic food chain : Some authors included the Parasitic Food Chains as a part of the GFC. As in the case of GFCs, it also begins with the producers, the plants (directly or indirectly). However, the food energy passes from large organisms to small organisms in the parasitic chains. For instance, a tree which occupies the 1* trophic level provides shelter and food for many birds. These birds host many ectoparasites and endo parasites. Thus, unlike in the predator food chain, the path of the flow of energy includes fewer, large sized organisms in the lower trophic levels and numerous, small sized organisms in the successive higher trophic levels.
iii) Detritus Food Chain : The detritus food chain (DFC) begins with dead organic matter (such as leaf litter, bodies of dead organisms). It is made up of decomposers which are heterotrophic organisms, mainly the fungi. and .bacteria.. They meet their energy and nutrient requirements by degrading dead organic matter or détritus. These are also known as saprotrophs (sapro: to decompose).
Decomposers : Secrete digestive enzymes that breakdown dead and waste materials (such as faeces) into simple absorbable substances. Some examples of detritus food chains are :
1. Detritus (formed from leaf litter) - Earthworms - Frogs - Snakes
2. Dead animals - Flies and maggots - Frogs - Snakes.
In an aquatic ecosystem, GFC is the major .conduit for the energy flow. As against this in a terrestrial ecosystem, a much larger fraction of energy flows through the detritus food chain than through the GFC. Detritus food chain may be connected with the grazing food chain at some levels. Some of the organisms of DFC may form the prey of the GFC animals. For example, in the detritus food chain giveri above, the earthworms of the DFC may become the food of the birds of the GFC. It is to be understood that food chains are not .isolated. always.
