In an ecosystem various trophic levels are interlinked with each through a food chain. (the transfer of food energy from green plants through a series of organism with repeated eating and been ea.... Later is referred to as food chain). It can be defined as transfer of energy from one trophic level (eg. Producers) to next trophic level (eg. consumers). A very simple food chain is shown here.
All types of ecosystem possess two types of food chains.
1. Grazing food chain:
It starts from producers (greenplants) through herbivores to carnivores. The shorter the food chain, the more is the amount of energy available to the last trophic level in an ecosystem.
(a) Terrestrial food chain:
(i) Grass-Grasshopper-Shrew-Hawak
(ii) Grass-Rabbit-Fox-Wolf-Tiger
(iii) Grass- Rat-Snake-Vulture
(b) Aquatic food chains:
(i) Phytoplankton-Zooplankton 
Small Fish Large Fish Shaark
(ii) PhytoplanktonZooplankton
Fish Crane Hawk.
2. Detritus food chain:
It starts from dead organic matter and goes to detritus feeding organism ( detrivores or microorganism) and on to their predators. The ecosystem exhibiting detritus food-chain are less dependent on direct solar energy but chiefly depend on the influx of organic matter produced. A good example of a detritus food chain based on mangrove leaves are shown here.
Mangrove leaves hawk. Detritus microorganism crabs and shrimps small fish large fish.
Food web: in nature food -chains are not isolated, but they are interconnected with each other and form food web. Thus ,food web is defined as" a network of food chains which are interconnected at various trophic levels, so as to form a number of feeding connections amongst different organism of a biotic community".
For example in a grass land there are five simple food chains which are interlinked.
(i) Grass- grass happer- hawk/vulture
(ii) Grass-grass hopper-lizard-hawk/vulture
(iii) Grass-rabbit-hawk/vulture
(iv) Grass-mouse-hawk
(v) Grass-mouse-snake-hawk.