The beginning of a food chain begins with the sun. Photosynthesis is the process that plants use to turn the sun's energy into food - sugar. An example of a food chain is rabbits eating grass and foxes eating rabbits. Food chains are complex and form food webs. 

Nothing in nature is wasted. Decomposers include worms, bacteria and fungi.
Vultures, hyenas, crows, racoons and blue jays are important to the food chain. 
When one plant or animal becomes extinct it affects the whole system.
Source: The Science of Life Food Chains by Carol S. Surges