From IslandWood Education Wiki
The food web is divided into two broad categories: the grazing web, beginning with autotrophs, and the detrital web, beginning with organic debris. There are many food chains contained in these food webs.Each organism in the food web is eaten by the next in line. Each position in the food chain is called a trophic level. Plants are at the first trophic level. Often times, food chains and webs are displayed as pyramids because the number of consumers at each level decreases significantly, so that a single top consumer, (e.g., a polar bear or a human), will be supported by a million separate producers.
A change in one part of the food web affects other populations. This is exemplified when we play a game like Owls, Mice and Seeds. We can increase this understanding by drawing attention when one population (ie owls) goes extinct, and by introducing pesticides to the seeds.
The higher up a food chain you go, the less food (and energy) that remains available. This is because energy is lost to the environment with each transfer as entropy increases. About eighty to ninety percent of the energy is expended for the organism’s life processes or is lost as heat or waste. Only about ten to twenty percent of the organism’s energy is generally passed to the next organism.
See also: The Food Chain Song