Food chains and food webs or food networks describe the feeding relationships between species in a biotic community. In other words, they show the transfer of material and energy from one species to another within an ecosystem.

A food chain describes a single pathway that energy and nutrients may follow in an ecosystem. There is one organism per trophic level, and trophic levels are therefore easily defined.

They usually start with a primary producer and end with a top predator. Here is an example of a food chain:

phytoplankton > copepod > fish > squid > seal > Orca (the largest member of the oceanic dolphin family)

This "chain" can be described as follows: Killer whales (Orca) feed upon seals, that feed upon squid, that eat small fish, that feed on copepods, that feed on microscopic algae. In this example, algae - autotrophs by virtue of their ability to photosynthesize - are the base of the food chain. It is always the case that numbers - or at least biomass - decreases from the base of the chain to the top. In other words, the number and mass of phytoplankton cells is much greater than the number and mass of copepods being supported by the phytoplankton.

Viewed another way: to support one Orca requires many seals, large numbers of squid, huge numbers of fish, and so on down the chain (see energy pyramid). Food chains are overly simplistic as representatives of what typically happens in nature. The food chain shows only one pathway of energy and material transfer. Most consumers feed on multiple species and are, in turn, fed upon by multiple other species. The loss of energy through the levels causes the species at the top to be less numerous. On average 10% of the organism's energy is passed on to its predator.