From Academic Kids
Food chains and food webs 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.
As usually diagrammed, an organism is connected to another organism for which it is a source of food energy and material by an arrow representing the direction of biomass transfer. Organisms are often grouped into trophic levels—from the Greek for nourishment, trophikos—based on how many links removed from the primary producers they are. Primary producers, or autotrophs, are species capable of producing complex organic substances (essentially "food") from an energy source and inorganic materials. These are often photosynthetic, but some, like those forming the base of deep sea vent food webs, are chemotrophic.
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 large predator. Here is an example of a food chain:
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 algae. In this example, algae—autotrophs by virtue of their ability to photosynthesize—are the base of the food chain.
A food web extends the food chain concept from a simple linear pathway to a complex network of interactions. The direct steps as shown in the food chain example above seldom reflect reality. Food sources of most species in an ecosystem are much more diverse, resulting in a complex web of relationships as shown in the figure on the right. In this figure, the grouping of Phytoplankton → Herbivorous zooplankton → Carnivorous zooplankton → Arctic char → Capelin on the far right is a food chain; the whole complex network is a food web.