Food web
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.
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 grouped into trophic levels—from the Greek for nourishment, trophikos—based on how many links they are removed from the primary producers. Primary producers, or autotrophs, are species capable of producing complex organic substances (essentially "food") from an energy source and inorganic materials. These organisms are typically photosynthetic plants or algae, but in rares cases, like those organisms forming the base of deep-sea vent food webs, can be chemotrophic.
Food chain
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:
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.
Food web
A food web or food network 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.