A
food web (or
food cycle) is the natural interconnection of
food chains and generally a graphical representation (usually an image) of what-eats-what in an
ecological community. Another name for
food web is a
consumer-resource system. Ecologists can broadly lump all life forms into one of two categories called
trophic levels: 1) the
autotrophs, and 2) the
heterotrophs. To
maintain their bodies, grow, develop, and to
reproduce, autotrophs produce
organic matter from
inorganic substances, including both
minerals and
gases such as
carbon dioxide. These
chemical reactions require
energy, which mainly comes from the
sun and largely by
photosynthesis, although a very small amount comes from
hydrothermal vents and
hot springs. A gradient exists between trophic levels running from complete autotrophs that obtain their sole source of carbon from the atmosphere, to
mixotrophs (such as
carnivorous plants) that are autotrophic organisms that partially obtain organic matter from sources other than the atmosphere, and complete
heterotrophs that must feed to obtain organic matter. The linkages in a food web illustrate the feeding pathways, such as where heterotrophs obtain organic matter by feeding on autotrophs and other heterotrophs. The food web is a simplified illustration of the various methods of feeding that links an ecosystem into a unified system of exchange. There are different kinds of feeding relations that can be roughly divided into
herbivory,
carnivory,
scavenging and
parasitism. Some of the organic matter eaten by heterotrophs, such as
sugars, provides energy. Autotrophs and heterotrophs come in all sizes, from
microscopic to many
tonnes - from
cyanobacteria to
giant redwoods, and from
viruses and
bdellovibrio to
blue whales.