<P> A food web (or food cycle) is a natural interconnection of food chains and a graphical representation (usually an image) of what - eats - what in an ecological community . Another name for food web is 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 . </P> <P> Charles Elton pioneered the concept of food cycles, food chains, and food size in his classical 1927 book "Animal Ecology"; Elton's' food cycle' was replaced by' food web' in a subsequent ecological text . Elton organized species into functional groups, which was the basis for Raymond Lindeman's classic and landmark paper in 1942 on trophic dynamics . Lindeman emphasized the important role of decomposer organisms in a trophic system of classification . The notion of a food web has a historical foothold in the writings of Charles Darwin and his terminology, including an "entangled bank", "web of life", "web of complex relations", and in reference to the decomposition actions of earthworms he talked about "the continued movement of the particles of earth". Even earlier, in 1768 John Bruckner described nature as "one continued web of life". </P>

What is meant by food chain and food web