<P> Linkages connect to nodes in a food web, which are aggregates of biological taxa called trophic species . Trophic species are functional groups that have the same predators and prey in a food web . Common examples of an aggregated node in a food web might include parasites, microbes, decomposers, saprotrophs, consumers, or predators, each containing many species in a web that can otherwise be connected to other trophic species . </P> <P> Food webs have trophic levels and positions . Basal species, such as plants, form the first level and are the resource limited species that feed on no other living creature in the web . Basal species can be autotrophs or detritivores, including "decomposing organic material and its associated microorganisms which we defined as detritus, micro-inorganic material and associated microorganisms (MIP), and vascular plant material ." Most autotrophs capture the sun's energy in chlorophyll, but some autotrophs (the chemolithotrophs) obtain energy by the chemical oxidation of inorganic compounds and can grow in dark environments, such as the sulfur bacterium Thiobacillus, which lives in hot sulfur springs . The top level has top (or apex) predators which no other species kills directly for its food resource needs . The intermediate levels are filled with omnivores that feed on more than one trophic level and cause energy to flow through a number of food pathways starting from a basal species . </P> <P> In the simplest scheme, the first trophic level (level 1) is plants, then herbivores (level 2), and then carnivores (level 3). The trophic level is equal to one more than the chain length, which is the number of links connecting to the base . The base of the food chain (primary producers or detritivores) is set at zero . Ecologists identify feeding relations and organize species into trophic species through extensive gut content analysis of different species . The technique has been improved through the use of stable isotopes to better trace energy flow through the web . It was once thought that omnivory was rare, but recent evidence suggests otherwise . This realization has made trophic classifications more complex . </P> <P> The trophic level concept was introduced in a historical landmark paper on trophic dynamics in 1942 by Raymond L. Lindeman . The basis of trophic dynamics is the transfer of energy from one part of the ecosystem to another . The trophic dynamic concept has served as a useful quantitative heuristic, but it has several major limitations including the precision by which an organism can be allocated to a specific trophic level . Omnivores, for example, are not restricted to any single level . Nonetheless, recent research has found that discrete trophic levels do exist, but "above the herbivore trophic level, food webs are better characterized as a tangled web of omnivores ." </P>

Where are producers located on the food chain