Food webs and pyramids are the most popular ways of representing the trophic structure (that is, the feeding relationships and related functional groupings) of ecosystems. Food webs show the connections between population systems at different trophic levels and within the same level of organization; food pyramids illustrate the proportions of the different feeding groups. Both are representations of the functional aspects of ecosystems involving transfers of energy and materials between the component populations—the connections that support life in species assemblages. Webs and pyramids can be used to characterize local systems, subdivisions of local systems isolated for emphasis, or larger, more inclusive eco-logic systems at the regional scale. Along with composition and diversity, trophic structure is one of the fundamental properties of ecologic systems of all kinds.
Food chains are linkages embedded within more extensive webs, teased out to emphasize relationships of particular interest or to highlight especially important organisms. An example is an intertidal marine food chain, which could consist of a bottom-dwelling green alga that imports the energy for this part of the overall food web by converting sunlight into chemical energy through photosynthesis. A grazing gastropod harvests this resource (representing the next position in the chain). A dominant species of crabs patrolling the intertidal zone preys on the gastropod (a third level), and occasionally a seagull preys on the crab and exports the energy away from this limited system. The algae-gastropod-crab-bird food chain is but one component or pathway in a much more complex web or network of trophic interactions within the same ecosystem. Such relationships may be stable over long periods of time, vary seasonally, or take place only during a certain stage in what is called ecologic succession; and the chain may be repeated regionally in numerous local ecosystems, or could be unique to one place.
It must be remembered that the flow of energy and movement of chemicals through ecosystems are closely coupled. Organisms that convert sunlight into available energy for an entire system rely on the presence of critical nutrients, especially nitrogen and phos-phorous—the universal fertilizers of ecologic systems that depend on photoautotrophy. Although chemical movements vary greatly among systems, in terms of available pools versus storage sinks, cycles versus unidirectional transfers, and the specifics of chemical utilization by component organisms, in general terms chemicals can be recycled (mass balance considerations apply), whereas energy is dissipated (the laws of thermodynamics are at work) as the transformed chemical energy passes through food webs.
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