Food Webs (or Food Chains)

Food webs are depictions of the feeding relationships that exist among species within an ecosystem, indicating flows of energy and biomass between trophic levels. Although a food web is a more complex conception than a linear food chain, it remains a relatively static and binary depiction: species either interact or they don't. Despite these limitations, food webs are useful conceptual tools, providing insights into the organization of communities and the interactions among different species within them.

Food webs are organized into trophic (or feeding) levels. Species are categorized as either producers or consumers. Producers or autotrophs, literally “selffeeders,” constitute the first trophic level—those species that synthesize their own food through processes of photosynthesis or chemosynthesis and includes most plants, algae, phytoplankton, and some species of bacteria. Photosynthetic species use carbon dioxide, water, and the light energy of the sun to produce sugar molecules as well as oxygen. Thus, these species are responsible for producing the relatively oxygen-rich atmosphere that exists on earth today. Chemosynthetic species produce carbohydrates via several different possible chemical pathways. Some use the chemical energy bound up in inorganic molecules (such as hydrogen sulfide), to produce carbohydrates from carbon (derived from carbon dioxide or methane), and oxygen.

Consumers, also termed heterotrophs, feed on other organisms, both living as well as dead. Those that eat the latter are decomposers or detritus eaters. All organisms eventually enter the detrital food web after they die and decompose or are consumed and their remains excreted. Herbivores, species that consume autotrophs, occupy the second trophic level. Carnivores are species that feed upon herbivores or other carnivores, with those that feed on herbivores occupying the third trophic level and those that feed on carnivores occupying higher trophic levels.

Food webs encompass a number of dynamic and interconnected food chains. A species may be an omnivore, consuming both producers and consumers, eat consumers from different trophic levels, and be preyed upon by a variety of species at different trophic levels, including fellow members of its own species. Some producers, such as Sundew and Venus Flytrap, supplement their primary production with the consumption of animals. There are also temporal dimensions to food webs. Predator – prey relationships may change both seasonally and through the life history of a species. For example, adult herring prey upon juvenile cod or eggs and may, in turn, be preyed upon by adult cod. In addition, some species, such as cod, cannibalize younger members of their own species.

Terrestrial and aquatic food webs are generally separated in space; however, some species facilitate cross-habitat fluxes of nutrients and detritus. Seabirds and some of the large vertebrate predators (humans, pinnepeds, polar bears) link marine and terrestrial food webs, transferring nutrients of marine origin to the land. Similarly, the migratory Pacific salmon grows to maturity in marine waters and returns to spawn and die in the freshwater environments where it was born, thereby connecting freshwater, marine, and terrestrial food webs. Salmon carcasses provide food for a wide variety of terrestrial animals, including bald eagles and bears, and are an important source of marine-origin nutrients in some freshwater streams.

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