Food Webs Essay ⋆ Environment Essay Examples ⋆ EssayEmpire

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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 “self-feeders,” 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.

The abundance of higher trophic level species is ultimately dependent on the productivity of autotrophs. A large proportion of the energy, as much as 90 percent, is lost in each trophic level transfer (as uneaten waste, feces, heat, consumer energy, and respiration, and so on). Because of this, there are limits on the absolute number of trophic levels found within an ecosystem. While the first and second laws of thermodynamics dictate that a substantial amount of bottom-up (nutrient driven) regulation of food webs exists, there is some evidence that top-down (predator dominated) control of food webs is or was important in some ecosystems.

Human Impact on Food Webs

There are few, if any, food webs, on earth that have not been significantly affected, or even dominated, by human activities. Humans impact food webs in two general ways: creating deficits by extracting organisms from ecosystems; and by producing subsidies, concentrating and transporting wastes from one system to another. Agricultural production is significant in both respects. Plant-based agricultural systems replace natural systems, substituting monocultures for greater species diversity and extracting most of the primary production from the system for human consumption. The application of pesticides disturbs food webs, creating secondary outbreaks and resurgences of the targeted populations. On the other hand, industrial animal husbandry concentrates manure and other wastes, which if not managed adequately, may pollute aquatic systems, overfertilizing them and ultimately leading to eutrophic conditions.

Commercial and recreational fishing and hunting constitute significant impacts to some ecosystems. Removing predators from ecosystems can lead to trophic cascades, changing food web structure and dynamics, perhaps irreversibly. Pollutants, too, move through the trophic levels of food webs as animals eat and in turn are eaten by others through processes of bioaccumulation and biomagnification, causing problems for organisms at high trophic levels, and especially in northern latitudes, including birds of prey, marine mammals, and babies fed human breast milk. Global climate change may pose the greatest challenge to the stability of food webs, altering growing seasons, changing the geographical ranges of species, creating unpredictability in predator-prey relationships and ultimately threatening many species with extinction.

Bibliography:

Kenneth T. Frank, Brian Petrie, Jae Choi, William C. Leggett, “Trophic Cascades in a Formerly Cod-Dominated Ecosystem,” Science (v.308/10 June, 2005);
Henry F. Howe and Lynn Westley, Ecological Relationships of Plants and Animals (Oxford University Press, 1988);
Daniel Pauly, Villy Christensen, Johanne Dalsgaard, Rainer Frose, and Francisco Torres Jr., “Fishing Down Marine Food Webs,” Science (v.279/6 February, 1998);
Stuart Pimm, Food Webs (Chapman and Hall, 1982);
Stuart L. Pimm, Food Webs, 2nd (University of Chicago Press, 2002);
Gary Polis, Mary E. Power, Gary R. Huxel, eds., Food Webs at the Landscape Level (University of Chicago Press, 2004).