Variations

Not only are food webs in different environments composed of different organisms with differing levels of connectivity, but such networks also change over time. Early stages of succession are dominated by organisms with high resource utilization rates, fast growth, and high rates of propagule production. Productivity is high at first, but as later colonists arrive and establish themselves, systems settle down to a more efficient functional regime, consisting of more trophic specialists that grow more slowly and have lower fecundity. Biomass builds up during the later stages, and the important chemicals can be recycled. Thus one of the important ways of identifying recently disturbed ecosystems is to document the properties of local trophic structure.

A recent breakthrough in food web theory involves the realization that adjacent local ecosystems actually interact with each other, comparable to the population systems making up their internal working parts. It is well known that adjacent systems share migratory species or predators with large hunting ranges, but it has been demonstrated only recently that significant amounts of chemicals and energy can be imported from neighboring systems, and that this may be more widespread than was once suspected. Food webs that extend over regional scales could be used in these cases to map the transfer of energy and materials, revealing the functional identity of regional ecosystems.

A final word is in order about the effects of humans on food webs, which have been pervasive. Human activity and by-products degrade and destroy trophic organization in many ways. Elimination of habitats of the dominant species, overharvesting of producers and consumers (especially top predators), and the intended or unintended introduction of exotic plants and animals all do great damage to food webs. If ecosystems survive at all, they may be less diverse, less complex in terms of trophic connections, and consist of monotonous copies of other systems similarly despoiled and invaded. Species at all levels of food pyramids have been affected, and few natural systems are likely to remain truly pristine as the biodiversity crisis continues to unfold.

—William Miller III

See also: Bacteria; Carbon Cycle; Carnivora; Coevo-lution; Communities; Ecological Niches; Ecosystems; Positive Interactions; Protoctists; Succession and Successionlike Processes

Bibliography

DeAngelis, Donald L. 1992. Dynamics of Nutrient Cycling and Food Webs. London: Chapman and Hall; Elton, Charles. 2001 [1927]. Animal Ecology. Chicago: University of Chicago Press; McNaughton, Samuel J., and Larry L. Wolf. 1979. General Ecology, 2d ed. New York: Holt, Rinehart and Winston; Paine, Robert T. 1966. "Food Web Complexity and Species Diversity." American Naturalist 100: 65-75; Polis, Gary A., and Kirk O. Winemiller, eds. 1996. Food Webs: Integration of Patterns and Dynamics. New York: Chapman and Hall.

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