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Trophic cascades occur when predators in a food web suppress the abundance or alter the behavior of their prey, thereby releasing the next lower trophic level from predation (or herbivory if the intermediate trophic level is a herbivore). For example, if the abundance of large piscivorous fish is increased in a lake, the abundance of their prey, smaller fish that eat zooplankton, should decrease. The resulting increase in zooplankton should, in turn, cause the biomass of its prey, phytoplankton, to decrease. This theory has stimulated new research in many areas of ecology. Trophic cascades may also be important for understanding the effects of removing top predators from food webs, as humans have done in many places through hunting and fishing activities. A Top Down Cascade is a trophic cascade where the food chain or food web is disrupted by the removal of a top predator, or a third or fourth level consumer. On the other hand, a bottom up cascade occurs when a primary producer, or primary consumer is removed, and there is a reduction of population size through the community. An example could include Paine's study from the University of Washington, where he removed several species in different plots along the North-Western United States coast line, and realized that ''Pisaster'', a common starfish, when removed, created a top-down cascade which involved a surge in barnacle and mussel populations, but a decrease in the populations of chitons, limpets, and whelks. This led to the conclusion that ''Pisaster'' was a keystone species in that food web. == Origins and theory == Aldo Leopold is generally credited with first describing the mechanism of a trophic cascade, based on his observations of overgrazing of mountain slopes by deer after human extermination of wolves.〔 Leopold, A. (1949) ("Thinking like a mountain" ) in "Sand county almanac"〕 Nelson Hairston, Frederick E. Smith and Lawrence B. Slobodkin are generally credited with introducing the concept into scientific discourse, although they did not use the term either. Hairston, Smith and Slobodkin argued that predators reduce the abundance of herbivores, allowing plants to flourish.〔Hairston NG, Smith FE, Slobodkin LB (1960) "Community structure, population control and competition". ''American Naturalist'' 94:421-425〕 This is often referred to as the green world hypothesis. The green world hypothesis is credited with bringing attention to the role of top-down forces (e.g. predation) and indirect effects in shaping ecological communities. The prevailing view of communities prior to Hairston, Smith and Slobodkin was trophodynamics, which attempted to explain the structure of communities using only bottom-up forces (e.g. resource limitation). Smith may have been inspired by the experiments of a Czech ecologist, Hrbáček, whom he met on a United States State Department cultural exchange. Hrbáček had shown that fish in artificial ponds reduced the abundance of zooplankton, leading to an increase in the abundance of phytoplankton.〔Hrbáček J, Dvořakova M, Kořínek V, Procházkóva L (1961) "Demonstration of the effect of the fish stock on the species composition of zooplankton and the intensity of metabolism of the whole plankton association". ''Verh. Internat. Verein. Limnol.'' 14: 192-195〕 Hairston, Smith and Slobodkin argued that the ecological communities acted as food chains with three trophic levels. Subsequent models expanded the argument to food chains with more than or fewer than three trophic levels.〔Oksanen L, Fretwell SD, Arruda J, Niemala P (1981) "Exploitation ecosystems in gradients of primary productivity". ''American Naturalist'' 118:240-261〕 Lauri Oksanen argued that the top trophic level in a food chain increases the abundance of producers in food chains with an odd number of trophic levels (such as in Hairston, Smith and Slobodkin's three trophic level model), but decreases the abundance of the producers in food chains with an even number of trophic levels. Additionally, he argued that the number of trophic levels in a food chain increases as the productivity of the ecosystem increases. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Trophic cascade」の詳細全文を読む スポンサード リンク
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