It also became evident, however, that while birds reduced the abundance of one of the limpet species, Lottia digitalis, as might have been expected, they increased the abundance of a second limpet species (L. ![]() In addition, the birds freed up space for algal colonization through the removal of barnacles (Figure 20.3).įigure 20.2 Variation in the pelagic ecosystem of the Great Salt Lake during three periods that differed in salinity. Overall, limpet biomass was much lower in the presence of birds, and the effects of bird predation cascaded down to the plant trophic level, because grazing pressure on the fleshy algae was reduced. Glaucous-winged gulls (Larus glaucescens) and oystercatchers (Haematopus bachmani) were excluded by means of wire cages from large areas (each 10 m2) in which limpets were common. But when salinity declined from above 100 g l-1 to 50 g l-1 in 1985, Trichochorixa invaded and Artemia biomass was reduced from 720 to 2 mg m-3, leading to a massive increase in the abundance of phytoplankton, a 20-fold increase in chlorophyll a concentration and a fourfold decrease in water clarity (Figure 20.2).Īnother example of a trophic cascade, but also of the complexity of indirect effects, is provided by a 2-year experiment in which bird predation pressure was manipulated in an intertidal community on the northwest coast of the USA, in order to determine the effects of the birds on three limpet species (prey) and their algal food (Wootton, 1992). Normally, the zooplankton, dominated by a brine shrimp (Artemiafranciscana), are capable of keeping phyto-plankton biomass at a low level, producing high water clarity. There, what is essentially a two-level trophic system (zooplankton-phytoplankton) is augmented by a third trophic level (a predatory insect, Trichocorixa verticalis) in unusually wet years when salinity is lowered (Wurtsbaugh, 1992). The Great Salt Lake of Utah in the USA provides a natural experiment that illustrates a trophic cascade. ![]() In a food chain with four links, a top predator may reduce the abundance of an intermediate predator, which may allow the abundance of a herbivore to increase, leading to a decrease in plant abundance. It occurs when a predator reduces the abundance of its prey, and this cascades down to the trophic level below, such that the prey's own resources (typically plants) increase in abundance. The indirect effect within a food web that has probably received most attention is the so-called trophic cascade (Paine, 1980 Polis et al., 2000).
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