Trophic cascades

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). 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. Of course, it need not stop there. 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.

The Great Salt Lake of Utah in the USA provides a natural experiment that illustrates a trophic cascade. 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). Normally, the zooplankton, dominated by a brine shrimp (Artemiafranciscana), are capable of keeping phyto-plankton biomass at a low level, producing high water clarity. 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).

Another 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). 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. 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. In addition, the birds freed up space for algal colonization through the removal of barnacles (Figure 20.3).

250 200 150 100 50 0

60 40 20 0

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Salinity

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Trichocorixa density

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Density of grazing Artemia

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1973

1985-86 Year

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Figure 20.2 Variation in the pelagic ecosystem of the Great Salt Lake during three periods that differed in salinity. (After Wurtsbaugh, 1992.)

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. strigatella) and had no effect on the third, L. pelta. The reasons are complex and go well beyond the direct effects of consumption of limpets. L. digitalis, a light-colored limpet, tends to occur on light-colored goose barnacles (Pollicipes polymerus), whilst dark L. pelta occurs primarily on dark Californian mussels (Mytilus californianus). Both limpets show strong habitat selection for these cryptic locations. Predation by gulls reduced the area covered by goose barnacles (to the detriment of L. digitalis), leading through competitive release to an increase in the area covered by mussels (benefiting L. pelta). The third species, L. strigatella, is competitively inferior to the others and increased in density because of competitive release.

Figure 20.3 When birds are excluded from the intertidal community, barnacles increase in abundance at the expense of mussels, and three limpet species show marked changes in density, reflecting changes in the availability of cryptic habitat and competitive interactions as well as the easing of direct predation. Algal cover is much reduced in the absence of effects of birds on intertidal animals (means ± SE are shown). (After Wootton, 1992.)

Birds present

Birds excluded

L. digitalis L. pelta L. strigatella

Barnacles

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ra 4

L. digitalis L. pelta L. strigatella

Barnacles Mussels

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Fleshy algal species
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