2.2 TROPHIC CASCADES120GRAZING RATE90THE INDIRECT EFFECT WITHIN A F...

20.2.2 Trophic cascades

120

Grazing rate

90

The indirect effect within a food web that has probably received

60

30

% day–1

most attention is the so-called trophic cascade (Paine, 1980; Polis

0

et al., 2000). It occurs when a predator reduces the abundance

Chlorophyll a

of its prey, and this cascades down to the trophic level below,

20

such that the prey’s own resources (typically plants) increase in

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abundance. Of course, it need not stop there. In a food chain with

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mg m–3

four links, a top predator may reduce the abundance of an inter-

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mediate predator, which may allow the abundance of a herbivore

to increase, leading to a decrease in plant abundance.

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Water transparency

The Great Salt Lake of Utah in the USA provides a natural

8

6

experiment that illustrates a trophic cascade. There, what is essen-

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tially a two-level trophic system (zooplankton–phytoplankton) is

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Secchi (m)

augmented by a third trophic level (a predatory insect, Trichocorixa

1973 1985–86 1986–90

verticalis) in unusually wet years when salinity is lowered

Year

( Wurtsbaugh, 1992). Normally, the zooplankton, dominated by

a brine shrimp (Artemia franciscana), are capable of keeping phyto-

Figure 20.2 Variation in the pelagic ecosystem of the Great

plankton biomass at a low level, producing high water clarity.

Salt Lake during three periods that differed in salinity.

But when salinity declined from above 100 g l

⁻¹

to 50 g l

⁻¹

in 1985,

(After Wurtsbaugh, 1992.)

Trichochorixa invaded and Artemia biomass was reduced from 720

to 2 mg m

⁻³

, leading to a massive increase in the abundance of

It also became evident, however, that while birds reduced the

phytoplankton, a 20-fold increase in chlorophyll a concentration

and a fourfold decrease in water clarity (Figure 20.2).

abundance of one of the limpet species, Lottia digitalis, as might

Another example of a trophic cascade, but also of the complex-

have been expected, they increased the abundance of a second

limpet species (L. strigatella) and had no effect on the third, L. pelta.

ity of indirect effects, is provided by a 2-year experiment in which

bird predation pressure was manipulated in an intertidal community

The reasons are complex and go well beyond the direct effects

on the northwest coast of the USA, in order to determine the effects

of consumption of limpets. L. digitalis, a light-colored limpet, tends

to occur on light-colored goose barnacles (Pollicipes polymerus),

of the birds on three limpet species (prey) and their algal food

whilst dark L. pelta occurs primarily on dark Californian mussels

(Wootton, 1992). Glaucous-winged gulls (Larus glaucescens) and

(Mytilus californianus). Both limpets show strong habitat selection

oystercatchers (Haematopus bachmani) were excluded by means of

for these cryptic locations. Predation by gulls reduced the area

wire cages from large areas (each 10 m

²

) in which limpets were

covered by goose barnacles (to the detriment of L. digitalis), lead-

common. Overall, limpet biomass was much lower in the pre-

ing through competitive release to an increase in the area covered

sence of birds, and the effects of bird predation cascaded down

to the plant trophic level, because grazing pressure on the fleshy

by mussels (benefiting L. pelta). The third species, L. strigatella,

is competitively inferior to the others and increased in density

algae was reduced. In addition, the birds freed up space for algal

colonization through the removal of barnacles (Figure 20.3).

because of competitive release.

Birds present Birds excluded

400

200

Number of limpets (m–2)L. digitalis L. pelta L. strigatella L. digitalis L. pelta L. strigatella

75

50

25

Percentage cover

Barnacles Mussels Barnacles Mussels

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

Fleshy algal species

birds on intertidal animals (means ± SE are

shown). (After Wootton, 1992.)