2.3 FOUR TROPHIC LEVELSPREYING ON PHEIDOLE ANTS THAT PREY ON A VARI...

20.2.3 Four trophic levels

preying on Pheidole ants that prey on a variety of herbivores

that attack ant-plants, Piper cenocladum (though the detailed trophic

In a four-level trophic system, if it is subject to trophic cascade,

interactions are slightly more complex than this – Figure 20.5a).

A descriptive study at a number of sites showed precisely the

we might expect that the abundances of the top carnivores and

alternation of abundances expected in a four-level trophic cascade:

the herbivores are positively correlated, as are those of the

relatively high abundances of plants and ants associated with

primary carnivores and the plants. This is precisely what was

low levels of herbivory and beetle abundance at three sites, but

found in an experimental study of the food web in Eel River,

northern California (Figure 20.4a) (Power, 1990). Large fish (roach,

low abundances of plants and ants associated with high levels of

herbivory and beetle abundance at a fourth (Figure 20.5b). More-

Hesperoleucas symmetricus, and steelhead trout, Oncorhynchus mykiss)

over, when beetle abundance was manipulated experimentally at

reduced the abundance of fish fry and invertebrate predators,

allowing their prey, tuft-weaving midge larvae (Pseudochironomus

one of the sites, ant and plant abundance were significantly higher,

richardsoni) to attain high density and to exert intense grazing

and levels of herbivory lower, in the absence of beetles than in

pressure on filamentous algae (Cladophora), whose biomass was

their presence (Figure 20.5c).

On the other hand, in a four-level

thus kept low.

four levels can

Support for the expected pattern also comes from the tropical

trophic stream community in New

act like three

lowland forests of Costa Rica and a study of Tarsobaenus beetles

Zealand (brown trout (Salmo trutta),

Large

Brown

Lizards

fish

trout

Figure 20.4 Three examples of food

webs, each with four trophic levels.

Web-spinning

Fish fry

(a) The absence of omnivory (feeding

Predatory

spiders

and

at more than one trophic level) in this

insects

predatory

North American stream community means

it functions as a four-level trophic system.

On the other hand, web (b) from a New

Zealand stream community and web

Herbivorous

(c) from a terrestrial Bahamanian

arthropods

Tuft-weaving

chironomids

community both function as three-level

trophic webs. This is because of the strong

direct effects of omnivorous top predators

on herbivores and their less influential

Algae

Seagrape

Filamentous

effects on intermediate predators. (After

algae

shrubs

Power, 1990; Flecker & Townsend, 1994;

Spiller & Schoener, 1994, respectively.)

1000

60

beetles

100

40 40

20

10

Herbivores

0

1

Herbivory

4

3

2

Leaf area

Ants

Site

trees

Figure 20.5 (a) Schematic representation of a four-level food chain in Costa Rica. Pale arrows denote mortality and dark arrows a

contribution to the consumer’s biomass; arrow breadth denotes their relative importance. Both (b) and (c) show evidence of a trophic

cascade flowing down from the beetles: with positive correlations between the beetles and herbivores and between the ants and trees.

(b) The relative abundance of ant-plants ( ), abundance of ants ( ) and of beetles ( ), and strength of herbivory (

) at four sites. Means and

standard errors are shown; the units of measurement are various and are given in the original references. (c) The results of an experiment

at site 4 when replicate enclosures were established without beetles ( ) and with beetles ( ). Units are: ants, % of plant petioles occupied;

herbivory, % of leaf area eaten; leaf area, cm

per 10 leaves. (After Letourneau & Dyer, 1998a, 1998b; Pace et al., 1999.)

reported for a four-level trophic terrestrial community in the

predatory invertebrates, grazing invertebrates and algae), the

presence of the top predator did not lead to reduced algal

Bahamas, consisting of lizards, web spiders, herbivorous arthro-

pods and seagrape shrubs (Coccoloba uvifera) (Figure 20.4c) (Spiller

biomass, because the fish influenced not only the predatory

& Schoener, 1994). The results of experimental manipulations

invertebrates but also directly affected the activity of the her-

indicated a strong interaction between top predators (lizards) and

bivorous species at the next trophic level down (Figure 20.4b)

herbivores, but a weak effect of lizards on spiders. Consequently,

(Flecker & Townsend, 1994). They did this both by consuming

the net effect of top predators on plants was positive and there

grazers and by con-straining the foraging behavior of the survivors

was less leaf damage in the presence of lizards. These four-level

(McIntosh & Townsend, 1994). A similar situation has been

trophic communities have a trophic cascade, but it functions as

Certainly, the accumulating evidence seems to support a

if they had only three levels.

pattern of overt community-level cascades in simple, especially

wet, communities, and much more limited cascades embedded

within a broader web in more diverse, especially terrestrial, com-