Four trophic levels

In a four-level trophic system, if it is subject to trophic cascade, we might expect that the abundances of the top carnivores and the herbivores are positively correlated, as are those of the primary carnivores and the plants. This is precisely what was found in an experimental study of the food web in Eel River, northern California (Figure 20.4a) (Power, 1990). Large fish (roach, Hesperoleucas symmetricus, and steelhead trout, Oncorhynchus mykiss) reduced the abundance of fish fry and invertebrate predators, allowing their prey, tuft-weaving midge larvae (Pseudochironomus richardsoni) to attain high density and to exert intense grazing pressure on filamentous algae (Cladophora), whose biomass was thus kept low.

Support for the expected pattern also comes from the tropical lowland forests of Costa Rica and a study of Tarsobaenus beetles preying on Pheidole ants that prey on a variety of herbivores that attack ant-plants, Piper cenocladum (though the detailed trophic interactions are slightly more complex than this - Figure 20.5a). A descriptive study at a number of sites showed precisely the alternation of abundances expected in a four-level trophic cascade: relatively high abundances of plants and ants associated with low levels of herbivory and beetle abundance at three sites, but low abundances of plants and ants associated with high levels of herbivory and beetle abundance at a fourth (Figure 20.5b). Moreover, when beetle abundance was manipulated experimentally at one of the sites, ant and plant abundance were significantly higher, and levels of herbivory lower, in the absence of beetles than in their presence (Figure 20.5c).

On the other hand, in a four-level trophic stream community in New Zealand (brown trout (Salmo trutta), four levels can act like three

Brown Trout Food Webs Wiki

Figure 20.4 Three examples of food webs, each with four trophic levels. (a) The absence of omnivory (feeding at more than one trophic level) in this 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 (c) from a terrestrial Bahamanian 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 effects on intermediate predators. (After Power, 1990; Flecker & Townsend, 1994; Spiller & Schoener, 1994, respectively.)

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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, cm2 per 10 leaves. (After Letourneau & Dyer, 1998a, 1998b; Pace et al., 1999.)

predatory invertebrates, grazing invertebrates and algae), the presence of the top predator did not lead to reduced algal biomass, because the fish influenced not only the predatory invertebrates but also directly affected the activity of the herbivorous species at the next trophic level down (Figure 20.4b) (Flecker & Townsend, 1994). They did this both by consuming grazers and by con-straining the foraging behavior of the survivors (McIntosh & Townsend, 1994). A similar situation has been reported for a four-level trophic terrestrial community in the Bahamas, consisting of lizards, web spiders, herbivorous arthropods and seagrape shrubs (Coccoloba uvifera) (Figure 20.4c) (Spiller & Schoener, 1994). The results of experimental manipulations indicated a strong interaction between top predators (lizards) and herbivores, but a weak effect of lizards on spiders. Consequently, the net effect of top predators on plants was positive and there was less leaf damage in the presence of lizards. These four-level trophic communities have a trophic cascade, but it functions as if they had only three levels.

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  • angelica
    What trophic level would a bettle be?
    1 year ago

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