Predation and Food Webs - Freshwater Ecology: Concepts and Environmental Applications - page 402

invertebrates decreased. Thus, even though the predator did not directly
kill the crayfish, the nonlethal effects on the crayfish and its food species
were significant.
TROPHIC LEVELS, FOOD WEBS, AND FOOD CHAINS
Food webs can be simplified into trophic levels for ecological analyses.
These levels were described briefly in Chapter 7. Traditionally, levels in-
clude primary producers (photosynthetic organisms), decomposers or de-
tritivores (consume dead organic material), primary consumers (herbivores
or grazers), and secondary consumers (eat primary consumers). This sim-
plification has been criticized because numerous organisms feed on several
trophic levels. Thus, some have suggested that species can be assigned frac-
tional trophic levels (e.g., primary producers are assigned level 1, herbi-
vores level 2, predators of the herbivores level 3, and animals that eat half
herbivores and half primary producers level 2.5). Others suggest that food
chains are too simple, and we should only consider food webs. However,
the idea of trophic levels has proven particularly important in analysis of
the effects of top consumers on organisms lower in the food web. These
cascading trophic interactions will be discussed next.
THE TROPHIC CASCADE
In 1880, Lorenzo Camerano postulated that animals can control the
biomass of lower trophic levels and diagrammed how a carnivore can con-
trol herbivore populations and this can have a positive influence on plants
(Camerano, 1994). The idea that predation at the upper level of food
chains can have a cascading effect down through the food chain is called
the trophic cascade . Control of primary production by abiotic factors such
as nutrients or light is called bottom-up control. Control of primary pro-
ducers from the upper levels of the food chain is referred to as top-down
control. These ideas entered modern ecology in a key paper by Hairston et
al. (1960) that argued that plants dominate terrestrial systems because
predators keep herbivores in check. These arguments were extended to the
idea that even numbers of links in food chains (Fig. 19.8) will lead to
higher biomass of primary producers (Fretwell, 1977). In this section, I dis-
cuss how the trophic cascade may apply in lakes, streams, wetlands, and
groundwaters.
In a highly cited paper, Brooks and Dodson (1965) documented that
increased predation pressure by a planktivorous fish led to much smaller
zooplankton species and that this resulted in increases in chlorophyll. The
idea that the trophic cascade was operating in lakes received some subse-
quent attention (Arruda, 1979; Shapiro, 1979), but a series of papers by
Carpenter, Kitchell, and coworkers (Carpenter and Kitchell, 1987; Car-
penter et al., 1987) stimulated abundant research on the trophic cascade
in lakes (Power, 1992). This research was stimulated because managers
may be able to use biomanipulation of lakes to improve water quality by
controlling the fish community. The general approach is to encourage the