Disturbance and Food Chain Length in Rivers

Abstract

“Are trophic cascades all wet?” asks Strong (1992), speculating that trophic cascades (sensu Paine (1980) and Carpenter et al. (1985)) might be exclusive features of aquatic communities because they are simple and poor in species relative to terrestrial communities. River food webs are as complex and speciose as food webs in any other ecosystem (Winemiller, 1990). Nonetheless, field experiments in rivers have documented chains of strong interaction that link predators through consumers to primary producers (e.g., Wootton and Power (1993), Power (1990a), and Power et al. (1985)). In this chapter, we discuss factors that appear to control the length of chains in river food webs. Here, we define the length of functionally significant food chains as the number of feeding guilds (trophic levels) that are alternately released and suppressed following removal of a top predator, with the criterion that changes in biomass at higher trophic levels must affect biomass of primary producers. We will use the impact of higher trophic levels on plants to distinguish threads of weak interactions from chains of strong interactions in food webs. For example, if removing predators increased herbivore biomass, but this herbivore response did not measurably change plant abundance, we would not consider this interaction to be part of a functionally significant food chain We do not necessarily advocate this as a general definition of food chain length, but use it to explore specific questions in this chapter. In general, it is important to distinguish when trophic levels are counted by functional (top-down) criteria, which require potential population regulation (Fretwell, 1977; Oksanen et al., 1981), versus descriptive (bottom-up) criteria, based on the number of energy transfers inferred from diet analyses (Cohen, 1978) or isotopic fractionation (Kling et al., 1992). These different criteria are likely to identify chains of different length in the same web.