Indirect Effects of Herbivores Modify Predicted Effects of Resources and Consumption on Plant Biomass

Abstract

Much recent research on community and food web dynamics has focused on the relative importance of consumptive (top-down) processes and resource (bottom-up) limitation, which operate vertically within food webs (McQueen, et al. 1986; Power, 1992; Strong, 1992; Hunter and Price, 1992; Rosemond et al., 1993; Osenberg and Mittelbach, this volume). For convenience of analysis, and to generalize results, organisms are typically lumped into trophic levels (e.g., plants, herbivores, predators). Both resources and consumers can have effects on the biomass of trophic levels via strong vertical effects through food webs. The existence of trophic cascades, in which the consumptive effects of top predators indirectly influence lower trophic levels, indicates the profound effects of vertical top-down control (Dayton, 1985; Power, 1990; Power et al., 1985; Carpenter et al., 1987; Elner and Vadas, 1990). In addition to top-down effects, bottom-up effects of growth-limiting resources to plants (e.g., inorganic nutrients, irradiance) can cascade up (sensu Leibold (1989)) food webs to affect plant biomass of higher trophic levels (Vanni, 1987; Hershey et al., 1988; Hart and Robinson, 1990; Rosemond et al., 1993).